ASHP sizing

[shuff27]

So I've plugged all my data into the @Jeremy Harris spreadsheet with this result.  Annual heat requirement comes out as 10852 kwh for average OAT & 13830 kwh for min OAT.  How do I use this data to correctly size my ASHP to heat wet UFH on the GF (110 sq.m. single zone) & DHW?  Upstairs I'm planning to do without heating in the bedrooms & have elec mat UFH & elec towel rails in the 2 bathrooms.  Plus whole house MVHR of course!

 

For DHW the occupancy will only be 2 persons for 95% of the time.

 

Thanks for any guidance.

Heat loss versus outside air temperature
Difference between room and OAT (deg K) =	0	5	10	15	20	25	30
Ventilation heat loss energy (J) =	0	199015	398030	597044	796059	995074	1194089
Ventilation heat loss power (W) =	0	55	111	166	221	276	332
Wall heat loss power (W) =	0	212	425	637	849	1061	1274
Window heat loss power (W) =	0	85	169	254	338	423	508
Door heat loss power (W) =	0	19	38	58	77	96	115
Roof heat loss power (W) =	0	75	150	225	300	375	450
Ground floor heat loss power (W) =	378	378	378	378	378	378	378
Fabric heat loss power (W) =	378	769	1160	1551	1942	2333	2724
Total heat loss power (W) =	378	824	1271	1717	2163	2610	3056

[Conor]

You look at the total worst case heat loss - 3056w = 3kW. So it's saying on the coldest day you'll need continuous 3kW of heating to cancel out they heatloss and keep the house at that temperature. That means you want a 5kW heat pump.

[shuff27]

6 minutes ago, Conor said:

You look at the total worst case heat loss - 3056w = 3kW. So it's saying on the coldest day you'll need continuous 3kW of heating to cancel out they heatloss and keep the house at that temperature. That means you want a 5kW heat pump.

 

Thanks.  Would 5kW also provide enough DHW?  I'm guessing a tank capacity of min 200L, max 300L would do us.

 

Is there any benefit of oversizing the heat pump?  The price difference between a 5kW & 7kW Vaillant Arotherm Plus, for example, is only around £350.

[ProDave]

I would say 5kW is marginal, by the time you have allowed time for DHW heating, a 5kW ASHP would be running practically 24/7 in the coldest period to keep up.

 

If like us you want a silent house at night I would go for probably an 8kW ASHP

[SteamyTea]

1 hour ago, shuff27 said:

Is there any benefit of oversizing the heat pump

Yes, several.

They will have to defrost less, will cope better with any unexpected (though temperatures are known to swing wildly) cold periods, will heat your DHW quicker and will have a better CoP.

 

The down side is you really need to fit a decent sized buffer tank, but you want to do that anyway.

So have a look at the minimum volume your chosen heat pump, work out the smallest 'heating loop', if that is less than the minimum volume, you need to fit a buffer to up that volume.  There will not be hardly any downside to doubling the minimum volume, it will reduce short cycling.

[Dan F]

You have about the same calculated heat loss as us.  We are using the 7kW arotherm plus, but the 5kW may have been sufficient.  Remember that the kW specified is nominal and the output will depend on air and water temperatures and if the AHSP is running in eco/quiet mode, max or somewhere inbetween.

 

There are the specs for 5kW Model

- A-2W35:   50%:  3.3kW max: 7.1kW

- A-2W55:   50%:  3.1kW max:  6.3kW

- 300L UVC reheat time from 10-40C:  1hr 32min

 

And the 7kW Model:

- A-2W35:   50%: 4.7kW  max: 9.4kW

- A-2W55:   50%:  3.9kW  max: 8.0kW

- 300L UVC reheat time from 10-40C:  1hr 10min

[shuff27]

Thanks for the replies.  I've asked my local ASHP contractor to quote based on my calcs & build details so let's see what they recommend.

[Thorfun]

this is an interesting thread for me as we're finalising our ASHP size and have a few options! I'll start with a bit of background. we have a basement, ground and 3 bedrooms on the first floor. at the moment the plan is to only heat the ground floor using wet UFH. the FF will have electric UFH and towel radiators in the en-suites and we have decided to use split/ducted AC units for cooling on the FF so can use that for supplementary heating if required.

 

our M&E guys have worked out that we should have a 11kW ASHP but that is based on all 3 floors having heating requirements.

 

here's my output from @Jeremy Harris's spreadsheet:

 

 

that is based on 1 ACH which is the most I'm hoping for so, fingers crossed, the actual figures will be less.

 

So I can see from that output that 6kW ASHP is too little and an 8.5kW ASHP might be pushing it a bit in the coldest days and I can understand why they suggested a 11kW unit.

 

But, after further discussions with them and with the fact that only the GF will be wet UFH they have said that my heating requirements for the GF is only 3.6kW and so we could potentially only require a 6kW unit. Although, as we've no idea how the house will actually perform and we may need heating in the basement I'm a bit concerned that the 6kW unit doesn't give us any potential expansion if we need it in the future. 

 

As such, I am thinking of simply going for the 8.5kW Ecodan unit. seems to be the sweet spot and will easily cope with our initial demands but also gives us a 'buffer' should our heating requirements change over the years.

 

From a cost perspective there's approx £500 difference between each unit.

 

What are other's thoughts on our situation and which heat pump size would you recommend?

[SteamyTea]

2 minutes ago, Thorfun said:

What are other's thoughts on our situation and which heat pump size would you recommend?

How many hours a year will you need the full capacity of the HP.

Then, how many hours a year will you need more power.

 

Then do the sums, allowing for the drop in CoP and the cost of running extra heating, probably a resistance heater.

Don't forget your DHW will take a chunk of time out of the day.

[Thorfun]

1 minute ago, SteamyTea said:

How many hours a year will you need the full capacity of the HP.

Then, how many hours a year will you need more power.

 

I can't see it being many. we're based in the South East so winter temps don't usually get too low for too long. 

 

2 minutes ago, SteamyTea said:

Then do the sums, allowing for the drop in CoP and the cost of running extra heating, probably a resistance heater.

Don't forget your DHW will take a chunk of time out of the day.

the drop in CoP is interesting. it looks from the spec sheets that at A-7/W35 the 8.5kW unit is the least efficient.

 

but doing the maths to calculate what that reduced CoP means in actual costs especially when we're planning a 10kWp PV array and the payback period for the extra £500-odd increase to the 11kW unit is beyond me.

 

 

[IanR]

7 minutes ago, Thorfun said:

But, after further discussions with them and with the fact that only the GF will be wet UFH they have said that my heating requirements for the GF is only 3.6kW and so we could potentially only require a 6kW unit. 

 

Your M&E guys must be assuming the energy losses of the FF (and basement) are then being covered by another heat source. I can't imagine you want to use the electric UFH and towel rads to fully heat the FF, but rather to top up those rooms if the GF heat has not permeated sufficiently.

Your ASHP needs to cover the total energy losses for the house and the GF UFH needs to emit sufficient heat for the whole house. This will then permeate to the 1st floor via convection, conduction and MVHR.

In a well insulated and airtight house, DHW is more likely to be the energy requirement that drives ASHP sizing. What size UVC have you spec'd?

I have a slightly lower energy loss than your calculation, and a 500l UVC. I went with an over-sized 12kW ASHP for a quick reheat time on the cylinder. I could probably have gone with 8kW ASHP and not noticed the difference as the DHW usage is not as big an issue as I expected, even when both space heating and hot water are being called for simultaneously.

Over-sizing the ASHP does of course require a decent size buffer for space heating.

[SteamyTea]

19 minutes ago, IanR said:

Over-sizing the ASHP does of course require a decent size buffer for space heating

Yes, but that is generally a height issue, not a floor area one.

All ASHPs should have a buffer fitted.

[joe90]

1 hour ago, SteamyTea said:

All ASHPs should have a buffer fitted.

Well I have fitted one on the advise of our plumber guru but some here have not. The advantage of a buffer is being able to have an immersion in case of ASHP failure or service to keep the UFH working, less inhibitor required, also said it can stop the ASHP from short cycling. Also my ASHP only heats water to one temp (mine set to 48’) so with a buffer I can differentiate as this may cause short cycling.

[SteamyTea]

16 minutes ago, joe90 said:

The advantage of a buffer is being able to have an immersion in case of ASHP failure or service to keep the UFH working,

Another advantage is one could try out the heating system for a year, just using immersion heaters. Then when you know how your house performs, buy the correct size ASHP. 

Does require 100 quid of data logging kit, but we should all be doing that anyway.

[joe90]

26 minutes ago, SteamyTea said:

Does require 100 quid of data logging kit, but we should all be doing that anyway.

Why, I use yours ???. I just look at the lecky bill.

[Marvin]

54 minutes ago, SteamyTea said:

Another advantage is one could try out the heating system for a year, just using immersion heaters. Then when you know how your house performs, buy the correct size ASHP. 

Does require 100 quid of data logging kit, but we should all be doing that anyway.

 

or a £26 meter in series...

[SteamyTea]

19 minutes ago, Marvin said:

a £26 meter in series

Assuming that is a standard meter, it don't log the time of use.

[Marvin]

5 minutes ago, SteamyTea said:

Assuming that is a standard meter, it don't log the time of use.

Your quite right!  I only wanted to know overall what was required during the winter. 

[Thorfun]

4 hours ago, IanR said:

Your M&E guys must be assuming the energy losses of the FF (and basement) are then being covered by another heat source.

yep. the FF can be heated by the AC units if required. The basement we have not actually decided on a heat source nor if we will need one. I guess that's another reason why the 8.5kW unit would be better so we have the option to put wet UFH in the basement should we require it.

 

4 hours ago, IanR said:

What size UVC have you spec'd?

300l.

 

4 hours ago, IanR said:

Over-sizing the ASHP does of course require a decent size buffer for space heating.

they've suggested a 25l buffer tank

[SteamyTea]

1 minute ago, Thorfun said:

4 hours ago, IanR said:

Over-sizing the ASHP does of course require a decent size buffer for space heating.

they've suggested a 25l buffer tank

What is the volume of the smallest circuit, and what is the minimum volume the ASHP can handle?

[Thorfun]

7 minutes ago, SteamyTea said:

What is the volume of the smallest circuit

I haven't had the ufh designed yet so can't answer this ?

 

8 minutes ago, SteamyTea said:

what is the minimum volume the ASHP can handle?

how would I find this out? the spec sheet on the units don't seem to mention it. https://library.mitsubishielectric.co.uk/pdf/book/Ecodan_PUZ-WM85VAA_Monobloc_Air_Source_Heat_Pump_Product_Information_Sheet#page-1-2

[IanR]

14 minutes ago, Thorfun said:

they've suggested a 25l buffer tank

My 12kW Nibe requires a minimum 80l buffer (or always open loop), and I believe the 8kW Nibe was the same.

 

I've gone with a 200l buffer, but for cooling reasons. ie. I can cool the buffer and let the UFH work off of that for longer while the ASHP is heating the UVC.

[SteamyTea]

10 minutes ago, Thorfun said:

how would I find this out?

Call the Mitsubishi technical help line.

 

01707 282880

Edited September 28, 2021 by SteamyTea

[Dan F]

6 hours ago, Thorfun said:

yep. the FF can be heated by the AC units if required.

 

Seems a shame to use AC units in a new build, or is it just me?   Are fan coils not an option?

 

Also I seem to remember you had external blinds and were planning to automate them with Loxone?  Do you have blinds in the FF rooms you are planning AC for?

[Thorfun]

2 hours ago, Dan F said:

 

Seems a shame to use AC units in a new build, or is it just me?   Are fan coils not an option?

 

Also I seem to remember you had external blinds and were planning to automate them with Loxone?  Do you have blinds in the FF rooms you are planning AC for?

I spoke to Enhabit about our cooling requirements and we are having the cooling modelling done (awaiting the results). we can use fan coils and it's something they would design but they say it complicates things and they've recommended keeping it simple and having the cooling as a separate system. plus the fan coils they've suggested aren't amazingly cheap and it's something that I just don't have time to play with as there's too much else to do.

 

We are indeed having external blinds on the FF (and many GF) windows and we think our cooling requirements will be very low but we both can't sleep if it's too hot and if we can have an 'invisible' ducted AC system in the bedrooms for the occasional use we both think that it's a price worth paying. The AC company we've spoken to are happy to simply first fix for AC now and then we can just live in the house and see if we even need it and then if we do second fix the AC units with minimal disruption if we can't find the funds in the budget to fit it now.

 

There are many options I'm finding with self-building and you have to, at some point, just make a decision and stick with it otherwise you could end up going around in circles. There's no reason why an AC cooling system can't be controlled from Loxone and so we can have a centrally managed temperature controlling system that can deal with hot and cold weather and whatever climate change throws at us, albeit for a price.

 

I'm having to choose my battles with the build and decide which to attack myself and which to delegate out, and cooling is one of those I'm delegating.