Calculating heatpump size (using gas readings to help?)

Hardware Heatpump
1

New to the forum and I’ve been trying to do as much reading as possible!

I’ve put my house plan into heatpunk and have played around with the air changes, but I don’t know exactly what our air tightness is, and obviously this makes quite a big difference to the heat loss calculations!

I was fairly happy with the idea that a Vaillant Aerotherm+ 7kW would cover out needs, however I’ve really been put off by all of the negative comments about noise in Vaillant Arotherm: excessive noise from compressor. And also the more I read about heat pumps, the more I realise that oversizing will mean that the unit can modulate down as well (leading to more cycling in certain conditions, which is bad).

I have started to wonder if I can get my heat loss low enough to be covered by the 5kW unit (which seems to have much better reviews?), however I have also seen the posts about baking in extra capacity for when the weather is both cold and damp, therefore I don’t want to go too low!

I was wondering if I could use my gas bill from previous cold spells to estimate my max heat loss requirements?

The most extreme gas usage I’ve seen since living in the property is:
18th Jan 2024 (between 0c and -6c outside, 19c inside)
124kWh gas used
85% efficient boiler = 105.4kWh of heat produced (including hot water)
105.4/24 = 4.4kWh average heat demand

Most of this usage is from 7am-11pm, 8kWh being the highest usage period (7-8am), fairly consistent 7kW/h between 9-12, and 6kW/h until 11pm.

So, based on this, would I be able to take the average hourly heat usage figure of 4.4kWh and go with a 5KW unit, or would I need to look at the max usage figure of 7kWh?

This year I have increased the loft insulation from ~150mm to 300mm and I have also been adding 100mm PIR between the floor joists (should be completed before this winter hopefully).

We also have a log burner which we could use in extreme periods.

The heatpunk calculations (when temperature is set to 20c and air changes set to 1) come out as a total heat loss of 5370 W, which is covered by the 5kW unit at 46C flow temperature ( 5676W), however I don’t think heatpunk take into consideration the drop outs caused by the defrost cycles Vaillant maximum output capacity testing.

Would love to hear peoples thoughts on any of this?

3

Hi @ash, and a warm welcome to the forum.

You are asking the right questions, and hopefully you will get plenty of learned answers here. I’d just make a couple of general points:

  • No matter what your heat loss actually is, remember that it’s what your installer estimates that’s important if he will be applying for the all-important BUS grant on your behalf, and that he has an incentive to oversize your heat pump. (Two incentives actually - he makes more profit from selling you a bigger HP, and you are less likely to complain that your house isn’t warm enough.) So be prepared for a debate with him if you think his estimate is much too high.
  • There is a (minor) downside to choosing a HP that just matches your maximum demand - you’ll have less scope for roomstat setback at low outside temperatures, as discussed in House Thermal Inertia and Roomstat Setback (some cautionary notes). (Personally, my economics favour a night time setback, and I’m glad I chose a HP with a couple of kW spare for house reheating in the morning.)
  • It may pay you to review several HP manufacturers/models, and then choose an installer familiar with the brand. If you choose your installer first - especially if it’s one of the nationwide ones - you’ll end up with his standard brand, come what may.
  • As you will soon see on this forum, there is no perfect heat pump (they all have good points and bad points). But in general, I suspect that you get what you pay for…

Keep us informed of your progress!

4

I think you’ve got it more or less right, other than you don’t need to factor hot water into your heat demand calculation - making your actual heat loss a little bit lower than you’ve suggested (depending on how much hot water you used that day).

Also, if you’re after the BUS grant then what you think your heat loss is will be more or less irrelevant, as the installer will do a heat loss survey to MCS guidelines and decide what they’re comfortable fitting. In this case you’re performing the calculation to satisfy yourself that they’re not putting in something crazy oversized.

5

An approach to getting a ballpark sizing from your annual gas consumption (yes, really!) is described in this article. Michael DePodesta provides supporting calculations, explanations etc. The rest of his stuff is worth checking out as well.

6

I have an oversized Vaillant- it’s the 10kW rather than the 7kW which would have been big enough. I have two observations 1. Vaillant is pretty good at copping with oversizing - my system is doing pretty well in the league table despite being oversized. 2. The cause of the 7kW having poor reviews is mainly under sizing. In frosting conditions it doesn’t maintain the documented output. This coupled with a big incentive for people on the margin to opt for the single fan 7kW unit over the double fan 10kW, that until recently needed planning permission, meant that quite a few people ended up with a unit that couldn’t keep up with demand.

On balance I would go with the 7kW as recommended by your installer I doubt you will regret it.

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