What numbers do I need to size a heat pump?

[Selfbuildsarah]

We have received a predicted energy assessment and a BREL compliance report today. Will I find the numbers I need in there to help size a heat pump? I can’t see anything re heat loss amongst the headings. We are meeting with an installer tomorrow. Over heating stuff is to follow.

[SimonD]

10 hours ago, Selfbuildsarah said:

We have received a predicted energy assessment and a BREL compliance report today. Will I find the numbers I need in there to help size a heat pump? I can’t see anything re heat loss amongst the headings. We are meeting with an installer tomorrow. Over heating stuff is to follow.

 

SAP is about calculating energy use not about sizing the actual heating system or emitters (Radiators / Underfloor Heating / etc) . But the new standards for heat loss and system design do use aspects of SAP (e.g. designed or measured air permeability). You do need a heat loss calc and system design.

[Nickfromwales]

10 hours ago, Selfbuildsarah said:

We are meeting with an installer tomorrow

Shouldn't they be providing input? If they can't you need better installers!?!

[Selfbuildsarah]

Seeing them this morning. I just wanted to prep. We can’t find any installers in our area (see previous post). We have full SAP report (all 16 pages) which means nothing to me. Heat loss comes up a handful of times relating to different things, all different figures. I see other people quoting a single number … 4, 5 heat loss or such like so was expecting something similar to be on the report. This is worse than windows 😂

[SimonD]

29 minutes ago, Selfbuildsarah said:

Seeing them this morning. I just wanted to prep. We can’t find any installers in our area (see previous post). We have full SAP report (all 16 pages) which means nothing to me. Heat loss comes up a handful of times relating to different things, all different figures. I see other people quoting a single number … 4, 5 heat loss or such like so was expecting something similar to be on the report. This is worse than windows 😂

 

Yes, so they should be telling you that they'll produce a heat loss calculation based on your drawings and the SAP report you have. Then you should get a figure of a total output for the heat pump and a report telling you the heat load of each room and that will feed into a design of the underfloor heating (UFH), radiators, fan coils etc. If you're going for UFH on its own or with fan coils, keep an ear out for if they ask about cooling too, as that could be very handy for you.

[SteamyTea]

There is a bit of terminology to learn.

Energy is measured in MJ (megajoule). A joule is the standard unit for energy and when converted to base units of kilograms, metres and seconds is very small. Why the M for 1,000,000 is added at the front.

Now no one, apart from us nerdy scientists, use MJs, and domestic energy is metered and purchased in kWh. MJs can be multiplied by 0.0007778, or divided by 3600 to end up with kWh.  The k just means 1000, W is watt and h is an hour, which is 3600 seconds.

 

Power (W) is the rate that energy is used. You can think of this as your miles per gallon in your car, with energy being how many gallons are in the tank. If your car does 40 MPG at 60 MPH, and your tank has 8 gallons in it, you can drive 320 miles over 5 hours and 20 minutes.

A unit of power is called a watt, which is actually a J/second. Again a small unit, so a k is added. A k is 1000.

This gives the more normal kW for a power rating.

Boilers, heat pumps and even wood burners all have a maximum kW rating i.e. 6 kW.

Radiators, UFH and fan heaters also have a power rating.

(This can get confused by some people talking old imperial units of BTU and BTU/h, but we went metric in ,'73, so tell them off)

 

Where it starts to get confusing is a house will need a varying amount of thermal power to keep it at a steady internal temperature. This is caused by external temperatures rising and falling during the seasons and even during the day.

This can be overcome in a number of ways. The easy way, and the way old heating systems were set up, was to fire up a boiler at full power, heat some water, pump it around the house to all the radiators, which then heated the air. When the house was up to temperature, the thermostat turned the boiler off. When the temperature dropped a few degrees, it turned the boiler back on.

These days we are a bit more sophisticated and try to deliver enough energy to match the losses. This keeps the house at a steadier temperature, and used less energy overall.

You will almost certainly read on hear about weather compensation (WC). This is just a basic feedback system that knows what the outside temperature is, how much power is being delivered to the house and for how long it may be needed.

 

All that can be boiled down to a few numbers.

The main numbers you need to know are the thermal losses for each room. Once those are added together, you get a number for the whole house. It is usual to size for a worse case i.e. -10°C outside. This gives you a maximum power requirement (heat source kW).

Most of the time, you will be heating the house when it is way warmer outside, so the second number you need is about how much the heat source can be turned down internally. This is called modulation. Gas boilers have quite a high ratio, sometimes 10 to 1 i.e. 12 kW down to 1.2 kW. Heat pumps are not, generally, so good and are often in the range of 3 to 4 to 1. There are ways around this, but that is for later.

 

The other thing that is important is domestic hot water (DHW). 

There are two ways to do this. 

Heat water only when it is needed (instantaneous), this requires a lot of power, often over 25 kW.

Or

Heat and store in a cylinder.

This method takes longer for the same amount of delivered hot water, but used less power, often in the region of 3 to 6 kW.

If the heat source is delivering both central heating and DHW, then this needs to be taken into account. 

With modern combination gas boilers, they are sized to deliver enough hot water, and rely on boiler modulation to deliver lower power for the space heating.

Heat pumps, generally, rely on being only slightly oversized (which improves efficiency most if the time) and run a space heating time slot and a DHW timeslot at different temperatures and different times during the day.

 

There is a lot of detail in heating design.

So questions to ask are.

 

Maximum house losses.

Room by room heat losses.

DHW reheat times.

[Nickfromwales]

1 hour ago, SimonD said:

 

Yes, so they should be telling you that they'll produce a heat loss calculation based on your drawings and the SAP report you have. Then you should get a figure of a total output for the heat pump and a report telling you the heat load of each room and that will feed into a design of the underfloor heating (UFH), radiators, fan coils etc. If you're going for UFH on its own or with fan coils, keep an ear out for if they ask about cooling too, as that could be very handy for you.

DHW capacity needs to be discussed properly too, as most will put a smaller cylinder in than is advisable which limits your capacity to store cheap rate electricity (or excess PV generation) as hot water.

[Mr Blobby]

Just to jump in here as our heat pump will be ordered before month end...

 

Our PHPP model gives a heating load (PH) of 2.8 kW. 

Heat pump will be also used for DHW and cooling.

Planing to have open circuit, no buffer, no zones.

 

We have two choices, 5kW or 7kW.  Which one's best?

[SteamyTea]

34 minutes ago, SteamyTea said:

0.0007778

Whoops, 0.00027778 (fat fingers on a small screen)

36 minutes ago, SteamyTea said:

hear

Whoops again, autocorrect.

 

[-rick-]

1 minute ago, Mr Blobby said:

We have two choices, 5kW or 7kW.  Which one's best?

 Neither?

 

Even 5kw sounds a bit big. Smaller heatpumps are available, Important to get one that can moderate it's output down a lot. Most of the year you'll need a lot less than 2.8kw

[SteamyTea]

3 minutes ago, Mr Blobby said:

Our PHPP model gives a heating load (PH) of 2.8 kW. 

Heat pump will be also used for DHW and cooling.

Planing to have open circuit, no buffer, no zones.

 

We have two choices, 5kW or 7kW.  Which one's best?

what size is your DHW cylinder?

[Nickfromwales]

5kw will suffice, just will take a bit longer to fully recover a depleted hot water cylinder.

 

If you plan on solar PV > diversion > immersion, or cheap rate grid > immersion then less of a worry.

 

5 minutes ago, -rick- said:

 Neither?

 

Even 5kw sounds a bit big. Smaller heatpumps are available, Important to get one that can moderate it's output down a lot. Most of the year you'll need a lot less than 2.8kw

DHW just takes an age to heat up off at <5kw, especially if you've a larger home and use a lot of DHW. Factor in icing and defrosting, and then you're getting very close to 'undesirable' performance under duress.

[Mr Blobby]

25 minutes ago, SteamyTea said:

what size is your DHW cylinder?

 

400 litre tank if we can squeeze it in.  I think the new Telford 400l HP tank is less than the old 750 diameter so it may just fit but the pipes behind will be inacessible (note to self: Get a square plant room next time, not a long thin one 🤦‍♀️)

 

 

17 minutes ago, Nickfromwales said:

5kw will suffice, just will take a bit longer to fully recover a depleted hot water cylinder.

 

If you plan on solar PV > diversion > immersion, or cheap rate grid > immersion then less of a worry.

 

DHW just takes an age to heat up off at <5kw, especially if you've a larger home and use a lot of DHW. Factor in icing and defrosting, and then you're getting very close to 'undesirable' performance under duress.

 

Yes lots of PV but SWMBO has asbestos skin and showers only on boil setting.  If the water is ever cold then my ass is grass so will get the 7kW for extra cooling and a quiet life, even if that means a bit of extra cycling.

 

Interestingly enough I think the 5kW and 7 kW panasonic monobloc look to have the same internals but a different flow rate to get more heat out.

 

 

Edited 36 minutes ago by Mr Blobby