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Measured heat use way lower than MCS calculation. ASHP sizing.


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Hi, perhaps someone with more experience than me can help. 

 

My gas usage for the last year is around half that suggest by a recent MCS ASHP survey.  I really don't want to pay for and live with a double height HP unit if its not needed, but don't want to undersize either. 

 

I live in 160m2, 4 storey mid terrace house in Bristol. C 1870 Extended to the rear and loft so half of the exposed walls are essentially new build levels of insulation and air tightness.  Floors are fairly well insulated. Mixture of rads and wet UFH. Fully double glazed, half the exposed walls are 300 and 400mm uninsulated solid brick/ stone.

as mentioned we are looking to install an ASHP. my measure gas usage does not match the ASHP installers calcs. 

 

My figures for heat loss at at -2c are  4- 5kw and theirs come out at ~9kw. (i calculated this from measuring gas usage over the year, used degree days to establish a heat loss per degree difference in/out. then see what it gives at -2c )

Annual usage is equally off. Our gas usage was 9000kwh for the year and the calcs suggest 19000kwh. (no calculation by me on these numbers)

For refence that gives a  measured value of 56kwh/m2 vs calculated  119kwh/m2

 

Finally I ran our combi at 40 degree flow for the past winter  (coolest average day was 2c ) and maintained 20degree internal temp fine. but the calculation suggest the radiators would only keep interior temp of 20c with flow of 55c and outside temp fo 3 degrees. which would be far from ideal. 

 

Do calculator tend to over estimate? I have worked hard to reduce drafts, perhaps the clacs are based on an assumption for an old drafty house ?  any advice on if my measured figures seem plausible? The renovation has been done well but not to enerPHit. 

 

opinions on whether oversizing or under sizing is the better option welcome. Our primary aim of ASHP is CO2e reduction not financial. 

 

Thanks in advance. 

 

Joe

 

 

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Are you planning to use any government scheme for ASHP installation, where you are at mercy of the supplier with them working hard to maximise their benefit? If not, are you planning to keep the gas or rip it out? If the former, then you have a backup and can optimise the ASHP size even better.

 

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Hi Olf, 

space does not allow keeping the combi boiler and we are looking to use the UK 5k grant when the gov get round to launching it. so we do need to use MCS fitter.  we have several people quoting so I'm not ties to this one and i think i could make them tweak the calcs to be closer to my readings to get the job done (e.g. increase the airtightness they have used. )

 

we do have back up of a small wood burner so there is back up allowing us to risk under sizing slightly. Although in Bristol there is the risk the wood burner will be banned at some point. 

 

 

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Hi @Jwenty

 

This is a very difficult question to give a good reply to.

 

I believe that there is no alternative but to go through all the major variables that affect the result in detail.

 

In my humble opinion, if you are not prepared to do this, as this is a lot of information, you are stuck with general assumptions.

 

For an example of details to consider, in order to reply people here would require clarity on some items already mentioned:

 

Are you comparing the 9000kWh of gas with the output of the ASHP or the input? What is the expected COP of the proposed ASHP?

 

You calculate down to -2C: Interesting that your in Bristol. As you know ASHP's work best running low and slow: The lowest temperature recorded in Bristol in the last 20 years I think was minus 7 or minus 8C although that is not an all day temperature.

 

A big source of heat loss in winter is to do with refreshing the air in the building. Are you aiming for a MVHR or is it trickle vents, and an airbrick for the wood burner?

 

The devil is in the detail.

 

Oh, and yes we fitted an ASHP to our renovated bungalow that has AIM (Airtightness, insulation, MVHR) and now APE (Air Source Heat Pump, PV and EV.) to a high standard.

 

Marvin

 

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Redo the sizing of the ASHP yourself, then shop around for a suitable unit, ask a plumber how much to fit.

Then ask yourself if the £5k grant is a genuine saving.

 

Most in here already know the answer, as I think you do.

 

What is stopping you fitting a gas boiler outside in a cabinet, or in the loft?

Not that I want to promote combustion technology. Just seems odd to take one out.

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15 minutes ago, Marvin said:

Hi @Jwenty

 

This is a very difficult question to give a good reply to.

 

I believe that there is no alternative but to go through all the major variables that affect the result in detail.

 

In my humble opinion, if you are not prepared to do this, as this is a lot of information, you are stuck with general assumptions.

 

For an example of details to consider, in order to reply people here would require clarity on some items already mentioned:

 

Are you comparing the 9000kWh of gas with the output of the ASHP or the input? What is the expected COP of the proposed ASHP?

 

i'm comparing the out put of the heat pump to the gas used. so the 9000kwh is an over estimate of the heat as it does not include the inefficenciy of the combi. 

 

You calculate down to -2C: Interesting that your in Bristol. As you know ASHP's work best running low and slow: The lowest temperature recorded in Bristol in the last 20 years I think was minus 7 or minus 8C although that is not an all day temperature.

this is what seems to be recommended for my location. as you say i suspect its becuse the thermal mass of the building means it to do with likely daily averages rather than a night time peak. 

 

we do not have heat recovered ventilation. we have extractors for the the bathrooms, and trickle vents. the closed off fire places have louvered vents so can be opened or closed manually. From memory the fire is small enough to not have required a dedicated air brick. 

 

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20 minutes ago, SteamyTea said:

Redo the sizing of the ASHP yourself, then shop around for a suitable unit, ask a plumber how much to fit.

Then ask yourself if the £5k grant is a genuine saving.

 

Most in here already know the answer, as I think you do.

 

What is stopping you fitting a gas boiler outside in a cabinet, or in the loft?

Not that I want to promote combustion technology. Just seems odd to take one out.

Thanks steamy, I'm having a go at the calculations myself, but I'm finding it slow going, its such a mix of materials and finishes. but I'll get there...

i will explore this plumber  option, but this means i need to be even more confident of the sizing if i take the decision solely on my shoulders. 

 

the cupboard where the boiler is, is also the best spot for the hot water cylinder and is not big enough for both. as its a narrow mid terrace there really are few alternative locations on external walls for a flue outlet. and really we are looking to leave gas behind. 

 

thanks 

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HOW did you calculate the heating need just from your gas usage?  What you probably have is an average heat input needed over the whole heating season.  What you need is a peak heating input on the very coldest day in winter, which could be twice your average, so the MCS sizing is probably reasonable.

 

What no company wants, is to estimate too small, install a 5kW ASHP then have a customer complaining that in January it just cannot heat the house to a comfortable temperature.

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42 minutes ago, JohnMo said:

Hi @JohnMo

Just ran my building through this.  Their calculations run as follows for our building:

 

(Floor area of building times assumed wattage per m2 of floor) 100m2 multiplied by 20-40 watts is 2000 to 4000 watts or 2-4kW.

I assume that it implies that the ASHP unit needs to be between 2-4kW input?? but this is not clear?? 

 

(multiply your answer by 2000 for heat loss and hot water) 2-4kW by 2000 gives 4000- 8000kWh. When we had a gas boiler in the same building for 3 years, after we renovated but before installing the ASHP, we used about 5500kWh per annuum. I would have expected the result to be in the lower half of their calculations because of the buildings more than average insulation and a warmer climate on the Isle of Wight.

 

We have the Cool Energy inverTech Air Source Heat Pump CE-iVT9 4.3kW-9.5kW and it runs fine. From Jan 5th 2022, when I started recording, we have used 635.41kWh over 58 days for heating and hot water. The ASHP will run at a maximum of 16amps which is about 3.5kW input.

 

For us this calculator gave a good indication of what we would need. It may or may not work with other properties - I don't know.

M

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Hi @Jwenty

 

Another item that may be considered in the ASHP calculations (which I did planning central heating) is to assume any wall connected to another building had a temperature the other side from your building of 2C in winter. This pushed the theorised heat requirement up but sometimes neighbours go away.

 

What does your less than 2 year old EPC (that you require for MCS ) say?

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3 hours ago, ProDave said:

HOW did you calculate the heating need just from your gas usage?  What you probably have is an average heat input needed over the whole heating season.  What you need is a peak heating input on the very coldest day in winter, which could be twice your average, so the MCS sizing is probably reasonable.

 

What no company wants, is to estimate too small, install a 5kW ASHP then have a customer complaining that in January it just cannot heat the house to a comfortable temperature.

I divided the heat use of the house by the total annual degree days, giving the heat needed for every degree difference between in and out then working of the coldest average day being -2 i multiplied by the temp difference of 22. so this should give the peak load rather than the average as i understand it.  Even if this is incorrect  the overall all annual usages are a factor of 2 different. 

 

agreed that all engineers want to lean towards oversize.  

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2 hours ago, Marvin said:

Hi @Jwenty

 

Another item that may be considered in the ASHP calculations (which I did planning central heating) is to assume any wall connected to another building had a temperature the other side from your building of 2C in winter. This pushed the theorised heat requirement up but sometimes neighbours go away.

 

What does your less than 2 year old EPC (that you require for MCS ) say?

hi Thanks for this it looks like i need more calculations to compare too. interesting point about assuming the neighbours stop heating. that would make a massive difference in our tall skinny mid terrace. in that case we might need a huge HP! 

 

we do not have an up-to-date EPC (it no longer has to be 2 years old it must not recommend loft of cavity insulation. ) but interesting his calculations are even higher than the old epc from before the improvement to the loft ,  extension and insulated basement slab. although i get the impression the EPC calcs are quite basic? 

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3 hours ago, TonyT said:

Didn’t think you could get a grant based system if you had a woodburner already installed?

thanks for the heads up, i will check this but none of MCS surveys have mentioned it yet.... 

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4 hours ago, ProDave said:

HOW did you calculate the heating need just from your gas usage?  What you probably have is an average heat input needed over the whole heating season.  What you need is a peak heating input on the very coldest day in winter, which could be twice your average, so the MCS sizing is probably reasonable.

 

What no company wants, is to estimate too small, install a 5kW ASHP then have a customer complaining that in January it just cannot heat the house to a comfortable temperature.

 

With a wood burner backup, as the OP has, would seem OTT to spec based on the worst case scenario perhaps? No point having an 18kW monolith out there if it's only needed for a few days in the year and you could have just thrown a few more logs on the fire...

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Not sure where the sizing for worst ever case weather comes from.  It is only likely for a few hours a year and maybe only once every few years.  So pretty meaningless.

 

MCS Standard MIS 3005, requires the unit to achieve 100% of the duty at an external temperature condition exceeded for 99.6% of the year.  So you basically ignore those few hours where it could be - lots of degrees.

 

MCS also states

supplementary heat is not permitted from direct electric at external temperatures above the design 
external temperature (“bi‐valent point” or “balance point”), but other alternative auxiliary sources of heat are permitted where this is not reasonable 
practicable in which case the system becomes a ‘hybrid’ system. 
Although additional supplementary heat may be required when the external temperature drops below the bi‐valent/balance point, this will occur 
for very short periods of the year and therefore does not significantly affect overall seasonal efficiency even when direct electric heat is utilised.

 

Heat pump sizing

standard rating condition for ASHPs used to be air at 7oC and water flow at 
35oC. However, in practice they may be required to produce water at approximately 55oC with design ambient temperatures of circa -2oC. At 
these latter conditions the output of many heat pumps could be as little as 60% of the output at ‘standard’ (nominal) rating. For example a 13kW 
ASHP may provide 13kW of heat at 7oC and 35oC flow temperature but it is unlikely to do this at an air temperature of -2oC and water flow at 55oC. 
The actual output could be as little as 7-9kW and hence, if relied upon for the design of the system, it will be vastly under-sized incurring the cost of 
expensive supplementary heating and/or lead to issues of poor comfort.

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30 minutes ago, JohnMo said:

Not sure where the sizing for worst ever case weather comes from.  It is only likely for a few hours a year and maybe only once every few years.  So pretty meaningless.

 

MCS Standard MIS 3005, requires the unit to achieve 100% of the duty at an external temperature condition exceeded for 99.6% of the year.  So you basically ignore those few hours where it could be - lots of degrees.

 

MCS also states

supplementary heat is not permitted from direct electric at external temperatures above the design 
external temperature (“bi‐valent point” or “balance point”), but other alternative auxiliary sources of heat are permitted where this is not reasonable 
practicable in which case the system becomes a ‘hybrid’ system. 
Although additional supplementary heat may be required when the external temperature drops below the bi‐valent/balance point, this will occur 
for very short periods of the year and therefore does not significantly affect overall seasonal efficiency even when direct electric heat is utilised.

 

Heat pump sizing

standard rating condition for ASHPs used to be air at 7oC and water flow at 
35oC. However, in practice they may be required to produce water at approximately 55oC with design ambient temperatures of circa -2oC. At 
these latter conditions the output of many heat pumps could be as little as 60% of the output at ‘standard’ (nominal) rating. For example a 13kW 
ASHP may provide 13kW of heat at 7oC and 35oC flow temperature but it is unlikely to do this at an air temperature of -2oC and water flow at 55oC. 
The actual output could be as little as 7-9kW and hence, if relied upon for the design of the system, it will be vastly under-sized incurring the cost of 
expensive supplementary heating and/or lead to issues of poor comfort.

Thanks for this tougher response. this what i had understood. good point about the variable heat output at varying out side temperatures. the ASHPs i have been looking at look to have good performance and maintain their rated power to the very low temps.  

 

I think all I can do now is re calculate the heat loss and compare the three figures and work from that. 

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4 hours ago, JohnMo said:

thanks for this too, it makes me have more confidence in my measured usage vs the calculated usages. As the house is both heavily renovated and mid terrace it seems the MSC value of 120w/m2 is OTT (heat geek sugesting 110 for an uninsulated single glazed detached house ).   my measured heat loss of 56 kwh/m2 seems to fit well on their scale. 

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