where in the country are you / what's the lowest external air temperature, and what temperature are you wanting to maintain the house at?

are you planning to have a secondary heat source, eg a wood stove? This would allow for a smaller heat pump as the wood stove could be used to provide extra heat on the coldest 5-10% or so of days, while the heat pump did it all the rest of the time.

Do you plan to have some form of a buffer tank? This can seriously reduce the size of heat pump you need, as it can smooth out the peaks of demand eg on first heating in the morning, so you can use a smaller heat pump.

stuff like this is important in determining what size heat pump you'd be ok with, as well as the actualy sap estimates.

Gavin_A - I am the Isle of Skye. It never gets really cold, probably about -6-7 at worst. It's wind and rain we get not cold!! Probably looking for 20 for a rough internal temp.
I had planned on a stove but after reading some things on here about I'm not so sure. I would need a room sealed stove as building as air tight as possible. Had been reading about the flue being a nice cold bridge. Now just thinking is it really worth the cost/effort to install.
Yes, planned to use a buffer tank or tank in tank.

Nick - yes did look and that's where I originally seen mention of box - x + box - y + z = demand! or something along that kind of idea.

Thanks for the input.

I just felt 25KW heat punp seamed way too big. I would rather it is oversized but not massively so.

Our house is 170ish m2 heated area, 1800s granite, so nowhere near as insulated or airtight as yours despite ongoing efforts. We have 16kw Ashp which is plenty big enough.

various parties tried to size the ashp from calculated loads, but in the end we went with 16kw cos it was only few hundred quid more than the 11 kw model.

One installer proposed an ashp sized to match the total capacity of all our radiators, which are oversized, so that proposal was for very big ashp.

Beware ashp ratings are quoted at standard +7 oC outside temp. At real winter temp, they will actually deliver about half their name plate rating, depending how cold it gets.

Thanks Kotpat, will email. Cheers

Targer,

After I built my house I recorded the energy use over a week and compared it with the SAP figures and my own calculated figures. According to SAP I should have used twice the amount of energy that I actually used whereas my own spreadsheet was within 10% of reality. As others have said, don't use SAP.

If I set the temperature difference to 30C (minus 10 outside; 20 inside) my home-made spreadsheet comes up with a heat loss of 59.3W/m^2 for my house which has higher u-values than yours (0.18) and is a stupid shape (only 100m^2 but high ratio of external wall & roof to volume).

Multiply 305 (your floor area) by 59.3 gives a total heat loss of 18kW. Your house should be less than that.

I'm on Skye as well so weather conditions should be similar.

My house is heated primarily by the sun.
From May to September the UFH doesn't run at all.
In April/October an 800W air to water heat pump collects solar energy from inside the house and redistributes it via UFH as required.
This works even in the depths of winter if the day is sunny enough.
In the proper cold and dark I have a 10kW solid fuel boiler in my workshop (so no cold bridge out of the house) that heats rads in two rooms from where the heat pump takes warm air and redistributes via UFH.

I don't know of anyone on Skye who relies solely on an ASHP. Most people have a stove of some sort (possibly for aesthetics as much as heating). I'd always want some sort of back-up in case the heat pump failed. Maybe a big immersion heater in your buffer tank would do the job.

Hope that helps,

Malcolm

Yes, I'm in the wooden shed at the end of the loch, and yes, I had the first ecocent on Skye (I think).
I haven't investigated how it works with MVHR but Nick at ESP seems to be pretty clued up - I'd be happy to follow his advice.

Posted By: Gavin_ADo you plan to have some form of a buffer tank? This can seriously reduce the size of heat pump you need, as it can smooth out the peaks of demand eg on first heating in the morning, so you can use a smaller heat pump.

Not sure I understand or agree that a buffer tank, which is usually fairly small in capacity of 100 litres or so can reduce the size of the heat pump. My understanding is that heat pump buffer tanks are mostly used for system hydraulics and flow rate purposes and being of such relatively small volume offer very little in the way of thermal storage.

Posted By: Nick ParsonsHeat pump buffers are as big as you make them. Yes, many suppliers suggest 100 ltr, but one installer I know won't use less than 300 ltr. If you are looking to buffer heat so as to benefit from E7, you'd want more than that, I think.

From memory I think I'm right in saying that buffers tanks are sized at around 10 litres per kW output of heat pump and thermal stores upwards of 50 litres per kW.