JohnMo

Wow that's a2a heat pump costs?

What 6kW of head room on a max 6kW heat demand? On a house with heat/time constant of 6 days? Not sure I could agree. But I will bow out, and leave you to it.

You need to for someone to do airtightness testing. The are testing the house as a whole for air leaks. Do a Google search in your area. The is not completed until the house is finished.

You would need to find out the modulation rate, inverter heat pump do modulate. So yes you could if the figure you quoted is in the modulation range. Heat pump deliver best running long and slow. My batch charge temp was 30 degs for 7 hours. My WC flow temperature at the moment 23 deg, during the day and 21/22 at night. Floor temp is 21-21.5, room 19-19.5, thermostat are moved to 22 to limit influenced the room heating. Your issue is when the average temp drops enough to require you to put the heating on, the heat demand is really low, and the big heat pump can't cope and short cycles.

Short cycling is where the boiler/ heat pump, starts runs for short period and trips on high return temps. It is then locked out 10 mins and then repeats. Lots of energy goes into heating all the metal in the system as this sucks the heat from the water. Not energy efficient.

I did batch floor charging end last years heating season, works fine, but not always great, as the energy you put into the floor, comes out but not always the way you want it to. If you have under charged the floor the house temp drops a little more than you plan, if you overcharge it will be a little warmer than you plan. To work well you need to constantly modify the charge time, to match the average temperature outside. This was a pain, so this year I moved to WC with a small overnight setback to match the house/floor inertia (offset about 6 hours). My boiler is oversized, but I have a huge buffer (160l), so have room to try different schemes. Current input into floor is varying between 0.5 and 0.8 kWh depending on outside temp.

Some down sides Turn down issues when not at min outside temp. Steady state has a higher minimum kWh loading due to larger motors. Bigger starting in rush current to get motors turning. Bigger cables, fuses etc all adding incremental cost increases Higher minimum water flow requirements. Requiring larger diameter pipe runs, considerable cost adder. Much more likelihood of short cycling, as min flow requirements are not met. Floor hot spots, due higher than ideal water flow temps More likelihood you will need a buffer, offsetting some efficiency gains elsewhere. Big ugly unit in garden instead of a smaller ugly unit in garden. Good possibility a small unit could run on solar in the summer, no chance of the big one doing so. There may be reasons

So font of all knowledge, what is the answer to your question? You have stated everything its not, those with years of building experience are all wrong. Even these (links below) aren't talking any sense, talking about having consistent temperatures (to stop mould forming), cold walls, allowing moisture to form by condensation, then mould growing. https://www.permagard.co.uk/advice/how-to-remove-mould-from-walls#:~:text=When warm moist air comes,are often the coldest surfaces. https://www.oxford.gov.uk/info/20271/guidance_for_private_tenants/1129/preventing_damp_and_mould#:~:text=Mould is likely to occur,house that is too dry! We are all intrigued to know the answer.

Maybe they had cold water storage to mitigate pressure fluctuations

Reality check Why are you sizing for an on time of 13 hrs. If you run on WC your on time is 24 hrs, but your loading is less during the set back time so your -5 degC DT becomes 20.6degC. 150kWh/d becomes(150/24)*13 = 81.25 spread over 13 hours or 6.25kWh per hour. The night time case is less than that, so no need to calculate. I think you need to go back to basics and calculate the heat demand for your house, use the spreadsheet on here. As I believe the way your are going you are talking yourself in to a much bigger HP than you need. Looks like you need a heat pump near 8kW to me.

Are you building a house, garden room or something else?

With Radians comments and Why would you want to increase the UFH flow temp. Your thermostat will be on and off and make things worse. Your making things worse not better.

I had a similar thing last winter, in that the min turn down of the boiler was above demand, even though I have a buffer there was issues transferring the excess heat to the buffer and way to many thermostats. Made some changes over the winter and summer to improve things. My gas consumption average last Oct was 44kWh per day, this year so far 15/16kWh per day (DHW and heating). First your heat demand kW, I assume are based on the coldest temp (design temp), it is unlikely you are at that temperature yet, so heat demand will be lower. Therefore cause you more issues. @markocosic says you can/should be able to set up the boiler to give weather compensation for the heating and fixed flow temp for the cylinder heating. To get the most out of what you have, you should get both upstairs and downstairs to heat at the same time, as single zone and have as much flow through the whole system all the time as you can, to increase demand on the boiler. It is likely your cycling is what using the gas, that almost exactly what mine did. Not on for enough to to heat everything up, off long enough to cool down. So goes into heating up lots of metal and water without doing much heating. You need to trim the flow temperature of the whole system downwards, so the trv and thermostats are not being made (set them a couple degrees higher than you need ( say 22-23). If individual room are too hot, decrease the flow through the UFH / Radiator loop, if it need to be hotter do the opposite. What you need to achieve, is a heating system that is just heating the room to the temperature you want and no more. Thermostats sound good, but not sure they do you any favours.

The heat pump I am referring to, is integrated in to the cylinder, not an outdoor unit such as this https://www.heatershop.co.uk/dimplex-edl200uk?gclid=CjwKCAjw7p6aBhBiEiwA83fGuv1lhBgRkNTKXeOCKrMAh88nzCQ7629o7blDnXEtkFHPAkp4AdBQrRoCP5gQAvD_BwE

Ali Johnston Ltd, trading as Timber Central based in Nairn (NE Scotland). But go to England etc. They are on facebook. All my Scottish larch came from them, good delivery, price etc. Very flexible on profiles and lengths. They do Douglas Fir and larch.

With cold connected to the top each time, or at least the blue isolation valve. So not consistent with the original statement

Last Oct I used an average 44kWh per day, mostly due to short cycling. Fixed that over the summer with some system modes and setting up WC with a few calculations and a bit of fine tuning. House is also very well insulated.

Bit confused by your photos as the flow meter move from top to bottom rail? No flow normally means air in the loops. Also the manifold being higher than the boiler is encouraging air locks.

Single zone with weather compensation. With night time setback of a couple of degrees. Have the thermostats set slightly higher than the target room temp of 20, to keep the UFH manifold pump on. Bedrooms temps are 18, set by slowing the flow rate to those rooms. Last year I worked out the how long the floor takes to heat up and set start time and end time of the day time temp around it, so starts at 0230 and at 1830. My current flow temps are around 23 degrees. According to the boiler datasheet, I should be getting around 110% efficiency. Not sure how true that is. Gas usage including DHW is averaging around 15/16kWh a day over the last 10 days in NE Scotland.

To look after any dips in pressure you could install an accumulator and a check valve upstream. The accumulator would be pressurised when the pressure is high and iron out the dips mostly. 50 or 60 L maybe big enough, other will chip in if I am talking rubbish. 12kW on demand heater, could be costly to run as it would use peak electrical, not off peak and still give a rubbish flow rate. Other option for heating would be an exhaust air heat pump (EAHP) cylinder, a little bigger than an normal cylinder, with heat pump incorporated. Accumulator, EAHP, would gibe you a pressure stable unvented cylinder, with a cheap way of heating it

Metal building is a conductor from roof to ground. A metal roof is top of a building that is generally a conductor on top of and insulator, so the risks are different No ideal if it makes a real difference. Ask your electrician he may know?

Sound like your spending thousands or a few hundred, to save a few pounds in running costs.

We have compacted type 1, then crushed lime stone. Any gravel type protects you use, make sure you specify a crushed rock and not sand washed (rounded) pebble. Crushed rock locks its self together, so it compacts and doesn't move much. Anything rounds just moves about and is a complete pain.

Sorry plumber is making assumptions and is wrong. Modern Air to air heat pumps, or Aircon as it also known, is really efficient. Here is a typical test report. EER is the cooling efficiency and CoP the heating. An EER of 1 is one unit of electricity is equal to one unit of heat, same for CoP. The higher the number the better. You will see a CoP of 7 at +7 degs. 1kWh input would give you 7kWh out. Eurovent-HP-MU2R15-UL0---MJ07PC-NSJ---MJ07PC-NSJ-1661239885-ba-9646a4f3.pdf A normal boiler and radiators has a CoP of around 0.8 or less.

The maths make natural ventilation a non starter as once your insulation is good, the ventilation heat loss becomes huge. The cost of running MVHR compared to heat loss from ventilation is low. Also building regs don't allow natural ventilation, unless you leak like a seize.