How to take fan out of a Ravenheat HE85 T combi boiler

[Slugbabydotcom]

The ch thermistor is the one in question. Resistance should be between 20k and 500 ohm.
At room temp 9k Ohm is typical and at armpit temp 37º 7k Ohm is typical
As this is the clip on type it will be immaterial when the system is flushed.

There should be a very thin smear of white thermal paste beneath the contacts of the thermistor. I would be inclined to clean the contacts and pipe then use a smear of fresh thermal paste before replacing the sensor.

 

[psarinuk]

I will get some thermal paste from Plumb centre and clean pipe/ CH thermistor as you suggested...

For info to all, the part number for clip on type thermistor is 0007son11010/0

Do I need to order this part only or the DHW thermistor as well (0007TER05005/0)?

Please bear in mind that CH is running fine, its the DHW thats intermittent...

Another thing that we noticed was that when the boiler is hot (i.e. CH has run for more than 10 minutes), DHW comes very hot continously without any issues...

 

[Slugbabydotcom]

For info to all, the part number for clip on type thermistor is 0007son11010/0

Thanks for that

Do I need to order this part only or the DHW thermistor as well (0007TER05005/0)?

Please bear in mind that CH is running fine, its the DHW thats intermittent...

The CH thermistor is the primary thermistor. It is in use whether HW or CH or both are called for. It does a lot more work than the DHW one:- Overheat, frost protection and temperature control of the primary circuit. The DHW one only reduces the flame to prevent overheating of the DHW thats it!
I replace roughly 50 CH thermistors to one DHW.

Depending on how much chew you have getting hold of the parts then its up to you whether you chance £15 or whatever on a part that has an outside chance of being the culprit. I am in a different situation to you. I can just get one out of the van because I use so many.

 

[psarinuk]

A little update:

I could not find this clip on thermistor (part number 0007son11010/0) anywhere. Ravenheat only sell it to registered companies..

I took off the existing thermistor, cleaned the thermistor and pipe thoroughly, got some thermal paste from maplins and reinstalled it back..

That does not seem to have made any difference.. I've a multimeter, so if I knew how (where) to measure, I could post some results to indicate if its a faulty thermistor...

 

[Slugbabydotcom]

I yahoo'd the part number and found THIS

I have given some testing info earlier in the topic. A new thermistor levels the playing field for the other things to be checked out and I find it is always the best starting point.

After initially mistaking the main HE for the plate HE and 2 weeks later you are unable to check a resistance of a component I feel that you are still at the start of a long road if you intend fixing this yourself.

If most of the experts who have already posted, even those who are not so familiar with Ravenheats had your boiler in front of them, I dare say we would have got to the root of most problems within an hour. A decent internet diagnosis is difficult at the best of times but much more difficult to make when the recipient has the limitations that you have shown.

Other things that need checking involve what we call the gas train which is best and safest if left to an RGI to check. A decent one will know how to test the thermistors, flue, fan, fan sensing, flame sensing devices as well as the max and minimum burner pressures, water flow rates etc.

My suggested course of action for you now is to change the thermistor as it is a high probability, low cost item, for a boiler part anyway. Flush the system and fit your magnaclean or boiler buddy as you seem confident in doing this. Because your thermistor is a clip on type it doesn't matter if you flush the system first.
If you still have any problems then it is time to get a corgi in who would be able to ring Ravenheat for technical advice on the number I gave earlier. Ravenheat approved RGI's again from that number have the best chance of finding your problem quickly.

 

[psarinuk]

Hello all

Sorry for the long post.
With special thanks to slugbaby, I think I've solved the issue..

Regarding the link for getting clip on thermistor, the delivery time was 7-9 days from them , which is why I hasn't bought it from there.. Got the part from parts centre in Barking, collected it for £17.80 inc VAT..

Smeared some thermal paste on the thermistor and on the copper pipe and put it on, connected the wires and started DHW with the boiler cold...
It did do some intermittent shut off in the beginning but the water never went cold or even lukewarm, in fact the water is coming out much hotter than before, so I've reduced the user DHW and CH thermostats as the radiators are getting over heated as well...

Would need to observe it for a few more days really to confirm as previously the problem was intermittent as well...

After that I drained the radiators and did another mains flush through every single one of them (fitted drain valves on return end of radiators)..

Got some more sludge out.... Then installed Magnaclean on the return flow pipe to the boiler (about 0.5 metre away)...

Also added inhibitor to one of the radiators. Its pricey at £25 and also a bit messy. Could I just get the inhibitor bottles from B&Q in future (about £5) and just fill up the magnaclean container with it?

Opened all iso valves, bled the radiators and had the CH run through the night... so a very warm night...

Cleaned magnaclean in the morning, not much sludge to report but washed out some iron oxide...

Thanks for your input and any comments are welcome...