knobblycats

Good question @Marvin I tested it a little while back and our current supply is delivering 26 litres per minute at about 5-6 bar.

Can you still get that heat pump for £1300?

That makes sense, thanks! You've really helped me to get to the flaws of my thinking and I've adjusted my spreadsheet to account. Looks like the payback time for an ASHP would be around 5 to 8 years depending on how it was used. Makes more sense now!

I'm assuming the DHW doesn't drop below 35 degrees - my thinking is that by the time the water is at that temperature it's already too cold for most usage so we stop using it. Is that right, do you think? Or do UVC work differently than I imagine?

Apologies, perhaps I didn't phrase this bit correctly- "Assumes 3kw heating for 1kw electricity for the ASHP" To rephrase, for the ASHP I'm assuming 3kw of heating out for 1kw of electricity in, not total heating load. If we take the £1.20 immersion cost per day and divide that by 0.08, it comes out as 15kw of heating, not far off what you're suggesting.

This is all really interesting, and at the risk of opening a can of worms, I wanted to share my thinking in going immersion only vs an ASHP for DHW. These are approximate numbers and it'd be good to understand what other people think and if there's a flaw in my logic? A 250/300l UVC is around £800. An ASHP with heat pump UVC would be approx £3,000 (can you get it cheaper?) Running the immersion on a night tariff would cost approx £1.20 per day. The ASHP on a night tariff would be £0.40 per day. The ASHP breaks even with the immersion after 8 years assuming no additional costs. If we use lower hot water temps with the ASHP, we'd also have to run it during the day on day tariff to top the hot water up. That's hard to work out, but let's assume we do 100% night 50% day it could cost £2.24. At that rate, due to the high cost of the install, it'll never pay back vs an immersion on night tariff only. Figures above use £0.08 night rate and £0.24 day rate. Assumes 3kw heating for 1kw electricity for the ASHP

That's fantastic! Thanks for sharing - are you happy with your system?

That's good to know, thank you.

We're using mini splits for heating/cooling. The heat load for the whole house is 2kw at -5 degrees C outside so it doesn't make sense to drop £££ on either a boiler or ASHP.

I'm anticipating 5-6 adults using the showers every morning. With that usage, most of the hot water will be at a much lower temperature by morning! Like you say, we can always run it at a lower temp if it turns out to be too much. If you wash or shave using the basin (and wait for the sink hot water to come through), then the hot water is now in the shared pipe ready for you to use the shower if you choose to use it (and vice versa). If we run two separate supplies (one for basin, one for shower) from the central manifold, then running one wont 'prime' the other and if you wanted to use both, you'd have to wait twice. It makes sense to me, I just don't know if it's better to do it that way. Yes, https://ebac.com/washing-machines/e-care-10kg-hot-fill What does that mean? Would that work if we're mixing the hot water down to a lower temp with a thermostatic valve? It won't circulate unless we're also adding cold water, and without any taps open, there's nowhere for that mixed water to go? Why? 10mm pipes use 0.034 litres per meter vs 15mm at 0.103 litres per meter. If we use 10mm single supply to a basin 10m away from the manifold, you'd have 0.34 litres in the pipe. If we use 15mm, we'd have 1.03 litres in the pipe. Theoretically, if you use the figures as above for a basin mixer at 4l/m, it'd take 15 seconds to heat up for a 15mm pipe. With a 10mm pipe it'd take 5 seconds to heat up. So a smaller pipe saves 2/3 the time, water and heating energy. Waste water heat recovery - https://powerpipehr.co.uk/

Yes, omitted from this design for simplicity and definitely will be running cold to washing machine/dishwasher. Good question, costs between £450 to £600 and between £80/100 claimed savings per year. A few other forum users seem happy with their power pipes.

Have a bit of downtime from the build due to an injury so I thought I'd try to design the hot water system. I'm planning to go with a manifold system of different pipe sizes suited to the end-use for each pipe run. I've chosen a direct unvented cylinder, probably around 250/300 litres. I did consider a thermal store and a Sunamp, would one of these be better? The laundry room, en-suite, and family bathroom are all within 4m of the cylinder. The shower room and ground shower room are up to 15m away but would only have occasional use, so I'm proposing running 15mm out to a small manifold within the room to serve the shower/sink. The idea is that running the shower then 'primes' the sink hot water, but not sure if that would really work? Run time for hot water to wash hands would be high without shower use. To maximise the amount of hot water, I'm proposing running the tank at about 85 degrees C and then blending the output down to a safer level before the manifold. Both the dishwasher and washing machine will take hot water, I'm hoping that by heating all the water on cheap night rates (and eventually PV), the running costs of washing/dishwashing can be reduced. Any other flaws with my design?

Here's some more info to put your mind at ease: Each end of the beam will be sitting on a rebar reinforced concrete column. The concrete strength of the wall itself (RC35) should be similar to that used in padstones, and the whole ground floor is monolithic - a single piece of concrete. Otherwise, it's a good point to raise, and I appreciate you looking out for those of us who may not know much!

I like this, thanks Russell!

Photo added!