Whole‑House Renovation: Need Real‑World Advice on ASHP + Fan Coils + MVHR Integration

[JohnMo]

1 hour ago, NetTechie said:

5kW or so does seem low to me

That's because everyone is brain washed by plumbers wanting to install 30kW+ boilers.

 

1 hour ago, NetTechie said:

Total heat loss for room 272, 18W/m2

Doesn't that far out. MCS start doing 2 or 3 ach which soon racks up the heat loss.

 

Your calculations assumes the room above is also heated, which is fine if it is. And should be with ASHP anyway.

[IGP]

4 hours ago, NetTechie said:

What's wrong with PIR cavity insulation?  It has a much better u value than rock wool, am I missing something?

The combination of EWI and cavity is due to the extensions and current building regs, whilst getting the external finish flush with a complete thermal envelope. I don't need to but then the extended walls will stick out on the side which I don't want. 

I would also sack the PIR off as a bad job, the U value will only be good on paper - it’s a nightmare to detail correctly and any mistake will undo your hard work and then some by all the air flowing through the gaps and around the PIR sheets. 

There are enough horror stories on these boards to chill your spine both literally and figuratively with regard to PIR as cavity wall insulation. 

 

Fill with EPS bonded beads/full fill mineral wool. 

Edited Tuesday at 16:40 by IGP
Missed word

[PhilT]

23 hours ago, NetTechie said:

the 5kW or so does seem low to me

how many people will be living in this house? 5kW is too low to keep up with family hot water demand in a property of this size. I have an oversized heat pump (11kW vs 6kW at -3degC) and it's great, I never have to worry about not being warm or getting hot water quickly. My experience, backed up by the data tables, is that oversized heat pumps are no less efficient overall, and more efficient during the coldest periods like we have been having over the last few weeks. Some headroom is essential for all sorts of reasons especially those situations where a strong recovery is needed - defrosts, power cuts, all the family off work and school with the flu, and any other situation where you need extra warmth, or the house temperature drops significantly and you need a fast recovery.

[NetTechie]

22 hours ago, PhilT said:

how many people will be living in this house? 

 

a typical family of 4, with a couple of guests staying over say once a month. 

[SimonD]

23 hours ago, PhilT said:

5kW is too low to keep up with family hot water demand in a property of this size.

 

This isn't the heat pump size, it's the property heat loss. A proper heat pump design would also look at DHW cylinder and re heat times in the context of property size and occupancy numbers. 

 

23 hours ago, PhilT said:

oversized heat pumps are no less efficient overall, and more efficient during the coldest periods

 

These assessment have to be considered in the round. Whilst it is sensible to consider heat pump size in relation to defrost, the coldest periods are usually very short so I would suggest the selected heat pump needs to be considered in light of the mean temperature it's going to be running at most of the time as well as its ability to cope with cold snaps. A heat pump twice the size may have poorer modulation. There's also the infrastructure costs where if you double the size of the heat pump, your pipe sizing will inevitably need to be upsized with consequential costs - a double in heat pump size can easily add £2k to the heat pump unit price itself. Input power is also likely to be higher with a bigger heat pump.

 

A properly sized heat pump is always going to provide the best overall efficiencies and balance with infrastructure costs.

[JohnMo]

On 14/01/2026 at 11:45, PhilT said:

oversized heat pumps are no less efficient overall

All depends on modulation and demand. A heat pump that runs without cycling will always be more efficient than one that cycles. First a heat pump that doesn't need to cycle can run a lower temperature - mine I am looking at 5 degs cooler at 8 to 10 degs outside. That's a big chunk of CoP increase. Then there is the needless heating of big chunks of metal at the start of each cycle. So basically I don't agree, unless of cause the large heat pump can modulate down to meet the 7 to 10 Deg outside temp with undue cycling, then there is just capital expenditure and large pipework etc to accommodate it.

 

Cylinder heating isn't a big issue, 210L from an indicator temperature of 6 degs only takes 50 mins with a 6kW heat pump to get to 50. Heat twice per day, no real impact on house temps, one heat is likely when your asleep anyway.

 

I have actually found heating by immersion has barely changed overall electricity consumption. On paper ASHP heating gives me a CoP of 3, but in practice the immersion does a good job, maybe due to the long pipe runs on my install.

[Dillsue]

Our house has a calculated heat loss of 8.5 kw at -2 but we put in a 7kw HP on the basis that it would be more efficient as the days of sub zero temps are few and far between and we could always run a fan heater or switch over to an LPG boiler if the HP couldn't cope. The HP has done 100% of the space heating since september and through the few cold snaps we've had recently with temps getting down to a degree or 2 below freezing. It worked very hard and guzzled eleccy whilst it was freezing but I'm hoping that the undersizing will pay efficiency dividends for the rest of the year??

 

An MCS umbrella/HP supplier wanted to sell us a 12kw unit!!

[PhilT]

1 hour ago, Dillsue said:

I'm hoping that the undersizing will pay efficiency dividends for the rest of the year

The data tables published by Mitsubishi and Vaillant, as examples, an 11.2kW vs an 8.5kW Ecodan, or a 10kW vs a 7kW Vaillant Arotherm+, indicate that the extra efficiency of the larger, slower revving scroll compressors* outweighs any inefficiencies in warmer times, but why would there be any significant losses for modern oversized scroll compressors cycling at reasonable intervals of no more than once or twice per hour in the warmer months anyway? Long gone are the days of fixed speed reciprocating compressors. Isn't the oversized low modulation inefficiency paradigm something of an urban myth now?

 

*(friction losses rise only in direct proportion to surface area, but rise exponentially with RPM)

[JohnMo]

27 minutes ago, PhilT said:

Isn't the oversized low modulation inefficiency paradigm something of an urban myth now?

Nope. Fact any cycling requires a higher flow temperature to compensate. Example my install is cycling 4 degs outside, runs for 1.5 hours off for a hour. Flow temp max 29.4 degs. Overall CoP so far today is 4.55, but heat input is approx 500W higher than it should be (2.5 compared to 2kW).  

 

Steady state running flow temp required is 23-24, also need 2kW heat input, reduced flow temp increases CoP to around 5.2 to 5.5, reduced electricity input by 500W, so should yield something like a 30% reduction in running cost, just by a using a heat pump that covers full heating range by modulation.  Apply that to 80% of the heating season it's not small change.