Unvented cylinder thermostats – plumbers?

Need to change the thermostat on my unvented water cylinder. The original clamped onto the pocket and had two probes, but the replacement is just a single probe on the end of some cable.

Any reason why I can’t just put a bit of thermal epoxy on the probe and slide it into one of the “channels” in the pocket, then secure it with some insulation? The plan is to temporarily have a thermocouple in the adjoining channel to make sure they’re reading the same.

It’ll be firmly secured to adjoining pipework so can’t fall out, but obviously I don’t particularly want to test the overpressure relief on the system…

Cheers,

Dave.

(It’s part of the Honeywell Evohome setup – only problem is that having coughed up for this I can’t afford to run the heating!)

perfectly safe.. absolutely no need for those plumbing folks to have completed all those pesky exams to be able to work on unvented cylinders.. i mean what damage can an overheating steel cylinder with 200 liters of boiling water do.

http://www.combustionsafety.com/secured/pdf/safetyalerts/star_spencer_lesson.pdf

If it is the type I think it is, then the thermostat does not touch the water, it is in a sealed pipe that goes into the tank.
No need to drain off etc, so no need for plumber involvement.

Having seen the appalling jobs done by three entirely independent plumbers I’m not willing to set much store by your “pesky exams”.

I asked a pretty simple question in the idle curiosity that there might be a specific adapter or similar to fit a probe like this. As usual on STW there’s the usual condescending answer which entirely fails to answer the original question.

In fact, I imagine you only read the title and ignored the rest of the post merely so you could be the first to wave your plumbing penis in front of everyone else.

Obviously lacking the secret handshake and the weekend course I’m not entitled to enter the special world of plumbing.

Edit: Thanks to Alani for the sensible answer.

Edit 2: Yes, I’m well aware that there’s not much chance of getting any more helpful advice following this rant but by Jove I feel better.

The honeywell evohome unvented kit is a thermistor/ntc. Thats why its sealed unit on the end of a cable rather than a switch wire.

The end of the cable needs to be press against the metalic inner surface of the cylinder. Wedged under the insulation (even better with some heat transfer paste)

Some cylinders even have a thermistor pocket on them, so check to see if yours has one.

The reason for the thermistor is so that it can give a water temp readout on the evohome controller.

Dont be so rude to Totalshell he talks sense and was trying to keep you from doing anything stupid. He’s far from a weekend course plumber.

p.s I have the evohome as well

Thanks flux. The tank (Tribune HE) doesn’t have a thermistor pocket so the original plan was to take out the existing dual thermostat and replace it with the Honeywell probe, then use a fail-open pipe thermostat in series with the valve / pump control to cut the power if it exceeded 70C.

Not keen on cutting into the tank insulation to tuck the Honeywell thermistor underneath, it’s a steel outer casing.

Edit: Ahhh… found the part I needed.

http://www.screwfix.com/p/dual-safety-thermostat-11/48571

This has fail written all over it.

This has fail written all over it.

How so? In order for the tank to explode the following needs to happen simultaneously:

* The wireless relay needs to fail in the closed position (fairly plausible)

* Two independent thermostats need to fail into the closed position (since their entire purpose in life is NOT to do this, it seems unlikely).

* The boiler’s independent temperature control needs to fail.

* The boiler’s second temperature cut-out need to fail.

* Finally, the over-pressure relief on the tank needs to fail to open. Again, this is its job.

Six independent failures need to occur before there’s a risk to life. I’ve seen the Mythbusters hot-water tank explosion.

Risk of injury? Certainly possible for the tank temperature to sit at 70-80C with a combination of the above, but equally this could happen to any system.

Arguably, I’m actually making the installation safer because I’m effectively adding another thermostat.

http://www.combustionsafety.com/secured/pdf/safetyalerts/star_spencer_lesson.pdf

Given the millions of unvented cylinders in use you’ve supplied a bit of a poor example TS as this incident was caused by a failure to use the correct safety valve which would have released the built up pressure – it’s got nothing to do with a thermostat.
I think we all know that the ‘special training’ the plumber has to do to become qualified to install unvented cylinders is all about the water pressure safety, not wiring up a thermostat.
Interestingly I had a used unvented cylinder fitted 6 months ago by a properly qualified plumber and he left me to wire up the thermostat!
I’m not saying that Flaperon is right to go ahead (he should contact the manufacturer or get on to the Navitron forum where there are plenty of knowledgable sorts.

That does have fail written all over it. If your insistent on bodge it the best thing you could do is leave the dual thermostat in place. Electric isolate the immersion heater. Remove the thermostat from that and place the sensor in the space provided. As the honeywell instructions suggest.

You cant reconnect the immersion heat if you go down this option. You also have the to wire the dualstat as a high temp stat only otherwise it will counteract the evohome.

Your plan will fail.

I’m not expecting it to explode I’m just expecting it to not work. That stat you link to is for a standard immersion element for a gravity system, it won’t work on your Tribune.

Also, if anything does go wrong I should imagine your house insurance will want to know that the person who did the work is qualified to work on an unvented system which obviously you are.

fluxhutchinson – thanks, that looks to be the best solution by far. The installation manual that comes with the system doesn’t talk about using that approach, it simply says that there needs to be independent protection.

That’s because I have the proper installation manual 😉

Thought I’d just tag on to this thread since the resident experts are obviously all in attendance! 8)

I’ve just moved into a flat with a Heatrae Sadia Megaflow High Efficiency 210 litre tank. Any time I want to take a long shower (after a long cold ride or something) it runs cold, even quicker if I use the second bathroom which has an astonishing flowrate out of the shower head. Either way, I doubt I’m getting more than 60 or 70 litres of hot water.

I noticed that of the two plastic coverings which I assumed covered the two 3kW elements, the upper one felt warm to the touch but the lower didn’t. I isolated the unit at the breakers and took the covers off.

I pushed the reset switch on the lower element and ran the hot tap for a while. Again, it didn’t take long to run cold. I could feel the upper element starting to warm, but nothing from the lower.

At this point I’m beyond my very limited expertise, but before I call the letting agency to call the landlord to call the plumber (assuming that takes about 10 weeks… 🙄 ) is there anything obvious I might have missed? The fuse box shows both elements are OK, the only other thing is that the switch to the lower element is in the ‘on’ position but the little red indicator LED isn’t lit, what does that suggest?

Are you on electric only? Economy 7 immersion maybe? If you do have economy 7 it should be wired to the bottom immersion and it sounds as if only the top one is working. Which really should be wired to a boost button.

Aha! You know your stuff, I just came across an old Money Saving Expert post suggesting something similar.

I’m on Economy 10, but it would seem as though my programmable thermostat lets me dictate how and when the water/heating comes on, and I just get charged high/low rate accordingly.

It would seem stupid for a two bedroom flat to have a hot water system incapable of providing enough water for two people to shower in the morning without manually having to ‘boost’ it beforehand.

Edit: I’ve just hit the ‘boost’ button on my programmable thermostat. Cue loud humming from utility room, nothing else appears to have changed though apart from the overflow/PRV dripping. I think I’ll just turn the boost of for now… 😕

You need to get onto your letting agent as it sounds like the expansion vessel/ internal air gap may need recharging and the wiring potentially needs looking at. HTH

I’ve just moved into a flat with a Heatrae Sadia Megaflow High Efficiency 210 litre tank.

I have one of those cylinders and chances are the lower thermostat has burnt out and needs replacing – there is a bit of history in this department.

You need to get onto your letting agent as it sounds like the expansion vessel/ internal air gap may need recharging

No need for the agents to be involved…. dead easy to fix and the instructions are on the side of the cylinder (open lowest hot tap, shut off cold supply valve by the tank, hold the upper valve open to allow air back into the top of the cylinder. Once gurgling stops, release the valve, open cold supply again and shut off the hot tap).
This will not affect the lack of hot water though.

Following the advice here the more I look at this the more uncomfortable I am about fitting the hot water side of the kit. I’ll leave it on the programmer and just use the Evohome side for heating.

Thanks for the help – and totalshell, I regret some of the things I wrote yesterday. 🙁