JohnMo

If you look at your manual for the boiler, section 5.12 allows to set the boiler with independent flow temps, for the cylinder heat and CH. Not sure if your installer did this or just what they normally do. If on S plan maybe not. If wired per manual you could lower your CH flow temp and the setting on your mixer. This would lower the output of the boiler to match more closely the mixer temperature. Also noted from the manual is your min turn down for CH is 4.3kW, this may be ok on a cold day - but any other time you may have some boiler short cycling. Low energy houses and normal plumbers don't mix very well, they try to do 'what they always do'

What boiler do you have? Any idea on what plan it's wired as?

The basics are For a given temperature difference between inside and outside the average heat loss will always be the same. If you increase the temperature difference i.e. its colder outside, you need to add more heat to floor to compensate for the heat loss. Your controller will measure flow and return and alter one or the other to a hotter set point as it gets colder outside, and less hot as the outside temp increases. Now if the sun comes out and even though its cold outside, if it heats the house through thermal gain, the thermostat will trigger and tell the heating to go off. Once the the houses cools the thermostat will then tell the heating to come back on again. Now the problems come form having a thick screed and react times. Our house has a 100m concrete floor and it takes about 6 hours to move a degree, so if I'm not careful the house temp can be all over the place. Its a bit of a process to find what suits your house. Trouble for most the controllers they are great for normal leaky houses, but low energy house with thick concrete floors it all a bit hit or miss with the control.

Comments on your layout Utility, very short run from manifold, not high flow rates, so not sure you require a double duct run. Similar comment to other extract duct runs. You could shorten supply duct runs everywhere by using coanda effect supply nozzles, these are wall mounted instead of ceiling, and the air will move across a ceiling for 4-6m, before dropping and then traveling across the room towards an extract. Something like these https://www.epicair.co.uk/collections/plastic-flat/products/supply-air-valve-coanda-effect-8960-90mm You seem to doubled up extract ducts, but supply runs seem longer and are single duct, so this needs sorting out.

For our master I did similar, one supply in bedroom 35m3/h, but a second one in a wardrobe/dressing room, think it was 15m3/h. Extract in ensuite set at 35 m3/h, so supply air over and above 35m3/h has to travel through the rest of the bedroom and down corridor to the next extract point, keeping the corridor supplied with fresh air also.

If the curve is set correctly the room temps manage themselves, with UFH the response is so slow feedback on room temp is pretty much meaningless for room compensation. As the flow temp is constantly being adjusted but floor takes hours to respond, when it the control changes the settings and it just chases its tail. Your thermostat just needs setting a degree or so over your target temp. If room temps are getting to high, compensation curve needs lowering, if individual room are getting too hot the UFH loop flow needs to be reduced.

I also had quite a few issues when I started using the UFH. Ours is 193m of UFH in 100mm concrete, which I think equates to 56 tonnes. So heat up times are long. Our first heat of the season uses loads of energy. You are doing the same every couple of days. Couple issues I see 1.Your flow temp in the floor is too high if you are overshooting that far. I have the temp set a 30 and batch charge each night, for a out 6 to 8 hours depending on outside temp, have a single thermostat set at 18.5, which in reality switches the heating of at 18.75 deg. The heat then settles at 19 an hour or so later. Once charged up the floor keeps the house stable for the next 16-18 hours, the process then repeats. 2. Can you reduce the firing temp of the boiler, it doesn't need to be that high, unless it that high for the cylinder? 3. Are you sure you are not short cycling - boiler running on off for short periods of 10min or less? 4. Do you have a buffer?

I'm still on gas but been doing lots of reading, I need a cooling solution as much as anything else. Heat pumps generally run a pressurised system same as combi, so if the freeze valves open the system would loose pressure. If the system is operation ie you have power, the pump would circulate to protect the system. Only in a prolonged power cut would the water drop below 3 deg and the valve open.

It has a lower heat transfer coefficient compared to water, so you have to run a degree or so warmer.

Two types of heat pump Monobloc, everything you need to get hot water is outside, water pipes connect to outdoor unit Split, smaller outside unit, but refrigeration gas pipes connect to indoor unit some of the stem is indoors, water connects to indoor unit. Hex unit, just a plate exchanger and pump in flashy box. Less efficient do to a temp drop across the heat exchanger. Glycol anti freeze, stops water pipes freezing. Without a heat exchanger the whole system has to have anti freeze in it. Split system refrigerant doesn't freeze at normal winter temp. Alternative to anti freeze is freeze valves, these open when they detect water temp at or below 3 deg, dumping water to the floor.

The above all run at 100% efficiency. So if 9kWh was available you would get 3 hours of heat for one the above rated at 3kW. Something like the below on eBay, would give you 9 hours or more based on the same output to the room and the same 9kWh eBay item number: 195191096004

If the heat pump is heating the air the ventilation efficiency at heat recovery is zero. You are not recovering any heat from the outgoing air. The heat added by the heat pump is being generated and used by the house the same as another radiator would. Efficiency improvement may be there but will not be massive. Assuming an ACH of 0.3 air transfer can deliver about 10W/m2 (if I am remembering correctly), without the air smelling of burnt dust.

I made a similar one. Then had someone with certified equipment come and 'official' commission so I got a test cert with calibrated equipment certificate. He remarked how close the initial setting were compared to final, I think one terminal was slightly adjusted. So works well.

PIV is a ventilation system, you are just blowing in, instead of sucking out. Assume ventilation rate is based on humidity CO2 etc? You may want to get humidity based trickle vents also, so if the humidity is below 50% the trickle vents close themselves and open as the humidity rises. I would use something like 0.3 ACH.

Couple of different ways to look at buffer and also what the heat manufacturer states as a minimum system requirements. Some manufacturer will have a minimum system volume and minimum flow rate. Some will specifically demand a buffer of minimum volume otherwise warranty is void. If you have UFH with a mixer valve you have no idea of the volume flow through system will always meet the min flow requirements as the mixer recycles a lot of the flow. Vaillant 8kW demands a flow between a min and max of 380 and 1400 l/h

I think it depends on make and model, some are just on or off others will modulate, but suspect most on the market are on or off. The only thing being smart is the internet connection.

This my system now. So basically a combi boiler that can take preheated water. It will do three showers at a time - just. Doesn't use much gas in the summer, uses mostly PV solar divert to heat.

You may be better with a big thermal store, size the boiler to charge the store, do as a DHW tank in tank or big DHW coil, seen them at 6.3m2 and 8.25m2. Charging coil about 2.5m2 in the same store, will work at low temps. 1000l? https://thermal-store.co.uk/1000l.php#1000l-lmt Then it would work down to about 50 degrees, or wack up the temp to 80 when you are full. No need for a buffer at all, just size to suit the storage you want and the DHW needs. UFH would come direct of the store, have a tapping for each manifold pump, would give you a simple system with full hydraulic desperation. Tonnes of hot water flow also.

Update October final figure So gas consumption compared to 2021 2021 - October used 1,370 kWh for DHW and UFH or (assuming 5kWh DHW) 1,220 kWh for UFH only 2022 - October used 562 kWh or 412 kWh for UFH only Saving 59% Interesting the Jeremy heating spreadsheet, for November predicts 884kWh for heating only. So basically used half that, 2 people in house plus dog and solar gain all help. November Issues and changes made at the start of November Weather Comp switched off New UFH mixer, now have an IVAR mixer, with low temp thermostatic mixing valve. Much better control than the previous Reliance one. Heat meter installed upstream of UFH. Had the heating off for a couple of days at the start of November. When I restarted it the gas consumption started getting much higher than I was expecting. Had a fiddle with WC curves, but this made no real difference. WC was dumped in favour of charging the buffer to set temperature based on a thermostat and running the UFH in batch charging mode. UFH starts at 1 am and continues running until the room thermostat (in hall) gets to 18.5 degs, it then is switched of the UFH. Basically all UFH loops are open except bedrooms on a single zone, bedrooms are heated by opening the doors to the rest of the house. Still fine tuning, but flow temp set to 30, buffer (thermal store) I have set to 50 degrees, this ensures the odd opening of tap doesn't fire up the combi as the water leaves the thermal store DHW coil hot enough to by passed around the combi. November gas consumption compared to 2021 2021 - November used 1,754 kWh for DHW and UFH or (assuming 5kWh DHW) 1,604 kWh or UFH only 2022 - November used 1,138 kWh or 988 kWh for UFH only Saving 35.1% Interesting the Jeremy spreadsheet, for November predicts 1102kWh for heating only. This is showing less of a percentage difference than October. Explained by more heat being lost to the building fabric, possibly less solar gain, but still some. Average temp below zero for next week, so will see how this goes.

So your heat pump needs to be no bigger than 8kW, with a small buffer. 11 kW would be huge.

People love reinventing the wheel and spending loads to replace something tried tested and works. Smart thermostats and Trvs everywhere is great in theory, but be careful they don't increase oil consumption when the boiler starts short cycling, when it's only got one circuit to manage and has little or modulation.

Just looking at your heat loss calculation at 10kW. This looks very large for your u values. Our house has very similar u values, but is 193m2. But at -5 we have a heat load of 3.2kW. So yours should be in the ball park of around 6 to 7 ? My biggest issue is a gas boiler too big, at low flow temps it will do a min of 6.7kW, against a min demand of 0.5 (ave 10 deg day) and max 3.2kW. I have thick screed so chuck loads of heat at a time and use as a storage heater if I want. With the amount of water you have in the floor, operate as a single zone and you will not need a buffer. I would double check your heat demand, get as smaller gas boiler as possible. Going big is a needless expense and is a pain in a low energy demand house. You have about 1km more pipe in the floor as me as I am on 300mm centres and 7 loops and with a floor u value of 0.09, a flow temp of 30 degrees for 7 hours is enough to keep the house warm until the next day. Would expect a flow temp nearer 25 would be enough for you. Not sure who did your UFH design, they are quoting 113W/m2, and 40 degree flow temps, that's getting to be about 40kW going in to the floor, bit bonkers.

Your calculation for heat loss is now 5.2kW, considerable drop

Perspective 3 person household consumption of DHW is 5kWh Number of design days per year, very few. 31% of 5 x very few, is not much, full cost electric for an immersion.

Per reply in other thread. I think if you get your ventilation heat loss sorted the DHW will be fine. Also this is a calculation for your lowest day temperature. You have an immersion should it be required to heat water. With a heat pump making it large/oversized to suit the worst day of the year, makes it huge at all other times. On a 10 deg day when your heat load is less than 1kW, a huge heat pump has no chance of being efficient without a huge buffer.