Posted by: @tobyg

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Ah well, so we've had our call with the installers. The calculations they are required to use (I assume required by ECO4) result in the 12kW heat pump in our case, and there isn't the flexibility to do anything else. They also said that the heat demand for a heat pump system isn't directly comparable to our energy use with a different fuel (i.e. gas) - which still doesn't make sense to me. But I'm sure part of what is going on here is that the target design temperature they have to use is higher than we have been having. The bottom line funding-wise is that it's this, or nothing, and we're going to go for it. (The only thing we're left debating is whether, with a potentially oversized heat pump, it's beneficial to go one size bigger with the hot water cylinder, to take advantage of spare capacity, as it were, or whether the reverse is true?)

Thanks for all the input everyone.

oh. not good. So "energy with gas isn't the same as energy with heat pump". completely wrong. energy is energy. heat energy is heat energy. 

They will have used the "MCS method" and come up with a vast overestimate of the heat loss of your property. There is no law or rule that says "the MCS method" has to be used. intelligent installers will listen to input (such as , I know what heat my house needs!)

you are very likely to end up with an oversized heat pump which will be a nightmare to get running stably in mild conditions. there are countless discussions on this and other forums with people with oversized units that give poor COP  when its mild (by which I mean 5-9C) which is exactly what you don't want as this is the vast majority of winter days and it will cost FAR more to run than you think. Read this for a saga of 1 person with an oversize daikin.

strongly suggest you think again before signing on the dotted line. 

@tobyg undersized doesn't mean hot water not hot enough. not at all . a smaller heat pump just takes longer to heat same amount of water to the same temp.

The risk of undersizing is that there isn't enough capacity to heat the house when its very cold, and some allowance has to be made for the negative effect of heat pump defrosts. Gloucestershire will be nothing extreme.

But: just to give you some idea of what the MCS method can come up with. for my house I got 12-14kw depending on how I did it. I measured 11kw by the gas  boiler method. I bought a 16kw heat pump (a 12 would have been fine, but for samsung the 12 is basically a 16 anyway). I actually have "design conditions" this morning for the first time since heat pump install. i.e. its -3C. My house is actually using only about 9kw to keep a steady temperature of 21C. Its the steady temperature thats important. I never let it cool down. fortunately my house (210m2) needs about 3.5- 4kw in mild weather, and thats within the modulation point of that heat pump, so my mild weather performance is ok.

I am fairly certain your house will need a lot less than 4kw in mild weather based on your quoted gas figures. which means you'll have a lot of difficulty getting the 12kw to run stably. 

I really suggest you do something to get a daily or hourly gas figure (all rooms heated), we've got great "test conditions" cold weather right now.

Here's an interesting article about the installation of the Samsung 5kW R32 Gen 6 ASHP by @bontwoody: https://renewableheatinghub.co.uk/samsung-5kw-r32-monobloc-gen-6-ashp

Sorry for going to ground after your last posts @iancalderbank - I've been doing some of the homework you suggested! As it's turned out, last week and this week have been good comparators - one cold, one mild. During the cold week (temperatures averaging around 0c in Stroud) we've been using approx 90 kWh per day; during the mild week (9c) approx 40 kWh per day. I don't know how that translates in terms of the numbers in kW you were quoting in your last post (e.g. "I measured 11kw by the gas boiler method".)?

I hear what you say about the risk that we will find that the heat pump is over-sized. We did raise this concern with the installers, and for whatever reason we are not being offered any flexbility on this; and as I said it seems clear that we either go ahead with this, or turn down the whole funded package. On balance we think the value of the works (partcularly the improvements to insulation) make it worth the risk that we will have some problems and high bills with the heat pump. The best I can say is that we are now going into this forewarned - so thank you for that.

@tobyg multiply by 0.85 for boiler efficiency then divide by 24 for a very simplistic heat loss figure.

@tobyg Installers seem very reluctant to listen to their customers. Im in the middle of a battle right now for a neighbour who needed 6kW of heat and got a 16kW heat pump! Beware once its in they are very unlikely to admit its oversized.

thanks @iancalderbank. So that's giving figures of 3.2 when it's cold, 1.4 when mild. When you say heat loss, what are these figures actually measuring? kW for one hour?

As I've said the heat demand figure the installers have given is 15716 kWh/yr; and they've given a figure for what they call 'capacity at design conditions' of 10.5 kW; I'm wondering if that's their version of my figure of 3.2 when cold.

@tobyg yes, its in units of kwh per hour. you had a figure of x kwh for the day, threw away 15% for boiler efficiency, then divided by to get kwh for the hour.  so you've got a number thats in kw. 

your heat loss by this method , when cold is 3.2 kw. not 10.5kw. 

A factor of 2 between "cold day" and "mild day" that you've got is not unreasonable. Some very simplistic maths to illustrate ( because its proportional to temperature difference)

mild day: 7c. house temp 21C. Difference 14C.

very cold day: -7c. house temp 21C. Difference 28C. Double the difference outside to in, double the heat loss. 

heat demand from an installer of an annual figure in kwh is the wrong way they should be quoting this it to you. it should be a detailed spreadsheet of room sizes, heat loss per room, adding up into a a figure in kw. there is then maths based on "degree days"  - how cold a typical year is - to map that to an annual figure. the figure that determines the heat pump size is the heat loss of the house at a specific worst case outside temp (usually -3C).

what this very rough maths shows that if you ran a different heating system 24/7 (which is how heat pumps work best), at 3.2kw constantly, it would  provide the same amount of heat into your house, and thus achieve the same average temperature over the whole day, as you just did with 90kwh of gas. I'm guessing you run your gas heating on a timer so this number is not exact at all its just a rough approximation, it could be a bit under because of the effect of you timing the gas, but it tells you their numbers are way out, as we all were trying to say up front!

there is also a rule of thumb out there (worked out by users with heat pumps who've been though all this "my heat pump is too big" grief) which says if you have a usable annual figure, divide by 2900 to get heat pump size. For you , this maths gives 5.4kw.

so we've got two rough methods, one got 3.2kw the other 5.4kw.  that should give you an idea where your unit size needs to be. A 3 doesn't exist but there are plenty of 5's.

if you buy a 12kw Samsung heatpump, you will just about be able to get it run at 3.5 to 4kw as a minimum. It will not go lower than that. you'll only get this minimum if your system setup and tuning is really good - which we know will be hard with an ECO4 install. So on very cold days it might be just about stable with your heat loss and not cycle, but it'll be at minimum power, which is not its sweet spot.

on milder days (where you only need about 1.5-2 kw), its output will not drop below this 3.5-4kw figure. So it will spent its time cycling on and off. Its performance, on the days that occur most of the year, where you want it to be running really efficiently, will be cr*p.

go smaller!

This sort of post really makes me think we have oversized our heat pump.  But even the one offering a smaller one never actually explained how and why.  The state of installers here in Scotland is really terrible.

I haven’t heard the divide by 2900 method before. It gives us 6.41, which is remarkably close to the 6.6 that the freedom heatpump toolkit churned out. I think heating engineer churned out 6.9 iirc, so all very close.

Ultimately our pump is sized to be able to keep the house warm at sub zero temps.  We had -15 for 3 nights last winter and regularly get -6 or below during winter.  The issue is that when it is +10 outside we just need a 4kw pump, whoever managed to make pumps which work well across the full range or else one big and one small pump.

Of course if we had a proper control system maybe we could just run a smaller pump continuously, whereas we have a larger one coming on for 10 to 30 mins depending, then off again for 50 to 30 mins, it isn't possible to set it running longer without rewiring everything.

THis is why they need to be tailored to local conditions when designing and setting up.  Someone in or beside say Edinburgh, 25 miles away, could probably use an 8 or 10 kW pump no problem because the weather is usually warmer.

It’s a real shame that the physics of gas compression mean that they don’t have a wider modulation ratio.