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Unvented Cylinder Range Tribune HE long term headache!

Discussion in 'Plumbing and Central Heating' started by Colin Gordon, 27 Mar 2021.

  1. Colin Gordon

    Colin Gordon

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    Hi folks,

    I am hoping there are some clever people on here who can guide me toward a solution with regard to a constant hot water discharge into a tundish.

    Initially, about a 9 months ago, we had a problem with water pressure dropping in our central heating system. We have an unvented cylinder with a gas boiler. An initial call out from our maintenance contractor advised simply topping up the pressure. We did this on an increasingly regular basis to the point (daily) where we got another company into advise.

    Since then we have had quite a few professionals in to change valves, replace expansion vessels, replace thermostats etc. Recently we have noticed the water discharge running constantly into the tundish. We have had intermittent leaks from the boiler also.

    Another strange symptom is that the boiler fires even when it is set to OFF on the control panel. Our thoughts today were that the boiler is heating too much water causing the pressure to increase and causing the PRV to pass. From research this suggested a faulty auxiliary switch? However, having switched off the boiler at the mains, the water is still running to the tundish even without heat, suggesting it is a valve failure.

    At this time, we think there are two issues - excess heat and valve failure. Does the valve activate the boiler? Any advice would be very welcome.

    Thanks for your help.

    Colin Gordon
     
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  3. Madrab

    Madrab

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    Only component on the unvented cylinder that can activate the boiler is the thermostat, if the programmer is on HW mode.

    If there's water releasing into the tundish then you need a qualified unvented engineer in to diagnose what valve is passing. It'll either be the temp pressure release valve on the unvented, the expansion relief valve on the unvented or the CH pressure relief valve, if it has one and they are all connected to the same tundish.

    It's no difficult to establish, nor is it difficult to fix. Even then, if you are losing boiler pressure and the CH PRV is connected to the tundish and then it could be that valve that's lifting.
     
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  4. oldbutnotdead

    oldbutnotdead

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    1 usual caveat- no DIY work on pressurised systems. Not even sure if you're allowed to check exp vessel pressure...
    2 PRVs are fairly cheaply made- if they activate too often they tend to stop sealing, plus you want the thing to fail safe anyway.
    3 Common failure mode- expansion chamber loses its bubble/becomes deflated, system pressure gets too high from thermal expansion, triggers prv. See above.
    4 Mains water pressure will be maintaining the flow from the tundish. If its warm water coming out it will be cooling the cylinder but shouldn't override boiler controls being off.
    Its possible that a combination of errors is causing dhw to be live all the time but cylinder and boiler stats should still regulate temperature.

    When you say 'boiler firing' is that a short cycle or a full (10-15min,) heating burn?

    The PRV needs replacing for sure. While engineer is there get them to check expansion vessels.
     
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  5. Colin Gordon

    Colin Gordon

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    Hi Rab,
    Many thanks for your advice. We are awaiting a new valve set on order to be fitted (although we've had valves replaced previously which made me think maybe this was the symptom but not the cause of the issue). That should hopefully resolve the tundish issue. I currently have the hot water tap running in the bath to stop it discharging and the boiler is switched off at the mains. I hope this makes sense. Colin
     
  6. ianmcd

    ianmcd

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    electricity will not come out of the tap, turning the boiler off wont make any difference, you have a failed PRV on your system
     
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  7. Colin Gordon

    Colin Gordon

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    Hello old but not dead,
    Thanks for your comprehensive advice. Yes it is a relatively short fire up of the boiler for 5 minutes or so even when the control panel has everything set to OFF. We have switched the boiler off at the mains just now but the water is still running which made me think it was an issue with mains pressure over riding the valves. I hope this makes sense as it is a very confusing issue! :)
     
  8. Colin Gordon

    Colin Gordon

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    Hi Ian,
    You are probably right about the PRV failing. What bothers me is that it keeps failing which makes me think there is another issue which is causing it to fail. I thought this might be too much heat causing pressure build up and over time the valves failing. many thanks, Colin
     
  9. oldbutnotdead

    oldbutnotdead

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    Do you just get one of these short cycles per day or do they occur regularly?
    Warning- modern boilers don't like being power-cycled too often, too much gubbins in there prone to failure at switchon/selftest.
    Have you tried adjusting (down) the cylinder stat while boiler is running when off? Or checked positions on any 2-port or 3 port valves?
    Is DHW temp abnormally high? Any kettling sounds?
     
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  11. Colin Gordon

    Colin Gordon

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    Hello OBND, the short cycles occur every 10 or 15 minutes and frequently in the middle of the night.

    this is something I would need to check with the plumber when he comes back.

    Yes the DHW Temp does seem high (you get a blast of heat off the cylinder) and there are gurgling sounds frequently from the cylinder. We had the thermostat replaced yesterday which hopefully will resolve this unless the actual main issue is at the boiler (which has been going through my mind recently).

    The valves are awaiting replacement but as I said, I think there is another issue which is causing the valves to fail and my instinct is that it is heat related from the boiler firing all the time unless the thermostat fixes it.
     
  12. Colin Gordon

    Colin Gordon

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    One other point to add in is that the boiler started banging when the heating pressure drops which is why we topped up initially and then got professional help in.
     
  13. oldbutnotdead

    oldbutnotdead

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    Might I delicately enquire if sir has had the unvented serviced annually (mandatory AFAIK) and maybe the boiler too? Sounds like you are having a cascade of small failures...
     
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  14. Madrab

    Madrab

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    Best advice is to anyone with an unvented cylinder and a boiler should service both at the same time. It might be a bit more expensive but certainly worth it when it comes to the family's safety.

    Some independents may offer a discount if both serviced at the same time.
     
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  15. Colin Gordon

    Colin Gordon

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    Hello, we have an annual service for the boiler but not sure about the tank. My bad for not being close enough to the detail. You may well be right though. Do you reckon it's game over or fixable? If we need to bite the bullet for a new system so be it. Many thanks for your advice which is much appreciated.
     
  16. oldbutnotdead

    oldbutnotdead

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    If it worked before it can work again, you need someone who can faultfind properly (not a board-swapping box shifter). You don't specify the age or make/model of the boiler but up to 10 yrs old should def be repairable, some last longer, some are a bit flakey
     
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  17. Colin Gordon

    Colin Gordon

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    Thanks again both, I really appreciate the advice. It's a 2013 installed Range Tribune HE cylinder and an Ideal Logic Heat 24 boiler. We've had a lot of professional folks look at the system over the last 6 months and I am aware that the issue is not easy to diagnose or fix. At the current time we have spent in the region of £1.5k on remedial work, so I was becoming a bit apprehensive about the longevity of the system and wondering if was game over.
     
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