JohnMo

I did nearly exactly the same. Not BnB, but increased insulation to 200mm and slab above that to 100mm concrete instead of screed.

Read the rules and state where it says anything with respect cooling not being allowed. Some rule about 10 years ago did exclude cooling but not the current ones.

I would be concerned with continuously running out of DHW. You are basically running the cylinder as thermal store but with DHW in it instead of heating circulation water. You would have to be careful with the coil placement to get meaningful energy available for the floor and DHW. New build, why an oil boiler and not a heat pump?

Why not posi rafters? Then either in fill or insulation on top.

Some photos of mine, left hand side are an inlet and out water connection. The controller is on the right hand side. Bottom image is the cover on

A paper sponsored by the government Hybrid_heat_pumps_Final_report-.pdf

No need, so why would you. You just need a small 4kW heat pump and it tees into existing system.

Bottom line is you have to follow manufacturers instructions. If the manufacturer says 600mm, as an example, that's what you have to do. Manufacturer instructions trump building regs. Choose log burner you are going to install follow those instructions.

That would work for me I'm on about my 10th house. But many just don't move from one decade to the next. So they would just stay as they are, no incentive for the many. What makes you say that? Zero need to switch to boiler until it hits about 3 degs. How many days a year do you get below that? daikin hydro split pump tee'ed to the existing system. Parts just over £2k. No need for a cylinder either.

The traditional way to operate a boiler (in the UK) was to run a boiler at an elevated temperature, have hot radiators and bounce of a the thermostat. So heating on heating off, repeat. When timer stops house gets cold repeat the next day. Boiler efficiency is dire, the boiler doesn't condense, so gets 80% if you are lucky. A heat pump operated that way takes a large hit in running costs. The best way is either Weather Compensation, so flow temp changes with outside temperature. The heat pump ticks away at a very high efficiency keeping the house at a stable temperature 24/7. The other version of this is to have a small setback at night, but essentially the same strategy for flow temp varying with outside temp. Your installer should really have left the heat pump operating this way. Search on here for weather (see attached) Heat-Pump-Guide.pdf

Really it makes very little difference with UFH. Once your at that level of output your talking 1 or 2 degs difference in flow temp. We have 300mm centres across 190m², just 7 loops, -6 last night our flow temp was 32. UFH spacing is more about reaction time to heat input. Chart to allow you to compare different spacing flow temps and outputs

https://en.econostrum.info/uk-homes-could-adopt-boiler-heat-pump-hybrids/ So at last the UK can use hybrid systems, hopefully with a proper grant. Easy to get the rubbish housing stock to reduce the CO2 emissions by a big margin. No internal heating system changes needed, 80 to 90% of the heating is done by heat pump the cold period by gas etc. No defrosting required on the heat pump killing efficiency and making it more expensive to run. Discuss

Can't see why they couldn't, floor output is just a matter of average flow temperature. So if they put 100mm centres in the heat pump doesn't really care, you just flow at close to 25 degs, even though they had a design flow. They would insist of heating in all room and all heated by the heat pump. Think you would get blank looks from a lot of installers if you mention fan coils.

Yes. You need to split some of thought processes out. New build is covered by express planning permission, a retrofit is normally covered by permitted development. Permitted development does not allow cooling. Permitted development is not applicable to a new build until sign off is complete. Also permitted development is not applicable if you want cooling, you need planning permission. None of the above has anything to do with grant. For grant applicability read up of Ofgem website. You need to read source information. If you find cooling isn't allowed I will happily eat my hat.

Spreadsheet is targeted at well insulated pretty airtight. But it's about being in the right ballpark, you will not get to exactly the heat losses to match the heat pump output. In 24 hrs you have moved from is 16kW plus ok, to now knowing that about 5 to 6kW is a better estimate. The 2 hrs off time for DHW gives you wiggle room, you can always flick the immersion on for DHW They will need as built test data to even consider it, some will not from what I have seen. So will not be interested in target figures. So everything just happens in the wrong order for the grant and getting a well sized heat pump. Just buy a Panasonic heat pump.

But can have damp, really good heat loss, especially when coupled with poorly installed PIR insulation. Any badly built building, is just that badly built. Seems a strange preference, you don't want a badly built anything, and accepting you may end that way is poor start to project.

Yes Correct they are different, PH is ACH, building regs is m³ per m². Can be very different numbers.

That's the figure to use, base on site exposure prevailing winds and exposed area. So number of Watts used will be very small.

2. Permitted development does not exist under planning permission. So if doing a new build permitted development only come after build control sign off. 5. Yes 6. Some need a widget adding such as Valiant, most will do it out the box, Daikin some will cool some will not. So be careful with choice. 7. You need to size for heat pump flow temps. 9. Yes 10. Thermostats mean you need a buffer, because you restricted flow from heat pump. Buffers unless very big are useless. Floor insulation, the more the better with UFH. Your floor is hotter than the room, so downwards heat loss can be high.

Why not to ads section?

I use a simple assumption You allow two hrs for DHW. So on the coldest day you need 4.5kW for 24 hrs, so 108kWh. To do this in 22 hrs you need 4.9kW of energy. So you need at heat pump that can put out 4.9kW at your lowest design temp.

No, but on a good airtightness house it is very small. If you want to calculate If the volume of a house is V m3, and the air change rate is n ACH (Infiltration rate) then the total amount of air passing through it per hour will be n × V m3. This air needs to be heated up through the temperature difference ΔT between the external temperature and the internal temperature. The energy required to raise one cubic metre of air through one kelvin is 0.33 watt-hours, i.e. its heat capacity per cubic metre is 0.33 Wh m–3 K−1. Thus the total ventilation heat loss, Qv , will be: Qv = 0.33 × n × V × ΔT watts Do not use the AP50 figure in this calculation, replace n for Infiltration rate in section 2 of the SAP.

No they are different things. The figure on the spreadsheet is ventilation rate via the MVHR. There is an additional not heat recovered leakage rate, this will also be in the SAP report near the airtightness score, it's about a 1/10th of the AP50 figure

Cell B39

I would be tempted to start a new thread titled with issue, most people will not bother looking with your current title. Everyone will see Grant setting and move on. The issue is relevent to any ASHP really. Your issue isn't you need to copy someone elses settings, you have an issue with defrost.