DIY Doctor

Cold feed pipe from loft tank to hot water cylinder getting hot

Postby Coupler33 » Mon Nov 23, 2020 5:43 pm

The domestic system is a heat-only gas boiler with an indirect and vented hot water cylinder in airing cupboard and a pressurised system for the radiators. The cylinder has a vent pipe at the top which usually remains cold and yet the cold feed pipe which has a column of cold water from the tank in the loft above heats up and sometimes gets hot enough to cause some turbulence as hot water slowly enters the cold water storage tank. Is this normal expansion or is it sign of a problem?
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Postby stoneyboy » Tue Nov 24, 2020 10:05 pm

Hi coupler33
Suggest you check that the vent pipe is clear of the water in the cold water tank - there should be an air gap of 50mm+.
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Postby Coupler33 » Wed Nov 25, 2020 9:42 am

Stoneyboy - thank you very much for your suggestion. I have checked that potential cause and the gap is fine. All best.
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Postby stoneyboy » Wed Nov 25, 2020 9:35 pm

Hi coupler33
The next thing to check is that the vent pipe from the top of the hot cylinder is clear especially of limescale deposits.
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Postby Coupler33 » Thu Nov 26, 2020 10:30 am

Thanks, yes I have checked that in so far as possible as it is a bendy pipe and difficult to assess any potential blockage at bends. I am considering replacing this pipe. Another idea occurs, it may be a faulty mixer valve. But that could be either sink or showers. Showers are gravity fed but with a pump fitted in airing cupboard.
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Postby Coupler33 » Thu Nov 26, 2020 9:03 pm

Is it normal and expected that the cold feedpipe should get at least warm in the lower part as heat from the cylinder extends part way up that pipe?
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Postby stoneyboy » Fri Nov 27, 2020 9:52 am

Hi coupler33
No, the cold feed pipe into the cylinder should generally be cold - assuming the pipe is fed into the bottom of the cylinder. If the vent pipe in top of the cylinder is blocked expansion can only take place through the feed pipe.
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Postby Coupler33 » Fri Nov 27, 2020 12:53 pm

OK many thanks. But maybe at the bottom of that cold feed pipe when there is no active draw off of water, some warming by conduction could occur counterbalanced by the downward pressure of water from the loft tank?
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Postby stoneyboy » Fri Nov 27, 2020 1:34 pm

Hi coupler33
Very unlikely - have you checked that the vent pipe is clear?
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