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Hi all.

I have a large house (ex commercial building)
It's heated by 2x Clyde Combustion CM-1 boilers rated at 220,000 btu each.

When the whole building is being heated, they work fine (although the fuel oil bill is ruinous)

My problem is - 70% of the building is currently being renovated and the rads are all turned off in that section, meaning the boilers are vastly over-sized for the rest of the building.

What is happening is - the boiler(s) light and the boiler temperature increases so fast that the temperature 'overshoots' the set temperature - to the point where the over-temp stat trips. This is obviously happening because the boiler stat can't react quickly enough to the changing temperature. The dual thermostat is in a brass pocket on the top of the boiler, meaning there is an air gap between the water in the boiler and the actual thermostat sensor.

So - my question is - is there a better thermostat I could fit (the current one is probably 30 years old), or should I fill the pocket with some sort of thermal compound that will more effectively thermally connect the water with the thermostat?

The only way I can keep the heating on at the moment is to set the boiler temp at about 65 degrees. That means that it has to cool down to effectively stone cold before it fires up again meaning luke warm radiators and no hot water (warm at best)
 

SimonG

Plumber
Advent Win
Run one boiler. Get a heat transfer paste into the thermostat pocket. Or a new thermostat, the original sounds well past it's best.
 
Run one boiler. Get a heat transfer paste into the thermostat pocket. Or a new thermostat, the original sounds well past it's best.
Thanks for that - the boilers run sequentially so there's only ever one producing heat at any given time - unless frost protection kicks in. Only see that happen once

I'll definitely do the thermal paste. New thermostat if that doesn't fix it.

Just wanted to make sure there isn't any reason why I shouldn't do it that I hadn't thought of
 

ShaunCorbs

Staff member
S. Mod
Plumber
Gas Engineer
Try the above first but if that fails you may be able to down rate the boilers output commercial/ decent oily engineer required for this he needs to know what he’s doing tho
 

Danb

Gas Engineer
I Assume your pumps on the rear of the boilers are pumping ok.
Does both boilers overheat?
If I remember rightly depending on the size each boiler has 2 burners?
Post automatically merged:

They old type atmospheric.
Or force draught
 
I Assume your pumps on the rear of the boilers are pumping ok.
Does both boilers overheat?
If I remember rightly depending on the size each boiler has 2 burners?
Only one burner - although it's a monster.
Pumps are fine - it's just that the flow is minimal because there's only a very small demand for heat.

There are 2 CH zones and DHW.

If the rads are all closed down on the TRV and the DHW is hot, the only flow is the bypass rad. Guaranteed trip out under those conditions - burner only runs for about 2 mins which nearly boils the water in the boiler.
 

Danb

Gas Engineer
Like the engineer previously said change your stat.
It shouldn't get to boiling point .
Minimal demand or not boiler should cut off when boiler stats happy untill it drops.
Is it the original boiler from when it was a commercial premises?
See if you can get a local commercial 'Registered Professional Gas Engineer' engineer
 

Last Plumber

Esteemed
Plumber
Gas Engineer
Advent Win
Hi all.

I have a large house (ex commercial building)
It's heated by 2x Clyde Combustion CM-1 boilers rated at 220,000 btu each.

When the whole building is being heated, they work fine (although the fuel oil bill is ruinous)

My problem is - 70% of the building is currently being renovated and the rads are all turned off in that section, meaning the boilers are vastly over-sized for the rest of the building.

What is happening is - the boiler(s) light and the boiler temperature increases so fast that the temperature 'overshoots' the set temperature - to the point where the over-temp stat trips. This is obviously happening because the boiler stat can't react quickly enough to the changing temperature. The dual thermostat is in a brass pocket on the top of the boiler, meaning there is an air gap between the water in the boiler and the actual thermostat sensor.

So - my question is - is there a better thermostat I could fit (the current one is probably 30 years old), or should I fill the pocket with some sort of thermal compound that will more effectively thermally connect the water with the thermostat?

The only way I can keep the heating on at the moment is to set the boiler temp at about 65 degrees. That means that it has to cool down to effectively stone cold before it fires up again meaning luke warm radiators and no hot water (warm at best)
If you are not shifting the right volume of water through them, then I am not surprised at them overheating to be honest. Boilers with too low a flow rate can back up a lot of residual heat and the temperature can continue to climb a fair way past the set cut off temperature as a result. Having said that, it does sound to me as though there may be other issues, even if that is the installation itself. A temperature of 65 Degrees C is quite hot and I would expect that to get your cylinder hot enough, definitely hotter than 'luke warm'.
I would advise you to get a Commercial Heating Engineer who works on oil boilers to have a look. They may be able to make some temporary adjustments for you. They will be able to test the stat to see if the calibration is out, maybe alter the control of the boilers on a temporary basis and see if there is anything wrong with the Plumbing side of things too.
 
Like the engineer previously said change your stat.
It shouldn't get to boiling point .
Minimal demand or not boiler should cut off when boiler stats happy untill it drops.
Is it the original boiler from when it was a commercial premises?
See if you can get a local commercial 'Registered Professional Gas Engineer' engineer
I know it shouldn't get to boiling - the problem (I believe) is that the stat isn't seeing the actual temp in the boiler because it takes too long for the water temp to permeate through the air gap in the pocket. When it does, it shuts off, and can't be reset for ages (20 minutes at least)
 

Danb

Gas Engineer
I take it there isn't 6" flat rod in there then that pushes it and holds in down in place. With alocking pin at the top.
 
If you are not shifting the right volume of water through them, then I am not surprised at them overheating to be honest. Boilers with too low a flow rate can back up a lot of residual heat and the temperature can continue to climb a fair way past the set cut off temperature as a result. Having said that, it does sound to me as though there may be other issues, even if that is the installation itself. A temperature of 65 Degrees C is quite hot and I would expect that to get your cylinder hot enough, definitely hotter than 'luke warm'.
I would advise you to get a Commercial Heating Engineer who works on oil boilers to have a look. They may be able to make some temporary adjustments for you. They will be able to test the stat to see if the calibration is out, maybe alter the control of the boilers on a temporary basis and see if there is anything wrong with the Plumbing side of things too.
Yep that's exactly what's happening - 20 odd degrees over according to the thermometer on the boiler which I don't trust to be accurate, but it does give an indication.

The cylinder has to be AT LEAST 60 degrees as it has a secondary circulation that (I'm told) the regs say needs to be that hot to kill legionnaires disease.

The cylinder gets plenty hot if I crank up the boiler temp - problem is that then after the burner cuts off, the temperature continues to climb far enough that the over-temp stat trips.
 

Last Plumber

Esteemed
Plumber
Gas Engineer
Advent Win
Yep that's exactly what's happening - 20 odd degrees over according to the thermometer on the boiler which I don't trust to be accurate, but it does give an indication.

The cylinder has to be AT LEAST 60 degrees as it has a secondary circulation that (I'm told) the regs say needs to be that hot to kill legionnaires disease.

The cylinder gets plenty hot if I crank up the boiler temp - problem is that then after the burner cuts off, the temperature continues to climb far enough that the over-temp stat trips.
You can test the Boiler stat. It should knock the Boiler off at a maximum of 82 degrees plus or minus 3 degrees (79 - 85). Test in the same pocket as the stat is sensing if you can. Failing that, test the flow off the top of the Boiler. You will get slightly different temperatures in different sections but generally it should give a near as damn it indication of the stats performance. If the temperature continues to rise after the stat is open, then the residual heat is continuing to heat the exchanger to the point of overheat as you already suspect. If it is Boiling, it is dangerous and must not be used. I am sure you know that but just thought I would say it anyway.
 
You can test the Boiler stat. It should knock the Boiler off at a maximum of 82 degrees plus or minus 3 degrees (79 - 85). Test in the same pocket as the stat is sensing if you can. Failing that, test the flow off the top of the Boiler. You will get slightly different temperatures in different sections but generally it should give a near as damn it indication of the stats performance. If the temperature continues to rise after the stat is open, then the residual heat is continuing to heat the exchanger to the point of overheat as you already suspect. If it is Boiling, it is dangerous and must not be used. I am sure you know that but just thought I would say it anyway.
Thanks for this - no not actually boiling although it's operating at 3 bar pressure so that would be north of 120 °. It's nowhere near that hot.
I think the stat is OK - it switches the burner off at about 85 ° but the temp continues to rise suggesting residual heat. I'll try the thermo paste to see if that enables the stat to react quicker.

Somebody suggested de-rating the boiler - I think that's the answer untill the renovations are finished and the whole system is back on.
 

king of pipes

Esteemed
Plumber
Gas Engineer
A good commercial engineer will be able come up with a solution is there a shunt pump fitted and a bypass to allow the the heat to dissipate ? Maybe a smaller boiler for the summer months when there's only a hot water demand or you will have similar problems. Kop
 
A good commercial engineer will be able come up with a solution is there a shunt pump fitted and a bypass to allow the the heat to dissipate ? Maybe a smaller boiler for the summer months when there's only a hot water demand or you will have similar problems. Kop
No shunt pump or primary bypass. Fitting one won't be trivial as the primary headers are in 4 inch iron pipe. Sounds like I'll have to get advice from a commercial heating engineer - if I can find one up here.

Thanks guys
 

Last Plumber

Esteemed
Plumber
Gas Engineer
Advent Win
Thanks for this - no not actually boiling although it's operating at 3 bar pressure so that would be north of 120 °. It's nowhere near that hot.
I think the stat is OK - it switches the burner off at about 85 ° but the temp continues to rise suggesting residual heat. I'll try the thermo paste to see if that enables the stat to react quicker.

Somebody suggested de-rating the boiler - I think that's the answer untill the renovations are finished and the whole system is back on.
Heat sink compound will help to improve the transfer of heat but if the stat is operating correctly, I can't see it solving the issue.

If the boiler is reaching a temperature over that which the boiling point would be at normal atmospheric pressure where it is situated and the pressure it is currently under (which I presume is created by head) was lost, you could end up with a boiler explosion/fracture/steam release/nasty indecent etc. Does that make sense? Unlikely but not impossible.

I obviously have not seen the set up so have not weighed up the probability of a disaster but as @king of pipes said, a good commercial heating engineer will be able to check over the safety and operation and advise you accordingly. I wouldn't use it until you have had that done to be honest.
 
Heat sink compound will help to improve the transfer of heat but if the stat is operating correctly, I can't see it solving the issue.

If the boiler is reaching a temperature over that which the boiling point would be at normal atmospheric pressure where it is situated and the pressure it is currently under (which I presume is created by head) was lost, you could end up with a boiler explosion/fracture/steam release/nasty indecent etc. Does that make sense? Unlikely but not impossible.

I obviously have not seen the set up so have not weighed up the probability of a disaster but as @king of pipes said, a good commercial heating engineer will be able to check over the safety and operation and advise you accordingly. I wouldn't use it until you have had that done to be honest.
That was my thought too - pressure is created by a pressurisation unit.

I'll get some advice from a commercial heating guy
 

Brambles

Advent Win
Phillip,

I would be very wary of trying to reduce the range rating of a Clyde CM1 they can be very problematic at low load. The thermocouple in the thermowell (for the cm1) should be loose and not held in with a steel or copper peg. I suppose you could add a thermal paste but the MI just shows a loose thermocouple to be inserted.

Oddly there are a lot of older Clyde boilers in Oxon, presumably their local salesman had great success in selling to the former US air bases!!

I do not want to break forum rules, but I refer to Carnot Consult when I get problems with (or need spares for) old Clyde boilers. Their advice is free and (in my opinion) is excellent
 

Ric2013

Plumber
That means that it has to cool down to effectively stone cold before it fires up again meaning luke warm radiators and no hot water (warm at best)
I would expect a boiler set at 65 to kick back in at 55ish at worst. Would suggest this isn't normal hysterisis for the thermostat and may need a new one. Caveat that I'm not familiar with commercial boilers, but cannot see why a commercial boiler would desirably run to lukewarm before refiring.
 

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