Why do I have to keep re-setting the immersion heater thermostat

[Unclefred]

Dear All,
My hot water cylinder (approx 45 years old) is fitted with a bottom mounted immersion heater controlled via a timer and an Essentials 7" TS220 thermostat bought from Screwfix. The cylinder itself is well lagged with two jackets and the timer kicks in every evening for 3 hours. Unfortunately the thermostat appears to have a mind of its own as the following day the water is frequently hotter than expected. Resetting the thermostat brings things back to normal for a while but the same then happens again. Any thoughts on what the most likely cause of this erratic behaviour is? Should I buy a new one? I look forward to some suggestions.
Thank you for reading.

 

[steadyon]

1. My first suspicion would be a build up of limescale in the bottom of the cylinder.
2. What do you mean by resetting the thermostat?
2a. Resetting the overheat thermostat button.
OR
2b. Adjusting the temperature setting dial.

 

[Deleted member 120897]

Welcome Fred,

Do you only have one immersion heater?

 

[SimonG]

Shot another thermostat into the immersion.

 

[Unclefred]

Being new to this forum I am not sure if my replies get posted. However to answer the questions, I keep re-setting by pressing the reset point with a paper clip or similar. I suspect that the thermostat occasionally - in fact too often for my liking - does not switch off and as a result the water gets too hot. I further suspect that it would be hotter still were it not for the timer kicking in to switch everything off. The water in our area is quite soft so we don't really have limescale problems. No gas central heating in our house, its all electric and we only have one immersion heater in the cylinder. Looks like I should take a chance on another and hope for the best.
Thank you for all your replies.

 

[Deleted member 120897]

Your replies get posted, but I think newbies are screened by moderators first.

 

[steadyon]

1. Modern immersion heater thermostats have:
1a. An adjustable thermostat, used to set the required water temperature. Once the set temperature is reached (not very accurately) the thermostat cuts off the electricity to the immersion element. The water starts to cool, and after a time the thermostat automatically turns the electricity on again and the cycle repeats.
1b. A non-adjustable thermostat, with a factory set limit, usually around 85 degrees. If the water (or rather, the thermostat) gets hotter than this set temperature, the thermostat cuts off the electricity to the immersion heater. Manual intervention, via the paper clip, is required to reset this thermostat. This thermostat is there as a safety device to stop the water getting hotter and hotter if the normal thermostat fails. Uncontrolled heating can lead to very hot water filling the cold water storage cistern in the loft as it escapes up the vent pipe. Because the CWSC is usually plastic, they can fail if filled with near boiling water.
2. I would suggest as a low cost diagnostic you replace the thermostat.
3. If this does not effect a cure, it would seem the water at the base of the cylinder is continually overheating. As previously stated, this is usually caused by an accumulation of limescale.
4. Unless your water is absolutely soft, 45 years is likely to have caused a good deal of limescale to have been deposited.
5. If you have a blanking plug (look at Screwfix 51855) fitted to the TOP of your cylinder, you could try the following:
5a. Turn off the cold water supply to the base of the cylinder.
5b. Open a downstairs hot tap to run off any hot water in the pipes.
5c. Remove the blanking plug. Expect some residual water, so be ready with old towels etc.
5d. Use a piece of hose pipe to syphon some water from the base of the cylinder. If this has a lot of limescale with it, you will have your answer.
6. Alternatively you could:
6a. Turn off the cold water supply to the base of the cylinder.
6b. Open a downstairs hot tap to run off any hot water in the pipes.
6c. Try undoing the immersion heater. The electricity supply MUST be off. Only slacken it a tiny bit until water starts to appear, then re-tighten.
6d. Drain the hot water cylinder. It should have a drain point at the base, normally where the cold feed enters. However, these drain points rarely work. You might have to syphon the hot water out, via blanking plug (see above) if you have one, or by disconnecting the vent pipe from the centre top of the cylinder.
6e. Remove the immersion heater.
6f. Clean out the base of the cylinder.
6g. Consider replacing the heater and thermostat if you go this far.
7. Warnings:
7a. Electricity off when doing anything with an immersion heater. If you have a double pole switch for the heater use this, but its much better and safer to turn all power off at the consumer unit. Do NOT rely on pulling the fuse for, or switching of the circuit breaker for, the immersion circuit.
7b. Immersion heaters and blanking plugs tend to be incredibly tight, particularly after many years. Always try to slacken them while the cylinder is full of water, as this makes it less likely to collapse.
7c. If you damage the cylinder in any way, you will need a new one.

 

[Unclefred]

1. Modern immersion heater thermostats have:
1a. An adjustable thermostat, used to set the required water temperature. Once the set temperature is reached (not very accurately) the thermostat cuts off the electricity to the immersion element. The water starts to cool, and after a time the thermostat automatically turns the electricity on again and the cycle repeats.
1b. A non-adjustable thermostat, with a factory set limit, usually around 85 degrees. If the water (or rather, the thermostat) gets hotter than this set temperature, the thermostat cuts off the electricity to the immersion heater. Manual intervention, via the paper clip, is required to reset this thermostat. This thermostat is there as a safety device to stop the water getting hotter and hotter if the normal thermostat fails. Uncontrolled heating can lead to very hot water filling the cold water storage cistern in the loft as it escapes up the vent pipe. Because the CWSC is usually plastic, they can fail if filled with near boiling water.
2. I would suggest as a low cost diagnostic you replace the thermostat.
3. If this does not effect a cure, it would seem the water at the base of the cylinder is continually overheating. As previously stated, this is usually caused by an accumulation of limescale.
4. Unless your water is absolutely soft, 45 years is likely to have caused a good deal of limescale to have been deposited.
5. If you have a blanking plug (look at Screwfix 51855) fitted to the TOP of your cylinder, you could try the following:
5a. Turn off the cold water supply to the base of the cylinder.
5b. Open a downstairs hot tap to run off any hot water in the pipes.
5c. Remove the blanking plug. Expect some residual water, so be ready with old towels etc.
5d. Use a piece of hose pipe to syphon some water from the base of the cylinder. If this has a lot of limescale with it, you will have your answer.
6. Alternatively you could:
6a. Turn off the cold water supply to the base of the cylinder.
6b. Open a downstairs hot tap to run off any hot water in the pipes.
6c. Try undoing the immersion heater. The electricity supply MUST be off. Only slacken it a tiny bit until water starts to appear, then re-tighten.
6d. Drain the hot water cylinder. It should have a drain point at the base, normally where the cold feed enters. However, these drain points rarely work. You might have to syphon the hot water out, via blanking plug (see above) if you have one, or by disconnecting the vent pipe from the centre top of the cylinder.
6e. Remove the immersion heater.
6f. Clean out the base of the cylinder.
6g. Consider replacing the heater and thermostat if you go this far.
7. Warnings:
7a. Electricity off when doing anything with an immersion heater. If you have a double pole switch for the heater use this, but its much better and safer to turn all power off at the consumer unit. Do NOT rely on pulling the fuse for, or switching of the circuit breaker for, the immersion circuit.
7b. Immersion heaters and blanking plugs tend to be incredibly tight, particularly after many years. Always try to slacken them while the cylinder is full of water, as this makes it less likely to collapse.
7c. If you damage the cylinder in any way, you will need a new one.

Thank you for your very detailed post.

Firstly, the water to our household I believe is sourced from the Lake District and is very soft.

As I said in my original comment the immersion heater is activated by a timer, which kicks in at 20:30 for a period of 3 hours. I judge this to be more than the necessary length of time to heat the water to the desired temperature and from the results that I have seen from our electricity provider, this has proved to be the case i.e. when operating normally, the thermostat switches the electrical supply off at the desired temperature and a significant fall in kWh units follows. The insulating jackets thereafter keep the water sufficiently hot for our usage during the day.

About 18 months ago the element in the cylinder packed in and the plumber who arrived on the scene syphoned the cold water from the top of cylinder by means of a long tube and I presume the plug to which you refer. I did not notice any obvious signs of scale in this water. NB as you say, the drain tap at the bottom of the cylinder did not work; even with our very soft water. The plumber advised that this is quite normal. Having drained the cylinder, a new element was fitted and from memory a new thermostat as well. By means of trial and error, the thermostat was then set to a provide the water temperature that we find to be acceptable. However, of late the thermostat has, on intermittent occasions, not apparently switched the heat off when the desired temperature is reached, thereby resulting in water that is hotter than required. I have also noted correspondingly higher kWh usage from our electricity provider. My puzzle is why the thermostat should behave in this mysterious intermittent way.

Your suggestion of a ‘low-cost diagnostic’ seems to be the easiest starting point and I intend to pursue this course of action.
Thank you again for your time and professional advice.

 

[Dan]

Hi and welcome to the forum,
Thanks for joining.

I wont get involved in the technical stuff as I am not a plumber.
If there is anything you need help with forum wise pleas ask.

 

[ronco66]

Thank you for your very detailed post.

Firstly, the water to our household I believe is sourced from the Lake District and is very soft.

As I said in my original comment the immersion heater is activated by a timer, which kicks in at 20:30 for a period of 3 hours. I judge this to be more than the necessary length of time to heat the water to the desired temperature and from the results that I have seen from our electricity provider, this has proved to be the case i.e. when operating normally, the thermostat switches the electrical supply off at the desired temperature and a significant fall in kWh units follows. The insulating jackets thereafter keep the water sufficiently hot for our usage during the day.

About 18 months ago the element in the cylinder packed in and the plumber who arrived on the scene syphoned the cold water from the top of cylinder by means of a long tube and I presume the plug to which you refer. I did not notice any obvious signs of scale in this water. NB as you say, the drain tap at the bottom of the cylinder did not work; even with our very soft water. The plumber advised that this is quite normal. Having drained the cylinder, a new element was fitted and from memory a new thermostat as well. By means of trial and error, the thermostat was then set to a provide the water temperature that we find to be acceptable. However, of late the thermostat has, on intermittent occasions, not apparently switched the heat off when the desired temperature is reached, thereby resulting in water that is hotter than required. I have also noted correspondingly higher kWh usage from our electricity provider. My puzzle is why the thermostat should behave in this mysterious intermittent way.

Your suggestion of a ‘low-cost diagnostic’ seems to be the easiest starting point and I intend to pursue this course of action.
Thank you again for your time and professional advice.

i live in dublin ireland and over the past 3 years. i have found that the thermostat the blue tiped one with the white round pointer with the numbers around keep on failing only lasting about two weeks before we get called back .i got onto the supplier in ireland and got no where with them their stat fails and the overheat stat kicks in they are a cheap stat made in china and we have difficulty sourcing a reliable stat i did not name the stat in case

 

[John.g]

That's incredible, I have replaced a few over the years in relations/neighbours, the all generally last 6/8 years and I just use whatever is available, ie Shel or Backer (16A) all in top mounted dual immersions.
The only problems I've encountered in a few was tht the actual cylinder temperature tended to drift upwards from the (measured) set point of 60C (to ~ 68C) that I set them to.
Are you talking about top mounted immersions as well?.

 

[Unclefred]

That's incredible, I have replaced a few over the years in relations/neighbours, the all generally last 6/8 years and I just use whatever is available, ie Shel or Backer (16A) all in top mounted dual immersions.
The only problems I've encountered in a few was tht the actual cylinder temperature tended to drift upwards from the (measured) set point of 60C (to ~ 68C) that I set them to.
Are you talking about top mounted immersions as well?.

In my case, the immersion (from local Screwfix store) is bottom mounted and as you appear to have experienced, the hot water in the cylinder sometimes drifts upwards from the set point. Hence the need to pull the insulating jacket to one side and poke the thermostat re-set button.

 

[John.g]

In my case, the immersion (from local Screwfix store) is bottom mounted and as you appear to have experienced, the hot water in the cylinder sometimes drifts upwards from the set point. Hence the need to pull the insulating jacket to one side and poke the thermostat re-set button.

When I say drift up, I mean just that, the hi limit stat doesn't operate, the temp might reach 66/68C and then
cut out.
I would remove the stat from its pocket and immerse it fully in a container at different temps, between say 55C and 70C and check the contacts (with a multumeter) opening/closing at different settings and turning the stat knob up and down.

 

[Unclefred]

When I say drift up, I mean just that, the hi limit stat doesn't operate, the temp might reach 66/68C and then
cut out.
I would remove the stat from its pocket and immerse it fully in a container at different temps, between say 55C and 70C and check the contacts (with a multumeter) opening/closing at different settings and turning the stat knob up and down.

Whenever the water gets too hot, I simply do as I indicated in the previous message i.e. pull the insulating jacket to one side then push the re-set with a ball point pen or whatever similar implement I have to hand. This seems to work OK for a while but I can't figure out why the water overheats at random. I do have a spare thermostat and will get round to replacing the erratic one when I run out of patience!