I have zero issue with a heat pump being oversized. Actual house loss at -3 is 3kW and our installed heat pump at -3 is 6kW. So able to deliver double the heat demand. However we get -9 a few days a year, so zero issue caused by that. Plus we have been able to also heat a summer house 24/7 for nearly no cost.

I could of quite easily installed a 4kW unit, but at -9 the output for heating and DHW would have a unit too small.

Yesterday we had an average daily temp of 8 degs, so heat delivered average was 1.4kW to heat the house and summerhouse over the day. Space heating CoP was 5.3, with the shortest run of 20 mins and longest just over an hour. This morning we had 3 degs and no defrosting was needed either.

Downside it took a bit of extra setting up. But a full open system helps so there is plenty of engaged water volume. System design was just simplified as far as possible, so nothing gets in the way of the heat pump controlling things.

I did my own heat loss calculations when sizing my boiler for renewal around 15 years ago, I also ended up over sizing the boiler because I didn't want to be in a position that I selected a boiler that was undersized. My cousin was a Corgi registered gas fitter, but old school with respect to boiler sizing, i.e. replace like for like... which I didn't want to do as I had an old floor standing Ideal Mexico RS80 I think 70-80,000 BTU's, it was a long time ago.

Can anyone recommend a free online heat loss calculator that is accurate and takes into consideration the room aspect wrt external walls their size, construction, window sizing, type and orientation, floor types, room type and temperature above/below, roof type and insulation. I would like to be able to play with different scenarios/materials i.e insulation, window types (triple glazed, type of gas and air gap sizes etc), loft insulation (different amounts of insulation, thickness and type plus location i.e roof line or floor of loft) etc etc. I also wonder about insulation to ground floor concrete screed and installation of UFH in order to be rid of rads.... but this might be 1 step too far?!

Ideally it would be useful if the model for house could be setup and these variables changed piecemeal to find the most economical next step to reducing my carbon footprint. I think I've done everything that is cheap and economical to do myself - i.e insulation, re-installation of replacement uPVC windows because they did such an appalling job (previous owners had some cowboys in), draughtproofing, insulation especially the draughtproofing of a raised floor garage conversion, etc etc.

I live in a southern 4 bed detached house Gas usage 7600kWh/Yr Electric usage 2000kWhr/Yr, 4kW Solar with 3.6kW inverter, plus a home grown PV diverter when the sun shines (no heating gas usage for most of around 8-9 months of the year)

I've been considering a Battery in addition to my Solar as well as increasing my array size - tried to get info of the consequences of doing this to my SEG (Jan 2012) from Octopus and i'm getting nowhere. Plus after the 1st 4kW of Solar install its quite difficult to cost in any upgrades...

I would be interested in examining the cost of a Heat pump given my Gas Bill (the gas only element) is only around £500/annum and gas is considerably cheaper than Electric per kWh...

Probably a lot to digest and take in here and it probably is not all relevant to heat loss calculations, but mostly it all ties in!

P.S. Thanks Mars the video was a good one!

Heatpunk is very good and I was able to get an accurate result from that https://heatpunk.co.uk which agreed with the sanity check of the annual gas bill.

if you want to do more sophisticated things like solar load or wind chill you need a passive house model (I got an old version demo model). No plumber model ever uses things like that.

The most unknown variable is heat loss due to air leakage. The defaults are tied to year of build & relevant building regs, which are not always achieved or you may have done work to make it better. We had a pressure for measurement, ~£300 and it was well worth it. Eventually potential installers (with some badgering) were able to use the true value.

Posted by: @judith

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The most unknown variable is heat loss due to air leakage. The defaults are tied to year of build & relevant building regs, which are not always achieved or you may have done work to make it better. We had a pressure for measurement, ~£300 and it was well worth it. Eventually potential installers (with some badgering) were able to use the true value.

The last sentence makes my blood boil!

Posted by: @johnmo

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I have zero issue with a heat pump being oversized. Actual house loss at -3 is 3kW and our installed heat pump at -3 is 6kW. So able to deliver double the heat demand. However we get -9 a few days a year, so zero issue caused by that. Plus we have been able to also heat a summer house 24/7 for nearly no cost.

That's fair enough and at 6kW being oversized isnt going to make much difference, and furthermore you probably want the extra oomph for DHW reheat.

However the typical 8kW retrofit is a rather different prospect.  Oversizing may make the difference between:

• needing planning permission or not
• needing pipe upgrades (pretty disruptive) or not
• radiators that fit and radiators that don't, or are obtrusive
• an outdoor unit which dominates the space available, or one which slips in nicely

These are in addition to any (so far unquantified) performance penalty,

If the customer wants to have some spare capacity, along with any consequences, then (s)he is obviously entitled to request it.  Equally if the customer wants a system with just enough capacity, (s)he should be equally entitled to request that.  Currently the customer isn't given the facts, accurate data or the choice, and key available information is ignored in favour of a spreadsheet based on input data which is at best suspect.  GIGO (garbage in, garbage out) is a well known phrase in the computing industry and I would venture to suggest it may apply here. 

Posted by: @jamespa

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The heating industry led us astray in the late 90s early 00s by setting up condensing boilers to run at high, constant flow temperature rather than at the low,

Interesting fact from 2003 - 2 years before condenser boilers became mandatory.

Defra; the heating industry; the Energy Efficiency Partnership for Homes; CORGI; and City and Guilds, is launching the ‘Energy Efficiency Installer Certificate’. The aim of this initiative is to provide 70,000 installers with the skills needed to specify and install condensing boilers and properly advise consumers on high-efficiency heating systems. Following the completion of pilot courses in 2003, a series of ‘Train the Trainer’ courses were held in February and March of 2004, and the first of the Installer courses commenced in March 2004. We aim to pass 45,000 installers through the programme by 1 April 2005 and a further 20,000 (bringing the total to 65,000) by the end of 2005. The Training programme will be funded from April 2004 until March 2005 by the Learning and Skills Council, though initial funding to get the programme started will be provided by the Energy Saving Trust (with Defra support). The training covers:

a brief introduction to climate change and the Energy White Paper changes to Building Regulations coming in April 2005.

the elements of high efficiency system specification (boilers, controls etc)

Those successfully completing the training will receive a Level 3 City & Guilds Certificate – ‘The Certificate in Energy Efficiency from Domestic Heating

So no idea what this training content was, but think it fell on deaf ears and industry not willing to change, otherwise Weather Compensation PDHW etc would be common, not on the fringes of being strange.

Posted by: @johnmo

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Interesting fact from 2003 - 2 years before condenser boilers became mandatory.

Defra; the heating industry; the Energy Efficiency Partnership for Homes; CORGI; and City and Guilds, is launching the ‘Energy Efficiency Installer Certificate’. The aim of this initiative is to provide 70,000 installers with the skills needed to specify and install condensing boilers and properly advise consumers on high-efficiency heating...

Thanks for confirming that the industry knew what it was doing (wrong).

I guess it was just too easy to whack in an oversized boiler, turn it up to 75C, fit trvs and let the system sort itself out whilst costing the customer an extra 10% on running costs and compromising comfort.

Now we see similar behaviour, with the added incentive of a 7.5k government grant.

How to fix it?

Posted by: @johnmo

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Posted by: @jamespa

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The heating industry led us astray in the late 90s early 00s by setting up condensing boilers to run at high, constant flow temperature rather than at the low,

 

Interesting fact from 2003 - 2 years before condenser boilers became mandatory.

Defra; the heating industry; the Energy Efficiency Partnership for Homes; CORGI; and City and Guilds, is launching the ‘Energy Efficiency Installer Certificate’. The aim of this initiative is to provide 70,000 installers with the skills needed to specify and install condensing boilers and properly advise consumers on high-efficiency heating systems. Following the completion of pilot courses in 2003, a series of ‘Train the Trainer’ courses were held in February and March of 2004, and the first of the Installer courses commenced in March 2004. We aim to pass 45,000 installers through the programme by 1 April 2005 and a further 20,000 (bringing the total to 65,000) by the end of 2005. The Training programme will be funded from April 2004 until March 2005 by the Learning and Skills Council, though initial funding to get the programme started will be provided by the Energy Saving Trust (with Defra support). The training covers:

a brief introduction to climate change and the Energy White Paper changes to Building Regulations coming in April 2005.

the elements of high efficiency system specification (boilers, controls etc)

Those successfully completing the training will receive a Level 3 City & Guilds Certificate – ‘The Certificate in Energy Efficiency from Domestic Heating

So no idea what this training content was, but think it fell on deaf ears and industry not willing to change, otherwise Weather Compensation PDHW etc would be common, not on the fringes of being strange.

When our condensing boiler was fitted 14 years ago (too big but who cared?) we were told very clearly that it only needed to be on high when winter was very cold.and at other times it should be lower.

But I forgot to turn it down again! This was the year before price hikes. So several thermocouple readings later we established we could save money by turning it down for most of the year. The boiler did not have weather compensation that came it 2 years later. The limit was 60C since it had no separate DHW setting,

BUT when we had some plumbing work done this year the plumber turned it up to 75 every time he changed anything. So some knew ages ago and some even now after all the advice on saving energy and bills still don’t know!

@judith Yeap, BG fitted a new gas boiler here some 15-16 years back and unbeknownst to me set the 24kWh boiler to 18 as the default setting. (After all, they were selling gas not paying for my consumption!) and as my house had a heat loss of less than half of that, it was not the best set up in the world. It was a condensing boiler but only one control for the DHW and heating 🤨 so I could not set it low enough. I am just a tad wiser now - and perhaps a little more cynical of so called surveyors from BG. Our ASHP runs a lot more economically than that BG spec’d boiler ever did!  Regrets, Toodles.