Ravenheat HE85T combi boiler CH constant re-firing

[psarinuk]

Hi all

I've a Ravenheat HE85T combi boiler which is also similar to CSI85.

WHen I put it ON for CH, the boiler comes on and then goes off. It carries on this cycle every 5-10 seconds, so I've turned it into DHW mode only (CH off).

DHW is running absolutely fine.

WHat could be the issue? I replaced both DHW and CH thermistors in Jan 08 when I had an intermittent DHW issue.
(The CH thermistor is a dry clip on type, the part number is 0007son11010/0 and more expensive than the DHW one which is a wet type with part number 0007TER05005/0)

Thanks in advance....

 

[Agile]

I think this may be a faulty fan relay PCB !

But it might also be a wrongly set up gas valve!

Tony

 

[psarinuk]

I think this may be a faulty fan relay PCB !

That sounds something major or is there a DIY way of idenitifying if its faulty?

But it might also be a wrongly set up gas valve!

The boiler has been running for 3 years, I guess if the gas valve was wrongly set up, it would have shown up by now in the services as well..
and the DHW is running fine...

Why do I suspect the thermistors again?[/i]

 

[Agile]

You suspect the sensors because they are cheap and easy for a DIYer to change!

Its also easy to measure their resistance!

I dont know how a diyer could identify if the fan PCB is faulty.

It could also be the main PCB !

However, you have only mentioned the CH and I have assumed the DHW is OK ! If that is the case then its a typical symptom of a faulty fan PCB.

However, the public immagine that its easy to repair boilers. The reality is thatit requires a considerable degree of skill and experience!

Tony

 

[psarinuk]

Agile

Thanks.. Could you please share the acceptable resistance of the thermistors for me to check?

I only have a multimeter, so can check resistance but how do I know what temperature it is against?

Also regarding fan PCB, if it comes to that, how much should be average cost of part and repair?

Ever since this morning the boiler has been running fine. However, I do not trust this boiler and am waiting (not really!) for the issue to come up again. The issue only happened first time last night when I turned the boiler ON after coming home in 3 days.

 

[Agile]

I think to be different the Ravenheat NTCs are about 25 K at room temperature.

Tony

 

[Slugbabydotcom]

I have cured cycling like this by renewing a thermistor on several occaions for this model. Once I have changed a thermistor its very rare that I have to go back to change it again.
Ravenheat say between anywhere between 500 and 20k ohm for resistance and as I recall it was about 9k at room temp [21ºish] and 7k after 10 mins under my hairy armpit. I dont know if you will get a good armpit reading with the clip on type so allow more leeway there.

After that there's a few things that are likely. For the early type with three lights on the front I've seen the blue SIT ignition boxes cause this behaviour when they go faulty. Its perceivable that the red honeywell variety could go the same, just that I havent come across it.

As Tony suggests I have seen low minimum gas pressure cause it and like yours it happened totally out of the blue. If theres not enough oomph in your minimum [starting] flame then the flame sensing device wont be able to sense the flame properly.

A lot of the fans on these seem to be biting the dust lately, noisy operation and/or slow start-up causing flame lift and again affecting flame sensing.

 

[psarinuk]

Hi slugbaby, yes it just happened totally out of blue, so maybe could have been low gas pressure.. (I hope)

I will see if it happens again and inform..

Mine is not an early type I think. I've 2 lights at the front: GREEN that stays ON, and RED that comes ON when overheats (there is a red light on the timer too that comes on when CH is ON). The manual and image for mine is:
http://www.ravenheat.co.uk/pdf/HE/HE85A(T).pdf

As the FANs seem to go on these like you and Tony say, would I need to replace the FAN PCB or the FAN or both (or do they come as one assembly)?

Thank you so much for your help..

 

[psarinuk]

Hi

The re-switching/ cycling is back again now with a fixed pattern ever since I took the CH sensor off to measure its resistance.. The resistance of CH sensor is fine and close to range that slugbaby advised, so I put a little bit of thermal paste and clipped it back on.

It only happens when the boiler is stone cold, like early in the mornings and when we come from outside after a few hours and turn it on.

It does not correct itself and carries on tripping, it starts, the flame comes on FULL, then the flame slows down a bit, then before it starts to run FULL ON again, something just cuts it off completely in 5-10 secs and the whole process starts all over again. I am not sure but I think the pump and fan are always running, can hear the pump and feel the fan faintly vibrating (could be wrong) when I place my hand towards the top of combustion chamber.

The easy fix that we are using at the moment is to turn DHW ON for about 30 seconds. Then turn the DHW OFF but the boiler won't cycle/ trip for CH again.

DHW as explained before runs just fine. The boiler was serviced by npower less than a month ago, and the wife suspects an evil hand there..lol

I so much regret buying this boiler now. My friends bought a Vaillant year before me and never had a slightest bother with it. My previous 20 year old boiler never missed a beat. This is a real false economy that I have bought myself now.

Any help to end my misery would be highly appreciated!!

 

[Agile]

To the CORGIs,

Isn't this the one with the APS sensing ring which has to be turned 180° to get it to pick up enough mB to reliably operate the APS ?

For some reason they have not put me on their list of approved engineers to give out my telephone number and I am hesitant to ask again to be added to it.

Tony

 

[psarinuk]

Could the CORGIs please reply to Tony and help me out!

This morning I heard the boiler cycling again when the timer started, so went downstairs and put the DHW ON in kitchen sink for 5-10 seconds, closed the tap and the boiler stayed ON, and did not trip at all after that.

 

[psarinuk]

The manual for my boiler (bought Feb 05) is on this link:
http://www.ravenheat.co.uk/pdf/HE/HE85A(T).pdf

Is APS sensing ring another name for flue ring by any chance (Part 7 on page 51)?

If there is way to check the APS sensing ring or its setting, please let me know so I can do that and report back.

Cheers..

 

[Agile]

If there is way to check the APS sensing ring or its setting, please let me know so I can do that and report back.

Cheers..

That is exactly what I did not want you to do !

Only a gas trained person, which means CORGI, should be opening the combustion chamber!

I do not give advice on gas work to unqualified people and anyway its against forum rules!

Tony

 

[psarinuk]

Could the setting of APS sensing ring be changed by the CORGI engineer who came to service the boiler last month?

The boiler had been running fine before the service. It still does and the fault is intermittent but more likely to occur when the boiler is cold.

 

[Agile]

If he is a very competent engineer who is familiar with ravenheat boiler he could have touched it to adjust or clean it.

They would be about 1% of CORGIs.

Most just poke about a bit and sometimes vaccuum a bit of dust out but their general knowledge and understanding of boilers is very poor on average.

Tony