ASHP - Size and Type

[richo106]

Hi All

 

Naively I thought it would be quite easy to decide on what size and type of ASHP i needed, spoke to a few different contractors/sales teams and they have all said something different..ranging from 8.5kw, 11kw and the latest one was 16kw.

 

Firstly my house is around 250m2 (125m2 floor area) it will be insulated quite well and have UFH both GF and 1st Floor. I will be focussed on air tightness (but not aiming for completely air tight but best i can with my building fabric) and will be having MHVR

 

I have a full SAP done on my proposed house (attached), what information is the critical information that I need to be looking at that to make a decision on size, this is something i am complete novice in.

 

My next dilemma is what make and type of ASHP. Either split or monobloc

 

The brands that have been mention to me are Valliant, Mitsubishi Ecodan, Daikin Altherma 3, NIBE

 

Any advice at all on this matter will be greatly appreciated

CombineReports_007400_001_2022-02-04_07-35-42.pdf

[Conor]

At the bottom of the report it shows the total heating demand for Jan and Dec, 2200kWh. Translate to a daily heating load figure and you're looking at about 7kW if you assume heating being on for 12hrs per day, every day in the coldest months. The peak may actually be higher than this, but normally it'll be much less.

 

Other comments will follow from people that have done in depth heat loss calcs that can give you a specific number. But from us being in a slightly bigger house but with the same heat load (but unused basement), our 9kW coolenergy monblock is doing a splendid job. Hardly broken a sweat yet.

 

Oh, monblock every time, less complicated, easier install, and all the noisy bits are outside.

[ADLIan]

Is this a new build house? Report refers to an existing house and therefore does not assess the fabric energy efficiency - DFEE and TFEE - which is part of Bldg Reg submission for new build and should be included. I’ve a feeling the dwelling may fail the Regs on this point and give an overall failure. Also SAP comes with a huge caveat that it is not a design tool!

[richo106]

15 minutes ago, ADLIan said:

Is this a new build house? Report refers to an existing house and therefore does not assess the fabric energy efficiency - DFEE and TFEE - which is part of Bldg Reg submission for new build and should be included. I’ve a feeling the dwelling may fail the Regs on this point and give an overall failure. Also SAP comes with a huge caveat that it is not a design tool!

Hi, We are adding another floor to the existing bungalow to create a house.

 

Does this help with building regs?

 

Many Thanks

[ADLIan]

That makes thing clearer. Probably classed as an extension to existing house rather than new build so ignore my comments on new build vs existing house.

 

[richo106]

Just now, ADLIan said:

That makes thing clearer. Probably classed as an extension to existing house rather than new build so ignore my comments on new build vs existing house.

 

Thank you

[JohnMo]

Just compared your SAP to mine, yours Jan 2100kWh, mine 1500kWh, my max heat demand (calculated) is 3kW at -5 degC.

 

Yours pro-rated, is about 4.2kW for a max heat demand.  But you need to calculate for local weather conditions.

 

You need to add an allowance for DHW, and the reduced performance as temperature outside reduces. Plus have decent modulation.

 

So on balance, as suggested above by @Conor would do a decent job or something similar. 

 

Ideally look for cooling ability built in and weather compensation.

 

Bigger is not better with heat pumps.

 

 

 

 

[richo106]

Thanks for the comments so far, my one concern is that the SAP is calculated with me achieving an air tightness of 3. If this is not achieved I am guessing this will add more demand to the heat pump

 

Cheers

[George]

I thought air changes were only be a small component of heat demand. 

[Spreadsheetman]

18 minutes ago, George said:

I thought air changes were only be a small component of heat demand. 

Quite the opposite. I did some ballpark numbers for the house that I am in the progress of buying and air changes were about 50% of the fabric loss assuming I can get them under 2, which is optimistic.

[PhilT]

The recommended model in that SAP document is an old tech 16kW twin fan unit which is hard to understand given the calcs. Go for one of the latest models featuring R32 or R290 and a scroll compressor in the 7 to 8kW range

Edited October 23, 2022 by PhilT

[richo106]

8 minutes ago, PhilT said:

The recommended model in that SAP document is an old tech 16kW twin fan unit which is hard to understand given the calcs. Go for one of the latest models featuring R32 or R290 and a scroll compressor in the 7 to 8kW range

https://www.theheatpumpwarehouse.co.uk/shop/heat-pumps/air-source-heat-pumps/mitsubishi/mitsubishi-8-5kw-r32-1ph-mono-ashp-uq-standalone/

 

Something like this?

 

Would it be worth jumping up to the next size brack 10-12kW if I struggle to get the air tightness i want?

 

Thanks again

 

[richo106]

5 hours ago, Conor said:

At the bottom of the report it shows the total heating demand for Jan and Dec, 2200kWh. Translate to a daily heating load figure and you're looking at about 7kW if you assume heating being on for 12hrs per day, every day in the coldest months. The peak may actually be higher than this, but normally it'll be much less.

 

Other comments will follow from people that have done in depth heat loss calcs that can give you a specific number. But from us being in a slightly bigger house but with the same heat load (but unused basement), our 9kW coolenergy monblock is doing a splendid job. Hardly broken a sweat yet.

 

Oh, monblock every time, less complicated, easier install, and all the noisy bits are outside.

https://coolenergyshop.com/collections/invertech/products/cool-energy-invertech-ivt9-air-source-heat-pump-ce-ivt9

 

This the one?

 

Thanks again

[Conor]

@richo106yep, and @dpmiller has one as well.

[PhilT]

1 hour ago, richo106 said:

https://www.theheatpumpwarehouse.co.uk/shop/heat-pumps/air-source-heat-pumps/mitsubishi/mitsubishi-8-5kw-r32-1ph-mono-ashp-uq-standalone/

 

Something like this?

 

Would it be worth jumping up to the next size brack 10-12kW if I struggle to get the air tightness i want?

 

Thanks again

 

Yes but even though I have an 11.2 version of that I would consider the latest offerings from Vaillant (R290) and Samsung (R32). The Coolenergy model appears to be using R410a, unless I am very much mistaken, so I would not recommend, but each to his own. Are you are able to assess the air tightness accurately enough before buying an HP?

[SteamyTea]

2 hours ago, richo106 said:

if I struggle to get the air tightness i want

Why struggle.  Make an air blower and constantly test and rectify it.

[richo106]

Yes I can do that

I will just wait until I have an plastered shell, I can then accurately measure and get my SAP altered accordingly 

 

Testing throughout sounds good to me! Sorry to be a pain but I thought I seen a method for that somewhere on the forum.

 

Is it just seal the door with an air blower and board and run the blower? 
 

what’s the best way to detect the leaks when the blower is running? 

 

thanks again for all the replies 

[Iceverge]

I cut a sheet of 18mm OSB to the sash size of one of our windows and fitted a pair of radiator fans from a Lexus is200. 

 

Then used a length of cable to connect it to the car battery with crocodile clips. 

 

Back of the hand for big leaks. 

 

A candle rammed in a length of stiff electrical conduit for the small leaks. 

 

I tried Joss sticks and a bee smoker too early on but they were various combinations of smelly and suffocating!  

 

 

[Andehh]

Question for everyone, most ASHP enable a decent degree of modulation esp with digital inverters (if marketing is to be believed) ... Surely this help protects to a degree of over sizing? 

Edited October 24, 2022 by Andehh

[HughF]

21 hours ago, PhilT said:

Yes but even though I have an 11.2 version of that I would consider the latest offerings from Vaillant (R290) and Samsung (R32). The Coolenergy model appears to be using R410a, unless I am very much mistaken, so I would not recommend, but each to his own. Are you are able to assess the air tightness accurately enough before buying an HP?

R410 makes for slightly simpler internal F-Gas plumbing as you don't need EVI, like you do with R32.

 

Chris, the MD of Cool Energy says that as soon as the drop-in replacement for R410 arrives, they are ditching the R32 models (which they do supply, albeit out of stock at the moment) and going back to the simpler unit.

[richo106]

I have spoken to the guy who did my SAP for me and he didn't include that I had MVHR so he has altered that. As he has also done 2 examples one with air tightness of 3 and one with 7

 

It shows a good difference and the impact of air tightness

 

 

3 - CombineReports_007400_001_2022-10-24_13-27-34.pdf 7 - CombineReports_007400_001_2022-10-24_13-12-49.pdf

[jack]

The relative impact of air tightness rises as the internal volume of the building increases. It's to do with the relationship between wall/ceiling/floor areas and internal volume. That's why you'll sometimes hear it said that poor insulation on very large buildings (think warehouses) is far less an issue than getting them airtight, whereas the opposite applies for, say, a small bungalow.

[SteamyTea]

[jack]

Never thought of it that way - interesting.

 

Must be more to it than that then (assuming the principle is even correct).

[JamesPa]

3 hours ago, Andehh said:

Question for everyone, most ASHP enable a decent degree of modulation esp with digital inverters (if marketing is to be believed). Surely this help protects to a degree of over sizing? 

Actually the degree of modulation for most models seems (according to spec sheets) to be typically only about a factor of 3 (eg 12kW modulating down to 4kW or thereabouts).  There are some exceptions of course, both worse and better.  That's not enough, even if correctly sized, to cover say from 15C (5 deg C temp difference between outside and target temp) and -2C (22deg C temp diff between outside and target temp) without resorting to on/off modulation.  

 

Incidentally the useful measure of modulation for most sizing purposes is between maximum output at coldest ambient temperature (eg -2 down south) and minimum output at warmest ambient temperature at which you require heating (typically 15/16).  That's a worse number than the manufacturers like to quote (if they quote it at all) because HPs are less efficient at low ambient and more efficient at high ambient.  There is a separate thread on this subject and data is not easy to find.

 

This is one reason why buffer tanks/volumisers are sometimes added to systems, to stop 'short cycling' (ie on/off modulation which happens too often in any time period).  Oversizing the unit simply makes it more problematic.

 

Edited October 24, 2022 by JamesPa