JohnMo

No not mixed up, look at the duct referenced. May be a big diameter but the duct is pretty rubbish and every guide says avoid.

You need to limit the use of that type of duct as pressure drops could be huge. You really should be doing MVHR to plenum then 75 or 90mm semi rigid duc to outlet. Then a standard 125mm inlet or outlet terminal. 100s of designs available If the duct needs insulation then add that on.

Really wouldn't waste your time with 12mm. Make them all 16mm. 12 and 16mm all install with Euro cone connectors so will install on same manifold, just a matter of ordering the correct parts. But as said 16mm output is better so don't waste your money. Hope you have plenty of insulation under the UFH.

Truth is, heat pumps for a new build are no more expensive than a good gas boiler. So that really should be be manditory now anyway. Boilers without weather compensation and Opentherm should be banned anyway. PDHW and weather compensation (or Opentherm control) should be mandatory for ALL boilers anyway. This should be mandated by making them only run that way. So have nowhere to plug on off control into. Smart meters made manditory (for suppliers and the public) to allow ToU tariff to be utilised for heat pumps. Then everything @ProDave said.

Do steps internal comply with accessibility rules? No idea of the answer but how would a wheelchair user get about. But answering your question I think people will need a sketch of what you propose. Can you not decouple from the ground and have within the thermal envelope?

Just get one with a big coil, external expansion. I have an Ideal no issues.

I just use a combined inlet outlet grill, so inlet and out are separated by about 50mm. Works great no min spacing required. Various sizes available https://www.paulheatrecovery.co.uk/product/directional-external-combi-grille-for-mvhr/

The other thing to think about is the heating system - UFH is very forgiving on heat source size as long as you set the flow temperature correctly. Our 3kW heat loss house, has a 28kW boiler I use in cold weather, it will run at min load for hours without stopping. Nice average zero degree day, boiler stopped with housecat 20.7, currently 20.9 (snowing all morning so no solar gains), boiler last ran 6 hrs ago, starting and stopping once overall. As @Bramco shows, batch charging on time of use tariff is the way to go. My 6kW is too small to do it successfully so I just use the boiler, until I get a smart meter that works.

Have you looked at the MCS scheme where you get to choose (to certain extent) and install. Umbrella scheme is what to look for. Cool Energy do it and many others. They design, supply the kit of parts to you, up to you it's installed and they commission.

Move the max flow temp a degree or 2 hotter at-5 and see what happens. I am assuming the tank temp is the DHW not your buffer, your plate heat exchanger inlet temp is the same as the gauge so that is your return temp. Leaving temp is 31 so that's the flow temp.

From title seems you may have a buffer, which will be the other cylinder. Are you running lots of zones? Also you may need to balance the flow rates either side of the buffer to eliminate mixing in the buffer. If your house is cold assume you are not trying to run for short periods? If running WC your curve at the cold ends needs to run hotter.

Would give zero insulation value unless installed with air gap either side and allowed to fully expand. Do PIR insulation of same thickness

Have given them the air test certificate to fix the high MCS air change rates? Then when they say you can't take credit for MVHR your heat loss actually reduces.

That is how mine is connected - no mixer or pump, also I have no radiators to demand a higher flow temp. Running fixed flow temp, although boiler thinks it's running WC, via a resistor, and a 0.1 hysterisis on/off thermostat switching between a normal and setback.

Is that based on a combi or system boiler? Combi has an additional heat exchanger so will take an efficiency hit.

This what my boiler data sheet says When I had my heat meter in the gas boiler circuit, I was recording efficiency in the 105 to 110% range on regular basis, that was based on a simple energy used to energy delivered calculation

Trying to learn something new. I know - Opentherm is a communication protocol between controlling thermostat and boiler and is a two way thing. So thermostat is say set at 21 and the control mechanism is to drip feed energy into house to keep house temp pretty constant. Also heard it mentioned that not all Opentherm control algorithms are equal, some manufacturers run boiler harder than others. Does Opentherm work in reality? Do you get lots of boiler cycles? Do you have cost savings in gas? Does it work for radiators and UFH? Assume UFH would need to be directly connected and no mixer valve at UFH manifold to get best gains?

There are are several ways to calculate the efficiency and that is where the higher number comes from, it's the same calculation used for all boilers sold in Europe and comes from a DIN standard I believe.

@John Carroll you have not tagged @j_s correctly. Type @ then carry on typing with no spaces and select from drop down that appears

Not sure how accurate that graph is in real terms. Believe it covers the boiler in isolation from a full system. System effects on efficiency are very different depending on system volume and type of heating (radiators, fan coils or UFH). When you fire up a boiler you have to heat the boiler metal work, interconnected pipes, the water within, then the radiators or floor, before any useful work is actually completed. A boiler running in a small loop, doing no useful work, my loose 12% efficiency on a 30 second run, but you wouldn't have a warm house, so real efficiency is zero. Trouble with most on/off control, is most boilers run full load to get to set point ASAP, so a short run is just full load, then off, so you have zero chance of utilising part load modulation efficiency gains, which comes from prolonged running, or something like Opentherm telling the boiler to slow down work less hard. Believe are also many differences in Opentherm on how hard it allows the boiler to run, Honeywell is run it hard to keep run time short, while Tado is run it slow and long - could be wrong?

Is that running Opentherm or on/off?

Do you mean compressed between two layers of wood?

If you are into condensation mode, so low return temps, then the steam from combustion is being condensed and you are gain latent heat, aiding to the heat transfer process, so yes not made up. Graph below. Read up on boiler condensation mode. All boilers are capable of this and have been for a couple of decades, just installer try their hardest to mess this up, because of the lack of proper training.

It really needs to made to work simply, but it's letting me map an flow average temp and effects based on OAT and flow temp. Straight WC is the answer but due to weather, have no (safe) access to reinstall the OAT sensor.

I am currently controlling my boiler burns, by finding the temperature it likes to run without stopping. Which is 39 degs. Found it was happy to run for hrs - charging a thick screed floor. Boiler min turn down is supposed to be around 6-7kW against a current heat demand of less than 3kW. Have had to fool the boiler in to thinking it's running WC, with a resistor. Found anything below about 36 Deg flow temp cycling was pretty bad. I then run a set of simple home assistant automations, which tells the boiler to fire up at a given return based on outside temp. I then tell boiler to run for between 1.5 to 2 hours. Bit like WC but less stop start cycling and on a fixed flow temp. I am also able to adjust the gradient of the startup so it adds 3 degs max to start up temp, so takes 11 to 12 mins to get up 39 degs generally. So far today (currently -5, and high of about +2 today) boiler has started 5x for heating and 2x for DHW. Have found the gas consumption pretty low and compared to cycling very low.