ASHP Quote

[soapstar]

Hello folks,

 

Given the wealth of knowledge on here I thought it might be wise for me to post up our quote for plumbing our new build and ask the question 'Does everything seem right?'

 

4 bedroom 1.5 storey house (250 sqm internal) with 3 bathrooms - downstairs (shower/bath combo, toilet, sink), upstairs (shower, toilet, sink), upstairs en-suite (shower, slipper bath, sink, toilet).

 

The quote is as follows:

 

 

Grants Air Source Aerona 3 16 KW
Grants 50 Litre buffer tank
Grants 300 Litre monowave heat pump cylinder
UFH to ground floor
Radiators on first floor
Dual fuel towel rails to showerooms
Manifold, actuators, pump centre and room stats for underfloor heating Plumbing for ensuite
Plumbing for showeroom upstairs & downstairs
Plumbing for kitchen and utility room sinks
Plumbing for dishwasher and washing machine
Plumbing for tap in garage
Lead flashings under soles of 2 dormer windows
Supply/Fit bathroom suites (6k allowance)
All copper pipes and fittings
All waste pipes and fittings
All gutters and downpipes (PVC)
Thermal fluid for heating protection

 

All for the price of £37k (including 6k allowance for sanitary)

 

Is the buffer tank necessary? Any implications of having one?

[dpmiller]

16kW is pretty big for an efficent new build.

[jack]

There'll be a minimum volume that your ASHP will need. A buffer tank is only required if your UFH system doesn't have at least that minimum volume of fluid.

 

Radiators will need to be UFH compatible (ie, designed to work with the relatively low temperature water provided by an ASHP). You'll likely be disappointed with ordinary radiators. Same with the towel rails - they won't output much heat if they're being supplied with water from the ASHP. 

[soapstar]

10 minutes ago, jack said:

There'll be a minimum volume that your ASHP will need. A buffer tank is only required if your UFH system doesn't have at least that minimum volume of fluid.

 

Radiators will need to be UFH compatible (ie, designed to work with the relatively low temperature water provided by an ASHP). You'll likely be disappointed with ordinary radiators. Same with the towel rails - they won't output much heat if they're being supplied with water from the ASHP. 

 

Thats some interesting information, thanks. We have received a quote from another company and they do not state the need for a buffer tank, in fact they quote for a smaller volume cylinder at 250 litres.

 

Regarding the towel rails this is something i haven't given much thought actually, they have quoted for dual fuel which i presume has the added benefit of electric if required.

 

Perhaps someone with a ASHP system and towel rails can let us know how they get on given the low temperatures of an ASHP? 

[jack]

I suspect that if you're using towel rails with an ASHP, the best you'll get is faster drying of towels and small amount of heat output.

 

5 minutes ago, soapstar said:

... in fact they quote for a smaller volume cylinder at 250 litres.

 

The cylinder won't form part of the fluid circuit to which the ASHP is connected, so the cylinder size isn't really relevant to whether you need a buffer tank. 

[PeterW]

I think the £6k for the sanitaryware is light - you can easily spend that on what you have listed.

 

I'm with @jack on this, a 16Kw is overkill even at building regs standards. That Grant ASHP is a bit lumpy on price too... nearly £7k plus another £1k for the cylinder..??

 

Copper all through..?? Why..?? 

[soapstar]

1 hour ago, PeterW said:

I think the £6k for the sanitaryware is light - you can easily spend that on what you have listed.

 

I'm with @jack on this, a 16Kw is overkill even at building regs standards. That Grant ASHP is a bit lumpy on price too... nearly £7k plus another £1k for the cylinder..??

 

Copper all through..?? Why..?? 

 

Hi @PeterW. Yes we did suspect the 6k for sanitary would be pushing it however we are not wanting to go all out to begin with, our aim is to achieve all our bathrooms to a basic standard with this amount, even if it means leaving out a couple things.

 

It would seem the consensus is a 16kw pump is overkill, what size would you normally suspect for a house of this size?

 

We have also been quoted for a Samsung ASPH from another plumber, total quote is around 2k less but without a buffer tank and a smaller cylinder (250l)

 

[Jeremy Harris]

You really need to try and work out your worst case heating requirement, assess how much hot water you need, then work from there.

 

You can get a pretty good estimate of the heating requirement using the spreadsheet I wrote a few years ago.  Just put in your external wall, roof and floor areas, window and door areas, and the associated U values, together with your estimated airtightness level and MVHR efficiency.  The spreadsheet will give close to a worst case heating requirement for an empty house, with no incidental heat gains, solar gain etc.

 

Heat loss calculator - Master.xls

 

Hot water capacity depends on how many are in the house, but for a house that size I think 300 litres is probably about right, I'm not sure that it would be sensible to go down to 250 litres or less, as an ASHP will be a bit slower than a boiler to reheat the tank, and the stored hot water temperature will be a bit lower, probably no higher than 55°C, so more hot water will be used when mixing down for showers or a bath than would be the case for a boiler heated tank, that may well be around 65°C or so.

 

I'd be surprised if you need more than about 10 kW of heating capacity, very worst case, in extremely cold weather, TBH.

[Benjseb]

I can’t see a breakdown on costs

 

Our house is 250sqm but it’s old stone build. EPC C/D (depending upon which one you read!) 

 

Our heat loss came back at 13kW. There’s no way you should estimate this. Do it properly

 

our 14kW Mitsubishi Ecodan plus 120l buffet plus 300l DHW tank was £10k ish plus vat. That’s with cloud monitoring and 5yr warranty. 

 

Also, I’d go for a biggish water tank then you can keep it at a lower temp (say 47c) which is more efficient to heat than a smaller tank at 55

[soapstar]

17 hours ago, JSHarris said:

You really need to try and work out your worst case heating requirement, assess how much hot water you need, then work from there.

 

You can get a pretty good estimate of the heating requirement using the spreadsheet I wrote a few years ago.  Just put in your external wall, roof and floor areas, window and door areas, and the associated U values, together with your estimated airtightness level and MVHR efficiency.  The spreadsheet will give close to a worst case heating requirement for an empty house, with no incidental heat gains, solar gain etc.

 

Heat loss calculator - Master.xls 92.5 kB · 40 downloads

 

Hot water capacity depends on how many are in the house, but for a house that size I think 300 litres is probably about right, I'm not sure that it would be sensible to go down to 250 litres or less, as an ASHP will be a bit slower than a boiler to reheat the tank, and the stored hot water temperature will be a bit lower, probably no higher than 55°C, so more hot water will be used when mixing down for showers or a bath than would be the case for a boiler heated tank, that may well be around 65°C or so.

 

I'd be surprised if you need more than about 10 kW of heating capacity, very worst case, in extremely cold weather, TBH.

 

Very impressive spreadsheet! Can i ask if you take the glazing from the windows into the 'window area' figure or are these calculated under the door areas? The french doors for example have glazing.

 

These calculations are way over my head as it is! Can you tell me what output figure I am looking at? 'Total heat loss' i assume? And how does this figure translate into what power of ASHP would be required?

[Jeremy Harris]

Just now, soapstar said:

 

Very impressive spreadsheet! Can i ask if you take the glazing from the windows into the 'window area' figure or are these calculated under the door areas? The french doors for example have glazing.

 

These calculations are way over my head as it is! Can you tell me what output figure I am looking at? 'Total heat loss' i assume? And how does this figure translate into what power of ASHP would be required?

 

Use the overall dimensions of the doors and windows, together with their U value (Uw in the case of windows and glazed doors).  Use the average Uw value for all the windows if they vary a bit from one window to another.  You should have all the information needed on your design SAP input worksheet.

 

The output figure is the total heat loss from the house, which will be the very worst case heating requirement, as the spreadsheet doesn't account for solar gain, or incidental heat gains from appliances, lights, the hot water tank waste heat, occupants, pets etc.  In practice every occupant contributes around 80 to 100 W of heat, the hot water tank roughly the same, and appliances may well add another 200 W to 300 W of heat gain (pretty much all the power going into the house ends up as heat).

 

You should be OK sizing the ASHP on the basis of the heat loss from this spreadsheet.  For example, for our 130m² the worst case heat loss, when it's -10°C outside, is about 1.6 kW.  In practice we've found that we never need more than about 1 kW of heating in very cold weather, and most of the time in winter we only need around 500 to 600 W of heating.  I fitted a 6 kW Glowworm ASHP, but it's massively oversized for the house (it happened to be cheap at the time, £1.7k delivered).  Our house would be fine with a very much smaller ASHP, but these aren't that common, about 4 kW seems to be the smallest readily available.

[soapstar]

4 minutes ago, JSHarris said:

 

Use the overall dimensions of the doors and windows, together with their U value (Uw in the case of windows and glazed doors).  Use the average Uw value for all the windows if they vary a bit from one window to another.  You should have all the information needed on your design SAP input worksheet.

 

The output figure is the total heat loss from the house, which will be the very worst case heating requirement, as the spreadsheet doesn't account for solar gain, or incidental heat gains from appliances, lights, the hot water tank waste heat, occupants, pets etc.  In practice every occupant contributes around 80 to 100 W of heat, the hot water tank roughly the same, and appliances may well add another 200 W to 300 W of heat gain (pretty much all the power going into the house ends up as heat).

 

You should be OK sizing the ASHP on the basis of the heat loss from this spreadsheet.  For example, for our 130m² the worst case heat loss, when it's -10°C outside, is about 1.6 kW.  In practice we've found that we never need more than about 1 kW of heating in very cold weather, and most of the time in winter we only need around 500 to 600 W of heating.  I fitted a 6 kW Glowworm ASHP, but it's massively oversized for the house (it happened to be cheap at the time, £1.7k delivered).  Our house would be fine with a very much smaller ASHP, but these aren't that common, about 4 kW seems to be the smallest readily available.

 

Thanks for clarifying @JSHarris 

 

The only figure I am unsure of is the MVHR efficiency - i have left this at 85%

 

The total heat loss is coming to approximately 7.5kw, which even if my figures are slightly off I do not see how this would reach 16kw?! 

 

Our location is coastal (east coast Scotland) so the weather/temperature here isn't as bad as some may think. We do however get a fair bit of wind from the sea (less than 0.5 miles from coast), combined with the large gable window which faces the coast I would assume this would contribute quite substantially to the heat loss.

 

Looks like I have to do further investigating with the plumber and query their calculations...

[Jeremy Harris]

5 minutes ago, soapstar said:

 

Thanks for clarifying @JSHarris 

 

The only figure I am unsure of is the MVHR efficiency - i have left this at 85%

 

The total heat loss is coming to approximately 7.5kw, which even if my figures are slightly off I do not see how this would reach 16kw?! 

 

Our location is coastal (east coast Scotland) so the weather/temperature here isn't as bad as some may think. We do however get a fair bit of wind from the sea (less than 0.5 miles from coast), combined with the large gable window which faces the coast I would assume this would contribute quite substantially to the heat loss.

 

Looks like I have to do further investigating with the plumber and query their calculations...

 

 

85% is fine for the MVHR, it's a pretty typical figure, perhaps slightly on the low side, but that's OK for estimating heat loss.

 

7.5 kW seems close to what I'd expect for a house the size of yours, built to typical building regs insulation and airtightness levels (ours is a great deal better than building regs requirements, hence the much lower heat loss).

 

As others have said, 16 kW seems way more than expected for a house of your size.  ASHPs come in set sizes, so I'd be inclined to go for one the next size up from the 7.5 kW you've calculated, say 9 kW, to give a bit of a margin (although the spreadsheet does really give the worst case when it's -10°C outside and the house is empty, with nothing on except the heating).  That should be fine, even allowing for the slightly higher heat loss from your windy location (@Stones has observed that wind makes a significant contribution to heat loss; he's up in Orkney).