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Heat Pump sizing - Further adventures of a boiler owner

Air Source Heat Pumps (ASHPs)

Last Post by JamesPa 10 months ago

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IanMK13

(@ianmk13)

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Topic starter 01/11/2024 4:47 pm

A couple of weeks ago I posted a graph of the gas consumption of my boiler versus heating degree day (HDD) that I created over eighteen months ago (here). Since I’ve learned quite a bit in the period since I originally created the graph, I decided to repeat the process for intervening years. I changed my approach this time, creating a separate plot covering the whole of each heating season rather than for a calendar year. I used weather data from my weather station to calculate local HDD data using the method detailed at degreeday.net using a base temperature of 15.5°C as derived by degreeday.net’s regression tool (which use my local airport’s HDD data). To validate my newly-calculated data and methods, I repeated my earlier plot. The trend lines were almost identical - y=12.784x+13.485 and R²=0.9237 this time against y=12.822x+13.658 and R²=0.9236 previously. My other new plots are as follows

It can be seen that 2021-22 was a relatively mild winter with the daily average temperature barely clipping 0°C on a couple of occasions. This was not the case in 2022-23; the second week of December and the third week of January being particularly cold (several degrees below zero). 2023-24 also had its share of cold days, although not quite as bad as the previous year. The boiler struggled this year as I had downrated the boiler and reduced the flow temperature at the beginning of the heating season. I was also playing with pump speeds and monitoring flow temperatures but was unable to come to any conclusions; I suspect the TRVs were affecting the situation. I also experimented with having no overnight setbacks last year.

My interpretation of the plots and trendline equations:

It looks like I may have turned-down my DHW thermostat in 2022.
There was quite a cold spell when I was away on holiday in October last year (Heating and DHW turned off). On checking, daily average temperatures dipped into single figures during my time away.
It’s not clear that my experiments at turning-down the boiler have been successful in monetary terms. Comparing the trendlines for 2021-22 and 2023-24 (cf y=mx + c), m is greater for the later year but I don’t believe it is fully offset by the reduced value of c.
It appears as though I need a 10-11kW heat source - but only for 2-3 weeks per year. A 7kW source would suffice at other times

Bearing in mind the disadvantages of a 10-1kW ASHP (in particular, the size), are there other viable options that merit consideration? Two smaller ASHPs? A gas hybrid heating system? How would radiator requirements be determined for a hybrid system?

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ashp sizing

Toodles

(@toodles)

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01/11/2024 5:03 pm

I have seen suggestions of using auxiliary electric heaters when it is only a matter of perhaps 10 - 15 days during a full winter season. Might this cover your needs?

Thanks to failures of British Gas to orchestrate prompt repairs when we were still heating with gas, we have two very compact 1 or 2 kW fan heaters plus a convection heater we bought years back. These all stand by in readiness for such occasions - though the Daikin 8 kW keeps us cosy at -7 or -8 degrees as we experienced last Nov/Dec for a few days. Regards, Toodles.

Toodles, he heats his home with cold draughts and cooks his food with magnets.

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IanMK13

(@ianmk13)

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Topic starter 01/11/2024 5:09 pm

@toodles Another option just occurred to me: If I get a little notice of the particularly cold snap then perhaps I could quickly book a beach holiday somewhere hot..

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Toodles

(@toodles)

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01/11/2024 5:15 pm

@ianmk13 Perhaps have a word with @davidalgarve 😉 Perhaps he could put you up! Though I should avoid Valencia area at the moment - they seem to have some flow problems.Toodles.

This post was modified 10 months ago 2 times by Toodles

Toodles, he heats his home with cold draughts and cooks his food with magnets.

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DavidAlgarve

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01/11/2024 5:31 pm

@toodles Are you going into competition with Booking.com? What are your commission rates?

I am rather occupied at the moment, with a big Birthday, a hip replacement operation and a cataract operation due. Otherwise OK!

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Toodles

(@toodles)

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01/11/2024 5:49 pm

@davidalgarve I’ve done several ‘big birthdays’ had cataract surgery (and much more too!) but still have my original hips! My commission rates are (behind hand in front of mouth - mumble mumble.)😉 Toodles.

Toodles, he heats his home with cold draughts and cooks his food with magnets.

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JamesPa

(@jamespa)

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01/11/2024 6:35 pm

Posted by: @toodles

↑

I have seen suggestions of using auxiliary electric heaters when it is only a matter of perhaps 10 - 15 days during a full winter season. Might this cover your needs?

There is sometimes quite a good argument for this and many heat pumps do have the capability to drive electric heaters inline. Check actual specs at design temp & FT, eg Vaillant 7kW is actually 8.6 at -2 and 45, but quite a few heat pumps only give their rated output at a rather restricted range of conditions.

That said the Mitsi 11.2kW R32 unit (and the R290 when they release it doubtless) or the Samsung 12kW unit arent much bigger than eg a Vaillant 7kW so consider these (or other manufacturers ) if they fit in space/noise envelope. Ideal also do single fan higher capacity pumps, sourced I think from Midea (the R290 ones anyway).

Remember the BUS requires that the unit can cover the entire heat load, but it seems that 'the unit' includes (at least according to the interpretation of some installers) any linked backup heater.

Finally how are you estimating your boiler efficiency. It could be more than 100% (up to 110%) if its actually condensing (most don't because FT is set too high) or 75% if its very old. This makes quite a difference to the conclusion.

2xheat pumps is of course possible, but you wont get it in under permitted development and it will cost a lot in ancillary kit which installers will want to put in, and there will be installers that wont touch it. Personally I wouldn't go there at 10-12kW, there are some perfectly good reasonably sized (ie single fan) solutions in this range.

Put these into the mix and you should be able to find a comfort spot. Feel free to ask more particularly as you move towards a conclusion. Then of course you have to persuade an installer to take notice of real measurements, which many wont (but some will).

As a word of warning - there are those (perhaps the majority if you include installers) who dismiss the 'what does my boiler use' method due to the uncertainties in boiler efficiency and how the boiler is used. MCS only formally recognises the fabric spreadsheet (although I do somewhere have an email from MCS recognising that it has limitations for retrofits). So you are a bit out on a limb relying on boiler measurements. That said its what I am doing for my install (which, following 2 years of hassle with planning, is now hopefully going to happen in the next couple of months).

This post was modified 10 months ago by JamesPa

This post was modified 9 months ago 2 times by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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