no hot water but cyclinder feed/return hot

11 Jan 2011
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United Kingdom
Can anyone help as I'm baffled. Why would the boiler feed/return to the hot water cylinder be hot and the thermostat be set to 60C but no hot water output?

My experience of domestic heating systems is limited. I'm in a 10 year old house with a gas, indirect boiler system. Central Heating works fine but the hot water has started to give a problem eg whilst showering I now have to keep moving the temperature up. And now there is no hot water at all. Next to the cyclinder there is a 3 way switch for CH/HW and I've replaced the controler for this 3 times now and the last replacement was a month or so ago. It was showing a light to indicate HW is on. The cylinder boiler feed and return pipes are both red hot. I removed the switch control and the valve switch was in the correct position for HW. The pump is also working. Yet the cyclinder output pipe at top of tank is cold. Not knowing what to do I've played around turning the thermostat up and down and turned the 3 way switch from HW to HW/CH to CH to HW and run taps. And now its working: I started getting warm water and now its hot but I noticed that the power shower is still not good the original cold/hot mix setting and needs more hot. Any ideas, suggestions.
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The coil may be scaled by the domestic water, and not allow heat transfer.

The motorised valve may be at fault. A hot flow and return will not guarantee a cylinder will be hot if the primaries are not circulationg corrctly.
I suspect a blockage in the return from the cylinder.

Mr. W
Thanks for replying misterdubya, BingoBOngo,

Sorry for the late response but I wanted to see what happened next and then had other issues to deal with - dont we all! I don't think the motorised valve is at fault as thats part of the new unit ie it fits onto the metal valve 'handle' which I can turn to the correct CH/HW positions by hand. But! though I can turn it with 2 left fingers it does not turn as smoothly as I would expect and think this explains why I've had to replace the Myson '3 way switch' 3 times in 10yrs (or is it 4?).

Re "coil may be scaled by the domestic water, and not allow heat transfer." I think scale may be the problem. The water went off again last week ie heat pipe going into immersion tank red hot but the hot water output pipe on top of the tank is cold, with hot water tap running. So I took my usual professional cool detached approach and turned all switches on/off and hit everything with a hammer and it started working. Ok slight exageration, what I did was:

1. turn immersion thermostat control to off and saw Myson 3 way switch
controller HW light go iff. Turned thermostat up and light came back on.
2 I dont think CH light was on (Myson 3 way switch) and the manual control lever on the Myson controller was not lose ie would not freely lift up to 'manual on'
3 I tapped the 3 pipes around the Myson control and one is the heater pipe into the immersion tank (which was red hot anyway). And then
hot water started coming out of the tap. Also the manual control lever on the Myson controller had become lose.

Trouble with doing things in a random manner you are not sure whats what. But then I didn't do much!

So maybe problem is pipes/tank furring/scaling up - we're in a very hard water area. But why would tapping the heater pipes make a difference.
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Can anyone help as I'm baffled. Why would the boiler feed/return to the hot water cylinder be hot and the thermostat be set to 60C but no hot water output?

Have you checked that the small head tank in the loft as got water in it, it should be about 1/3 full.
I had a problem with no HW.

.. but when I turned the m/v manually (from "Normal" to "Flush") , hot water rushed through to the cylinder.

Then I replaced the whole m/v and the value it sits on top of and it fixed the problem.

During the post mortem, I found a slight build up of a black-ish deposit inside the mechanism was, intermittently, stopping it from turning freely.
different problem. If the motorised valve was stuck shut, the flow and return pipes to the coil would not be hot

You say the hot water pipe coming out of the top of the cylinder is cold. This often happens if cold water is passing into the HW pipes. This often happens when a mixer tap, often at a shower or kitchen sink, and usually a ceramic disk type, is fed with mains pressure cold and tank fed hot.

Try turning off the water mains stop cock for half an hour, then run the hot tap and see if it is then hot. If so, examine all your mixer taps. Turn the mains back on, then run the hot tap only, then turn it off and feel the hot pipe. If it is back feeding cold mains water, you will find the hot pipe at the faulty mixer tap goes very cold very soon.

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