Worcester 15/19 oil boiler, problems with new thermostat

[Peter.N.]

I wondered why the water was so hot and found the adjustable thermostat was not turning off, it was only the over temperature stat that shut it down.

The new thermostat arrived yesterday and I duly fitted only unfortunately the water was still getting very hot, the thermostat was cutting out at about 80C but it didn't cut in again until about 40C. I suspect that the sensor is not making good thermal contact with the boiler, it is smaller on the replacement stat. Would anyone have any suggestions, might using some heat sink compound help? - John I wonder?

Thanks

Peter

 

[Burnerman]

Definitely a good move Peter, to ensure the stat is fully secure in it's housing and a sinking compound is an excellent idea. It's the only way to get the stat to operate within it's limitations. The stuff I've used is called Servisol I think.....maybe RS components or Maplins will stock it.
Is it a genuine stat that you have used?
Regards
John

 

[Peter.N.]

Thanks John. Its a 'Ranco' stat looks similar to the original except that the last two numbers are transposed, it ends in 90 the replacement is 09, I can't find any mention one with my number so I assume it has been marked incorrectly - very unusual I would have thought. Once it has cut out the thermostat will switch on and off around the middle of the range so it seems that the heat is taking a long time to 'soak in' it fits inside a capsule screwed into the boiler so you can't see what's going on.

Peter

 

[Burnerman]

The only stat I've had to replace was the 89161423900 Ranco and it came from www.heating-parts.co.uk
which is a very reliable firm.
Certainly if the new stat is a little slack in it's housing, a heat soak compound is essential.
Regards
John

 

[Peter.N.]

Thanks John

This one was listed for the boiler, it was: 87161423290 but mine ended in 09 which as i said I could find no trace of. Mine came from ebay of course.

Peter

 

[Peter.N.]

Hi John

Today I put a wooden wedge between the sensors so it was pushed hard against the side and it still didn't turn off until it way way to hot, I pulled the sensor out and it was to hot to touch so conduction doesn't seem to be the problem.

Are these thermostats adjustable? The old one appears to have a small philips screw which looks like an adjuster but can't see anything on this one.

Peter

 

[Burnerman]

Sorry I've never had the need to adjust a stat.....normally I just listen for the click and then test for continuity across the terminals if I have any doubts.
I really cant think what else could cause this.....presumably its the front panel knob you are adjusting, and turning it down should shut the burner down, once it has some heat generated in the water.
John

 

[Steelmasons]

Immerse the stat phial along with temp gauge into a pan of water and bring to the boil , should rule out stat or not..

 

[Peter.N.]

Problem solved (I think) I phoned Worcester gave them the number on the thermostat and they say its the wrong one. The number they gave me doesn't seem to relate to any of the Worcester oil boilers but it does look identical the the original, so I have ordered one.

Peter

 

[Peter.N.]

Still got problems John. Fitted the new stat which looks like the correct one but its still not switching off until its very hot, could it be possible that there is air in the top of the boiler? I can certainly hear air in the pump if only hot water is on. The water is circulating fine, I can't see that I have another duff stat I think the problem must be elsewhere, never met this one before but I can only think that the top of the boiler where the sensors are is running at a lower temperature than the rest.

Peter

 

[Burnerman]

Is there an air bleed point where the feed from the boiler enters the copper cylinder coil, Peter?
I can't remember if there are any bleed points at the top of the heat exchanger, but if you remove the white boiler top panel they will be visible with the usual square screw.
Have you tried removing the large chrome screw in the centre of the water pump too?
Good installation / servicing manuals are available from the Worcester website.
John

 

[Peter.N.]

No bleed points on the cylinder - I installed it I cant find one on the boiler, only a 1" blanking plug on the opposite side to the sensors. It is possible that there is a slight fall on the flow pipe where it leaves the boiler as I had to knock through an 18" stone wall, as I did for the flue, and although it looks fairly level its very difficult to measure the height both sides especially as one is lower!

If you suspect that could be the problem I will fit a bleed point at the top of the boiler. Its been working perfectly since I installed it about 8 months ago. I've installed a number of boilers and central heating systems over the years but never come across this problem before.

I have ordered a clip on temperature gauge so I can see exactly what's going on.

Peter

 

[Burnerman]

Do forgive my ignorance here Peter - working on memory again!
Your boiler is the Danesmoor series 15/19 - correct? If it is, it has 3 thermostats in series, high limit, manual reset then the one on the front panel.....all connected in series. I don't think there are any components to actually fail on the PCB - they are just plug in connectors so failure is very unlikely.
One issue I get from time to time on my own ancient system is air in the top coil connection to the copper cylinder - I overcame that by teeing into the feed pipe and then extending it with 15mm about 18" upwards, with a bleed valve at the top. This certainly helped with filling the dry system!
Another one was the blockage of the F/E tank outlet pipe feeding the system......this showed up by only a dribble at the radiator bleed valves. Replacing this pipe brought the system nicely back to life again. Have you tried this with yours?
John

 

[Peter.N.]

Hi John

Its a very basic utility boiler, no electronics at all, suits me down to the ground, you are correct regarding the wiring of the thermostats.

On thinking about the cylinder bleeding issue I remembered why it doesn't need it, the flow from the boiler goes to the top of the coil and then straight past to the expansion pipe over the header tank in the loft so its continually vented.

I have noticed this cavitation noise but only since we stopped using the radiators, if I turn one on the noise diminishes, I can't understand how air is getting into the pump unless its being released by the boiler at high temperature but then it should be vented straight up into the loft. When this temperature gauge arrives I will do some more precise checks.

Thanks for your help John

Peter

 

[Burnerman]

I guess you have checked the F/E tank for water Peter......are you happy that the pipe into the system is clear?
John