ASHP size (don’t sigh I’ve done the calcs)

[Kelvin]

We had a comprehensive SAP report done and from this the timber kit company had a report produced with recommendations. The SAP report was based on the as designed at the time but since then I’ve improved the floor insulation and gone with 3G instead of 2G throughout. It also assumed an as designed air change of 3.0 where my aim is to get under 1.0. I also plugged the numbers into Jeremy’s spreadsheet and the total heat loss in the worst scenario is 3.6kW which more or less tallies with the detailed SAP report.  
 

Consequently the ME guys recommended a minimum size ASHP of 6.8kW so we specced an 8.5kW Ecodan and this is what the installer also recommended. However the installer emailed me today and said that they’ve reviewed how they do their calcs after feedback they received and it means they now recommend we go to the next size Ecodan up so 11kW, the difference in price is only a few hundred quid. I called the ME guy that did our report and he said that doesn’t make any sense to him and recommends we stick to the 8.5kW. I’ve got a copy of the spreadsheet the installer uses so will plug our figures into that and see what it says but my expectation is it will match what I already have. 

Edited February 8, 2023 by Kelvin

[akjos]

How are you going to distribute the heat? UFH, rads something else? What flow temperature was it designed at?

 

also how big is your property?

 

If your heat loss is 3.6kw, a 11kw unit seems unnecessarily big. Anyway it’s worth knowing your whole design.

Edited February 8, 2023 by akjos

[Kelvin]

UFH in screed with 200mm insulation. No rads upstairs we’ll wire for electric panels and then decide after a winter if we need them. 40^oC albeit we’ll run it lower. There’s only two of us most of the time so hot water demand isn’t great. 

Edited February 8, 2023 by Kelvin

[ProDave]

It sounds like some installers are getting nervous about specifying under sized units so are going the opposite way and specifying larger than needed on the basis nobody complains.

 

I would stick to your original size if 2 independant calculations agree.

[Kelvin]

That was the impression I got from them too. 

[JohnMo]

If it's going via a grant, the heat loss calculation does not take any credit for MVHR, as it just takes the room air changes from a table within locked cells on the spreadsheet.  My heat loss is about the same as yours.  Have run the house for about 15 months and the heat losses and water usage is close to calculations. So I am about to install a 6kW ASHP. Key is to check at the lowest likely temp you have enough output to heat the house and out some heat into DHW.

[SteamyTea]

1 hour ago, Kelvin said:

However the installer emailed me today and said that they’ve reviewed how they do their calcs after feedback they received

What was the feedback.

Was it cold houses or lower than expected CoP.

 

We have had a couple of winters that were extreme for the UK in recent years.

 

[Kelvin]

46 minutes ago, SteamyTea said:

What was the feedback.

Was it cold houses or lower than expected CoP.

 

We have had a couple of winters that were extreme for the UK in recent years.

 

He was a bit vague but the summary seemed to be their calcs weren’t detailed enough when considering all the various factors. This seemed counter intuitive to me on why it would subsequently push them to recommend the next size of heat pump for every install. I don’t know the typical house they are installing them in so it could be cold houses. 

[JamesPa]

50 minutes ago, SteamyTea said:

What was the feedback.

Was it cold houses or lower than expected CoP.

 

We have had a couple of winters that were extreme for the UK in recent years.

 

It's easy to imagine that customers,  used to switching heating off eg during the day or overnight and the fast warm up time of a well overspecfiied gas boiler, find it difficult to adjust to the much more gentle response of a well matched ashp.  Oversizing will mitigate this to some extent, particularly if combined with a higher than strictly necessary flow temp.  I would be willing to bet some instalers would opt for the easy life this would deliver and probably many customers ditto.  Yes there is an efficiency penalty but many will sacrifice this for perceived comfort, convenience and responsiveness.

 

That could well be the reason.

[ReedRichards]

Surely there may or may not be an efficiency penalty?  If a smaller unit can modulate down to lower output powers than a larger unit then yes, there will be an efficiency penalty with the larger unit.  But if minimum power is the same either way then I don't see anything to cause an efficiency penalty.  

[Andehh]

I always thought that if an ASHP can modulate down low (ie some Samsung quote down to 10% I think??) then no penalties for a slightly bigger unit. 

 

Obviously if it can't modulate down to the lower what demand then there would be effieincy impacts. 

[JamesPa]

8 hours ago, ReedRichards said:

Surely there may or may not be an efficiency penalty?  If a smaller unit can modulate down to lower output powers than a larger unit then yes, there will be an efficiency penalty with the larger unit.  But if minimum power is the same either way then I don't see anything to cause an efficiency penalty.  

Agreed as regards HP size if it modulates down (which some will).

 

I also speculated that some may also turn up flow temp as a further bid for faster response time, which incurs an efficiency penalty of perhaps 2-4% per deg C based on some modelling posted in another thread.

[PhilT]

I was in exactly the same position and only went for an 11.2kW because we are planning on extending the house at some point, however the 11.2 coped so well in Dec when the night time temps got down to -11 I'm really glad I did. During milder times the compressor pauses maybe once or twice an hour when the heat requirement gets below the minimum continuous output (c. 30% of max power) however this doesn't seem to affect the recorded COP or efficiency in a measurable way

[Iceverge]

Will the larger unit have an easier life because rarely outputting it's max power or a harder life because it's cycling more? 

 

My guess is that a well sorted larger unit with a good ability to modulate will defrost less in winter and last longer too. 

 

 

[Nickfromwales]

1 hour ago, Iceverge said:

Will the larger unit have an easier life because rarely outputting it's max power or a harder life because it's cycling more? 

My guess is that a well sorted larger unit with a good ability to modulate will defrost less in winter and last longer too. 

My thoughts exactly. It becomes quite dependant on running at an optimal flow temp and an UFH system that doesn't have actuators / stats per room etc.

 

[Kelvin]

They are both Ecodan units. What do they typically modulate down to? 
 

PUZ-W112VAA-BS (R32) - 11.2kW

 

PUZ-WM85VAA-BS (R32) - 8.5kW

 

 

[DanDee]

22 hours ago, Kelvin said:

They are both Ecodan units. What do they typically modulate down to? 
 

PUZ-W112VAA-BS (R32) - 11.2kW

 

PUZ-WM85VAA-BS (R32) - 8.5kW

 

 

Download the pdf for this google search: Ecodan_ATW_Databook_R32_Vol5.5

 

https://library.mitsubishielectric.co.uk/pdf/book/Ecodan_ATW_Databook_R32_Vol5_5