ASHP Sizing from SAP/PHPP

[Conor]

Need to order ASHP soon enough... just checking size requirements. I've always assumed a 5kW ASHP would be sufficient. 200l DHW tank. two adults and one child. UFH mostly on ground floor only plus two rooms in basement - so three loops/zones only.

 

I have a preliminary SAP - summary attached. What here would allow me to pick ASHP size from? Also PHPP (PHPP slightly out of date as floor are increased)

 

Thanks

 

Conor

 

Edited August 20, 2020 by Conor

[arg]

As I understand it:

 

The "heating load" is what you want.  It's expressed in watts per square metre of floor area, so multiply by the floor area 11*241.9 = 2660 = 2.7kW.   So a 2.7kW output heatpump, running flat-out 24x7 and with no other inefficiencies (and not running the DHW) would just be enough to maintain their assumed temperature on the coldest days.  You obviously need one bigger than that.

 

The difference between "heating demand" and "heating load" is other assumed heat inputs to the building not from the heating system: cooking, electrical appliances, warm bodies etc.  So you'd need the larger "heating demand" number if for some strange reason you wanted to be sure you could keep the house fully up to temperature when you weren't living there.

 

There's other adjustments needed to the raw: obviously you've got to add the DHW requirement; a heatpump might not actually produce its full nominal output under the conditions you want (datasheets often quote several figures for combinations of output water temperature and outdoor temperature: since the max input power is typically fixed by the size of the compressor, the output power depends on efficiency, which will be lower on cold days and lower the hotter you need the output water to achieve the heat transfer); and you probably want more than the bare minimum to hold the temperature, just in case you've let the temperature drop for some reason and want to bring it back up within a reasonable time.

 

 

 

[Dan F]

15 minutes ago, arg said:

The difference between "heating demand" and "heating load" is other assumed heat inputs to the building not from the heating system: cooking, electrical appliances, warm bodies etc. 

 

That's not quite right;  both the heating load and heating demand both take into account "internal heat gain" as well as "solar gain".  The difference between the two is that the "heating demand" is in kWh and is per year (annual energy usage for heating) wheras the "heating load" is in W and is the energy input needed to keep the house at 20C on the typical coldest day of the year.

 

You'll want to take a slightly more pessamisitic stance on heating load than PHPP though, PHPP tries to be realist which is good for modelling, but not ideal for specifying ASHP.  Why?
i) because the building might not perform as well as modelled in PHPP in practice
ii) occupancy and other heat inputs might not meet PHPP assumptions
iii) PHPP uses a design temperature that isn't particularly low (-1.5C in south east).
iv) assumed solar gain might not be as much in reality due to shading, neighouring buildings etc.

 

Agree on the other points, another thing to take into account is DHW cyclinder reheat time.

 

Edited August 20, 2020 by Dan F

[arg]

1 minute ago, Dan F said:

That's not quite right;  both the heating load and heating demand both take into account "internal heat gain" as well as "solar gain".  The difference between the two is that the "heating demand" is in kWh and is per year (annual energy usage for heating) wheras the "heating load" is in W and is the energy input needed to keep the house at 20C on the typical coldest day of the year.

 

You are right of course.

[Conor]

Thanks guys.

 

So floor area has increased to 280m² since initial PHPP modelling, and we've added solar control glass. That would mean heating load would be closer to 3kw, or more.

 

Currently eying a 6kw Mitsubishi ecodan, seem this would fit the bill? Next one up is 8.5kw

 

[dpmiller]

Better to go 8.5kW and modulate down even on the Gold Coast...

 

The various different calculators all come out between 2.5 and 4.5kW for our place IIRC but I'll be going somewhere between 8 and 10kW to ensure plenty of headroom for DHW.

[Dan F]

Our PHPP "Heating load" comes out at about 3kW.  Our traditional method "heat demand" came out at 6kW. 

 

We plan to use a 305L Mixergy tank (reheat time isn't quite so important) and we are looking at using a 7kW ASHP minimum.  That said, we're being advised that using 10kW to give additional headroom wouldn't be a bad idea either..

Edited August 20, 2020 by Dan F

[MikeSharp01]

1 hour ago, dpmiller said:

Better to go 8.5kW and modulate down even on the Gold Coast...

Don't forget to look at the COP curve as you want best COP for the majority of your needs so trying to max out the COP for average heating day seems like a good approach to me. So although an 8.5Kw unit has stacks of grunt (Units not determinable) when you need it how much COP will it be (Pardon the pun) when running at 3Kw? Which it perhaps will be most of the time.

[joth]

2 hours ago, Dan F said:

Our PHPP "Heating load" comes out at about 3kW.  Our traditional method "heat demand" came out at 6kW. 

 

We plan to use a 305L Mixergy tank (reheat time isn't quite so important) and we are looking at using a 7kW ASHP minimum.  That said, we're being advised that using 10kW to give additional headroom wouldn't be a bad idea either..

 

Similarly, our PHPP say 2kW, but traditional room-by-room would like 4.7kW

Also looking at a 300L tank.

We were going with a 8.5kW Ecodan but had the offer today to use 6kW R32 model instead, as it's about £500 less than the 8.5.  I am awaiting the numbers on what this will do to DHW reheat time