Ravenheat RSF820/20T

[gavndee]

I wonder could someone could possibly help?
When I run the hot water for some time either for a shower or to fill a bath, once i shut the hot water tap and the boiler fires down, the pilot light goes out and will not light for about half an hour. It only does this if the hot tap is running for a decent bit of time. Is it overheating or something?
If anyone knows what would cause this and what might be wrong and how to fix it, it would be a great help.
Thanks in advance
Gav

 

[Slugbabydotcom]

Theres a number of component faults that could cause this.
It does appear that a safety stat is operating due to an overheat situation.
Whats the pressure at?
Pump is number one suspect. Is it noisy?
Is there a triffid growing up your flue?
perhaps the PCB is not providing pump over-run to clear the hot water from the heat exchanger after the boiler has shut down?
Perhaps the minimum gas pressure needs setting/ checking.
Loads of stuff really. You needs someone who is familiar with ravenheats to get to the bottom of this for you.
01132527007 ask for a recommended local engineer.

 

[gavndee]

Theres a number of component faults that could cause this.
It does appear that a safety stat is operating due to an overheat situation.
Whats the pressure at?
Pump is number one suspect. Is it noisy?
Is there a triffid growing up your flue?
perhaps the PCB is not providing pump over-run to clear the hot water from the heat exchanger after the boiler has shut down?
Perhaps the minimum gas pressure needs setting/ checking.
Loads of stuff really. You needs someone who is familiar with ravenheats to get to the bottom of this for you.
01132527007 ask for a recommended local engineer.

Thanks for the quick reply.
Do you mean mains water pressure?
The pump doesnt sound noisy
The flue is clear, its second floor height.
and I dont know about the gas pressure

 

[Slugbabydotcom]

Do you mean mains water pressure?

No I mean the system pressure as shown on the guage. It should be 1 to 1.5

 

[holty]

can you see if the piolt light is still lit when fired up.could be dhw limit stat causing the prob but the prob might be caused by somthing else e.g pump etc.

 

[gas4you]

If you intend 'experimenting' yourself, to which I would add that as slug says, you should really get an RGI in, I would suggest first port of call is to replace the ch themistor, as this controls the overheat stat as well, and is very common for these to fail, especially on dirty water systems.

 

[Agile]

I dont remember this one having pump over run but perhaps thats just because luckily they are now rather few and far between.

A common cause of overheating immediately after a hot water demand ( and there is no pump over run or its not working ) is lime scale on the surface of the primary part of the heat exchanger.

This causes poor heat transfer and so the fins get hotter than normal and when the demand stops this latent heat is transfered into the water and causes it to get very hot and trip the o/h stat.

Nice to see you again at the dinner on Saturday Dave, and to see Dan and John as well and to meet some others for the first time too.

Tony

 

[gavndee]

Thanks for all the advice.
Pressure is at 1.5 on the dial.
Where will i find the themister? I will have a look at this, and if it does not fix the problem then I'll get a gas man to look at it.
thanks

 

[gavndee]

can you see if the piolt light is still lit when fired up.could be dhw limit stat causing the prob but the prob might be caused by somthing else e.g pump etc.

I have just quickly run the hot water and the ch and the pilot light stays lit when the bioler is fired up

 

[Slugbabydotcom]

I have yet to see a rsf820/20 with a thermistor. I believe they do exist as I have a couple of black ones and never had occasion to use them. All the later ones that are wet pocket type are green in colour.
I'll go along with Agiles idea as being highly likely. I live in a soft water area so limescale is not at the forefront of my thoughts sometimes.

Just had another thought. This sounds a bit of a jackanory but it does happen on the odd occasion. If the combi was put in after a conventional system theres sometimes a long dead leg of 22mm pipe leading to the airing cupboard. The pipe where the hot water used to come from.
If this is capped off and is full of air then it will de-pressurise as you run your tap.
When you turn your tap off the air in the dead leg is re-pressurising. The diaphragm in the DV still detects a flow as the air is re-compressed and keeps the burner on causing there to be water overheated in the heat exchanger.

So get someone to turn the bath tap off and see if the flame lingers for a while afterwards

 

[gavndee]

I have yet to see a rsf820/20 with a thermistor. I believe they do exist as I have a couple of black ones and never had occasion to use them. All the later ones that are wet pocket type are green in colour.
I'll go along with Agiles idea as being highly likely. I live in a soft water area so limescale is not at the forefront of my thoughts sometimes.

Just had another thought. This sounds a bit of a jackanory but it does happen on the odd occasion. If the combi was put in after a conventional system theres sometimes a long dead leg of 22mm pipe leading to the airing cupboard. The pipe where the hot water used to come from.
If this is capped off and is full of air then it will de-pressurise as you run your tap.
When you turn your tap off the air in the dead leg is re-pressurising. The diaphragm in the DV still detects a flow as the air is re-compressed and keeps the burner on causing there to be water overheated in the heat exchanger.

So get someone to turn the bath tap off and see if the flame lingers for a while afterwards

Thanks for that. Just tried what you suggested, when the tap turns off, the combi shuts down straight away, within 1 second. The pilot light only goes out when it has been running for a full bath or a shower.

 

[Slugbabydotcom]

If the pilot looks weak then it could be that the injector is slightly blocked.A common thing on these
An RGI job that because you need to disconnect the pipe that goes to it and either blow the jet out with a compressor or renew it.
If the pilot noisy and appears to be lifting off then it could be that the aforementioned combustion chamber seal is not made properly. Are the 13 screws all tight? [don't overtighten them]. Is there a whistling noise?

 

[gavndee]

If the pilot looks weak then it could be that the injector is slightly blocked.A common thing on these
An RGI job that because you need to disconnect the pipe that goes to it and either blow the jet out with a compressor or renew it.
If the pilot noisy and appears to be lifting off then it could be that the aforementioned combustion chamber seal is not made properly. Are the 13 screws all tight? [don't overtighten them]. Is there a whistling noise?

I think we are on to something here.
when i run the hot tap and the burner kicks in all is fine. leave it on for a min or so then close tap, the burner goes out as it should. I can hear a noise similar to air sucking in or blowing out until pressure is equalisied, similar to opening a fizzy bottle of pop. it lasts about 5-10 seconds.
I have checked the the screws on the front panel that covers the burner and one of the threads has stripped so the screw won't tighten.
I can put a larger screw in to fix this. I will let you know how it goes.
Gav

 

[Agile]

This is something which should only be looked at by an RGI!

Is the fan running at low speed when the boiler is resting? In this situation the fan is fed via a 560 ohm resistor at right hand side underneath combustion chamber.

Is the air inlet clear of built up dust at the inlet part of the flue pipe connection when viewed from the underside from within the combustion chamber?

Tony

 

[Slugbabydotcom]

I was working on one of these yesterday so with it fresh in my mind I thought I'd have another look at your problem

pilot light goes out and will not light for about half an hour.

Theres a overheat stat inside the combustion chamber at the top left. This is part of your thermocouple. It breaks the current from your thermocouple in an overheat situation which in turn switches the pilot off.When the temperature drops again the stat re-sets itself.
Also no current flows when theres not enough heat from the pilot.
The combined thermocouple and overheat stats hardly ever go wrong on these.
The pilot burners always get blocked and I just so happened to change one yesterday.

I get the feeling that the noise you can hear is the sound of water boiling up.
That noise is often a feature on these.

So its a case of figuring out why the water boils up enough to trip that overheat stat.
One idea is that if the auto air vent becomes blocked there will most likely be air in the system water side of the heat exchanger. It could be other things causing a blockage ie. radiator grease, debris or crud coated pipes.

Your DHW stat could be faulty.

Your boiler could be overgassed meaning theres too much heat left in the heat exchanger at the end of the cycle. This is registered gas installer only territory.

Tony's right BTW there is no pump over-run. Thats why the low speed fan is important to clear the excess end of cycle heat.