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My only experience with low energy input is when my last boiler failed and I used the immersion. 3kw rated immersion seemed to have no trouble getting my 170L tank up to temp within an hour (from too cold to shower to plenty warm enough - can't be more precise than that).

Thanks for the update and I'm glad you are trying this. I assume with the controller still in the upper bedroom? If our guesses about your building are correct, it will take some time to warm. So while you are running this experiment I suggest the following: 1. Keep internal doors open - you want the air to be able to move around to equalise all the internal space* 2. Daily, open the windows for a good ten minutes to ensure the place has a good blow through (I think you said you did this anyway). 3. Regularly check the radiators with a thermometer ideally something like this: https://www.amazon.co.uk/ERICKHILL-Thermometer-Non-Contact-Adjustable-Emissivity/dp/B0C72MDQ2Z?crid=2V242L59HQZZ8&sprefix=ir%2Bthermom%2Caps%2C225&sr=8-6&th=1 The radiators should be warm and at approximately the same temperature (guessing around 50C) if the room with the thermostat in it is colder than 16C. If some radiators are colder than others or if the radiators are often not hot then there may be a problem with the system that needs fixing before things will warm up. *If you get hot at night or if you leave the bedroom window open at night, close the bedroom door to isolate the room from the rest of the house. You can also turn that specific radiator off at night and back on in the morning if it helps with bedroom temp (a good idea if you have the window open anyway). I know you said you will try this for 2 months but if you don't feel things improving at all in 2 weeks then maybe come back then.

My remaining thoughts are that I think I would want a duct, rather than direct bury. I'd want to be able to swap things out if there was a problem. If you are willing to put on top of the 150mm eps, then insulation not so needed. Could run individual pipes in conduit then so you could always pull/replace the pipes (like they do in many european countries). Much lower profile this way too.* I'm sure you've got another duct going in for electric stuff but I would try and put at least one more in for future needs. Probably won't use it but if a future need develops and you haven't got it then you are in a world of pain. * No expert so don't know if there is a flaw in this approach I'm not thinking of.

https://barkeepersfriend.com/

DHW definitely the limiting factor, depends how you design your system whether it's an issue. If the tank is big enough to handle the morning (or evening) routine in one hit with room to spare then reheat even if long shouldn't matter too much, just comes down to scheduling. If heat loss is 2.8kw, a 4kw unit can spend several hours a day on hot water (not all at once) and still have capacity to heat the house. Or have I missed something? This depends a lot on the heatpump though doesn't it? @JohnMo was talking about how his new 4kw heatpump had a much bigger external heat exchanger/radiator which should lead to a lot less icing/defrosting than his previous 6kw (downrated 9kw?) unit. Edit: I should add that Nick and Simon are the experts with experience, I'm not, just someone who is interested in this and trying to solidify my knowledge.

If these use the same internals then it's likely the minimum modulation is the same on both. If that's true then there is no benefit for going with the smaller model. Which means with a max heat loss of 2.8kw and a 7kw heatpump you will likely see a lot of cycling which is not great for efficiency. I feel like I'm channelling @JohnMo, surprised he hasn't spoken up yet.

Neither? Even 5kw sounds a bit big. Smaller heatpumps are available, Important to get one that can moderate it's output down a lot. Most of the year you'll need a lot less than 2.8kw

Any thought given to putting in a continuous layer of the 150mm eps then putting the pipe with PIR insulation around/40mm? on top (foamed in)

and I'm not stopping... This is stuff you have actucally bought and checked is 13mm? I ask, because the photo shows a pipe with insulation that looks thinner than the inner diameter of the tube. 16mm tube, 12mm inner diameter. Edit: Basically the photo looks identical to the ones advertising pipes with 7mm

Agree, not how I look at it but I think most people think about touch points, so expensive fixtures and fittings that feel luxury (eg solid metal rather than plastic and name brand 'premium' appliances, etc.

My reading was lots of insulation on top of the duct, none below. I assume there would be free air in the duct also. Edit: and the MLCP stuff I was looking at was 7mm insulation. Not sure I saw any when I was looking with 13mm. Link please?

I think what they are getting at is that plastic pipes have some minimal insulative value compared to copper pipes. Nick was talking in another thread how condensation doesn't tend to form on uninsulated plastic pipes. But that by no means they are insulated. If you are putting these pipes on an uninsulated ground bearing slab then putting some decent insulation on them seems very sensible to me. Someone here warned me off the pre-insulated MLCP pipes as the insulation isn't very thick. So fine for some uses but not if you want a lot of insulation.

I read your post as suggesting a solution to @Dunc I can see that happening over a long period of time and I'd guess the glazing industry has data on this. But I very much doubt it's relevant on the timescales discussed in this thread.

That's not replacing the argon + seal. So even if it clears the view the window is still performing far worse than designed.

How many of you are living there now? Is this a case where they are designing to be plenty for a fully occupied house but most houses are occupied a fair bit below their theoretical maximum. I'd guess flats are often occupied much closer to their theorectical maximum given house sharing and property costs in cities. So maybe there have been some problems in those types of properties leading to a rethink.