Hot Water Cylinder not Heating after a Bath

Discussion in 'Plumbers' Talk' started by HarveyH, Dec 1, 2017.

  1. HarveyH

    HarveyH New Member

    We have a Potterton Suprima boiler providing heating and hot water with a hot water cylinder.

    There is a pump providing water to the shower and bath to give a power shower.

    After a bath, the water pipe is hot from the boiler and through the motorised value and then cold when it reaches the cylinder. I would have thought that the pipe from the boiler would be separate from the water system and should not be affected?

    A heating engineer has replaced thermostats and the motorised value and this has not helped.

    However, if we use the immersion heater to heat the water cylinder this clears the problem and the boiler then heats up from the boiler....... till the next bath.

    Any advice gratefully received.
  2. Wow, that's weird.

    I guess it's a 3-port motorised valve? And you have the programmer set to DHW 'ON' whilst you fill the bath?

    In which case the pipe from the 3-porter to the cylinder coil should be hot.

    Could there be an air-lock in that coil? Seems highly unlikely, but if you could provide a photo of your cylinder/valve/pump setup, that would be cool (as it is now...)

    If it is down to an air lock, then the 3-port and pump should still be trying to operate, and the boiler should still come on, but the chnaces are the boiler will then turn itself off as it 'overheats'.

    The plumber replaced the 3-porter - was that the whole unit or just the motorised head?

    Anyhoo, I guess the thing to try is to see if the 3-porter is behaving as it should. As you undoubtedly know, it has three positions - DHW, CH and the middle which provides both. It 'defaults' (ie returns under spring power when the leccy power is turned off) to the DHW position, and the manual lever on the side should not have any loose movement in it.

    Ok, check that the manual lever is 'firm' and then turn on only the DHW on the prog (get someone else to operate the prog while you listen to the valve). Does the pump (and boiler) fire up? Does the outlet from the valve to the cylinder coil become hot?

    If you then turn off the DHW and turn on the CH only, the valve should 'whirrr' for 2 or three seconds followed by a 'click' and the pump (and boiler) should fire up again. The manual lever should be found to be completely loose now, and the outlet to the rads should heat up.

    If that's all ok, then try also turning on the DHW on the prog (keeping the CH on). You should hear a very brief 'whirrr' as the 3-port goes back to the middle position, and both outlets should become hot. (The manual valve will still be loose, tho' it might not have the full length of travel in it)

    That all good? Ok, now turn OFF the CH so only the DHW is left on - there should be a further short whirrr as the valve goes back to its original position - the pump and boiler should keep on running and only the cylinder's outlet from the valve should remain hot.

    Please report back.
  3. HarveyH

    HarveyH New Member

    Many thanks, here are some photos:

    It is a three way motorised valve and has been fully replaced more than once. It seems to be operating fine when the timer switches the water and heating on and off.

    I will try filling the bath and check again, and also try the shower.

    It is the copper pipe half way down to the right of the cylinder that goes cold towards the cylinder after a bath, but is hot up to, through the motorised value and within a few inches of the cylinder after a bath.

    I don't know if it is relevant but the pump at the bottom of the picture that goes to the the bath and shower rumbles several hours after use.

    Attached Files:

  4. Soz - I don't understand that system. It doesn't help that the pipework to the right of the cylinder is partially hidden, so I can't trace it all.

    What I can see is that there's an auto air vent at the top of each outlet from the 3-port valve, one for the DHW side and one for the CH - these are the two nice brass devices on the right hand side at the highest points of the pipes. Perhaps check that these are venting air properly.

    What I can't get my head around is the way it appears as tho' the DHW outlet from the 3-port valve is going to the booster pump on the floor, and that's then going in to the side of the cylinder 3/4rs of the way up. I must surely be seeing that wrong?!
  5. The Teach

    The Teach Well-Known Member

    Did the heating engineer :D recommend any further work ?

    The first tee above the pump is sending boiler heated water somewhere,it could be a bypass and needs checking to ensure its not starving the cylinder.

    Some basic checks carried out by your heating engineer would include checking the wiring is correct,the cylinder coil is not restricted,system set up correctly for hot water priority so the cylinder contents reheat quick enough to match the pumped bathroom demands.
    How many hours per day is the hot water set for ? Do you actually run out of heated bath/shower water when running on the gas boiler ?

    good luck,
  6. HarveyH

    HarveyH New Member

    Thanks Devil's Advocate.The air vents were both replaced earlier this year as one was leaking.

    Sorry the photo wasn't that clear, it is a bit like spaghetti junction with pipes going in and out everywhere. The DHW outlet from the 3-port valve goes into the tank 50cm up from the bottom of the cylinder and the pipe to the bathroom pump comes out about 80cm up from the bottom, so they are separate systems.

    I have been trying various combinations of the timed boiler controls. We didn't have a problem over the summer and it was fine filling the bath with the hot water turned on. I will try again with the hot water turned off.

    Thanks The Teach. The hot water is on for an hour and a half in the morning and 5 hours in the evening and it does sometimes run out of hot water during the day.
  7. kiaora

    kiaora Well-Known Member

    The bypass pipe is 22mm, if you trace the pipe, is there a valve on it?
    If not, that can cause problems, need it cut a wheel valve in that section, and balance the system.

    The heated water from the Boiler, will take the path of least resistance, why go thro’ a 3 port valve and a cylinder coil, when it can go straight back to the boile via the bypass!
  8. Dave does Gas

    Dave does Gas Well-Known Member

    Looking at those pictures I totally agree with Peter, It certainly looks as though the pipe installed where a bypass would be installed has no valve on it.
    With it being a Y plan you dont strictly need a bypass on the system so you could cut and cap both ends, Best practice would be to fit an automatic bypass or you could just fit a gate valve and reduce it down (least effective option).
    Having said that the pipe does need to be confirmed as a bypass and not heating a radiator/towel rail directly off the primarys in a bathroom, as its not clear from the photo.
  9. HarveyH

    HarveyH New Member

    Thanks everyone, sorry about the delay in responding. I attach a drawing of the hot water pipes.

    I'm beginning to think that perhaps it is the heating and hot water timer. The problem with the hot water not heating up after a bath or shower happens during the day and didn't happen over the summer.

    I have tried turning off the heating and then switching on the hot water and it works. Will try further checks to confirm.

    The timer is about the only part of the system that is original.
    Hot water cylinder.jpg Hot water cylinder.jpg

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