Heat pump for new build

[ProDave]

1 minute ago, deuce22 said:

I didn't have any detailed drawings or any other details.

 

I have had SAP calculations completed, so would the info I need be in there?

Yes they should be, though my SAP calculations seemed to over estimate the heat required compared to reality.

[shuff27]

2 hours ago, ProDave said:

Yes they should be, though my SAP calculations seemed to over estimate the heat required compared to reality.

Mine also over estimated as did the MCS certified heat loss calcs so a 11kW heat pump was proposed by my installer.  I persuaded them to change it to a 8.5kW unit backed up by my own calcs from the Jeremy spreadsheet.

[crispy_wafer]

Just to add my tuppenny worth, I've had two MCS accredited installers quote, one estimated an 8kW unit, another 11.2kW.  the smaller unit was a Midea, the larger was a Mitsi Ecodan.  No real discernable difference in costs, but will run through the calcs and see what the spreadsheet suggests. 

[SBMS]

3 hours ago, shuff27 said:

Mine also over estimated as did the MCS certified heat loss calcs so a 11kW heat pump was proposed by my installer.  I persuaded them to change it to a 8.5kW unit backed up by my own calcs from the Jeremy spreadsheet.

Out of interest - with a modern unit that modulates output, is it a big issue over specifying? Is it the additional capital cost of a bigger unit you were trying to avoid?

[jack]

Just now, SBMS said:

Out of interest - with a modern unit that modulates output, is it a big issue over specifying? Is it the additional capital cost of a bigger unit you were trying to avoid?

 

I believe they only module down to a certain point though. If that point is too high, you could have cycling problems.

[JohnMo]

49 minutes ago, SBMS said:

Out of interest - with a modern unit that modulates output, is it a big issue over specifying? Is it the additional capital cost of a bigger unit you were trying to avoid?

 

Typical turn down is around 2:1 on an inverter heat pump, some are a little better.

 

So if you look at the heating characteristics of the house at say -5 and 10/12 degrees, will your chosen heat pump do max demand and min turndown (kW) without being too big for the heat demand at 10/12 degrees, so that you don't need a buffer?  

[joth]

1 hour ago, JohnMo said:

So if you look at the heating characteristics of the house at say -5 and 10/12 degrees, will your chosen heat pump do max demand and min turndown (kW) without being too big for the heat demand at 10/12 degrees, so that you don't need a buffer?  

While it's nice to not oversize it vs the heat demand, as that allows the heat pump to operate at the lowest flow setting for longest period (so best COP),  I think the most critical thing is not to oversize it vs the minimum emitter size, as that's when egregious short-cycling will occur.

 

Looking at it my own setup is borderline on a 2:1 modulation ratio, as we have 3.5kW of UFH emitter and a 8.5kW ASHP.  I see a few quotes that 3:1 modulation is typical so that puts us within bounds, so long as I don't chop up the UFH into smaller separately controlled zones.