Overheat stat on Grant boiler

[harryarcos]

Following on from my question about monitoring F & R temperatures, I needed a measuring device because the overheat stat. frequently trips and then needs to be reset. It normally happens when the boiler has switched off and is no doubt due to the hot water rising to the top of the boiler, but I did wonder if the overheat stat. was faulty, because the F & R temperatures are as they should be. Perhaps it is a problem only experienced in bungalows, as the top of the boiler is the highest point in the system. It happened all the time with the old boiler, but the overheat stat. just illuminated a neon bulb and did not need resetting. Having found these probes on a well known site, I have seen the temperature go as high as 108°C as shown in the photo. and can imagine that it could get to the stat. cut out point, which I believe is 110°C Perhaps I could find a suitable replacement overheat stat. which does not need manual resetting?

I would welcome any comments on the problem.

 

[SimonG]

It's a really good idea to not be messing with a boilers safety devices.

 

[exedon2]

As above don't play with safety equipment.
Get a competent oil engineer to have a look.

 

[harryarcos]

Noted, but you should have added "unless you are confident that you know what you are doing". The same goes for taking apart laptops, removing load bearing walls or tracing faults in hybrid vehicles, all of which I have successfully done several times.
The main "safety device" is the boiler thermostat and it is clear that this is working perfectly. The fail safe is the high limit stat. and as this was only removed when the boiler was off, there was no danger.

If you have any advice regarding the problem, please let me know.

 

[SimonG]

Noted, but you should have added "unless you are confident that you know what you are doing". The same goes for taking apart laptops, removing load bearing walls or tracing faults in hybrid vehicles, all of which I have successfully done several times.
The main "safety device" is the boiler thermostat and it is clear that this is working perfectly. The fail safe is the high limit stat. and as this was only removed when the boiler was off, there was no danger.

If you have any advice regarding the problem, please let me know.

Now you see it doesn't matter how confident you THINK you are it's all down to how competent you are.
In this case you are not.

Just as a learning point for you, the boiler thermostat is not in fact a "safety" device, it is a control device. The high limit stat is the "safety" device.

Hence, even if you are a rocket scientist or some retired engineer, don't mess with boiler safety devices when not competent to do so.

 

[Brambles]

It sounds like insufficient flow through the system to satisfy the boiler. Operation of the overheat stat should be a rare event and when triggered, the cause identified (hence the requirement to manually reset. Your boiler is probably cutting out on the thermostat and the flow is insufficient to take the residual heat away from the heat exchanger fast enough.

If you had this with an earlier boiler - that possibly points the finger at your system configuration.

It is possible that the boiler is oversized for your system or not rated correctly. A good starting point is to check that the minimum flow rate through the system is large enough for the boiler and that the bypass loop is large enough and coming into operation when required.

Thereafter look at the pump to overrun temperature settings.

You really do not want temperatures of 108 degrees C in the boiler / system - whilst rare, the risk of a phase change from hot water to steam in the event of sudden pressure loss is there.

There are a number of cost effective portable instruments around to quickly and accurately measure temperature and flow.

The boiler MI’s will state the minimum flow rate required

 

[harryarcos]

It sounds like insufficient flow through the system to satisfy the boiler. Operation of the overheat stat should be a rare event and when triggered, the cause identified (hence the requirement to manually reset. Your boiler is probably cutting out on the thermostat and the flow is insufficient to take the residual heat away from the heat exchanger fast enough.

If you had this with an earlier boiler - that possibly points the finger at your system configuration.

It is possible that the boiler is oversized for your system or not rated correctly. A good starting point is to check that the minimum flow rate through the system is large enough for the boiler and that the bypass loop is large enough and coming into operation when required.

Thereafter look at the pump to overrun temperature settings.

You really do not want temperatures of 108 degrees C in the boiler / system - whilst rare, the risk of a phase change from hot water to steam in the event of sudden pressure loss is there.

There are a number of cost effective portable instruments around to quickly and accurately measure temperature and flow.

The boiler MI’s will state the minimum flow rate required

 

[harryarcos]

It sounds like insufficient flow through the system to satisfy the boiler. Operation of the overheat stat should be a rare event and when triggered, the cause identified (hence the requirement to manually reset. Your boiler is probably cutting out on the thermostat and the flow is insufficient to take the residual heat away from the heat exchanger fast enough.

If you had this with an earlier boiler - that possibly points the finger at your system configuration.

It is possible that the boiler is oversized for your system or not rated correctly. A good starting point is to check that the minimum flow rate through the system is large enough for the boiler and that the bypass loop is large enough and coming into operation when required.

Thereafter look at the pump to overrun temperature settings.

You really do not want temperatures of 108 degrees C in the boiler / system - whilst rare, the risk of a phase change from hot water to steam in the event of sudden pressure loss is there.

There are a number of cost effective portable instruments around to quickly and accurately measure temperature and flow.

The boiler MI’s will state the minimum flow rate required

Thank you for your sensible and useful advice. The last boiler lasted over 30 years and was only replaced because spares were unobtainable and the efficiency of new boilers so much better. It is clearly a problem of when the pump stops, so no matter how large the bypass loop is, it would not reduced the temperature unless it was above the boiler - if there was some electronic device to keep the pump running for a few minutes after the boiler stopped firing, then it would not be a problem.

I have measured the temperature of the water going through the pump and it does not exceed 70°C. Perhaps the answer is to put a small radiator in the loft!

 

[oz-plumber]

- if there was some electronic device to keep the pump running for a few minutes after the boiler stopped firing, then it would not be a problem.

There are plenty of devices like this available from any electrical supplier.

Basic Term - Run on timer.

Most boiler have these built into the PCB or boiler controls.

It's strange that a new boiler doesn't have this pump connection as standard.
Not a big issue to fit one if you require one.
Quite simple to install also

 

[SimonG]

People join these types of Forums for advice and information, not to be insulted. You have no idea how competent I am. Of course the thermostat is a "safety device" even if it is not identified as such.Who said "those who think they know it all, are a constant irritant to those that do"?

Your lack of competency is quite apparent.
Luckily you're not irritated 😉

 

[harryarcos]

Thank you for your sensible and useful advice. The last boiler lasted over 30 years and was only replaced because spares were unobtainable and the efficiency of new boilers so much better. It is clearly a problem of when the pump stops, so no matter how large the bypass loop is, it would not reduced the temperature unless it was above the boiler - if there was some electronic device to keep the pump running for a few minutes after the boiler stopped firing, then it would not be a problem.

I have measured the temperature of the water going through the pump and it does not exceed 70°C. Perhaps the answer is to put a small radiator in the loft!

There are plenty of devices like this available from any electrical supplier.

Basic Term - Run on timer.

Most boiler have these built into the PCB or boiler controls.

It's strange that a new boiler doesn't have this pump connection as standard.
Not a big issue to fit one if you require one.
Quite simple to install also

Yes, of course. I did not really think it through before making the statement. Something like that used in bathrooms as below should do it. I believe the pump runs at about .5 Amps - Note the "nuisance trips" of any safety thermostat"! Thank you Oz.

The ROT240X1800 provides simple, low cost run-on / lag off timing functionality for bathroom / toilet exhaust fans where it is desirable to run the fan longer then the room is occupied. Other uses include element heating applications where the fan is continued to be run to remove any residual heat before turning off, therefore helping to prevent "nuisance trips" of any safety thermostat.

 

[gpbeck]

People join these types of Forums for advice and information, not to be insulted. You have no idea how competent I am. Of course the thermostat is a "safety device" even if it is not identified as such.Who said "those who think they know it all, are a constant irritant to those that do"?

Your sensitivity, arrogance and ignorance are a powerful mix. Stay off the flight deck.