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Discuss Oil Central Heating but no hot water in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

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I have a two storey house with an oil boiler zoned heating system and two motorised valves. There is a horstmann H37XL control unit that has three buttons, one for downstairs and one for upstairs and one for hot water.

My first simple query is whether you should get hot water only when you hit the water button? would you not have hot water anyhow when the heating is on?
Or is this button for hot water without heating the rads?

Anyhow, both motorised valves work and the zoned heating works perfectly. But no hot water, whether I hit that water button or not.

I believe the system is a gravity feed system and we have a small header tank beside the main water tank.

Is there a valve somewhere that could be closed either on the pipes somewhere in the hot press.

Our immersion works fine.


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If upstairs rads OK then unlikely to be a circulation problem.
Check for balancing valve (gate valve with red wheel) on cylinder coil return, shut it, then open it fully, shut it again and open it ~ one full turn. Possibly air in pipes, slacken the coil top entry pipe and pull it back slightly to release any air then let water trickle out for a few minutes.
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Have checked there is water in header tank? And are you absolutely certain the flow and return to cylinder are gravity? No third motorized valve somewhere?
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Have checked there is water in header tank? And are you absolutely certain the flow and return to cylinder are gravity? No third motorized valve somewhere?
Hello, apologies for not replying before now. Here are pictures of the setup. I cannot think where a third motorised valve would be, cannot locate one. There is water in the header tank. Could the issue be one of these red valves in hot press?


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Normally, there are three motorised valves, one for downstairs, one for upstairs and one for the DHW cylinder coil. You should then be able to use any combination of the three.
As suggested above, you may have a gravity fed DHW system so the "water button" would have no effect, how did it work before the problem?
Suggest tracing the cylinder coil flow and return pipework back from the cylinder.
What are those two green items located near the cylinder?, one is mounted high up and the other near the cylinder base.
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With you having a danios cylinder thermostatic valve I would say you have gravity circs check the TRV isn’t stuck off or been turned low
sorry, are you referring to the unit in cylinder 1 photo or cylinder 2 photo please? and is it danfoss rather than danios?


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Take the thermostatic head off, wiggle the pin and give the body a couple of sharp taps.
Hello, I removed the stat head and the pin was a bit stiff but it does move up and down and was in the open position. The upper pipe at the point of entering the cylinder, i.e. after the stat valve is hot and the pipe coming out of the cylinder at the top is cold. The valve works as when I press it in the pipe coming down from the valve gets hot i.e. water is being diverted away from the cylinder. So there must be an airlock in the coil? and I suspect this has been there for a long time. Was going to use a hosepipe on the vent pipe in the attic and the other end on a tap to push the air out.

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