Heat Pump Sizing?

michaels

So now we are allowed to put the heat pump where I want it on the side of the house less than 1m from the boundary I have got another quote.

1. They only do Vaillant and are suggesting the 7kw is big enough - its specs show it can output about 8.5kw at -7 with a 45 flow rate which is probably slightly more than we need which is about 6.5-7 kw at a design temp of -3 and a flow temp of 43. The 10kw Vaillant is too noisy.
2. We use about 20kwh of hot water a day so basically an extra 1kw of output per hour needed to cover hot water usage which I would say is quite tight on this size Vaillant?

But the more fundamental issue is should we go ahead. Currently we are using a second hand heat pump (10 year old LG 12kw) that I had originally purchased to heat the kids pool, and immersion for the hot water.

Obviously the COP on the hot water is 1 currently and might go as high as 3 if we switch to heat pump? 20 x 365 x 6.5p per unit is £475 a year, reduce this by 2/3rds will save about £300 per year.

Space heating we used (input into heat pump) about 5500kwh last year at a cop of say 2.5. If this improves to 3.5 then that would save about 1600kwh at 6.5p = £100 per year.

Total saving £400 a year, cost with grant - perhaps £5000 seems to have a very long payback period. Of course currently we have a large battery so use 6.5p per unit night rate electricity most of the time, we can't be certain that will always be available. We don't know how long our already old heat pump will last, there aren't really any servicing or even repair options for a heat pump with no provenance - then again we also save at least £100 per year on servicing!

I really want to go for new and shiny but I can't make a sensible economic case to do so until either the current system fails or it looks like the grant might be going away or I guess cheap night rate disappears.

Anyone want to persuade me otherwise or point out something I have missed? Thanks

Alnat1

Why would you expect to get a heat pump grant if you already have a heat pump?

michaels

https://forums.moneysavingexpert.com/discussion/comment/81909760#Comment_81909760

Because it is just used to heat the kids pool.

shinytop

https://forums.moneysavingexpert.com/discussion/comment/81909741#Comment_81909741

I agree. I also can't see the 7kW Vaillant being enough. Your DHW isn't going be a steady 1kW load; it'll be much peakier Also when it's cold you'll lose output because of defrosts. I think you're likely to end up with a cold house and/or using the immersion. I doubt you'll get a DHW COP of 3; unless you have a huge cylinder, with your use you'll need it as hot as possible to dilute down.

Is there a bigger, still quite enough alternative?

NedS

What about the new Grant R290 range - they do a 9kW model. I know the 6.5kW model has been well received and has a great modulation range, but not seen much on the 9kW model yet.

michaels

Thanks, there are as far as I can see the following that are 57db or quieter and at least 8kw

LG 9kw 57db

Samsung 8kw 56db

Viessmann 8kw and 10kw 51db, 56db

And the Grant 9kw and 12kw 54db, 52db(? sounds too low)

Another to add, Ideal Logic Air 8kw, 10kw 55db, 55db

Finally Mitsubishi 8.5 and 10kw R290 which some sources seem to say are 55db but others say louder?

Am thinking the Grant 9kw or Ideal Logic 10kw - any one have any comments or feedback on these? Thanks

unknownuser2u

Your lived data approach is spot on and much more useful than EPC-based heat loss calcs in my opinion.

One thing worth factoring in — your boiler's gas input of 8.2kW isn't the same as heat output. A modern condensing modulating boiler running at low 50s flow temp is probably 90-95% efficient, so actual heat into the house is closer to 7.5-8kW at 0°C. Your extrapolation to ~10kW at -5°C with a higher indoor target still sounds reasonable though.

The issue you've spotted about COP and cold weather output is the real one. Nominal ratings on heat pumps are typically quoted at something like 7°C outside / 35°C flow — not the conditions where you actually need the most heat. At 0°C and a flow temp in the low-to-mid 40s (which you'd need to hit that output), many units also start to derate — you might only get 85-90% of the nominal figure. So that 11.2kW could realistically be delivering 9-9.5kW when you need the full 10kW. Not disastrous but not much headroom either.

Your hot water demand is also on the high side — 25kWh/day is a lot to ask alongside space heating. The key question is how the install is designed to handle both at once. Most setups will pause space heating to prioritise DHW cylinder reheat, which is fine in mild weather but on a cold morning that pause could be felt. Worth asking Octopus specifically how they're handling that.

I'd push them on a few things before agreeing:

• What is the actual rated output at 0°C and -5°C at your likely flow temp, not just the nominal figure?
• Is there a buffer tank in the design?
• What's the cost difference to the next size up? Often surprisingly small relative to the total job.

On your numbers I'd want a bit more headroom than 11.2kW gives you, especially with that DHW load on top.

Magnitio

https://forums.moneysavingexpert.com/discussion/comment/81911121#Comment_81911121

I realise this thread has been dormant for a few weeks, but thought you might be interested in the new Vaillant heat pumps that are starting to become available for installation via Heat Geek:

https://www.myvaillantpro.co.uk/arotherm-pro

They are quieter, smaller and more efficient. The 11kW now has a single fan.

michaels

https://forums.moneysavingexpert.com/discussion/comment/81983552#Comment_81983552

Thanks Heat Geek proposed a zero disruption install of a 7kw vaillant but even based off the Czech output tables I don't think this would be enough. I am also concerned it would be noisy with having to run max out and possibly having tonal issues.