Faulty Thermostat or faulty installation

[Chris B]

Next door neighbour has just had their HW tank replaced under maintenance contract by BG. The old tank was copper but I think the new one is SS - it is certainly very flat topped (still open vented).

They transferred the immersion heater (the immersion heater is not used and is a back up for boiler failure only) and thermostat from old tank to new but now they seem unable to get satisfactory HW temperature.

One minute it is scalding hot, reduce the thermostat setting by 1Deg and the water ends up dropping to luke warm before the boiler wakes up to warm it up.

When moving the thermostat manually there is about a 5C "backlash" between switch on/off operations.

Any suggestions as to whether this is

1) a thermostat that didn't like being moved and has failed due to the disturbance

2) Lack of thermal connection between the new tank and the thermostat (should there be any form of heat transfer compound between them) - I think this would be my first point of call if it was my tank)

3) Incorrect positioning of the thermostat

4) any other ideas.

He has the maintenance contact with BG so they will be called in to fix it but he would like some confidence that the problem can be fixed.

[Andy Burns]

Is it the strap-on with curtain wire, or shoved in a pocket/tube type?

[Chris B]

Its held on with a flat metallic strap about 5mm wide that goes right round the tank.

[Tricky Dicky]

Whereabouts on the tank is it? It should normally be about 1/3 of the way up, too high and because hot water rises it will constantly monitor the water temperature where it is hottest so the bottom half of the tank does not warm up.

Richard

[Chris B]

Well I haven't actually measured it but yes at a guess I would say its about 1/3 of the way from the bottom

[alan_m]

Is that just the normal hysteresis of 5 to 10C?

It is probably a simple bi-metal strip with little to go wrong - except in moving it the installer has created a high resistance electrical connection - wire connected with screw through the insulation and only just making electrical contact.

The strap may not be tight enough to hold the thermostat tightly against the tank. As far as I'm aware no compound is used between the tank and thermostat

Where is it positioned? The position may only be relevant to what temperature you set it to. Currently on by tank I have installed two (Ebay special) temperature probes/displays. One half way down the tank where the thermostat is fitted and one at the very top. While water is heating from cold there is a approx a 10C difference in reading with the top of the tank being hotter.

[John Rumm]

Is the tank insulated? Normally on a foamed copper cylinder, you would cut a pocket in the foam to allow the stat to make contact with the metal. Many of the SS cylinders are dual skinned with a metal outer cover over the insulation, which in turn is over the inner cylinder. This could make the stat unable to "feel" the change in temperature effectively, and also make it such that even a quite substantial temperature change in the cylinder is reflected in a small change visible to the stat.

[Brian Gaff (Sofa]

Surely the thermostat is measuring the water temp not the tank temp is it not?. Why did they not fit a new one? They are cheap items after all, compared to a new tank. Brian

[Brian Gaff (Sofa]

The first type are as useful as a chocolate fireguard in my experience, do these still exist? Brian

[Brian Gaff (Sofa]

Yes but does not the water come from the top and in to the bottom? Brian

[Brian Gaff (Sofa]

If its the old one where was it before?

Brian

[Andy Burns]

Yep 15 quid from screwfix, if you've got a hot water cylinder with separate lagging, there's nothing wrong with a strap-on stat, it's what I have here.

[Brian Gaff (Sofa]

If it is a double skinned tank then surely it has the pocket type thingy for one of those thermostats?At least then it will be in the standard place and no bits of left over insulation will be causing issues. Brian

[Chris B]

To answer all the questions at once, it seems to be foam covered (best equivalent picture I can find on the net is like this - although not this brand)

The thermostat is definitely measuring (or trying to) the outside temperature of the tank and not the water.

the fitters have cut out a section of foam about half a mm bigger than the thermostat (just like the thermostat fitment on my own clyinder). Might they have not fully cleared the foam from the side of the tank?

[Tim+]

I think only you can answer that! Remove the strap and have a look.

Tim

[Andrew]

once you have made sure that all the foam has been removed from the cylinder where the thermostat makes contact, ensure that it makes good mechanical contact with the cylinder.

if you have some thermal grease, the stuff used with CPU cooling 'radiators', put a smear of that on the tank before refitting the thermostat.

[newshound]

BG installers could not be so stupid...could they?