Hot water overflow from F&E tank

Discussion in 'Plumbers' Talk' started by MikityP, Dec 17, 2019.

  1. MikityP

    MikityP New Member

    Hiya everybody,

    I've only got a limited knowledge of plumbing, so I'm sorry if I leave out any important details! I'm wondering if anybody out there would know why the water in my F&E tank overheats so readily?

    We have our hot water to come on twice a day for half an hour each once in the morning, once in the evening. On a frequent basis, although not every day, just before the hot water is due to go off in the morning you can hear a rumbling noise from our F&E tank and it fills with hot water. It is virtually always at this time. The water had been gushing out of our overflow pipe until recently and we've only managed to stop it by adjusting the position of the ballcock so that the tank isn't so full.

    A week or so when this happened, air obviously got into our system and some of the radiators stopped working. Our very old Myson CP63 pump was also making a louder noise than it usually does, so I thought it might be a problem with the pump. We've replaced the pump with a Grundfos UPS3, but it doesn't seem to have solved the problem.

    As far as I know these kinds of things can be caused by several things, including a small hole in your hot water cylinder. However, I thought this steadily caused water to leak into your F&E tank and the water gushes in. Also, I made a bit of a hash of replacing the pump and the F&E tank was empty for a couple of days and no water dripped into it. I thought it could also be a thermostat issue. The one on the hot water tank seems to be working OK. The hot water just came on this evening. Before it came on the thermostat clicked on around 40 degrees, by the time the 30 mins was up the thermostat was clicking off at 60 degrees where it's normally set. We've also tried the boiler thermostat on our Ideal Classic FF boiler. It's currently set to 77 degrees (as it generally is over winter). I turned it down this morning to the summer setting (66 degrees) and put the heating on. The boiler was clicking off before the radiators had heated the house properly, which suggests to me that the boiler thermostat is working OK. Does anybody have any ideas what's wrong? I assume I'm missing something really obvious!
  2. Mike83

    Mike83 Screwfix Select

    Have you set the new pump to it’s minimum setting? I would try that first.
  3. MikityP

    MikityP New Member

    Ahh, so the pump may be too powerful? It was on constant speed 3. It sounded a bit too strong to me, so I have adjusted it to constant pressure 2 (out of 2). I will change it down to 1 and see if that works. I think constant pressure is supposed to work best with our heating system? We have an ABV fitted
  4. MikityP

    MikityP New Member

    I am afraid lowering the pump setting hasn't worked. We were woken up this morning by the F&E tank rumbling at virtually exactly the same time as always (6.38, about 2 minutes before the hot water was due to go off). The thermostat on the hot water cylinder had pretty much reached 60 degrees, and clicked off at about 62 degrees, so that seems OK. I went up to the loft and steam was rising out of the F&E tank. Could it be a boiler thermostat problem or maybe something else that I am missing? Any ideas would be great!
  5. MikityP

    MikityP New Member

    Also, the boiler had overheated, if that helps at all? Although, it's working fine with the heating on now...
    Last edited: Dec 21, 2019
  6. Severntrent

    Severntrent Screwfix Select

    What makes you say the boiler has overheated?
  7. MikityP

    MikityP New Member

    Sorry, should have put that. I had to push in the overheat thermostat reset button on the side

    Just thinking of other things. Could my ABV be on totally the wrong setting? Does that matter if the pump is on constant pressure mode?
  8. MikityP

    MikityP New Member

    I am probably really clutching at straws here. Another thing is...

    The hot water cylinder thermostat is clicking on at about 25 degrees and the cylinder itself is fairly cold to the touch. Should the temperature have gone down so much just 2 hours after the hot water went off?
  9. Severntrent

    Severntrent Screwfix Select

    Interesting that your boiler trips out as so does mine which is a FF250. This only occurs when central heating is on and my supposition is that when the house is up to temperature the room thermostat tells the boiler to switch off and at the same time the pump is stopped, this means that water stops circulating through the boiler which since it has some residual heat in the casing means the temp within the boiler rises and hits the trip temp. I used to think that the pump carried on running and circulating the water even when the boiler was not firing but fitted a new Grundfos UPS3 last month with the running light indication and now know that this is not the case, I assume if there was of couple of overrun minutes to the pump the boiler would not trip out.
    Like you I turned the boiler thermostat down and this appeared to cure the problem, I assume because of the lower temperature of the water being circulated meant that the trip temp within the boiler is not reached when the water ceases to circulate, alas like you I found that the radiators did not give out the necessary heat to warm the rooms sufficiently. My workaround at the moment is to put the room thermostat quite high so that, whilst the boiler will switch off when the water being circulated reaches high temperature the pump keeps on running and keeps the contents of the boiler below cut off.

    Your tripping seems to occur when you are heating the water? but I assume the above hypothesis is of similar value.

    Not quite sure where to go from here, is it a common fault on *** and that the cut off temp malfunctions after a few years, I have had my boiler for several years and only have had the cut off operate a few times prior to this year.

    Re your over flowing F&E tank, water expands when heated so I can imagine the top water level in the F&E tank rising as the system warms up thus reaching the overflow pipe. Obviously by lowering the top water level you have now managed to contain this expansion within the tank. When you get the rumbling from the F&E tank is the expansion pipe within the system discharging water into the F&E tank, if so again this could be because of water expansion issues, pump speed and frictional resistance within the central heating system being more when the TRVs (which I assume you have got) on the radiators close down. Raising the height of the systems expansion pipe discharging into the F&E tank may eradicate this problem. When I say raising the height I don't mean the actual discharge end that goes into the tank but the top most height of this pipework where it bends over 180 degrees to discharge into the tank.

    Hope the above gives you food for thought. Good luck.
  10. Severntrent

    Severntrent Screwfix Select

    Re your hot water cylinder drop in temperature, is it insulated?
  11. MikityP

    MikityP New Member

    Thanks Severntrent, really great reply!

    Sounds like you have similar issues to us. Could it be a problem with the boiler thermostat not working properly, due to it's age, and overheating the system? Our boiler is turned up higher in winter, and although it is happening miles more now, I don't really ever remember it happening in the summer when the boiler thermostat is on a lower setting.

    My workaround that I am going to try tomorrow is to lower the time the water comes on in the morning from 30 mins to 20, maybe that would work?

    The thing that I really don't get is that it happens at the same time all the time. Why doesn't it happen in the afternoon? I ran the hot water for an hour earlier this morning, no overheating and the hot water cylinder thermostat switched the boiler off.

    I am not sure about the expansion pipe. I don't make it into the loft in time to see what's happening! However, I will try and have a look if I can catch it at the right time.

    The hot water cylinder is insulated. Maybe the lower temperature is normal? Oh and yes we have TRVs on the radiators apart from a heated towel rail and the radiator where the room thermostat is.
  12. Severntrent

    Severntrent Screwfix Select

    Ditto I don't have problems in summer

    20 mins sounds good way forward although you may end up with luke warm tank of water

    Would not have thought that insulated cylinder could loose heat that quick but are you deducing what the temp is via the thermostat, I assume thermostat is about 1/3 from bottom of tank ,maybe after 2 hrs hot water risen to top of tank and cold to bottom, how hot does the HW pipe comimg out the top of the cylinder feel.
  13. MikityP

    MikityP New Member

    Yep, I am a bit worried about it being luke warm. Oh well, only my wife has a shower then, so the rest of us will be OK... ;)

    Our thermostat is about a quarter of the way up, which I think is still OK. I will check the hot water pipe out of the cylinder when the hot water is on next.

    Another thing that occurred to me is that the pump was still trying to run when the F&E tank was overheating. So something was still telling it to keep pumping even though there was too much heat going to the tank. The problem is that the boiler overheats too quickly, and shuts everything down, to be able to tell what's causing the problem...
  14. MikityP

    MikityP New Member

    Hi Severntrent,

    Yesterday evening I really gave it some thought about your timing point. I originally said I was going to adjust the time down to 20 mins, but that obviously wouldn't get the hot water cylinder up to temperature. So, I thought I need to go above the amount I had it on before. This way the cylinder should get up to temperature and switch the pump off and the boiler stays ready to fire, but isn't needed. So, I tried 50 mins. I think the cylinder got up to temperature after about 32 mins, I was watching the boiler at the time, and then the boiler just sat there until it clicked off after 50 mins. Hopefully this should rectify our problem now. It seems like I have created this issue by having the water only on for 30 mins in the morning and evening, but I worked out after we moved in that this was enough for our needs and besides this year, we have only had overheating problems a handful of times (we have lived in our house for 10 years). I also had it on this timing as our old pump used to occasionally rattle a bit and wake us up and I thought we could cope if I had the hot water coming on just before we got up.

    Either way, should this really create a problem anyway? I mean, surely a central heating system should be able to cope with the pump switching off at the same time that the thermostat reaches the required temperature. As we both have similar types, maybe it's a quirk of Classic FF boilers? When we have ours serviced, the heating engineers say it's in great condition given its age, so I am not keen on replacing it at the moment.

    Thanks for your replies on this and I really hope your issues get resolved soon!
  15. Mike83

    Mike83 Screwfix Select

    If the stat is fitted 1/4 up the tank then the water will always be a lot cooler than nearer the top after 2 hours.
    If the stat is fitted quite low it won’t detect the temperature correctly. This could cause the boiler to remain on longer than expected.
  16. Mike83

    Mike83 Screwfix Select

    Changing the ntc (thermostat) on the boiler is a simple job. I would probably change this first before doing much else.
  17. MikityP

    MikityP New Member

    Thanks Mike83. Yes looking at it again, it is quite low. I will look into moving it up a bit
  18. Mike83

    Mike83 Screwfix Select

    I normally go for 1/3 up the tank.
    The cylinder stat only knows the temperature of the tank where it’s in contact.
    The top of the cylinder can be a fair bit hotter than the cylinder stat setting just before the stat gets satisfied.
  19. MikityP

    MikityP New Member

    Morning everybody,

    OK, so we are still experiencing the same issue.

    This morning, the rumbling from the F&E tank woke me up at 6.14 (just what you need after a late Christmas night ;)). This is now 24 minutes after the hot water goes on. When I got downstairs, the boiler had overheated again. I pressed the override switch back in. The boiler didn't come back on because the hot water in the cylinder had reached the required temperature and the thermostat wasn't asking for heat.

    So, a couple of other thoughts I've had. Am I setting the pump to the totally wrong setting? Owing to the problems we have had, I phoned Grundfos when we first put the new pump in and they said the settings don't overly make that much difference. The pump has been on constant pressure 1. Yesterday, I thought the radiators were taking a while to get hot, so I turned it up to constant pressure 2. I personally wouldn't have thought this would make a huge difference, but I could be completely wrong. Also, I thought I heard a slight noise yesterday morning with the hot water when the pump was on constant pressure 1, so maybe the pump settings aren't causing the problem? Although, yesterday morning, I thought the pump was having to work harder with the hot water on compared with the heating on, when I would have thought this should be the other way round? A
    Is this because of constant pressure being on?

    I am now going to try fixed speed 1 and see how that goes. I am not sure if this is a problem, but the pump was still running when the rumbling was occurring, but it had stopped by the time I went down to the boiler and pushed the override button back in. So, is the pump running when it shouldn't be? If so, why does this only cause a problem in the morning?

    I apologise if this is a naive question, but does my system need to vent itself in some way and something is blocked and the only other venting route is to push hot water up to the F&E tank? If so, why would this need to occur at the same time every day, if at all?

    Any other ideas would be fantastic!
  20. Severntrent

    Severntrent Screwfix Select

    Re the rumbling, I would be inclined to run pump at full speed to see if clears any air pockets at start of heating cycle, when doing this check and make sure vent pipe not constantly discharging into F &E tank. Re boiler cutting out like you I'm still at a loss understanding this and will live with it till spring/summer comes when things seem to settle down

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