@hughf  I particularly wanted to point this out with regard to the samsung 12 that was being proposed. because it is essentially a 16, with some firmware level change that derates the maximum, but that makes no difference at all to the behaviour at low output, because its the same hardware. I have a 16. Today, around 12C , its been running at 3.8kw output off 780watts input (links in my sig if interested). that is squeezing it absolutely as low as it'll go. That overheats the house but not by lots in my case, I get long cycles not short. A smaller / better insulated house such as the OP's will  not play nicely with this unit.

the 2900 rule of thumb originates on protonsforbreakfast.

@iancalderbank, going back to your last post, thanks again for the enlightening explanation. I attached part of the calculations provided by the installers, in case you'd like to see them (though I'd understand if you've had enough - this does feel like a lost cause!). The material

fits to some degree with what you say about heat loss per room etc, and they come to a figure of 9kW for the house (I'm not sure how that ties in to their 10.5kW figure), so again, obviously very different from the figures I'm getting based on current gas usage. Strangely, they actually seem to be under-estimating the room sizes, and also they don't include a loft room, which I'm thinking may count in our favour slightly in soaking up a bit more of the heat (though counter to that we prefer the house a touch cooler than the design temperatures).

I'm also wondering if we could mitigate at all by not running the HP 24/7, even though as you say, that's not how they work best. I reckon on the cold days a couple of weeks ago we had the heating on roughly 10 hours a day.

With ECO 4 we don't get to buy the heat pump, the funding all goes straight to the installer, and as I say it seems to be highly prescriptive about what can be specified and how; that's why we're stuck.

@tobyg 9 these estimates are often out by a lot, double isn't unheard of. are you able to get any other heat loss estimates done? have you done one yourself with the publicly available tools? design temps you have to go with those to get any grants, as its specced for a generic occupier.

on the cold days this system should be fine, its the mild ones I'm worried about. mild days are a large percentage of the days in a typical winter, you don't want to spend lots of money in those days, which you will if you're fighting to control an oversize system.

in terms of schedule vs 24hrs. there is loads of threads on here, have a browse. the think you have to remember in any calculation is that your house loses heat 24 hrs a day. 

Posted by: @tobyg

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we prefer the house a touch cooler than the design temperatures).

I'm also wondering if we could mitigate at all by not running the HP 24/7, even though as you say, that's not how they work best. I reckon on the cold days a couple of weeks ago we had the heating on roughly 10 hours a day.

We have a 12kW Gen 6 Samsung ASHP in an old stone cottage with a heat loss calculated at 9.8kW at -3.7, which I found to be similar using one of the heat loss spreadsheets. The design temperature for the living room was 21C and 18C for the others. However, when it was really cold and I had the heating input balancing the heat loss, I worked out my heat loss at an IAT of 18C and OAT of -3.7C was closer to 7kW. My minimum flow temperature is around 34C when it is mild outside, otherwise the heat pump starts to cycle every 8 minutes as my radiators cannot emit the minimum heat that the heat pump modulates down to.

Like you, we prefer the house cooler and  operate a number of setbacks. The Samsung wired controller thermostat is currently set to 17C from 05.00 to 07.30, 13.00 to 16.00 and 19.00 to 22.00. The rest of the time it is set at 16C, which is effectively off for most of that time unless it is colder outside. So the heat pump may only be running for 9-12 hours depending on how cold it is outside. This is not the recommended way to run a heat pump as the flow temperature needs to be higher when running intermittently, than it would be if running constantly. The COP therefore is lower, but we use less electricity overall and the short cycling is eliminated, which are two big advantages. If we were to run it constantly, then the minimum daily electricity consumption would be 0.75kW x 24 = 18kWh. We only use this much when OAT drops below 4C. If you decide to go ahead with getting the 12kW Samsung, then this strategy may work for you, although it sounds like your real heat loss is a lot lower than 7kW. Let us know how you get on and try to ensure that you get a single loop system with no buffer tank and valves/filters that are not unduly flow restricting.

ok folks, so after the various comments here, we decided over the weekend to have one more go at emailing the installers, raising the sizing issue (not really expecting any change, but to feel we had done all we could, and to have the matter clearly on record). Thanks particularly @iancalderbank for your help and explanations with the figures for the gas boiler method (I think that's what you called it), which I put to them.

They phoned back today - they've calculated this in line with ECO4 requirements, and they stand by their design and heat loss calculation of 10.5kW. But somewhat to my surpise they've said that if we're still worried, as an alternative they would be able to provide a Grant 10kW Aerona, providing we sign a disclaimer in case it's not quite powerful enough in very cold weather. They said Samsung don't do anything between 8kW and 12kW, which I knew, hence the Grant. For what it's worth my impression is that the poeple we're dealing with are conscientious and are trying to be helpful within the parameters as they see them (they've been reasonable on other issues).

So that's what we need to decide - Samsung 12kW or Grant 10kW! I've started looking up the Grant, including on this forum; if anyone here has an opinion I'd be very grateful for it. Thanks once again.

And thanks for the comments in response to my question about trying to mitigate oversizing by not running the HP 24/7; it's reassuring @mike-h to hear how you've approached this, and as @iancalderbank suggested I've looked at some threads on this which also indicate that although it's sub-optimal, it can help matters.

@tobyg lots of grant users on this forum so suggest to get engaged with them . maybe needs a new thread to get their attention.  One thing - check what the actual output of the proposed unit is. The "nameplate" number is not always a true reflection of actual output capacity. The experience with some has been that its not true when you most need it to be i.e. when its sub-zero.  This varies from manufacturer to manufacturer: I'm not saying the grant isn't a 10kw at -3C, I'm just saying please check the databooks.

@iancalderbank - thank you - I'll try to find out the actual output, and start a new thread if I can work out how!

@iancalderbank, in the Grant 10 kW specifications it says:

"Heating Capacity (kW) (BS EN 14511 - air 7oC/ Water 35oC): 11.1", (the placement of the dash there is a bit confusing, I assume that's plus 7c. 35c seems low for the flow temp (radiators or hot water); I was being given figures for 50c for the Samsung. Forgive my ignorance, but might that mean in reality the output would be less?

It also says: "Power input (kW) (BS EN 14511 - air 7oC/ Water 35oC): 2.10"

@tobyg that means it will produce 11.1 kw of heat output, when the outside air is at 7c, and the heat pump water is at 35C. whilst consuming 2.1kw of input electricity. This implies its very efficient (11.1 / 2.1) = 5.3  .  But, a COP of 5.3 is highly optimistic for a real world installation. Only the people with super-super-optimised ASHP heating systems , of any type, get close to that. So, a pinch of salt needed.

if you need to run at 50C then the number will be different. If its -3C outside the number will be different. you need a capacity data table.

I do suggest you make contact with other grant users they will hopefully have been through this. I only know samsung in detail as that's what I have.