HW Cylinder over-temperature resettable thermostat

The immersion heater on our HW cylinder is the old-fashioned sort, without an over-temperature safety cut-out. It's not practical to replace the immersion with a modern type, because the plumber who installed the HW plumbing, cleverly arranged it to pass straight over the top of the immersion heater housing, thus preventing removal of the heater without a significant re-plumbing operation. Added to that, removal of immersion heaters that have been in place for decades can have its problems, AIUI.

As a second best, I'm considering using a secondary external thermostat clamped to the outside of the tank near the top, wired in series with the immersion heater and set to say 90C. While I can find a thermostat that has an appropriate power rating for the immersion heater (Horstmann HCT2, e.g.), ideally I would like one that is resettable so that it stays switched off if the set temperature is exceeded, but I can't immediately find one. Does anyone know of such an item?

Reply to
Chris Hogg
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Usually the 'stat slips into a sealed pocket in the immersion heater so it's not necessary to remove the heater from the cylinder or drain it down.

Drain (or freeze) the length of pipe across the heater, cut and remove a segment, put the new stat in and reinstate the segment with couplers. Better still, insert a U in place of the straight segment to curve round the heater cap.

Owain

Reply to
spuorgelgoog

If all else fails, that, or something similar, will be the way to go. But I'd prefer the simpler route of an external resettable thermostat.

(How many 't's in resettable? Two doesn't look right! resetable? Looks worse!)

Reply to
Chris Hogg

Can you not just swap the stat? eg for

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or one with the length you require. You can usually bend it slightly around small obstacles.

Reply to
ARW

Send us a photo of the obstruction. It should give us something to comment on. Once a heater is unscrewed (if it was required) it can usually be tilted to clear any reasonable obstacle.

Reply to
DerbyBorn

Youdon'twannadothat.

It's a safety device, the last fail-safe preventing boiling water coming out of your taps, shower or through your ceilings.

Clamp on thermostats have a habit of becoming unclamped, which makes the control system see the air temperature as the water temperature.

Drain down, cut the pipes and rejoin them with compression connectors when you've done.

Get a proper thermostat in a pocket or turn it off. That is all.

Reply to
Onetap

From what's already been said, and my own poking around taking the photos, I realise the heater doesn't need to come out.

Photos:

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shows a side view of immersion heater cover and offending pipe.

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shows the view from above.

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shows the partially removed thermostat rod hard up against the underside of the offending pipe.

If I only wanted to remove the t'stat, I could probably do it by bending it (swiveling it around doesn't help), but then I'd never get a replacement back in in one piece. I don't have good thoughts towards the plumber!

ATM the immersion is on a timer overnight (Economy 7), so isn't on for long enough to boil the tank. OK, so timers can fail, but the probability of a timer failure and the immersion t'stat failing together must be low. My thinking about an external over-temperature t'stat is as a third line of defence, fairly simple to fit and unlikely ever to be needed.

Reply to
Chris Hogg

On 13 Jul 2014, you wrote in uk.d-i-y:

I think I would be looking for an opportunity to loosen the immersion heater (to make sure I can) and then either when it is rotated to a different position change the stat or loosen it completely to tilt it to allow the stat to be changed. However, if it ain't broke don't fix it. Don't bend the thermostat.

Reply to
DerbyBorn

Depending on how long that pipe is there's a fair chance you could just loo sen the compression fitting to the cylinder and push it back slightly enoug h to withdraw the 'stat. Or if the other end is also compression can you ju st pop it off and then put it back.

The wiring should be outside the insulation and the first metre or so of th e pipe from the cylinder should be lagged.

Owain

Reply to
spuorgelgoog

Do what I do when faced with such spelling conundrums, google the word! I copied 'resetable' and pasted it into the search engine box (paste and go) which gave me the first three hits for 'resettable'. Clearly, the correct spelling by consensus (btw, I googled 'consensus' to make sure of the spelling :-).

Reply to
Johny B Good

Now that's a possibility I hadn't really considered. The pipe in question is about 3 ft long, with a right-angle bend halfway along, teed (soldered) into a vertical pipe, so there might be just enough 'give' in it to replace the 'stat.

Wiring normally outside the insulation; see the other pics. Pipe left uninsulated to provide warmth for the airing cupboard, in which it all sits.

Thanks for the suggestions.

Reply to
Chris Hogg

Onetap posted

Does this objection also apply when using a clamp-on thermostat to control an ordinary gas boiler based hot water system?

Reply to
Big Les Wade

I'd watch out for that: Trying to remove an immersion heater can easily end in tears (well, at least having to fit a new cylinder) if the threads are tight and gunked up. I'd have thought the simpest fix is to re-route the pipe or put service valve just to the left (in the first photo) of the heater. That way that section of pipe can be removed easily for access.

Reply to
GMM

No.

Reply to
Johny B Good

Johny B Good posted

Why not? One would expect the results to be the same, unless the gas boiler based system has an additional way of measuring the HW tank temperature. .

Reply to
Big Les Wade

With Gas CH&HW, the risk of the hotwater tank boiling or exploding simply doesn't exist. The consideration with such a failure in the case of an immersion element is the risk of the tank boiling.

The worst that can happen with gas heating is that you land up with a tankful of unusually hot water which aught to clue you into the possibility of a failed or loose thermostat without invoking the risk of boiling the tank and filling a plastic header tank with almost boiling hot water which could result in its failure and expensive flood damage.

Reply to
Johny B Good

The boiler would have/should/must have its own thermostat and, if operati ng in a sealed system, a high-level manual-reset cut-out thermostat, so the safety considerations don't apply. The boiler stat temperature set-point w ould be higher than that of the control system.

Having said that, clamp on sensors and thermostats are farly crap and nee d to be insulated over to get any reading that resembles that of the fluid. Immersion sensors in a pocket are the proper way to control it.

Reply to
Onetap

Sorry, misread that. Yes, the clamp comes loose, there's usually no heat-conductive paste between tank and thermostat.

Reply to
Onetap

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