I have a townhouse, an unvented hot water system and a combi boiler. My cols taps on the top floor run HOT for around 5 minutes. I have also noticed that the water in the tiolet is also hot to the point that if you sit on the toilet you can burn your back.
Anyone have any ideas? Is this issue a health problem.
Plus as my hot water is on 24/7 this could cost me in more ways than one any ideas on how I could calculate the cost?
Cheers
Kris
I'm not sure if you mean you have an unvented cylinder, or just a combi boiler. If it is an unvented cylinder, turn it off immediately. Ensure that the immersion is off and the heating coil isolated. Then get a specialist plumber qualified on unvented systems to look at it. When unvented cylinders go wrong, they can explode with dire conseqences (i.e. total destruction of the house). This doesn't sound like one of the dangerous failure modes, but it pays to be careful.
If it is a combi, I suspect that you either have misplumbed hot and cold water, or more likely, a shower valve without check valves that is passing water between the hot and cold systems. The same problems would also affect an unvented cylinder system, but combis are fundamentally safer and less likely to explode.
Christian.
Oh yes. It is very bad. On the plus side, it seems to me that the failure mode you are seeing is unlikely to be one of the ones leading to explodion. However, there is no guarantee of this and you should get a specialist in very quickly.
I still think that the most likely cause is a malfunctioning mixer valve, probably a shower.
Christian.
Yes, I agree that you need proffesional help. In the meantime, here's a couple of things to try:
I've seen this on an un-vented system where the one-way expansion checkvalve on the cold inlet had stuck open, allowing hot water to expand back to the balaced cold tee-off. It couldn't expand further back upstream into the mains, due to another checkvalve in the 3-bar pressure reducing valve.
Identify the valves as follows:
Follow the cold feed back from the cylinder inlet. It will come to a valve with a knob on it. This is the Expansion checkvalve / expansion relief valve.
It discharges into a plastic cone thing called a tundish. It is essentially a visible overflow.
Observe the tundish for flow. Is their either continuous or intermittent flow in the tundish? That indicates a problem.
Check that the expansion relief valve is not stuck shut by twisting the knob and checking for a discharge of water in the tundish. Carefull! it will be hot!
Feeding into the tundish will be a second valve with a knob on it, directly on the cylinder. This is the Temperature / Pressure relief valve. Give it a quick twist and ensure it discharges too.
If both of these discharge on twisting, and neither is discharging unless you twist them, then the thing is most likely safe at least. So you have some peace of mind untill a qualified person can check it.
Feel the cold inlet pipes at the unvented cylinder. It will be hot at the point where it enters the cylinder. If it get hot upstream of the expansion check valve, then there's a problem there.
Another possible problem is the possibilty that the air volume in the expansion unit has become depleted, the usual symptom of this is intermittent discharge from the expansion relief valve. This is not a big problem to fix if it's happening. In any case, it should not expand back through the expansion check-valve, however.
A possibility that no-one has mentioned is bad pipe routing. Does the cold pipe run hard up alongside a hot pipe anywhere, allowing a pipe-volume to heat up? That would normally run off in a few seconds, not a few minutes.
If it's taking a few minutes, then it sounds like an expansion volume, and that as I say points to a problem with the expansion check valve. Probably not a safety issue, but in need of qualified attention.
Several possibilities.
(i) the cold water is sitting in uninsulated pipes in the roof and getting pretty hot.
(ii) you have a connection between hot and cold somewhere..possibilities include an overheating tank that is forcing hot water back into the cold feed, a faulty mixer tap that is on, but with the outlet blocked..and so on
(iii) The tank itself is faulty.
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