Advice on oil boiler thermostats

[Steve-T]

I've got a problem with an oil boiler overheating the water, I have a solution and if anybody has any relevant advice, I'd be obliged.

I have a Firebird 90S oil boiler, which was installed about 10yrs ago. It has a Cordice 542788 thermostat fitted, which has a range of 60-90c apparently. The water temperature tops out at 68c when the thermostat is on its lowest setting so either it is not working properly or there is too much leeway in the manufacturers settings.

The problem is that the hot water has damaged the shower pump. The easiest fix being a new thermostat which actually works and a better range.

I have found this on eBay which looks like it might do the job:

Reliance - Dual Combined Control and High Limit Thermostat (30¡C - 70¡C) | eBay

Products will not be covered if misused or not serviced as per maintenance instructions. Useful Link. Ending Soon.

www.ebay.co.uk

I've then had a look at the manufactures and found that the only one that they supply is for a hot water cylinder, although it looks like you can buy copper pockets for it, which I understand to be sensors on wires rather than a ridged spike that goes into the cylinder.

Cylinder Thermostats | Reliance Water Controls

Reliance Water Control's Cylinder Thermostats allow temperature control of central heating systems in domestic & commercial environments.

www.rwc.co.uk

I have contacted Firebird who are of the opinion that the wiring is compatible, I'm just awaiting a reply from the eBay supplier to confirm the length and diameter of the pockets to ensure that they will fit into the boiler.

The industry standard for boilers appears to be 60c< and the for shower pumps its 60c> so there is a very fine line of tolerance. I do understand that stored water should be around 60c to prevent Andy bacteria forming. I'm guessing that Thermostats are juts not very accurate, so having a wider range of operation is surely an advantage to overcome this.

Many thanks in advance,

Steve

 

[SJB060685]

Google imit stat, phials should fit in sleave ok. Can always pack out with thermal paste.

 

[John.g]

I've got a problem with an oil boiler overheating the water, I have a solution and if anybody has any relevant advice, I'd be obliged.

I have a Firebird 90S oil boiler, which was installed about 10yrs ago. It has a Cordice 542788 thermostat fitted, which has a range of 60-90c apparently. The water temperature tops out at 68c when the thermostat is on its lowest setting so either it is not working properly or there is too much leeway in the manufacturers settings.

The problem is that the hot water has damaged the shower pump. The easiest fix being a new thermostat which actually works and a better range.

I have found this on eBay which looks like it might do the job:

Reliance - Dual Combined Control and High Limit Thermostat (30¡C - 70¡C) | eBay

Products will not be covered if misused or not serviced as per maintenance instructions. Useful Link. Ending Soon.

www.ebay.co.uk

I've then had a look at the manufactures and found that the only one that they supply is for a hot water cylinder, although it looks like you can buy copper pockets for it, which I understand to be sensors on wires rather than a ridged spike that goes into the cylinder.

Cylinder Thermostats | Reliance Water Controls

Reliance Water Control's Cylinder Thermostats allow temperature control of central heating systems in domestic & commercial environments.

www.rwc.co.uk

I have contacted Firebird who are of the opinion that the wiring is compatible, I'm just awaiting a reply from the eBay supplier to confirm the length and diameter of the pockets to ensure that they will fit into the boiler.

The industry standard for boilers appears to be 60c< and the for shower pumps its 60c> so there is a very fine line of tolerance. I do understand that stored water should be around 60c to prevent Andy bacteria forming. I'm guessing that Thermostats are juts not very accurate, so having a wider range of operation is surely an advantage to overcome this.

Many thanks in advance,

Steve

I have one of those boilers (15 years) as well with a Imit boiler stat, its range is 65C/85C and is very accurate, I generally run my boiler at 70C but I have a motorized valve/cylinder stat on the HW cylinder set to 60C, you seem to be running your cylinder uncontrolled so this is why you have to control its temp with the boiler stat?, it's still a bit surprising though that the pump is damaged by 65/68C hot water temperatures as the manufacturers are well aware that the temperature must be > 60C for legionella protection.

 

[SimonG]

I have one of those boilers (15 years) as well with a Imit boiler stat, its range is 65C/85C and is very accurate, I generally run my boiler at 70C but I have a motorized valve/cylinder stat on the HW cylinder set to 60C, you seem to be running your cylinder uncontrolled so this is why you have to control its temp with the boiler stat?, it's still a bit surprising though that the pump is damaged by 65/68C hot water temperatures as the manufacturers are well aware that the temperature must be > 60C for legionella protection.

Pump manufacturers specify water must be below 65C. Anything above that distorts the impellors. Seen it a few times.

Cant see why pump manufacturers cant design for hotter temps.

 

[Steve-T]

I have one of those boilers (15 years) as well with a Imit boiler stat, its range is 65C/85C and is very accurate, I generally run my boiler at 70C but I have a motorized valve/cylinder stat on the HW cylinder set to 60C, you seem to be running your cylinder uncontrolled so this is why you have to control its temp with the boiler stat?, it's still a bit surprising though that the pump is damaged by 65/68C hot water temperatures as the manufacturers are well aware that the temperature must be > 60C for legionella protection.

The cylinder is quite old with no separate thermostat for boiler heated water sadly.

I take your point about the shower pumps failing with such a small margin, very frustrating!!

It may be that my stat is working within the parameters that it should e.g. +/- 10%, however the shower pumps appear to have much finer tolerances. To that end, my thinking is get a stat that has a far greater range above and below 60c then I should stand a better chance of actually achieving the correct temperature.
[automerge]1586868532[/automerge]

Pump manufacturers specify water must be below 65C. Anything above that distorts the impellors. Seen it a few times.

Cant see why pump manufacturers cant design for hotter temps.

I can see a market for boiler manufacturers to make shower pumps that work with their kit!!!!
[automerge]1586868704[/automerge]

Google imit stat, phials should fit in sleave ok. Can always pack out with thermal paste.

The Imit Stat is pretty much ident with what I've got. If the lowest setting is 60c and there is some leeway in the setting, then the water could got higher and I would be in the same position. If I get the Reliance unit, it affords more temperature control unto and around 60c.

 

[SimonG]

Convert cylinder to fully pumped. 2 port valves, cylinder stat and room stat. S plan.

 

[SJB060685]

You need independent thermostatic control of heating and hot water. As above before looking to replace the dual stats at the boiler look into a fully pumped system, or at least a c plan

 

[Ric2013]

I think you're going to be disappointed with that high limit stat clipped to the boiler flow. Depends how quick it is to react. May be okay if if has a mercury phial.

I once tried to govern a boiler, that had a very high hysterisis internal thermostat, using the Honeywell pipe stat (which was a basic bimetallic cylinder stat which clips to a pipe). The problem was that the boiler raised the water temperature faster than the Honeywell stat could react. So the boiler would overshoot before the stat became aware of it. If it set the Honeywell stat lower, it would prevent overheating, but then the boiler had to run almost stone cold before the stat would allow it to refire. Eventually it turned out that the boiler internal thermostat, thought to be obsolete, was available, and replacement solved the problem.

I would agree with others that the solution would be to fit a thermostat to your water cylinder and convert to fully-pumped. It'll be of the cheap and nasty bimetallic strip design, no doubt, but the quantity of water involved means that the stat will have time to gauge the temperature. You'll find a cylinder stat may not be especially accurate to the marked settings, but once you have got the setting where you want it to be, it should be surprisingly consistent.

 

[John.g]

Would one of these TapStats (I think they are called) be a cheap(er) option if the HW temperature is the only concern, these connect directly via the strapped on cylinder stat to a valve fitted in the cylinder coil and have no other (electrical or otherwise) requirements. Drayton controls make them.

 

[SJB060685]

Would one of these TapStats (I think they are called) be a cheap(er) option if the HW temperature is the only concern, these connect directly via the strapped on cylinder stat to a valve fitted in the cylinder coil and have no other (electrical or otherwise) requirements. Drayton controls make them.

Looking at the specs John i can't see how this cuts power to burner? Although this will govern the HW temperature if its not cutting power to burner then you will still get cycling with nowhere for the heat to go, unless the 3 way also acts as a bypass?

 

[John.g]

No, it won't cut power to the burner but I was under the impression that there are no controls on the boiler apart from a on/off switch and maybe a timed on/off so the whole system, CH+HW), runs as one?.

 

[SJB060685]

His current setup is gravity HW and pumped CH. At present when the programmer comes on for HW that's a direct live to burner and no temperature control other than boiler stat governing HW temperature. When the programmer comes on for CH it brings on HW as well for live to burner and the roomstat just controls the pump, a very primitive and dated way of doing this. In the back of the programmer there will be a plug for fully pumped or gravity.
He needs to consider updating to fully pumped with independent control or at least a C plan as I mentioned above, both get rid of the short cycling

 

[Steve-T]

I think you're going to be disappointed with that high limit stat clipped to the boiler flow. Depends how quick it is to react. May be okay if if has a mercury phial.

I once tried to govern a boiler, that had a very high hysterisis internal thermostat, using the Honeywell pipe stat (which was a basic bimetallic cylinder stat which clips to a pipe). The problem was that the boiler raised the water temperature faster than the Honeywell stat could react. So the boiler would overshoot before the stat became aware of it. If it set the Honeywell stat lower, it would prevent overheating, but then the boiler had to run almost stone cold before the stat would allow it to refire. Eventually it turned out that the boiler internal thermostat, thought to be obsolete, was available, and replacement solved the problem.

I would agree with others that the solution would be to fit a thermostat to your water cylinder and convert to fully-pumped. It'll be of the cheap and nasty bimetallic strip design, no doubt, but the quantity of water involved means that the stat will have time to gauge the temperature. You'll find a cylinder stat may not be especially accurate to the marked settings, but once you have got the setting where you want it to be, it should be surprisingly consistent.

Thanks for all the replies thus far.

It appears the only replacement thermostat is another firebird unit because of the length of wires to the probes:

My understanding is that most of these dual thermostat units have a leeway of +/- 0.5c so I would imagine fitting a new unit would solve the problem.......alas you have had experience of the boiler heating up the water faster than the thermostat can react to, so this fix might not work.

A understand that the system is old in design so a new cylinder with separate thermostat valves for the hot water would be the answer, but the is not an easy fix during Covid-19 and I'd imagine I'd be throwing £1000 at the problem too.

 

[John.g]

His current setup is gravity HW and pumped CH. At present when the programmer comes on for HW that's a direct live to burner and no temperature control other than boiler stat governing HW temperature. When the programmer comes on for CH it brings on HW as well for live to burner and the roomstat just controls the pump, a very primitive and dated way of doing this. In the back of the programmer there will be a plug for fully pumped or gravity.
He needs to consider updating to fully pumped with independent control or at least a C plan as I mentioned above, both get rid of the short cycling

I must be missing something but where does it show the above set up?

 

[SJB060685]

I'm assuming from comments above that's the case

 

[John.g]

Oh, fair enough, as I said above, even though I have a motorized valve controlling my cylinder temperature via a spare output on my solar controller and a PT1000 probe in the cylinder which gives pinpoint temperature control I don't use the motorized valve end contacts to switch the boiler, I just use the programmer to switch the boiler on/off wired through one (main living room) roomstat, the other 10 rads have TRVs and this gives excellent results. If the OP's system is fully pumped and he wants to keep it simple, he could also just install a cylinder coil motorized valve & a cylinder thermostat.

 

[Ric2013]

My understanding is that most of these dual thermostat units have a leeway of +/- 0.5c so I would imagine fitting a new unit would solve the problem.....alas you have had experience of the boiler heating up the water faster than the thermostat can react to, so this fix might not work.

Not what I meant, Steve. I have experience of a boiler heating up faster than a Honeywell Pipe Stat can react. Purpose-designed boiler stats are probably fast enough to react - provided, that is, that the boiler isn't massively oversized for the number of rads in use.

A understand that the system is old in design so a new cylinder with separate thermostat valves for the hot water would be the answer, but the is not an easy fix during Covid-19 and I'd imagine I'd be throwing £1000 at the problem too.

You almost certainly will be able to keep your existing cylinder. You'll benefit from faster cylinder heatup times if you switch to a fully-pumped design. Depending on how accessible your system is and where you live, I think we're talking less money than you think. If not a Tapstat or Cyltrol may be a cheaper option if converting to fully-pumped proves difficult for some reason.

May be worth getting some prices from local plumbers when possible.

 

[Steve-T]

Gents, thanks for all your advice, I've gone for a TMV as the solution rather than mess around with the boiler thermostat.

Having spoken to the Firebird, they declared that their thermostat (60-90c) usually worked at 65-78c with a +/- 3c tolerance. It's a bit like using pig iron on a job that requires titanium to that end, my thermostat might have been working 'correctly'.

The TMV is currently pushing out 41c water through the shower via gravity. This might change once the Stuart Turner 3.0 bar pump is installed upon its arrival.

 

[John.g]

The problem I've seen with gravity fed TMV's is that they restrict the flow rate, I assume that you are installing a double ended shower pump and that you are increasing the hot end temperature from 41C to ~ 50/55C to mix with the cold water in the shower mixer or are you just installing a single ended pump to just pump the 41C water through the shower?. Whichever, I would suggest ensuring that the flow rate from the TMV is adequate for your requirements as a 3 bar pump will give/require a very high flow rate.

 

[Steve-T]

The problem I've seen with gravity fed TMV's is that they restrict the flow rate, I assume that you are installing a double ended shower pump and that you are increasing the hot end temperature from 41C to ~ 50/55C to mix with the cold water in the shower mixer or are you just installing a single ended pump to just pump the 41C water through the shower?. Whichever, I would suggest ensuring that the flow rate from the TMV is adequate for your requirements as a 3 bar pump will give/require a very high flow rate.

I have been in dialogue with Stuart Turner, who have recommended the 3.0 bar pump based on the system/pipework that we have..::

Sadly - This forum won't allow me to upload a diagram of what was sent to them

​

Hot Water Cylinder ========= 3m (22mm)========== PUMP ======== 2m (15mm)
||
||
||
||
2m (15mm)
||
||
||
||
SHOWER​

Hopefully this will format, basically the hot water cylinder is 3m from the existing pump site, with 22mm pipe. The pipe reduces to 15mm just before the pump and continues a further 2m and then turns right (90 degrees) and continues another 15mm to the 20x20cm shower head.

They recommend that the pump is located as close to the hot water cylinder as possible, which is where it now is. The old pump was there prior to this new bathroom. The reason for relocating into the bathroom was because the airing cupboard/hot water cylinder is located in my daughter's bedroom and way back when I was getting up at 4.30am...not that she complained about the pump noise!!! The airing cupboard also negates issues with boxing the pump in and air flow.

The old pump was gravity fed by 22mm hot and cold from the airing cupboard to shower mixer (virtually same location) mixed and then sucked back by the pump along another 22mm pipe to the pipe and then pumped back to the shower head under pressure along a further 22mm pipe. Apparently new shower mixer units of this design are hard to find, hence the change. Yup.....hindsight would advice to use two of the 4 existing 22mm pipes for a new pump at one end and new shower mixer at the other

The new pump is a Stuart Turner (ST) Monsoon 3.0bar Twin Universal, as we need a negative head pump. They recommend 3.0 bar because of the distance, reduction from 22>15mm halfway along the length and also a few twists and turns where the old pump was removed the 15mm pipework rejoined.

The gravity flow was deemed to be sufficient by ST, with discussions about pumps pushing water not pulling it, hence the pumps is better off being at one end of 6m rather than in in the middle. Concerns were raised about the the extra pressure at 3.0bar draining the tank, however they explained flow v pressure, stating the pressure increases not the flow, so it should be fine. They were aware of the TMV too and the restriction from 22 to roughly 15 through the valve, by nature of its design.

Anyway, the long and the short of it all is that the system works fine!!

The max water temperature is 44.5c at the shower head. The 38c safety limit is at 38.6c so no drop in water temperature from the TMV. The pressure is efficient and not excessive. the noise is around 62dB compared to the Salamander CT60BU pump at nearly 80dB........the difference between somebody talking to you or shouting at you.

Three showers were had concurrently this morning, two were female with hair washes and at the risk of balancing the sexism, mine was with that just for men hair dye shampoo stuff that you either know about or will do someday (leave in for a minute before rinsing) All showers were run on the 38c setting without need to increase the temperature due to hot water usage.

Changing the boiler thermostat was not the correct way to go. Oil boilers just aren't made to be that accurate, hence you need some better kit like a TMV.

​

 

[SJB060685]

So you've installed a temperature mixing valve to solve your problem? The problem with any thermostat is the hysteresis, hence the +/- factor. Unfortunately a lot of these stats use gas or liquid which expand a certain amount at set temperatures to cut it in and out, there will always be a degree of lag

 

[John.g]

I have been in dialogue with Stuart Turner, who have recommended the 3.0 bar pump based on the system/pipework that we have..::

Sadly - This forum won't allow me to upload a diagram of what was sent to them

​

Hot Water Cylinder ========= 3m (22mm)========== PUMP ======== 2m (15mm)
||
||
||
||
2m (15mm)
||
||
||
||
SHOWER​

Hopefully this will format, basically the hot water cylinder is 3m from the existing pump site, with 22mm pipe. The pipe reduces to 15mm just before the pump and continues a further 2m and then turns right (90 degrees) and continues another 15mm to the 20x20cm shower head.

They recommend that the pump is located as close to the hot water cylinder as possible, which is where it now is. The old pump was there prior to this new bathroom. The reason for relocating into the bathroom was because the airing cupboard/hot water cylinder is located in my daughter's bedroom and way back when I was getting up at 4.30am...not that she complained about the pump noise!!! The airing cupboard also negates issues with boxing the pump in and air flow.

The old pump was gravity fed by 22mm hot and cold from the airing cupboard to shower mixer (virtually same location) mixed and then sucked back by the pump along another 22mm pipe to the pipe and then pumped back to the shower head under pressure along a further 22mm pipe. Apparently new shower mixer units of this design are hard to find, hence the change. Yup...hindsight would advice to use two of the 4 existing 22mm pipes for a new pump at one end and new shower mixer at the other

The new pump is a Stuart Turner (ST) Monsoon 3.0bar Twin Universal, as we need a negative head pump. They recommend 3.0 bar because of the distance, reduction from 22>15mm halfway along the length and also a few twists and turns where the old pump was removed the 15mm pipework rejoined.

The gravity flow was deemed to be sufficient by ST, with discussions about pumps pushing water not pulling it, hence the pumps is better off being at one end of 6m rather than in in the middle. Concerns were raised about the the extra pressure at 3.0bar draining the tank, however they explained flow v pressure, stating the pressure increases not the flow, so it should be fine. They were aware of the TMV too and the restriction from 22 to roughly 15 through the valve, by nature of its design.

Anyway, the long and the short of it all is that the system works fine!!

The max water temperature is 44.5c at the shower head. The 38c safety limit is at 38.6c so no drop in water temperature from the TMV. The pressure is efficient and not excessive. the noise is around 62dB compared to the Salamander CT60BU pump at nearly 80dB....the difference between somebody talking to you or shouting at you.

Three showers were had concurrently this morning, two were female with hair washes and at the risk of balancing the sexism, mine was with that just for men hair dye shampoo stuff that you either know about or will do someday (leave in for a minute before rinsing) All showers were run on the 38c setting without need to increase the temperature due to hot water usage.

Changing the boiler thermostat was not the correct way to go. Oil boilers just aren't made to be that accurate, hence you need some better kit like a TMV.

​

Good to know that the TMV is supplying sufficient flow for your needs, would you mind posting a link to it please.
You might also indicate its location relative to the cylinder/pump.