Unvented cylinder system (2)

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Hi
My nieghbour has had a problem with his boiler over heating, an engineer has been to try and sort it out, and has concluded it's his unvented tank.
I'm not an expert, but how many things can go wrong with this type of tank? My understand ( practical not experience) tells there are only the two valves plus the expansion tank that can go wrong, but I'm sure you will put me right on this
Th
 
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His engineer is clearly clueless, the unvented cylinder is not a heat source, so cannot cause a boiler to overheat.
It's possible however that the cylinder zone valve may be stuck open, causing the boiler to continue firing, a few simple tests done by a competent person will soon establish that.
 
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Thank you for this advice, I think the engineer should have carried out to see if this motorised valve was faulty.
 
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Engineers eh....

The stat controls the zone valve so check that is also working.
 
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As stated I'm not an engineer, how do I check this other than taking the cover off and see if it rotates.
 
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Over heat is often caused by insufficient flow. Increase pump speed or reduce thermostat on the boiler, or both will make a difference.
 
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Yes, I'm aware you have to have G3, I've no intention of doing anything, but was only enquiring on behalf of my nieghbour. I just want to see him having to pay for a new tank if it wasnt needed.
I will suggest he has 2nd opinion look at it.

Once again thanks for you great advice.
 
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I have a unvented system in my own house, other than the engineer checking the filter on the one valve and the pressure in the expansion tank, that's all he does. And this costs me £85. But guess it has to be done.
 
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Could you/he check whether the flow pipe from boiler, is hot where it enters the cylinder and the return close to the cylinder?
 
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I have a unvented system in my own house, other than the engineer checking the filter on the one valve and the pressure in the expansion tank, that's all he does. And this costs me £85. But guess it has to be done.

Christ.

Why?

Get a engineer to service the system and boiler and the tank at the same time?

I'm in the wrong job....
 
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