System Design and Ground Source

Sizing a boiler is pretty straightforward, isn't it? If it's a traditional radiator system with a cylinder, you just tot up the outputs of all the rads (or make an educated guess!), add a few kilowatts for the cylinder, and that's it. Of course, that's assuming the rads are the right size to start with.

Sizing for a heat pump is a bit different - for several reasons:

• If it's oversized, the heat pump won't be as efficient as it could be
• If it's undersized, the house will be cold and your client will be complaining
• You have to size the ground arrays to match, and undersizing those will cause problems
• Heat pumps don't (always) modulate like boilers, so they have to be sized more carefully
• It's expensive to put it right if you get it wrong
• The MCS scheme dictates how to size them to avoid problems
• Often heat pumps are fitted into new properties with new heating systems, so you have to design the whole system from scratch
• Heat pumps work at lower temperatures than boilers do, so you have to account for this when you design the system.

No wonder people get confused. It's a minefield, right? So let's simplify things a bit.

First things first - there are some rules we're bound by, that are dictated by MCS and laid out in the Installer Standards known as MIS3005. You can find these here.

Basically, we must size the heat pump to handle 100% of the building's "peak load". Peak load just means "worst case scenario" - so the amount heat which is needed to heat the whole building, on the coldest day of the year.

It is possible to have something called a "Bivalent" system, which is where a heat pump is supplemented by another heat source (normally a boiler). We'll cover these in a seperate module because it's a big topic!

You size a heat pump to meet the heating load of the house. It's almost the other end of the scale from sizing a boiler for an existing system! First you work out what the heat losses of each room are. That way, you can size emitters for each room. Then you can add up the heat loss of all the rooms, and that tells you what size heat pump you need. Remember, with a heat pump you are "trickling" heat into the property at a rate that matches the rate that heat is lost - because the flow temperature from a heat pump is comparitively low, you run a heat pump for longer than a boiler.

It's a detailed and long winded calculation. Essentially, the heat losses are worked out on a room by room basis. The external temperature used depends on where your property is, and the internal temperatures depend on what sort of room you're trying to heat. Generally, bedrooms are heated to 18°C, living areas to 21°, and bathrooms to 22°. 

Transmission of heat via the walls, windows, floor and roof is calculated by using their U-values. These values are a measurement of how thermally efficient a material is. Also calculated is how much heat is lost through ventilation. Air change rates depend on the type of room and whether the ventilation is natural, or mechanical.

You can do this calculation yourself if you really want to, and if you have time - there is a tool available here.

Alternatively, get Kensa to do it for you. We'll need a set of plans, plus elevations (and sections if you have them). if you have a SAP Worksheet, that's really useful because it provides us with the U values we need.

You cannot use SAP or an EPC to calculate a heat pump size, because SAP is not an "elemental" sizing tool - it's a measure of predicted carbon savings.

Right, now you know the loads for each room. And you also know, because we've covered it in the Heat Pump Basics module, that the lower the flow temperature coming from a heat pump, the better they work, and the better the efficiency. You also know that the better the efficiency figure, the more the householder gets paid under the RHI. So your next goal is to design a system based around a flow temperature that's as low as you can sensibly go.

Generally speaking, domestic heating systems in the UK use radiators, underfloor heating, or less often, fan coils.

Underfloor heating is a preferred option in many new builds because it's a lifestyle choice - no ugly radiators on the wall interfering with where you can put the furniture! But underfloor is a really good partner for a heat pump because underfloor heating, like a heat pump, is made to work at low flow temoeratures.

Radiators (and fan coils) are perfectly OK to use with a heat pump. A lot of people use these upstairs and UFH downstairs because it's cheaper. This makes perfect sense, just remember that the radiators have got to be sized around the lower flow temperature. They must be big enough to match the heating load for that room, even when running at (for example) 40°.

As a rough rule of thumb this means radiators are around 2.5 times bigger than they would be for a boiler.

Many radiator catalogues these days will give you rad outputs at the correct delta T for a heat pump. Underfloor designs can be done to your stipulated temperature, but often, your suppliers will design around a standard 45° flow anyway, depending on the type of system.

ADDITIONA READING
HEG
DHDG
TIS SHEETS SYSTEM DESIGN
LINK TO HEAT LOSS TOOL

 

A buffer tank is a store of heating water. The same water that's going round the heating system. Don't get it confused with drinking water; this is not a hot water cylinder!

You need one because they help to maintain system efficiency. How?

Starting up a heat pump takes a fair bit of energy. They are best left to run slow and steadily for as long as possible. A heat pump that short-cycles on and off won't be running at its best - this exaggerates normal wear, and it isn't efficient.

If your heating system is satisfied really quickly (perhaps it's not that cold, or maybe some zones are closed off) then the heat pump would cycle. The buffer helps to smooth this out and minimise the cycling.

It also provides a minimum flow across the heat pump. It takes time to distribute heat around a heating system - if there's nowhere for it to go, the heat pump just can't perform. So you use a buffer to provide this - either that, or you need to leave around 30% of the heating system uncontrolled (i.e. no zone valves, no actuators - bathrooms and hallways can be good for this).

What Kensa suggest is that you treat the buffer a bit like a bypass valve on a traditional system. Use two connections on the buffer. Some manufacturers will use four connections - a flow and return out from the heat pump into the buffer, then a flow and return from the buffer to the heating system. This works - but why spend the time heating up a store of water prior to heating up the system? You could argue that the buffer is stored heat ready to go, the next time there is a call for heat - but with the thermal mass of an underfloor system, you're unlikely to notice a drop off in room temperature anyway.

SEE ADDITIONAL READIG TIS SHEET ON BUFFERS

DAZ'S NEW BUFFER LAYOUTS

USE NEW ANIMATION

 

Heat pumps work on low temperatures. So how can you get a heat pump to charge up an unvented cylinder to 65°?

There are two answers to that.

1) Don't.

2) Do it with a high temperature heat pump.

Number one seems a bit contentious, but all we mean is that you let the heat pump do most of the work. A normal heat pump will get the water in that cylinder up to around 55°. That's perfectly usable - bathing temperature is 45°. But for legionella prevention, use an immersion to bump this up to 65° for an hour once a week. That's all that is needed to kill off the legionella bacteria.

Number two - Kensa make something called a Hybrid heat pump. These units contain two compressor circuits, one running normal refrigerant and giving flow temperatures of up to 55°. The other uses a different refrigerant that allows the heat pump to generate flow temperatures of up to 65°.

So far, so good.

Can I keep my old cylinder? Probably not.

For the same reasons that heat emitters have to be oversized, so does the coil inside the hot water cylinder. A lower flow temperature from the heat pump means that more surface area is needed to get the water in the cylinder up to temperature. Roughly speaking this is about 0.2m² per kW of heat pump. When you consider that most cylinders use a coil that's about 1.5m², you can see that once you go over about 8kW, you're going to need a specially designed cylinder.

Coil surface area isn't the only factor though - again, you have to maintain a minimum flow rate across the heat pump for it to work, and if the tappings on the cylinder, or the internal diameter of the cylinder coil is too small, then you will never get the flow rate you need - the frictional resistance is too great.

This is why Kensa supply unvented hot water cylinders that are specifically made to work with heat pumps. They have larger coils with the correct diameter. If you want us to cover the MCS on your project, you need to use our cylinder - because we know that they work! - or provide evidence that the one you have chosen meets the specifiations of the equivalent Kensa model.

MIS3005 suggests you use a figure of 45 litres of hot water per person, per day.

If you look at the number of bedrooms in the property, then add one, you can multiply this by 45 to get a requirement.

We find it easier to use 50, rather than 45, because most cylinders are supplied in volumes divisible by 50 anyway.

So let's say you have 3 bedrooms. Add one, that's 4. 4 x 50 litres = 200l. You need a 200l cylinder (or as close as you can get!).

SEE ADD READING TIS SHEET

Adding solar to a ground source system is entirely possible and generally, pretty straightforward.

All of our unvented cylinders are available with an extra coil, so that you can easily add solar thermal. It's that simple - pick the twin coil version of the cylinder you need. Speak to the solar panel manufacturer to ensure that you're getting the right number of panels for your roof conditions and cylinder size.

Solar PV will also work well - solar PV will never "power the heat pump", but there's no reason why you shouldn't use PV power to run the immersion in your cylinder (using an Immersun or similar). Any unused power is then sold back to the grid provider.

Generally we don't recommend using solar to contribute towards space heating; this means using a thermal store, and controlling the heat pump when using one of these can get awkward. Besides, when you are most likely to need extra space heating is when your solar system is at its lowest performance.

Normally when you have a hot water outlet from a cylinder, it's just that - an outlet. There is no return back to the cylinder.

But when the cylinder is located a long way from the taps, you can add a return to it and have a long loop. This means you can pump the water in this loop, which minimises the delay between opening the tap and hgetting hot water.

In traditional systems, the secondary return used to go straight back into the cylinder itself, and indeed many cylinders have a tapping for just this purpose. Kensa recommend you don't use it - you are putting cold water straight back into the top of a nicely heated cylinder. You end up ruining the stratification in the cylinder because everything mixes up, rather than the warmest water being at the top.

We also recommend that you use a flow boiler or Willis heater (a small 1kW electrical inline heater) or trace heating wire to keep the secondary return warm near the outlets. Use a timer to ensure that this doesn't remain on 24/7, otherwise electricity bills will be high.

A bivalent system is one that has two (or more) sources of heat, for example a heat pump and a boiler.

To be truly bivalent, each heat source has to be connected to the same system.

For example:

A large house where half the property is heated by a heat pump. The older part of the house has an oil-fired boiler running the heating. The two heating systems are seperate, they are not plumbed together at any point. They may as well be in different houses. This is NOT a bivalent system.

Another example:

A farmhouse, renovated, but the large vaulted living area is hard to heat. The heat pump heats everywhere via underfloor heating, but to boost the living area, a wood burning stove has been fitted. The stove is a standalone appliance, no back-boiler, and only heats the living area. This is NOT a bivalent system.

How about this one?

The owner of a large barn conversion wanted a heat pump, but the peak load was 16kW. The DNO would not allow the connection of anything larger than 12kW.

So, a thermal store was installed alongside the heat pump, as well as an LPG boiler. The heat pump is sufficent to heat the property unless it is colder outside than 4°C.

Both the boiler and the heat pump feed the thermal store, which in turn feeds the heating circuit. An external sensor switches the heat source over, from the heat pump to the boiler, if the external temperature drops below 4°.

This IS a bivalent system.

It has a "bivalent point" of 4 °C.

It's important that the "backup" heat source (in this case the boiler) is sized to handle 100% of the heating load - there would be no point having the boiler only make up the last 4 kW.

MIS3005 tells us that a bivalent system has to have an "integrated" control system, such as the external sensor described here. If the control systems for the boiler and heat pump were distinctly seperate, then the system would not be covered by MCS.

Another point of note is that this system would have to be metered if the client wished to claim RHI payments. This is because not all of the heat produced qualifies as "renewable". So the amount of heat produced by the heat pump has to be measured - the system can no longer rely on the "deemed" figure from the EPC.

 

Well, given the explanation of bivalent systems in the previous section, saying "no you can't" would be counter-productive!

You do have to be a bit careful though.

Take that previous scenario of an LPG boiler alongside a heat pump, both feeding a thermal store. That boiler has a flow temperature of around 70°C. The return back to the boiler would be around 50°C.

If the heat pump is set with a return temperature of only 45°, what will happen when the boiler has been running? Ignore the external sensor for a moment.

The boiler would run, and heat the thermal store back up to 70°. The heat pump's return sensor is never going to drop below 45°! So it's permanently satisfied. That heat pump will make no contribution to the heating system. So the boiler will continue to do its job - and cost the customer a fair bill in LPG.

Hence you have to be clever with controls. That external sensor will help - it will turn the boiler off completely, and once the temperature in the store drops down, eventually the heat pump will kick back in.

This is why using an uncontrollable source of heat (say, the back boiler on a wood burning stove) is inadvisable with a heat pump.

The next reason to be careful with thermal stores is hot water.

There are plenty of thermal stores on the market that contain a small coil. Mains cold water flows through the coil, picking up heat from the primary water in the store, and there you have it - mains pressure hot water, at a good flow rate! Just what everyone wants.

Except there's a catch. Remember the lower flow temperatures heat pumps work at? If your thermal store is designed to work, and give hot water, with a primary temperature that's more closely associated with a boiler (70° +) then there's a pretty good chance that the coil in that store isn't going to have sufficient surface area to get a good heat transfer.

You are having to heat water at a mains-water flow rate - 8 to 10 litres a minute - and heat it up to the householder's expectations, which is going to mean 50° at least. Your heat pump will only get the thermal store up to 55° (unless you use a high temperature variant). At the speed it flows through the coil, there's no way that mains water will pick up 40° or more from the thermal store. It's just not possible unless the coil is made substantially larger, or unless the flow rate is slowed down (which the householder won't want).

Kensa have tested units like this and we've found that they don't perform well with standard heat pumps. Occasionally the conditions are such that they will operate within their design parameters with a High Temp unit, but we advise you to avoid them and consider alternative systems instead.