ReedRichards

That's not wrong but you would have to carefully balance every radiator in the house for it to work. Efficiency is great, it saves you money. But you can save even more money by wearing lots of clothes and not bothering to heat your house. So be aware that you may be sacrificing convenience for the sake of efficiency.

That seems like a perfectly valid observation. But if the heat loss is at least 2800 W, how on earth is it currently being heated by a 1700 W (@ Delta T = 50 C) radiator?

So there are some radiators where it isn't? What differentiates these ones?

Just a thought but I have an open plan lounge and dining room. This was originally heated by three radiators and I had to add a fourth one we we moved from oil to a heat pump. The heat loss of the two rooms combined was calculated to be 3900 W but the calculation split this into 3 "areas" and assigned a heat loss to each. So the difference between Company A and Company B could be down to a difference of opinion as to what they think constitutes the lounge, if it isn't clearly separated from another area by a door.

No, it will be hotter because you are heating it with your UFH. Or even if not you will still be losing heat through the floor but if it's 10% with UFH it must be about 5% without; given the same floor covering. I suppose the worst case would be if the heat that penetrates the insulation can escape laterally to something that is colder than 8 degrees C. So you would have to level the ground, cover that with an appropriate membrane then insulation but around the edge of that insulation you need to dig a trench down to where the ground really is 8 C and fill that trench with insulation.

This is absolutely correct so there are two issues: Why do the two companies quote vastly different values for the heat loss? Why do both companies quote heat loss values far higher than your current radiator is capable of matching with its current heat output?

Surely it depends on how much insulation you have under your slab? And you can probably run your UFH at a lower temperature than radiators which should reduce the running cost? Otherwise why on earth does anyone go to all trouble of installing UFH and the risk of having to dig it up later should anything go wrong?

Not wet UFH, surely?

The Stelrad model of such a radiator has a quoted output of 1672 W at Delta T = 50 C. So the 1732 W figure must be the rated output and not what you will achieve with a heat pump, which will operate at a much lower delta T than 50 C. But it could be roughly what the current radiator produces at your current operating conditions with an oil boiler (80 C flow, 60 C return perhaps?). Either way, if this is the only radiator in the lounge then the calculations suggest it is currently insufficient to keep the lounge warm enough in colder weather. If that's not true then you need to consider why.

Have you tried Octopus?

This is for an earlier version of the software but that might not affect the wiring: https://midsummerwholesale.co.uk/pdfs/homely-installation-manual--mini--en-a5-lg-v1.2.pdf

In winter the field mice like to take up residence in my loft. There isn't much to amuse them up there so they like to chew on any foam pipe insulation that they can find, whereas so far they haven't touched the aluminium-clad stuff. I was concerned that your insulation looked too much like the type that is susceptible to rodent damage.

My feeling is that this feature is a gimmick and it is better to control your DHW recirculation pump independently, as I do.

I'm not sure that foam insulation is suitable for outdoor use. It's certainly not the type that is usually used.

I'm not @Dan F but my thought is that this is more about the temperature you want to achieve. If you like your house to be a bit cooler overnight then use the set-back mode. If you like the same temperature 24/7 then don't.

Sorry, you've lost me there.

Surely it's just about the amount of warmed water you have per unit area of floor? The closer you make the pipes then the more warmed water per unit floor area so the more heat it can give out for any given water temperature. Since what you want is a fixed amount of heat for any given outside temperature then the closer the pipes the cooler you can make the water so the more economically you can run your heat pump (provided it's not also having to provide heated water for radiators elsewhere in the building).

I'm curious, @TheMitchells, what happened at this discussion? You had some rooms where you would certainly need to increase the size (or number) of radiators and that would take some ingenuity.

Really? My heat pump is right next to a drain which carries rainwater off the roof to a soakaway. Hitherto I had thought that precluded my getting an R290 heat pump when my existing one needs to be replaced.

In the spreadsheet the installer has (stupidly) entered 100% in the "% Demand met" column instead of the actual figure, which is the number in the "Total Output" column divided by the number in the "Power loss " column. So the worst case is the Bed 1 En-Suite where the figure is 293/290 = 101%. The lounge isn't much better 2391/2306 = 104%. On the other hand the kitchen 1176/768 =153 %. You would need to add a third radiator into the lounge of much the same size as the other two and squeeze a second radiator into that ensuite with about half the output of the towel rail and you would be on your way to achieving the capability to run at a lower flow temperature. Aim for all the numbers, calculated properly, to be 150% or greater. Another option (as suggested by others) is the use of fan coil radiators but they need both plumbing and electrical power so they make the installer's job quite a lot more involved. If you want to talk about anything with the installer tomorrow (i.e. at short notice) ask them what they think of those.

If you retrofit an ASHP then under many circumstances it qualifies as a Permitted Development. But that only applies if you don't use it for cooling. Of course if it is part of a new build then the planning permission will encompass the heat pump and you can do what you like with it, heat or cool. I presume @Dave Joneswas in that situation so his ignorance can be forgiven.

I would be surprised if you can get a quote for fan coils as part of the installation. As it's a retrofit you could not use them for Air Conditioning without planning permission. And you have to not mind the fan noise.

For some reason I'm not able to download the pdf files attached above. Is there a trick to do that?

You mean kWh but yes, it means that the heat demand calculation is either wrong and/or it's based on keeping the house warmer than you actually do and/or last year was milder than the calculation suggests. Most of your electricity usage will have been for heating and hot water and I don't think these calculations ever agree perfectly with actual numbers.

What you are trying to achieve is the best compromise between (maximum) flow temperature and radiators you can fit in the available space. The lower you can make the flow temperature, the more economical your ASHP will be to run but the larger will be the surface area of the radiator(s) needed to heat each room. A radiator specified to run at an average of 15 degrees above room temperature needs to be 2.5x bigger (in surface area, essentially) than one specified to run at an average of 30 degrees above room temperature. The latter is roughly what you would have with a 53 C flow temperature.