Cylinder Stat

10 Feb 2005
Reaction score
United Kingdom
Central heating is not quite my usual field but I was asked to look at a cylinder stat today. The system is fully pumped with individual zone valves for central heating and hot water. The customer complained that the DHW was getting too hot. He had already had a new cylinder stat fitted but it had not helped. I checked that the stat was correctly wired up then checked how the zone valve reacted to different stat settings. Even at the minimum setting the zone valve stayed open but if the stat was turned to the off position the valve closed. I swapped back to the old cylinder stat and repeated the test. This one behaved in exactly the same way despite the water in the tank being piping hot. I've not had much to to with these cylinder stats but would have thought, like with a room stat, you should be able to use it as a crude form of thermometer by listening for the clicks? These ones only clicked when they were below the marked temperature scale but as I said, the water was already hot. :confused:
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the water was already hot.
But was it hot at the point on the cylinder where the cylinder stat was attached? And did the cyl. stat make good contact with the cylinder surface?
The stat is about half way up the cylinder. I must admit that the hot water from the taps felt hotter than I expected after feeling the bare patch on the cylinder. As for the contact, I did wonder about using some heat conducting grease, as used for power transistor heatsinks (I have some somewhere).
I did wonder about using some heat conducting grease
Shouldn't be necessary, but wouldn't do any harm. You didn't say how hot cylinder surface was at stat contact point (I hate people who ignore my questions, when I'm trying to solve their problem!).

The question is "does the cyl. stat respond to the temp. of cylinder wall at point of contact as it should?" If it does, what is the problem? Bear in mind that it is normal for hot water to stratify so that it is first hot at top of cylinder only, then getting hot progressively lower down, until it reaches level of stat. Stats are usually placed 1/4 to 1/3 way up cylinder, so yours sounds rather high!
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I did not take any means of measuring the cylinder temperature but when I go back I will take a roasting thermometer and the conductive grease. The customer is going to get another new stat so we will have another go in the next few days.
Cylinder stats rarely fail, so I wouldn't bother to change it again. The fact that two have behaved the same virtually rules out a stat fault.
Has the problem only shown itself since the central heating started being used?. Could be the hot water zone valve is passing and heating the water up to the temperature on the boiler stat.
Went back this morning and fitted another new stat, Danfoss this time. It now works fine, set it to 45 degrees and zone valve is closed, set it to 55 degrees and the zone valve is open :cool:

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