JohnMo

At least 50mm is my vote

Just reread the £24 or 80kWh per day. Are you sure it's insulated? Or is it really big? and the doors open all day? If you put a log burner in, you would get no work done, you would be forever loading it up with wood.

Or if you want a high efficiency A2A heat pump. Mitsubishi Heavy Industries SRK25ZSX-W/SRC25ZSX-W 2.5kW

I think the thermostat calling for heat while weather comp is operative goes against the basic principles of weather compensation. The weather compensation basically runs 24/7, so isn't looking for a thermostat. The controller follows the heating curves as outside temp varies, and if you have a setback it just lowered the flow temp a few degrees.

If you look at some of my recent replies to other threads there are some basic heat calcs that illustrate the heat loss with and without insulation

For clarity is the builder suggesting not to insulate? Other that what are the differences?

Another alternative is a cylinder incorporating a heat pump as linked to above, for hot water only. And a small heat pump for heating/cooling.

I had heard the same, they have been saying the same for years. I have preheated water going in to the boiler via a solar PV heated cylinder, so was using the combi, topping up DHW to 48, and still using bout 8-9kWh of gas a day. I read a bit about Alpha Boilers Superflow (preheat cylinder on a combi) and the instructions made a big thing about making sure the DHW was turned up to the max, so gave it a go. Changes made early June. May monthly gas usage 269kWh (no CH heating, just DHW) June 105kWh, used loads extra messing with settings over a couple of days as a trial. Should have used around 70kWh for the month. July 64kWh August 104kWh, less solar output going spare for preheated water. So seems to have made a quite big impact, £10-14 per month lowering of DHW costs.

You've been busy.

May try upping to 70 also

How do you do it?

Wow that only an average of less than 9kWh per day. Are you bulldozing a passivhaus?

I just paid £250 for airtest, MVHR commissioning done at same time for £200.

If this is a combi I have found the same. I upped my temp to 65 and have a significant drop in gas usage. Tried at 48 and up, so 65 has been the best for gas usage.

We put 12mm2 in. Doing water pump treatment plant compressor, and discharge pump. Since installing have taken a power supply to garage, and will possibly have a small ASHP connected also.

I would check your solar gains at this time of the year. As said once the heat is in its difficult to get it out with out Aircon. Have you thought about a split Aircon system, high does heating and cold very efficiently.

We have a 192m2 floor area, UFH pipes on 300mm centres. But with a smaller area you have watch flow rates as they can easily be way to low for a heat pump. Ours is about 12-13l/m so ok for a 3to 5kW heat pump. If the floor area was smaller you need to looking at 100-150 spacing to ensure you have a suitable flow rate to keep the HP happy. Don't base any calc on having solar gain, as it may not be there. Couple of overcast days with no gain can make a big difference.

I just had a look at my boiler instructions and it states the flue should run with a slope back towards the boiler. So the flue is always going up hill from the boiler to discharge point. This is to allow the condensation to run back to the boiler.

I am in complete agreement, it's a logical easy enough plan to follow. Only thing I would add is to take all the timers off the central heating, set the thermostats to around 24 and trvs fully open. Start the testing at a weekend when you home for a couple of days.

I just used 70mm PIR upstands on all external walls.

But your only moving the heat a degree or so either direction. No different to an overnight setback, then adding the temp back in a couple of stages, which allows the heat pump to gently heat the house up. Which is pretty much what heat geek and others and your general common sense says is the right thing to do for the best CoP. Basically slow and gently does it. The OP hasn't or isn't doing that, he has most of the house heating off, which doesn't let the heat pump flow at the correct rate, and the rooms he is trying to heat, cannot heat up well because the heat is moving to anywhere colder in or outside the house. Those two things on there own will give a rubbish CoP.

Me also, wife wasn't keen

Would work, but Something like this would be much better or something from Duco or passivent https://www.ventilationland.co.uk/product/44535/itho-daalderop-cve-s-s-co2-optima-inside-package-hp-415-m-h-plus-build-in-rv-humidity-sensor-and-co2-sensor-plus-4-ventilation-valves-plus-auto-rf-control-perilex-plug.html Then you just demand controlled inlet and you are good to go.

Are you just reading my first reply and rewriting it?

You don't have to have trickle vents, things like this are also available from various manufacturers https://www.aereco.co.uk/products/air-inlets-uk/eht/ https://www.bpdstore.co.uk/glidevale-energy-saver-humidity-sensitive-trickle-ventilator/p/182