Heat Pump Specialist

This module contains some slightly more advanced knowledge, and practical "how-to" sections on a variety of subjects. It's intended for information only, so there are no assessment questions. If there is anything you would like to see included here, please let us know!


Cooling with a ground source heat pump

Can it be done? How?

Cooling can be achieved with a ground source heat pump but there are a few things to remember.

There are two types of cooling:

  • Active cooling

The compressor circuit is essentially run backwards, so that chilled water is fed to the heating system in order to provide cooling. Heat gleaned from the property is dumped back into the ground. In order to run the compressor in this way, modifications must be made to it, so we need to know that you want to do this at the point of order.

  • Passive cooling

The heat pump is essentially taken out of the loop here - the cold heat transfer fluid from the ground arrays is circulated around an additional cooling heat exchanger, which is interfaced with the existing heating system. Basically you are cooling the heating system.

Points to bear in mind:

Cooling is generally done best by blown air systems. Cooling is all in the name of comfort, and if you consider it, a cool draught over your skin is far easier to detect than a cold floor.

Even using a simple desktop fan can make you feel more comfortable, and there is no chilling of the air occurring here  - so with passive cooling, certainly it will be more effective with heat emitters such as fan convectors or air handling units. 

Passive cooling with underfloor might only knock 1 or 2 degrees off the ambient room temperature. - and you need an extra heat exchanger.

Cooling, when used with an underfloor system, can cause condensation to form.


What about MCS and RHI?

Things can get a little sticky with MCS and the RHI regarding cooling.

Firstly, if you change a heat pump so that it is capable of active cooling, you are altering the heat pump to the point that it is considered a whole new model. So any unit that was previously MCS approved without cooling would not be approved any longer.

Secondly, if you want to run a unit for cooling (whether it's active or passive), any hours of cooling the heat pump does are ineligible for the domestic RHI. This means that to count the hours when the heat pump is heating, you have to add metering. If that cooling isn't hugely effective (perhaps you are using a radiator based system), then you're paying for metering, and not getting any real benefit from the cooling either.


What does MCS say?

MIS 3005 (the MCS installer standard) does actually lay out guidelines for systems used for cooling.

As long as the system is predominantly used for heating, then MCS would still apply.

In the Non-Domestic RHI, there is a clause that says that heat from the ground arrays must be "naturally generated" - for example you cannot directly use warm water that is a by-product of an industrial process or from space cooling as a heat source, then expect to get better RHI payments because your inlet temperature is so good.

This does not cover heat that's stored in the ground.

It is proposed (as at end May 2016) that ground source and water source systems will be eligible for non-domestic RHI when using "waste heat" or "recovered heat",  provided they use some heat from the ground as well.

At present, if this waste/recovered heat is stored in the ground or in ground/surface water, the system is eligible for non-domestic RHI.

The proposal suggests that the heat will no longer have to be circulated into the ground first. This makes sense - any extra circulation means loss of heat and therefore efficiency, so it's sensible to remove this step.

How to: repressurise the ground array (for homeowners)

make a little video! do one for the system too.

Wiring, Controls and Weather Compensation

What controls can I fit?

You can fit any controls to a Kensa heat pump, as long as they give you a 240v "run" signal/enable.

So, this can be traditional heating controls (timer and room stats), it can be 230v controls from your underfloor system, or it could be advanced heating controls from a third party. 

You can even get remote control switching systems that use a mobile phone or the internet to switch the heating on and off, or change the thermostat settings.

Any controls can be fitted

Do Kensa units have weather compensation?

All Kensa units are shipped with an external sensor, and are capable of weather compensation. It's just turned off in the unit's software.

Why do we do this? Well, Kensa believe that for the majority of installations, weather comp has actually very little benefit, at least for the first year's life of the system.

A lot of systems where heat pumps are fitted use underfloor heating at least on the ground floor, which is invariably in screed. One of the advantages of underfloor is that it produces a low, steady heat. If you turn off the heat pump totally, you wouldn't notice the underfloor slab starting to cool - perhaps not even for 24 hours.

So why use a weather compensation device that turns the heat pump down or off, perhaps several times in one day?

Add to this, new buildings take time to settle and dry out. So the heating load in real life may be higher over the first 12 months or so of the building's life. You may find that the system needs recalibrating later on for this reason.

Most underfloor systems are controlled on a room by room basis using room thermostats. Why then use a system that changes the heat pump's flow temperature according to external temperature, when every room could have a different requirement anyway? It makes more sense to use one or the other, rather than the two systems conflicting with each other.

Weather compensation can be confusing for an end user too - when they just need one room a few degrees warmer just for a few hours, they end up making fundamental adjustments to the way the system runs, when all they really needed was to turn up a room stat!

It's even possible that you may have a room stat calling, and an UFH circulating pump running, when you are getting very very little from the heat pump because of the weather compensation. That's not efficient - it takes an age to see any noticeable change in temperature, and all that time, the pump's running, using energy.

Weather comp has its place. You get good ratings under SAP for using it and in the right building, on the right system, it can be a "fit and forget" solution. We just urge its use with caution.


Where to wire in the weather compensation external sensor.

Note: This will not activate weather comp; you will have to turn it on via the controller. See the heat pump manual for instructions on how to do this.

Wiring diagrams


Wiring for a single zoned radiator system

Wiring for a system with underfloor and radiators, plus DHW

Wiring for a system with underfloor and radiators, plus DHW - using a three channel programmer

Controls for a bivalent system

Electrical phases and start-up currents


In the UK, most domestic homes use single phase power. Most commercial and industrial premises use three-phase. Some industrial sites (primarily in the North-East) use something called Split Phase, which is occasionally mistaken for three phase.


It's a rough generalisation but down the average suburban street will run three phase cabling:

  • Phase 1
  • Phase 2
  • Phase 3
  • Neutral
  • Earth

So 5 wires altogether. The norm is to keep the phases balanced by connecting every third house the the same phase - so for example house number 40 will be on P1, house 41 will be on P2, house 43 will be on P3, etc. This sometimes accounts for why when you have a power cut at home, you peek outside and some houses still have the lights on!

Between any phase and Neutral, you'll see 230 volts. But between each phase, you will find 400v. This is due to the the way power is generated right back at its source. The extra voltage is a boon to distributors because the more voltage you have, the lower the amperage needs to be. If the whole country's supply was distributed on 240v cabling, it would have to be ridiculously thick cable to cope!

Single phase is simply phase, neutral, earth. That phase could be any one of the 3; it doesn't matter, and you will probably never know (or need to!).

Split phase is a bit like "cheating" 3 phase. You get 4 wires - Phase 1, Neutral, Phase 1A (that's the odd bit) and an Earth.

Phase 1A is just a copy of Phase 1. The waveform is not offset. 

So between either phase to Neutral, you'd see 230v. Between Phase 1 and Phase 1A you would see 460v.

In this type of system, it makes sense to use a heat pump made for single phase, and wire just the heat pump to P1, and everything else in the house to P1A.

There is no reason why you should not fit a single-phase heat pump to a 3-phase supply in the same manner.


Start-up current and soft-starts

Start-up currents are a pretty important aspect of heat pump installation.

When heat pumps first hit the market, there were no soft starts. The compressors started up "Direct On-Line" - this meant you got a big spike in current for the first split second of running, while the motor in the compressor swung into action. This was noisy, and could cause issues with "flicker" - flickering lights in the property, or in properties wired to the same phase.

This spike would only last for a couple of cycles in a second (remember UK supply is 50 hertz, or 50 cycles per second).  So this wouldn't trip breakers - it's historically the reason why heat pumps used type D breakers (they're less sensitive).

As technology improved though, electronic soft starts were added and the startup spike, whilst still present, was smoothed out. This is the reason why many Kensa units will run happily on a type C breaker.


On three-phase units, the start-up current is shared across phases, so what would ordinarily be a 60 amp start-up is split down to 20A per phase. remember the quoted start-up currents for Kensa twin compressor units have got to account for one compressor already running, and one starting.

This is because the compressors never start simultaneously, so the worst case for current draw is one starting, and one running.

Kensa's Plantroom models come with soft start modules.

On the 3-phase Compact models it is an optional extra (being 3-phase, the start-up load is split across phases anyway). Without it, the 3-phase units are simply DOL (Direct on-line).

The Shoebox models don't have a soft start at all - the load is so small it's not required.

Running Currents

Why does the running current change depending on the flow temperature? Isn't the compressor a fixed speed type?

Compressors in all Kensa units are fixed speed, as they are for most ground source units. Generally speaking modulating compressors are most often used on air-source heat pumps where the incoming air temperature does vary greatly.

However, as the flow temperature of a heat pump changes, so does the temperature  - and therefore the pressure - of the refrigerant gas in the compressor circuit.

This includes the compressor. Any change in pressure means a change in the load on the compressor - so to do the same amount of work, the current draw is higher. hence the running current changes, depending on the heat pump's running conditions.

Heat pump fault codes and their meanings

Fault codes

Display flashes tP:

Low system pressure (ground loops or heating system)

May be due to ground pipework relaxing. Check gauges and repressurise. If this continues to occur check for leaks.


Display flashes LP:

Low refrigerant pressure (cold side).

Can occur if ground arrays are under filled/have air in/frozen, if pump is not spinning, or no circulation (blockage).


Display flashes HP:

High refrigerant pressure (hot side).

Occurs when no circulation around heating circuit - failed pump, system plumbed incorrectly, pump airlocked.



Freeze protection activated. 

No flow around ground arrays - bleed any air from pump, check any filters fitted. Check antifreeze is at correct concentration.



Sensor error

Contact Kensa Technical Support




Sensor error

Contact Kensa Technical Support



Sensor error

Contact Kensa Technical Support




Sensor error

Could be weather compensation sensor - check wiring.

Contact Kensa Technical Support



Other codes


Heating return temperature



Temperature of heat transfer fluid returning from arrays




Temperature of heat transfer fluid going out to arrays



Refrigerant pressure (bar)

Blank display

The display of the heat pump will be blank unless there is a call for heating or hot water. This is entirely normal.

If this happens whilst you are commissioning the unit, check that you have a call for heat or hot water, or ensure that the link between terminals 1 and 4 (heating) is still in place.

How to: Commission a heat pump

Getting started

Your first job is to get the ground arrays filled and purged of air, then pressure tested. There's a section on how to do this job, elsewhere in this training module.

Once you've done that, you can circulate the glycol mixture for a while to let it mix - just run the heat pump's internal pump. Do not turn on the main breaker yet though!

If you need help, our Technical Support team will happily commission the unit with you by phone. Ideally, give them a call with 24 hours' notice so that a member of the team is able to find the project paperwork beforehand, and give you their fullest attention. Call 01392 350128 and speak to Darren, our commissioning engineer, between 9 and 5 weekdays.

Health and Safety:

Ensure that the area you are working in is safe for you and others.

Make sure it's well lit and well ventilated. Keep your work area tidy to guard against slips, trips and falls.

Clean up any spilled antifreeze mixture immediately.

Bear in mind general manual handling guidelines.


If you are going to do the commissioning yourself without support over the phone, then you need to make some notes. 

In the back of the installation manual is a commissioning record. Make sure you complete it as you go along. Enter the details regarding the property, ground arrays and the system before you start.


Make sure all the breakers on the heat pump are OFF.

Make sure arrows have been marked on the manifold(s) indicating flow direction.

Is the cover off the heat pump?

Make sure both of the pump impellors are free to spin (take the cap off and test).

Is the heat pump installed on a stable, level surface? Does everything look safe?

Are the controls fitted correctly? Does all the wiring look safe?


Switching the heat pump on

If you are happy with all the checks, switch power on to the heat pump at the main consumer unit. 

Ensure you have a call for heating or hot water, or that the link between terminals 1 and 2 is still in place.

Switch on the 4 amp MCB in the heat pump. If it's a single compressor unit, there will be one 4A breaker. If it's a twin, there will be two breakers, one per compressor circuit.

This 4A breaker is for the pumps only. Don't touch the 25A breaker(s) just yet.

The load side pump should run, and you should see a drop on the system pressure gauge (about half a bar). If this doesn't happen, ensure you definitely have 240v on the incoming supply.

What do you see on the display?

A temperature reading Good - that's the return temp and it's what you should be seeing. Proceed with commissioning.
Nothing, it's blank Do you have a call for heat or hot water? If not, the display will stay blank. If your controls aren't working, put a link between terminals 1 & 2.
A flashing LP fault Low Pressure switch has tripped (ground side). Check the ground side pump runs and bleed out any air. If you have fitted filters, check they are not blocked.
"A1" is flashing Ground side heat exchanger probably frozen, or air in system. Switch off immediately and bleed any air from the heat pump using the Schraeder valves. Cover the valve with a rag to prevent spillages. Double check the pump is not stuck.
"E1", "E2", "E3" or "E4" is flashing This is a sensor error. Have a quick look inside the heat pump and make sure everything appears to be connected properly and there are no loose wires. E4 is related to the weather comp sensor. If you can't see anything wrong, call Kensa Technical Support on  0845 680 4328.
"HP" is flashing High Pressure Switch has tripped - normally low flow or no flow around heating system. Ensure system has open zones and that the pump is running. Bleed the pump.





Check for air in the pumps

When you first switch on the 4 amp MCB, the load side (heating) pump should run.

The pressure gauge should drop by about half a bar. Do this 3 times.

If the pressure on the gauge does not drop, this normally means that there is air in or near the pump. Shut off the 4 amp MCB and use the Schraeder valve at the top of the heat exchanger to bleed any air out. Cover this up with a rag to prevent spillage.

Schraeder valve air bleeds

After the pump has run for about 5 seconds, you should hear a loud "clunk".

This is the contactor for the ground side pump. 

You should see the pressure gauge for the ground side increase by about half a bar.

Again if this doesn't happen, turn off the MCB and bleed out any air using the Schraeder valve.

Change the antifreeze alarm setpoint

Heat pump controller (the display inside the heat pump is the main "brain", this is simply a slave  display)

The antifreeze alarm point on all Kensa units is set to 0°C at the factory (we do this to help prevent possible freezing on commissioning).

When you are happy that the correct amount of antifreeze has been added, and you have tested to concentration, then you can change this setpoint to -5°C.

If you don't change this setpoint  then the heat pump may show an A1 alarm during cold weather, and it will turn itself off.

Important - if you are working on a twin compressor unit, you will need to change this setpoint on both sides.

Only change this setpoint when the correct quantity of antifreeze has been added, and you have checked the concentration.

  • On the upper, external controller, press and hold SET until 0 is displayed. You can do this process on the internal controller if you prefer - on this one, press and hold SEL and PRG together.
  • Press and hold the "UP" button (^) until 66 is displayed - let go when you see 66.
  • Press SEL. The display should show "S-P" (this means "set parameter").
  • Press SEL again, and you will see "-/-"
  • Press SEL again.
  • Press the UP arrow again until "-/04-" is shown.
  • Press SEL
  • Press the DOWN arrow button to change from 03 to 00.
  • Press SEL..
  • ...Then press PRG.
  • Press the UP arrow button until "-d-" is displayed
  • Press SEL
  • Press  the UP arrow button until "d03" is displayed
  • Press SEL again. The display should now show the antifreeze setpoint temperature.
  • Now press and hold the DOWN arrow button to change from 0 to -5. Follow this with a press of the SEL button (again!).
  • Press PRG
  • Press the DOWN arrow button until "-/-" is shown, and press SEL
  • Press UP until "-/04-" is shown and press SEL.
  • Press the UP arrow to change from 00 to 03, and hit SEL to confirm.
  • Press PRG twice until "S-P" is shown.
  • Finally press and hold PRG until the display returns to normal.
  • Record the setpoint on your commissioning sheet.

Change the heating return temperature setpoint

This parameter is factory set at 30°C. With the usual heat pump delta T of 5°C, this would give you a flow temperature to your heating of 35°C.

This may be fine for a screeded underfloor system, but generally you will need to increase this temperature to 40° to 45° depending on the underfloor design, or possibly more for radiators (50° to 55° but again, this depends on how they have been sized).

For a system that is mixed, use the higher temperature (the underfloor manifold will blend the water down to the appropriate temperature).

  • On the external display, press and hold SET until "0" is displayed. If you want to, you can do this on the lower controller - press and hold SEL and PRG together
  • Using the UP arrow button, go to "11" then press SEL.
  • The display should show "S-P" for Set Parameter.
  • Press and hold SEL until "-/-" comes up.
  • Press the DOWN arrow button until "-r-" is shown.
  • Press SEL to confirm and you should see "r01" in the display.
  • Press the UP arrow until you see "r03", and confirm by pressing SEL.
  • The display should show the return heating water setpoint. Change this with the UP or DOWN arrows, then press SEL to confirm it.
  • Press PRG twice until "S-P" is shown.
  • Now press and hold PRG until the display returns to normal.
  • IMPORTANT - For twin compressor units, do this on both sides. Set the return temperature one degree higher on the right hand controller to ensure staggered starts of the compressors.
  • Record the setpoint(s) on your commissioning sheet.

Switch on the compressor

Now that you have changed the relevant settings and everything looks in order, you can turn on the compressor.

This is probably the most critical part of commissioning, so keep a close eye on your readings and if anything's wrong, turn off the MCB immediately.

Turn on the 25A MCB (on 3 phase models, switch them all on). If you are working on a twin, do each compressor one at a time.

If the compressor is unduly noisy, switch it off.

On 3-phase units, it's possible that the 3 phase wiring has been wired back to front somewhere (not necessarily at the heat pump either!) - this will make the compressor run backwards, and it will sound like a bag of spanners. It should purr like a fridge.

If it's anything other than this, call us.

Run the compressor for 5 minutes or so. If you see any errors, make a note of the code and then check this in the Fault Codes section of this module.

If you get no fault codes, you can go ahead and start checking readings.

Take sensor readings ("b readings")

The "b readings" are measurements taken by various sensors in the heat pump. They give an idea of how the heat pump is performing, and are helpful for fault diagnosis. Record all your readings on the commissioning report.

If the compressor is running happily and there are no signs of problems, you can check the readings via the controller.

The first one to check is b01, which is the return heating water temperature. This is a live measurement, not the change of setpoint you did earlier on.

  • Press and hold SEL until you see "-/-".
  • Press the UP arrow until "-b-" is shown.
  • Press SEL and you will see "b01". Press SEL to select it.
  • This will display the heating return temperature. 
  • If you have a flow restriction (a blockage, air or a stuck pump) then you will see this temperature climb really quickly.

Next, check b02, which is the temperature of the antifreeze solution coming from the ground arrays.

  • From where you were, just press SEL once.
  • Then press the UP arrow until the display shows b02. Press SEL to go into the reading.
  • This should read between -5°C and 15°C. If the reading is around room temperature, the probe may have come loose - double-check.
  • If this temperature drops rapidly, it is likely that there is air in the ground arrays - turn off the compressor! You may see it drop to -5° and then give an A1 alarm if you aren't quick enough to catch it.  In this instance, go back and re-purge the arrays.

The next reading to check is b03, which is the temperature of the antifreeze solution going back to the ground arrays.

  • From where you were, just press SEL once.
  • Then press the UP arrow until the display shows b03. Press SEL to go into the reading.
  • If b03 is showing a reading of below the antifreeze setpoint (which you will have changed to -5°C), but the heat pump has not yet gone to an A1 alarm, then go back and check that the antifreeze setpoint has actually been correctly set.

The last reading to check is b04. This is a measurement of the refrigerant pressure on the low side (the ground side) of the refrigeration circuit.

  • From where you were, just press SEL once.
  • Then press the UP arrow until the display shows b04. Press SEL to go into the reading.
  • The display will show the refrigerant pressure in bar. It should read between 2.6 to  4.6 bar. If it is too high, contact us. If it's very low (you may see an LP alarm as well) then this could be a symptom of something called "flat battery building" - the property is really cold and needs to be heated up slowly, one zone at a time.
  • If weather compensation is fitted, you will not be able to read this pressure.

Get the display back to normal by pressing PRG twice until S-P is shown, then press and hold PRG.


Enable weather compensation (optional)

Commission the heat pump fully before you add weather compensation - this will allow you to take the reading at b04 before unplugging the pressure sensor, because the weather compensating sensor uses the same connection.

Weather compensation can be a bit of a touchy subject - in some cases it can be a really useful feature. It needs to be correctly set up and the system and building themselves need to be stable.

We don't recommend the use of weather comp in brand new builds, because the building needs time to settle and stabilise.  Temperatures that are comfortable when the building is new may not be right once the building has dried out and settled down, a year or so later!

It can also be argued that weather comp is less useful on a system that is all screeded underfloor heating. This is because the thermal mass of the floor is so great that you'd never notice fluctuations in flow temperature over a short period of time. It can take a day or two for an underfloor system like this to get up to temperature, and the same to cool off, so a weather compensating system that is changing the heat pump's outlet temperature all the time is never going to be effective.

There's a section on weather comp and controls elsewhere in this module.

Where to find the weather compensation connection

To enable weather compensation:

  • Unplug the pressure sensor, and plug in the external weather compensation sensor (this needs to be fitted to a north facing wall, out of direct sunlight and away from heat sources like extractor vents).
  • Press and hold SET, and release when you see "0" on the display.
  • Press and hold the UP button until you get to "66", then let go. Press SEL to enter this parameter.
  • The display will show "S-P".
  • Press SEL again and you should see "-/-". Press SEL again.
  • Press the UP arrow until you see "-/04-", and press SEL.
  • Press the DOWN arrow, to change from 1 to 3. Press SEL.
  • Press PRG twice. Then press and hold PRG to get the display back to normal.

How to: Diagnose common system faults

The hot water is not very warm

There are many reasons for poor hot water.  In our experience, a common cause is when the return pipe from the buffer vessel is connected incorrectly into the system. This draws the hot water away from the cylinder and into the heating system. The heat pump then runs in hot water mode for a lengthy time trying to satisfy a never ending DHW demand. (see schematic)

Another common cause for poor DHW is how the secondary return pipe is connected. The secondary return pipe on a heat pump system needs to be connected back in to the hot outlet pipe at the cylinder. Usually on a gas or oil system this return pipe can connect into the bottom of the cylinder, however due to the lower heat pump flow temperature and need for stratification within the cylinder then it must go back in to the hot water outlet pipe only. (see fact sheet)

Check the type and specification of the unvented cylinder that's fitted.

If the coil size is too small, the hot water performance will be affected. Unvented cylinders not specifically designed for ground source heat pumps will not have a sufficiently large enough coil - minimum 3m² coil surface area.

Ensure that the hot water is not produced by a thermal store with inbuilt coil, for the same reasons

The heating is not very warm

Usually if all the design calculations have been carried out and the heating system is sized and installed correctly, then this problem should be fairly straightforward to rectify.

The most common causes are air in the system or poorly balanced systems. If a system is not set up and balanced correctly, the heat pump flow will always take the shortest route back, potentially not heating rooms that are further away.

There could be a simpler answer to a cold house.

It could be that at commissioning the flow temperature was set up to dry a new floor screed. Some installers like to heat the screed up slowly for the first time preventing excess cracking and movement in the floor. It may be that the flow temperature has been set too low for the designed system. This can be checked quite easily. 

The heating does not come on at all

Usually this is down to lack of call signal from your timeclock or programmable room stat.

Check the heat pump for a fault code. There are lots of safety features on the heat pump that will show a fault code before the machine causes itself any damage.

My electricity bills are very high

First let's eliminate other possible causes of high power usage:

  • Immersion heaters - make sure they are only used for an hour's legionella prevention every week, and not on 24/7
  • Zip taps - Qooker etc (taps that produce boiling water) aren't cheap to run
  • Secondary return systems on DHW - ensure these are properly controlled and that the flow boiler and bronze pump is not permanently on, and that the pipework is insulated with good quality insulation. Long runs of well insulated pipe will reduce heat lost to the atmosphere and hence reduce running costs.
  • Building stage - If the property is part built then don't be surprised if the bills are high! We have had this complaint from a customer using the heat pump for drying screed, when the windows hadn't yet been fitted. His building team were using power tools during the day and the heat pump was running as well - just because he wasn't using power, didn't mean nobody else was!
  • System issues - if the system has been plumbed incorrectly then there is a chance that supplementary heating sources are being used to make up any shortfall in DHW or space heating.

So, if everything else checks out OK and it looks like the heat pump might be the cause...

  • Check the heating return temperature is correctly set in the controller menu (b01). 
  • Is it stopping and starting a lot (short cycling)? Check for a correctly installed buffer cylinder, or that 25% of the system has been left open (uncontrolled, i.e. no zone valve/actuator/TRVs)
  • Is the heat pump too small? If it's undersized, it will be running at full tilt all day long to try to meet the demand. All you can do is try to establish how it was sized - look for a SAP, or full heat losses. Make sure that the property was well insulated to begin with! The efficiency of a heat pump is dependent on the outlet flow temperature. In a poorly insulated building a higher flow temperature is required, hence the running costs will increase. 
  • Are the ground arrays at fault? If the arrays were sized incorrectly, then the heat pump will eventually go to fault. The householder may simply have started to use alternative methods of producing heat - remember Kensa units do not have electrical immersions in them, but it's possible that the hot water cylinder does, and it's even possible that a back-up immersion has been added to the buffer tank.
  • Is it a controls issue? Have they been correctly wired in? Are the controls behaving as you would expect? If weather comp is fitted, try disabling it - explain this to the householder.  Poor control over your system can cause the heat pump to work harder than need be.  Separate programmable room thermostats will give you better control and reduce the heat pump run time, reducing your costs. 
  • Is the householder changing any settings? Make sure the end user knows to leave the heat pump alone once it's set up properly. Educate them in the use of the system controls, and make sure they know to change room temperatures with these, not the heat pump.
  • Are the heat emitters sized correctly? Poor design of the heat emitters will ultimately result in high running costs. The heat pump will run on for longer hours trying to heat a room from an undersized emitter.

How to: Fill, flush and purge a ground array

First - de-air the array

The first, and most vital part of filling the ground arrays with antifreeze, is to make sure that all the air has been purged.  The number-one installation issue that we see is air in the system.

Air can get trapped in slinkies and straight pipe, and is most likely to gather at high points in the system. Even vertical arrays (boreholes) should be de-aired.

Because of the length of the average ground loop (remember a 50m slinky is 300m of pipe), you need to ensure that the pump you use is powerful enough. The pump needs to be at least 1kW, and in addition must be able to produce a flow rate of at least 60 litres per minute at a pressure of 1 bar.

A normal cold water mains supply will not achieve this flow rate.

For most domestic installations, Kensa suggest the Clarke SPE1200SS pump, readily available from most plumber's merchants or directly from us. 

This pump comes ready to take 1" BSP fittings, but to connect to 28mm Speedfit on the slinkies, you will need to pick up two 1" male BSP to 28mm compression fittings.

You purge the air from the system using plain water. Don't put antifreeze in at this stage, it just makes life harder for the pump to circulate the liquid because of the increased viscosity.

Put a filter over the end of the pipe that returns water to the bin, to catch any debris from the pipework. An ordinary kitchen sieve will do.


  1. Ensure all valves are closed including the main valve to the heat pump (V1 above).
  2. Half fill the bin with clean water.
  3. Put a filter over the end of the pipe that returns water to the bin, to catch any debris from the pipework. An ordinary kitchen sieve will do.
  4. Now open the valves in order:
  • Slinky A return
    • Slinky A flow
      • Both purge ports

5. Hang onto the return pipe going into the bin, and start the purge pump .

6. If the water level in the bin doesn't drop, prime the pump again.

7. No water should be going through the heat pump or the other slinkies (you should have shut that valve in step 1).

8. Run the pump until air stops coming from the return pipe. It should be pretty obvious - if you leave the pipe below the waterline, the fluid in the bucket should be clear and undisturbed by bubbles, instead of foamy. This will take at least 10 minutes per slinky, probably longer.

9. When you have got all the air from the slinky, close the flow valve first, then the return.

10. Repeat for each slinky.

11. Once all the slinkies have been purged of air, open the valve to the heat pump in order to purge any air from here too. Hang on to that return pipe - air will be expelled from here pretty quickly.

12. Now reverse your connections and re-purge each slinky in the opposite direction, as per MCS instructions.

Pressure testing

Once all the air is out, you can proceed to fill the ground arrays with the correct glycol mix, and pressure test them. It's sensible to pressure test with water, rather than mixed glycol, because if there is a leak, you would only lose water (which is cheaper and safer than glycol!).

Pressure testing

The ground array should be leak tested in order to comply with BS EN805 (this is required under MCS). If this wasn't done when the arrays were installed, then do it now.

It is safer to pressure test with water, rather than air - and if you have spent ages getting all the air out of the system, why put it all back in again?

The pipework should be tested to more than 7.5 bar, but less than 10 bar. 

  1. First leave the arrays to rest for an hour.
  2. Then increase the pressure in the arrays to the test pressure. Use the isolating valves to hold the arrays at this pressure for ten minutes.
  3. Leave the arrays to rest for a further hour. Then take a pressure reading. This reading  will be a drop, but it should be no more than 30% of the initial test pressure.
  4. Now drop the pressure in the arrays quickly, by about 10% of whatever the initial test pressure was.
  5. Hold the arrays at this pressure for half an hour, and take readings at 10 minutes, 20 minutes and 30 minutes. The most pressure you should lose between these 3 measurements is 0.1 bar.
  6. If you lose no more than 0.1 bar on this final reading, the test is a pass.

Add the glycol

Adding the glycol

Now that the arrays have been de-aired and pressure tested, you can add the antifreeze.

Only do this once you are sure all of the air has been eliminated from the system.

Open the flow and return valves to one of the slinkies and run the purge pump. Empty out some water using the return pipe that goes back into the dustbin (you're getting rid of fresh water here so it can go into a drain - you are making room for the glycol). The water level in the bin needs to drop by 200 - 250 mm.

Make sure the suction pipe stays underwater, otherwise you will drag air into the system.

Turn off the pump and close the slinky valves.

Carefully pour a drum of antifreeze into the bin. Try not to spill it - it's slippery and hard to clean up. Wear gloves.

Allow the solution to settle for a few moments to get rid of bubbles.

Open valve V1 to the heat pump and start the purge pump, to circulate around the heat pump. Keep going until the fluid coming from the return pipe changes colour - this lets you know that all of the header, and the heat pump, has been filled with the glycol/water mix. This can take a few minutes depending on how long your header pipework is.

Close valve V1 and turn the purge pump off.

Now do the same thing for each slinky - draining off clean water and replacing it with a drum of antifreeze each time. Run the pump for 5-10 minutes each time.

Pressurise the system

Pressurising the system

Open all valves except the discharge purge connection.

Run the purge pump.

Watch the level of water in the bin - if this drops suddenly, there's air somewhere in the system still.

Running the purge pump will pressurise the system. Run it up to around 5 bar, and leave it there for 15 minutes or so just to double-check for leaks.

It's perfectly normal for the pipework to stretch a bit over time, so you may find that over the coming months, the pressure will fall. Anything under 2 bar, and it will need topping up.

Instruct the householder on how to do this, using the filling loop under the heat pump. Make sure that they know that if they have to do this regularly, they should contact you.

Sizing for water source

Some hints on open loop - flow rates andheat exchangers.


Firstly let's look at how much energy you can realistically get from the water.

Some basics: 

  • 1kWh = 3600kJ
  • Specific heat capacity of water = 4.2 joules per gram °C

Let's look again at our COP model:

In this instance, we're looking at water, not ground.

So we need 8kW - or 8kJ per second.

Now, you can't really extract anything from water colder than about 3°C - we'll see why later.  Let's say we have a spring fed stream we want to use. It's generally around 4°C (and our outlet temperature is 3°) and has a flow rate of about 2 litres per second.

2 l/s x 1° delta T (between the inlet and outlet) x 4.2 (constant - SHC of water) = 8.4kJ/second.

So we can see that from this example, this stream would be a suitable source for the above heat pump - we can extract 8.4 kJ per second from it, when we only really need 8. If the stream was warmer (and as a spring fed stream, it's likely to be), then we could extract even more energy.

Finding out an average temperature can be a nuisance. The householder has got to take the responsibility for this. There are river temperature logs available though, and OS maps will often tell you if a water source is spring fed.

Freezing in an open loop heat exchanger starts to happen at about 2°C. This is because parts of the heat exchanger will be at a lower temperature than this - you will be taking a reading from a sensor. Small ice particles begin to form on the face of the plate heat exchanger itself (where it might actually be 0° or less) and these act as "seeds". The ice spreads unbelievably quickly and within a matter of minutes there's slush inside that heat exchanger.

This is why you can't have a discharge temperature of under 3°C - because the heat exchanger begins to freeze at 2°!


Sizing an open loop heat exchanger:

You will need

Water side:

flow rate, source temperature, load (the first tow you'll have to measure or take the client's word for it, the second bit you can calculate as per the above)

Heat pump side:

flow  rate, temperature difference (always 4°K across the heat exchanger), load (you can get the flow rate from the heat pump manufacturer, the delta T is always the same, and the load is already known).

Controls and permissions

Freeze protection

The simplest option here is to set the heat pump up so that the antifreeze set point is programmed as 0° instead of the normal -5°.

Sometimes a flow switch and time delay relay are used to ensure that the heat pump only runs when there is definitely flow through the open loop side of the heat exchanger, but setting the antifreeze set point to 0° renders this redundant (don't forget that the 0° set point is measured in the heat pump, and the open loop heat exchanger will be around 4 ° warmer).

What is needed is a control method that only runs the open loop pump when it's needed, and stops it when there is no requirement for it to run - otherwise the pump will run 24/7 which uses a lot of energy and can cause over-abstraction (taking out more water than necessary).


If you "abstract" (remove) more than 20m³ of water per day from a water source, you will need an abstraction licence. In practice this is anything larger than around 12kW.

Licences are obtained from the Environment Agency. 

20m³ is 20,000 litres!

Water metering may also be required. The EA aren't always crystal clear on what is and isn't covered by the licence, so it's best for the client to speak directly to them.

You may also need a discharge licence.

Notes on pond mats

Pond mats can be useful for water that is cooler than a spring fed source.  Rivers aren't always that good for open loop systems because they tend to be really cold, thanks to field run off and lower flow rates,  so pond mats can be ideal - from a sizing point of view, they are a bit more forgiving than open loop, because there is a lot more surface area in a pond mat pound for pound!

The limiting factor for pond mats are surface area, and the thermal resistance of the plastic used to make them (which would limit their ability to collect heat to around 7kW per mat) and then more realistically, flow rate through them (which limits the real-world ability to more like  5.5-6kW per mat).

In stagnant water, you will need around the same surface area as you would if it were a slinky in land - around 250m² of pond surface area per pond mat.

Metering for heat pumps

Do I need a meter?

On 99% of domestic installations, no, there's no requirement for a meter.

You must, however, leave the installation "meter ready" - i.e. there must be a space on the primaries to fit a meter. You should really leave isolating valves there so that  fitting one retrospectively is easy, and there are prescribed lengths of pipework that have to be left - see

If the system is bivalent, then you will need to meter the heat coming from the heat pump, and the electrical input to the heat pump.

If the heat pump only heats part of the house, it will require metering (heat and power).

If any part of the heating distribution pipework is run outdoors, you will need to meter this to account for losses - unless the losses are under 3% of the total system load.

If the property is a second home, or holiday home, the system will need to be metered.

On projects where the owner is planning on applying for non-domestic RHI, meters will always be needed, because non-domestic RHI does not rely on deemed figures from an EPC.

Generally you will need a power meter and a heat meter for each heat pump.

Remember when designing your metering strategy that the whole idea is to calculate how much of the heat generated is from renewable sources, and also to calculate how efficient the system is (hence the requirement to measure electrical input).

What meter?

The normal electricity meter in a dwelling won't do it!

Power meters must be MID class A (or better).

Heat meters must be MID class 3 (or better).

Meters should be installed by an MCS Accredited installer. If you need to have metering fitted to a project covered by Kensa's MCS Umbrella scheme, your original installer can fit them and we will cover the MCS.

Ensure that meters are installed in accordance with manufacturer's instructions and OFGEM's guidance documentation.

Where does it go?

It varies! The meter location on the system will depend on what you need to measure. Normally you are trying to measure the renewable heat output so it's best to meter the flow of heat into a buffer or store, and before any tees come off the primary pipework.

Installing the meter should be done to manufacturer's instructions.

Remember for Non-Domestic RHI, you will need to include the metering arrangements on your system schematic.


Useful links

Metering for Domestic RHI

Factsheet - Do I need metering?

Essential guide to metering

When metering is needed on Domestic RHI

Guide to optional monitoring

Helpsheet - submitting meter readings

Metering for Non-Domestic RHI

Easy guide to heat pumps on non-domestic RHI

Easy guide to metering requirements for non-domestic RHI

Metering - Your ongoing responsibilities

Easy Guide to Periodic Data Submissions

Non-domestic RHI FAQ

Unusual system configurations

More than one hot water cylinder

More than one heat pump

​Two heat pumps, hot water on both units

Two heat pumps, hot water on one unit

Low loss headers

Cascading muliple units usng a low loss header

Multiple heat sources

Wiring for a bivalent system