Cylinder domestic hot coil.

[LeakFinder's]

Hi Guys,
I wonder can someone enlighten me why the last 3 jobs i have visited have the flow and return to the coil in 15mm. We had one job which was a bungalow and the flow and return run alongside the main 28mm for over 15m. We had another where the 22mm arrived up to the cylinder and around 1m away dropped to 15mm. The coils are usually 22mm. Are they just being misers or is it ignorance. The people have all complained about long periods of time to get hot water. I just hope i have been giving the right advice that best practice is the 22mm continually through to the system. Thanks for any advice.

[steveb]

maybe it was done to throttle down the feed to the coil because the heating was being starved. sometimes it is done with a gate valve on the return from the coil

[mylesy]

the specific heat of water is 4.18 j/g degrees celcius. so it can carry a lot of heat in a small amount. 15mm can supply the heat requirment for a cylinder especially if its only a short run. as said before, its better for the pump, as it doesnt have to force more litres around the system un-necasarily. thats why radiators can be fed in 10mm also. it can have the required heat and its less strain on the pump (less balancing)

[LeakFinder's]

Thanks steveb and mylesy Fair point i visit a number of sites where they only run hot water in the summer and complain like mad about the time it takes to get a decent volume of hot water. I guess the best way as you say is a gate valve used as a throttling valve is a good idea and then in the summer crack it open to get the domestic hot quick and back it off a bit coming back into the winter. I realise I am being optomistic when I say summertime given its been rubbish the last few years.

[steveb]

another factor in time to get decent hot water is the size of feed pipe from cswt

[fuzzy]

does it get hot?

[LeakFinder's]

fuzzy, yes it eventually gets their but its like a time lag

[mylesy]

what area is it in? there are a number of factors that could prevent poor hot water. if pipes are furred up or system has iron oxide in it, then it could prevent the conduct of heat from the coil. the cylinder insulation could be poor and if its in a draughty attic it could be losing heat. the cylinder stat could be set too low, the overall system btu requirements may be too high for what the boiler is kicking out (you could try and see how long it takes to heat on hot water only) the boiler may not be performing adequatly. undersized cylinder for the house (or the amount of people in it) it really is hard to say without investigating but some of these areas iv mentioned are some places to start looking. i could be something iv not said too. hope this helps anyway pal.

[tamz]

22mm is always the best way as the heat transfer rate will be up around the 20 odd kw (if the coil can take it)
The std allowance for hot water used to be taken as 2 - 3kw and the older coils or blades had a heat transfer of not much more than that. Hence the use of 15mm to feed the coil (around 6kw) Some jobs are piped in 8mm (FEP) so you might come across that one somewhere.
(Withoput calculating this), the re heat time for a modern cylinder fed by 22mm might be in the region of 20 odd minutes whereas the recovery time will be nearer an hour for an old cylinder or longer with a conversion coil or blade.
Just depends what they get used to but if they want it quicker take the cash and repipe it in 22mm

[Best]

That's correct, as far as i am aware, newer cylinders had to comply with a heat recovery rate in ideal situations of about 25min, so the coils had to be made bigger than a lot of manufacturers were making (we were being cheated).
Only way to pipe cylinder is in 22mm - especially if a fair distance away from boiler & fit a gate valve to balance it which is critical as these pipes will short circuit the entire heating as you are just really joining flow to return!
Pipes to coils are often getting blocked up on open systems often at return pipe to feed tee, if done this way. Usually blocked solid & may need pipes cut out. To check if water is flowing through coil, hold the return pipe to check for some heat. Remember that if Either pipe is blocked, even almost fully, the flow will remain hot, but will not have much circulation.

[LeakFinder's]

best,
I didn't realise the coils were of a reduced cross section i just remember when i was at the training centre some 30 years ago that we were taught to cut a cylinder and how to thread the coil and it appeared to be a 22mm section as opposed to 15mm section which is why i was likely struggling to get my head around it. We had a job last week where the vent was blocked and the wet vac was used we removed a long slug og brown gunge from the vent problem fixed

[mylesy]

i agree best, a super coil or something will really speed up the conduction. great point!