How to size a heat pump
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.
How do you calculate heat losses?
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.