HW cylinder not heating up

Postby edwardtsmith » Tue May 15, 2012 7:37 am


Just moved into a new house and have CH but the HW tank fails to heat up - it did the very first time we switched the system on but not since then. We have a system boiler (I believe - no external pump that I can see and most pipe work hidden from view so far).

The hot water cylinder has the 22mm flow and return pipes, the upper one always being hot when the boiler fires, the lower one always stone cold. There is no expansion tank in the loft and the cylinder is vented into the main water storage tank in the loft. Old disused galvanised tanks in the loft!

Firstly, do we have a direct cylinder or primatic cylinder?
I assume we don't have an indirect cylinder as we have no second feed and expansion tank!
If direct cylinder, why does the lower return pipe remain stone cold - I would have thought some warmth would be returning to the boiler...
If primatic cylinder, probably the air bubble has gone as the previous owner installed 2 power shower pumps off the cylinder - but wouldn't the colour of the water show the contamination of water supplies?

Any advice/info greatly appreciated....
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Simply Build It

Postby j1ll » Wed May 16, 2012 7:05 am

If it is a modern system boiler you may have a pumped hot water circuit with motorised valves on the feed to the cylinder controlled by timer and cylinder thermostat.

In this set up, I think your symptoms could be caused by a cylinder thermostat set to a low temperature, or by a faulty motorised valve.

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Joined: Tue May 15, 2012 2:39 pm

Postby edwardtsmith » Wed May 16, 2012 8:45 am

Hi Jim,

The first time it worked the cylinder thermostat was set low so I turned it up but now it doesn't heat the cylinder at all.

If it was a motorised valve or pump then wouldn't the hot water not get to the tank at all - the flow pipe to the tank is nice and toasty!

Discussing with a friend, they have suggested that the cylinder might be direct and be clogged with limescale - thus the return pipe is now blocked - is this possible?

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Postby Atticus Finch » Wed May 16, 2012 12:32 pm

Is there a valve on the return pipe leaving the cylinder? If so, it may have been closed by previous owners. Try opening it about one turn.
Atticus Finch
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Postby edwardtsmith » Thu May 24, 2012 8:40 am

Thanks for the help so far....

I have now dug around the system enough to realise that I actually have a indirect water cylinder but with a sealed system (hence no feed and expansion tank!).....There are no valves that could have been closed between the cylinder and the boiler....

So the hot water from the boiler (after passing through the 3 way valve) reaches the hot water cylinder (quite slowly incidentally) but does not seem to heat up the water inside the cylinder.....Is there any way that the pipese to and from the boiler and cylinder can become blocked?

Is it worth me checking the three way valve isn't always pushing the hot water down the CH side,i.e. dodgy three way valve?

Thanks again,
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Joined: Tue May 15, 2012 7:08 am

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