I’ve just read through your previous post.
Let’s clear some Rubbish up.
If you’ve got 220m2 of underfloor heating and we use a middle ground of 70w per m2, that’s around 16.5kw. Obviously when the system is warmed up, that load reduces.
Looking at the cylinder data, it’s cool is around 17kw, and if provided the full 17kw (and there are no other issues (cold cross over)), it should reheat in about 50 mins.
You’ve currently got the cylinder coming on for 2 hours in the morning, when the underfloor heating is likely calling for all available heat to warm the property after a cooler night time period.
So if the underfloor heating is calling for anywhere between 10kw-16.5kw, you’re left with nothing for the hot water cylinder.
What is available, will do very little in the 2 hours it’s set for.
This next bit regarding cylinder timings is a bit controversial.
Old copper cylinders with crappy insulation, lose a lot of heat. This is part of the reason people timed the heat up periods.
Newer cylinders are better insulated but still lose heat. I think your cylinder loses 2.2kw per 24hr. It loses this wether the cylinder is programmed to heat for 2 hours, twice a day or left on constant.
If the cylinder water is cooler, let’s say 50c, because it heated in the morning for 2 hours and now it’s 3pm and it’s lost heat, it’s standing losses will be less than if it was kept at 60c.
Now, saying that; I personally feel that the difference in loses are so small, there is very little money to be saved by having the hot water timed, rather than on constant.
Now looking at your cylinder design, the thermostat is 1/3 up the cylinder and the hot water return is connected at the top. This is creating stratification in the cylinder and the bottom 1/3-1/6 of the cylinder is in essence not contributing to your capacity.
To resolve this, I would have connected the hot water return to the cold inlet, this would ensure max use of the cylinder capacity.
The return also looks like it’s in 22mm, when 15mm should have been used. You’re losing a lot more heat (especially if uninsulated) with 22mm over 15mm.
So, what would I do to maximise the hot water situation.
A bigger boiler would help and should really have been specced during design. However, I wouldn’t change it until it breaks.
Firstly, I’d leave the hot water on 24/7 unless the noise of the boiler disturbs sleeping (then I’d set it off for say 11pm-4am.
This change isn’t going to make a huge difference to your efficiency. With your current timed periods, you lose around a kw of standing loses between reheats.
With constant, you’re still going to lose roughly the same standing loses but rather than reheating all the loses in one go at the next timed period, you’re reheating the loses a little bit at a time throughout the day.
You could get it rewired for hot water priority. Although I wouldn’t bother, just leave it on 24/7. However, without a rewire, you will still struggle with hot water if you empty the tank throughout the day as it will struggle to reheat with the underfloor heating on. Hot water priority stops heat to the rest of the system until the hot water cylinder is at temp.
I’d get the recirc pump connection moved and power it from a timer set for the morning/evening busy periods. This way you’re reducing the amount of pipework heat loses.
Do your underfloor heating manifolds have bypasses/mixing valves to control the temp of water being circulated? If so, set the boiler temp higher 70c/75c. This will give the system more heat/energy and mean the cylinder can take more.
I’ve had enough typing on my phone for one evening. Hope it makes sense.
