Ravenheat RSF84 Works intermittantly

[stuart_adair]

Morning all,

I've got a problem with my RSF 84E. When it comes on it works in short bursts of about 30 seconds before the burner turns off. It stays off for about 20 seconds and then comes on again. The rads get hot and when they are up to temperature the cycle shortens so that the boiler only works for a couple of seconds before the burner goes off.
Sometimes this doesnt affect the performance and it goes through the heating cycle in this way, sometimes the orange burner fail light comes on and the boiler trips out and sometimes the red spanner light comes on and I have to press the reset.
Suspect a thermostat but I'm open to suggestions ?

Stu

 

[gas4you]

What about when the hot taps are on? The same?

 

[stuart_adair]

No only the central heating. I can run a bath of hot water without any issues at all.

 

[weareleeds]

Try changing thermister first

 

[stuart_adair]

Thanks. I've just done a search on the LPS supplies site for Thermister and got no results. Also searched the Ravenheat RSF84 Adobe Acrobat maintance guide and couldnt find anything called a Thermister.
Where do I find it ?

 

[gas4you]

Try searching for this; RHEAT 0007TER05005/0 DHW SENSOR; its the same one for the ch I beleive.

 

[stuart_adair]

Brilliant - I'll give that a try. Thanks

 

[Agile]

This is a problem I have noticed on several of these boilers that they do not seem to have an anticycling delay. So far no one has wanted to pay me to investigate the problem.

Cycling like yours may be as a result of the gas valve setup being incorrect.

If it does modulate back it could indicate a restricted CH flow. Check all the valves are fully open.

You need to see if the flame turns down or just stays on at full power. I thought that the boiler turns off if the sensor goes open circuit?

Tony

 

[stuart_adair]

Thankyou for the replies. I've ordered a Thermister as this sems to be the easiest thing to try. Do I need to drain the boiler to fit ?

 

[Agile]

Its easier to just measure the resistance of the sensor! You dont have to drain the boiler to do that or even order a part which is probably not required!

Tony

 

[stuart_adair]

Excellent. I've got measuring equipment. What sort of readings should I expect to see ?

 

[stuart_adair]

I've just tested the resistance of the thermister. At cold I get a reading of 6.50 ohms. When I turn the central heating on the resistance falls to 6.10 ohms but then rises again to about 6.35 ohms and at this level the boiler trips off. The resistance then rises again to 6.40 ohms when the whole cycle starts again.
It doesnt appear to me that the resistance of the thermister changes logically, ie falling to a level when the boiler heats up, causing a trip which then increases the resistance until a level is reached and the cycle starts over.

 

[Agile]

I doubt you are measuring it correctly.

I think the value is a nominal 10K being about 13k at 15 C FALLING to about 2K at 80 C.

Hopefully you are measuring it with the PCB disconnected???

Tony