Posted by: @ian-s

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@jamespa many thanks for your thoughts, my gas usage for last year was 9309 kWh, 2024 was 9655 kWh, 2023 was 9011 kWh. Most of this would of been heating as we use air fryers and slow cookers a fair amount.

@ian-s 

Assuming your house is tolerably warm most of the time (ie that it doesn't cool  more than say 2-3 degrees during and 'setback' period), that clinchs it. Those consumptions indicate something around 3 to absolute max 4.5kW; the 5kW device should be fine (the classic calculation, which is science based and applies to much of mainland uk, is to divide annual consumption by 2900 to get loss).  If your house does cool a lot then it might be more, and if you give some idea of the amount and pattern a rough correction can be calculated.  Most likely it will still work out sub 5kW and you will likely end up with a FT below 49.

This may also help us understand why not many rad upgrades are needed.  The rads may be sized to fit the windows not the required output, so are currently well oversized.

With the caveat about house temperature, and a couple more questions as @mars suggests, i would say this is sufficient evidence to proceed.  If you want more assurance still you need to plot daily gas consumption against average outside temperature, preferably in a year when you operate your gas boiler as close to like a heat pump as it allows.  That's actually what I did, but my house was borderline 7kW not (subject to the comment about temperature) obviously well sub 5.

Posted by: @ian-s

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Heat Geek have been round and worked out our heat loss as 6.5kW which tied in with Aria, what I found interesting is the recommended heat pump is a 5kW Vaillant aroTHERM plus (Anthracite) which I could swap for a 7kW Vaillant aroTHERM pro (Anthracite) for £5 extra. The installer said the 5kW was more than capable of doing the job and it was the premium heat pump offering. Aria quoted a 6kW heat pump.

We would upgrade our cylinder, currently have a 180L slimline to the Vaillant uniSTOR PURE 175L Slim. If we kept our current rads the guaranteed efficiency would be 365% we could change 2 rads from type 11 to type 22 for 375% change another 2 from type 21 to type 22 for 380%.

The flow temperature would be 49°C which to me seems a little high so would be interested in people’s opinions.

49C seems high indeed. 

We had Heat Geek even if we found their heat loss assessment approach problematic. The sizing of the pump matched what we saw from the peak 30mins slot gas consumption for the last couple of years. 

If your cylinder is functional and could be reused, it should not be a problem even if in the case of Arotherm it could reduce the manufacturers' warranty time. We had quite a few iterations with heat geek configurations before we were happy. It seemed more of an art than science. The installation itself was very thorough and the SCOP guarantee for heating and hot water reassuring. I am pretty sure we will not get anywhere as low as the 3.75 they offered. 

Posted by: @batpred

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The sizing of the pump matched what we saw from the peak 30mins slot gas consumption for the last couple of years. 

Well at least they actually looked at gas consumption, most dont.  I suspect they also did the mandated calculation and this was just a sense check.  I wouldnt personally base a sense check on a single half hourly reading, an average over 3-6 hours+ is more appropriate.  

With these things its worth bearing in mind that, if you are going to end up with 5kW because thats the smallest available and anyway you want this for DHW, it really doesnt matter much what the actual loss is so long as its not greater than the capacity of the unit.  And if rads end up oversized because you have designed for 5 when in fact its 3, just turn down the FT and enjoy savings.

The precision needed in the 8-16kW range really isnt necessary at this loss point.  A mature industry would recognize this and have a separate, radically pared down, process, for such houses that more or less any local plumber could implement.  The industry isnt yet mature and, I fear, still values the fact that your local plumber is excluded! 

Posted by: @jamespa

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Well at least they actually looked at gas consumption, most dont.  I suspect they also did the mandated calculation and this was just a sense check.  I wouldnt personally base a sense check on a single half hourly reading, an average over 3-6 hours+ is more appropriate.  

Yes, I meant the 30 minute raw data as the input.

The 3-6 hrs is how your formula worked, I believe. 

Thanks! 

Posted by: @batpred

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Posted by: @jamespa

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Well at least they actually looked at gas consumption, most dont.  I suspect they also did the mandated calculation and this was just a sense check.  I wouldn't personally base a sense check on a single half hourly reading, an average over 3-6 hours+ is more appropriate.  

Yes, I meant the 30 minute raw data as the input.

The 3-6 hrs is how your formula worked, I believe. 

Thanks! 

 

OK, fair enough then. 

I calculated loss based on 24 hour average temperature and gas consumption (actually 12 hours offset because it was a better fit).  I checked against peaks of average 3, 6, 12 hr consumption,  All derived from the 30 min data.  

Hi all

Following from your comments I asked Heat Geek the following questions, their answers are HG - 

1. Why do you prefer the 5kW heat pump.

HG - Our recommendation of the 5kW plus is based on a combination of things including efficiency, cost, warranty, etc.

2. How will the 5kW heat pump cover the calculated 6.5kW heat loss.

HG - The output for the 5kW plus is 6.76 kW so it can handle your heat loss.

3. As we get older would the suggested heat pump cover an increase in house temperature to 22°C/23°C.

HG - We by default design our systems to maintain a house temperature of 21 degrees. If you want higher indoor temperatures in the future, we'd need to redesign the system now to make sure it meets the desired temp of 23 degrees. From there, you can adjust the temperature down. With that said, our heat geek guarantee is based on designing a system that meets 21 degrees (+ or - 1 degree). If you decide to go for a higher house temperature this may result in more time/costs to get your system designed correctly.

4. What does the design flow temperatures become with each of the radiator change options.

HG - 4 rads is 49 degrees; 2 rads is 51 or 50 degrees; 0 rads is 55 degrees

Based on their response I no longer think a Heat Pump is for us, we have been running our serviced every year 15 year old boiler at 60°C so don’t see any benefit especially as we can currently control our house temperature very quickly and easily. I will keep an eye on the heat pump world as technology moves at a very fast pace and who knows how quickly the will advance.

Many thanks for your thoughts and advice.

Cheers

Ian

@ian-s 

I am not sure by what logic you have reached your conclusion.  Just in case there is any ambiguity 'flow temperature' will mean design flow temperature at design OAT, most of the time it will be operating much lower, which your boiler presumably also does if it has weather compensation enabled on it. 

Based on what you have said I am also unsure what your motivation was in the first place for considering a heat pump and why this has changed, as you appear to have quite satisfactory answers to all the material uncertainties and have been offered a performance guarantee.  

However I am also not minded to waste any more time. 

I wish you luck. 

@ian-s We are chilly mortals who revel in a temperature of 22.5 degrees 24/7 throughout the house. Our Daikin ASHP is controlled by a Homely smart controller and on very cold days (-~6 degrees) our system copes adequately at no higher than 50 degrees flow temp (that is our max setting on the pump). We have radiators intended for our heating load and this necessitated the replacement of 8 out of ten existing emitters but the design temperature was to the usual MCS guidelines where we are expected to set a max. of 21 degrees in some rooms and just 22 in the bathroom. We are happy and our consumption is ~25-35 kWh on such a day. Regards, Toodles.

Posted by: @ian-s

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...

Based on their response I no longer think a Heat Pump is for us, we have been running our serviced every year 15 year old boiler at 60°C so don’t see any benefit especially as we can currently control our house temperature very quickly and easily. I will keep an eye on the heat pump world as technology moves at a very fast pace and who knows how quickly the will advance.

...

If I might make one further suggestion, @ian-s....

It is clear and entirely understandable from reading through the development of this thread that you are being cautious. I agree with that approach.

One of the major concerns for most people delving into heat pumps is the feeling of making a leap of faith. Since boilers are normally oversized and set up to heat in short, aggressive bursts, moving to a heat pump is not only a change of technology but also a change of heating approach and that much change is daunting. It is, of course, possible to make the change in two steps rather than one by getting a heat pump installed that can run like a boiler, but that is horribly inefficient and therefore expensive - not a great idea.

However, people don't seem to talk much about the fact that fossil fuel boilers don't HAVE to be run in an on/off way, and indeed in some European countries there is an obligation to install them with weather compensation. Even on this forum, we have bemoaned several times why the UK doesn't set up boilers that way, but I can't remember anyone here ever actually suggesting someone try doing that as a stepping stone towards getting a heat pump.

In your case, I suspect there may be some benefit in doing that, however. I don't know what boiler you have and so I don't know for definite you can do it but in many cases the process is only really a matter of getting an extra module fitted (so the boiler's flow can be modulated as well as turned on or off) along with an external temperature sensor. Given you have been keeping up to date with your boiler servicing, I'm going to assume you have a reasonable relationship with your boiler engineer, so it might be worth having a chat with him or her about whether the boiler can be switched to run under weather compensation.

My reason for suggesting this is threefold. Firstly, if it is possible then running the boiler that way is more efficient and will save you money. Secondly, if you are able to adopt that new heating approach, it'll in many ways add confidence that a heat pump can be an appropriate swap a little later down the line. Thirdly, if you are able to keep track of some stats under that new approach, you've got ideal information to throw at your research into a heat pump when the time comes and can compare much more accurately the real world with what the heat pump installers are estimating; the heat pump at that point isn't a leap of faith so much as a simple swap of heat source.

@jamespa our thought process was to reduce our carbon impact further and with our boiler getting older it might of been be a good time to make the jump. Not saying we won’t but just not now, the info and knowledge you and others have shared will be used when we look again as it has been very helpful. Who knows an even better heat pump might be just around to corner.

Next winter, if you haven't installed a heat pump, I second James' recommendation: condensing boilers can run at quite low flow temperature, which is more efficient and uses less fuel. And it also allows you to pretend you have a heat pump so you can actually measure what power and flow temperature you actually need.

Non condensing boilers have a relatively high minimum flow temperature, below that they corrode, so you must check first which one you have.

@ian-s 

Thank you for responding, at this point we all to frequently are ghosted.

Fwiw my own heat pump journey took 2 years.  Mostly this was due to planning issues but the same technical questions and uncertainty arose.

Having done the switch Im in no doubt it was the right decision and, as importantly, so is my wife.  The house is much, much more comfortable and we are paying the same or less for the heating.  I'm addition we are contributing a factor of 4 less carbon.  If we had batteries we would be saving a lot more, but we don't because we can't yet make the business case work, a situation that will change the moment plug in becomes legal.

I frankly doubt there will be ground breaking improvement in heat pumps in the next few years, but controllers will certainly improve and self-adjusting will become the norm.  That said the adjustments are mostly once only, thereafter it's a case of just leaving it to do it's thing 24*7 throughout the whole season, in a way that I never came close to with gas.

If you want further to derisk, @majordennisbloodnok has some good suggestions.  Collect 30min gas readings also and you can work out a measured heat loss.  However the info you already have is 95% there so that's not essential, it's within the capacity of the proposed 5kW Valiant and that's really all that matters, because additional rad upgrades can be done easily later if you wish.