Ravenheat RSF84 - DHW goes hot and cold

[richw]

Are you 'FCA', and your last two phone number digits '61'? If so, I think I'll give you a bell on Monday! Failing that, I'm not sure if Ravenheat will help me when I ask for 'slugbabydotcom'

Yep that's me. You can catch me in on that number between 11 and 12.30 ish

OK, thanks for the confirmation.

I've been experimenting with the HW temperature control on the front of the boiler (instead of leaving at max). It's now at the 12:30 position, about half way between the tap and the dot. Running the hot water is now much better - it doesn't cut out, although I don't think it's coming out of the tap quite as hot. Before I replaced the HW and CH sensors, messing around with this dial didn't stop the cutting out at all.

So... it's working better, but I don't think it's 100%. Surely I should be able to set the dial to whatever I want? Cutting out can't be normal! And the boiler used to work fine with it on max.

I'll see how I get on today and tomorrow, but I think I'll give you a bell on Tuesday. Cheers!

 

[Slugbabydotcom]

The first of these clip on thermistors I ever saw went wrong within a week. I replaced it and its worked fine since. I haven't been able to get hold of any whenever I tried but I dont have the faith in them that I do with the newest wet pocket thermistors that I have been supplied with.
The clip on type need a thin smear of thermal paste. Can you confirm that you used this?

 

[wizbongre]

Are you 'FCA', and your last two phone number digits '61'? If so, I think I'll give you a bell on Monday! Failing that, I'm not sure if Ravenheat will help me when I ask for 'slugbabydotcom'

Yep that's me. You can catch me in on that number between 11 and 12.30 ish

OK, thanks for the confirmation.

I've been experimenting with the HW temperature control on the front of the boiler (instead of leaving at max). It's now at the 12:30 position, about half way between the tap and the dot. Running the hot water is now much better - it doesn't cut out, although I don't think it's coming out of the tap quite as hot. Before I replaced the HW and CH sensors, messing around with this dial didn't stop the cutting out at all.

So... it's working better, but I don't think it's 100%. Surely I should be able to set the dial to whatever I want? Cutting out can't be normal! And the boiler used to work fine with it on max.

I'll see how I get on today and tomorrow, but I think I'll give you a bell on Tuesday. Cheers!

Sounds familiar to how mine ended up. Strangely, as soon as I move the temp control even a fraction further towards it's max setting it stops modulating and overheats! I'm still tempted to try and replace my sensors and the controls but it doesn't sound like it will guarentee a fix!

Gareth.

 

[richw]

The first of these clip on thermistors I ever saw went wrong within a week. I replaced it and its worked fine since. I haven't been able to get hold of any whenever I tried but I dont have the faith in them that I do with the newest wet pocket thermistors that I have been supplied with.
The clip on type need a thin smear of thermal paste. Can you confirm that you used this?

Interesting! Ravenheat told me that the wet ones were obsolete, and since 2005 they have recommended the dry ones.

I didn't use any thermal paste, as the copper piping seems to transfer heat amazingly well and the clips are tight. I'll see if I can get hold of some and give it a quick go.

 

[Agile]

The designers of the boiler make the response time constant as a function of the electronic components and the thermal transfer characteristics of the sensors.

Most boilers use wet sensors as they give the fastest response. They are rather more expensive though !!!

A dry sensor is by its nature going to respond considerably slower and the thermal paste is required otherwise the boiler will give a less good performance. ( OK then, it will give an even worse performance! ).

Tony

 

[Slugbabydotcom]

CH and HW potentiometers [controls] are the same so they can be swapped around to determine if one is faulty.Often just the leads can be changed over. You can swap thermistors too.

I was told the same about the thermistors but have always had the availablility problem. The newer wet pocket ones are green like the others but have a blue ring at the base.

Thermistors and 'pots' can be tested with a meter set to ohms readings. I dont bother much with that method as I always have spares.

 

[richw]

Sounds familiar to how mine ended up. Strangely, as soon as I move the temp control even a fraction further towards it's max setting it stops modulating and overheats!

Hi Gareth,

Yes, that's exactly where I am! It's working really well with the dial set to JUST before the black blob at the 1 o'clock position. If I move it to ON the 1 o'clock position, it will return to the nasty cut-out behaviour.

According to the manual, I should be able to adjust the dial to whatever temperature I want, so I can't believe that the current setup is correct. Mind you, it's working really well, so I'm tempted to leave it alone! I think I'll see how it goes over the next few days or a week.

 

[richw]

CH and HW potentiometers [controls] are the same so they can be swapped around to determine if one is faulty.Often just the leads can be changed over.

Hmm! I wanted to try that, but the leads have different mouldings on the end to connect to the PCB. I didn't want to start chopping and choc-blocking the wires, so left them alone. Or are you suggesting the leads will come off from the potentiometer end? I didn't think they would (without breaking it!).

I was told the same about the thermistors but have always had the availablility problem.

I did have trouble getting them from anywhere apart from LPS Supplier (who I believe are next-door!). Even then, their web site was out of date and didn't have them under the RSF84 section. You can look them up if you put 'sensor' in their search box, though.

Thermistors and 'pots' can be tested with a meter set to ohms readings. I dont bother much with that method as I always have spares.

I don't suppose anyone's got any idea what resistance or voltage values to expect? I guess I could compare the voltage or resistance range across both potentiometers, and check they are the same, couldn't I? (assuming they are the supposed to be the same!)

I'm starting to think that I'd be wasting your time getting you to look at it, as it's currently in a working state (apart from the niggling doubt in my head that something's not quite right!).

Thanks to everyone for your comments. This forum is great!

 

[Slugbabydotcom]

Slugbabydotcom wrote:
CH and HW potentiometers [controls] are the same so they can be swapped around to determine if one is faulty.Often just the leads can be changed over.

Hmm! I wanted to try that, but the leads have different mouldings on the end to connect to the PCB. I didn't want to start chopping and choc-blocking the wires, so left them alone. Or are you suggesting the leads will come off from the potentiometer end? I didn't think they would (without breaking it!).

Pots show as the same part number but the original ones have a red and blue wire CH and DHW respectively. They go so rarely that I havent changed one yet but I do have a vague memory of putting my meter across the terminals and watching the numbers go up and down when I twiddled it. If I was in your situation I would suspect them last and concentrate on the supernotorious thermistors. These have to be totally ruled out before you can look at other causes. A rough check is that they should be between 500 ohm and 20 k. Trouble is this varies according to the heat so I stuck 2 new wet pocket T/stors under my armpit for 5 mins and the readings are 6.84k and 6.92k. 2 mins later at this room temp of 21º and I'm getting 7.9k from both.
What a kerfuffle ! Another way is to stick a 10k resistor across the thermistor connector [testing purposes only] and see how it runs. I have the luxury of being able to pull a T/stor out of my pocket and try it. I fit so many!

If you are happy with how it is running then just do the thermal paste thing, remember a thin smear and leave it at that until the day when you are again unhappy with its performance.

 

[richw]

A rough check is that they should be between 500 ohm and 20 k. Trouble is this varies according to the heat so I stuck 2 new wet pocket T/stors under my armpit for 5 mins and the readings are 6.84k and 6.92k. 2 mins later at this room temp of 21º and I'm getting 7.9k from both.

I've just tested the clip-ons with a multimeter, and I'm getting similar resistance values. And I was quite amazed how quickly it responded when moved to a chilly wondow-cill! The sensors in the boiler also seem to have logical readings - the wet and dry ones are the same value. Perhaps I didn't need to buy the new sensors after all!

Moving on to the potentiometers, I think I've discovered the problem... assuming they are supposed to be identical:

I unplugged the HW and CH potentiometers from the PCB, and measured their resistance range as I turned the knobs. The HW pot was 0 to 10.15 kOhms, and the CH 0 to 8.08 kOhms. So one must be wrong! And funnily enough, moving the HW knob to the 'magic' position where it works (just before the blob at 1 o'clock), yields a resistance of 8.08 kOhms! So, I assume the CH pot is OK and the HW one is broken.

Gareth: if you're reading, I would be really interested in knowing if you can see the same pattern (if not the absolute numbers).

I may order myself a new HW potentiometer. Then again, as it works, I might just save myself the thirty quid. I still can't believe a bloomin' potentiometer is broken, though!

If you are happy with how it is running then just do the thermal paste thing, remember a thin smear and leave it at that until the day when you are again unhappy with its performance.

I think I might do that. I'll certainly put your name/number on the boiler manual, though! Many thanks.

 

[Slugbabydotcom]

Well done ! Give yourself a pat on the back and treat yourself to a parmo

 

[wizbongre]

A rough check is that they should be between 500 ohm and 20 k. Trouble is this varies according to the heat so I stuck 2 new wet pocket T/stors under my armpit for 5 mins and the readings are 6.84k and 6.92k. 2 mins later at this room temp of 21º and I'm getting 7.9k from both.

I've just tested the clip-ons with a multimeter, and I'm getting similar resistance values. And I was quite amazed how quickly it responded when moved to a chilly wondow-cill! The sensors in the boiler also seem to have logical readings - the wet and dry ones are the same value. Perhaps I didn't need to buy the new sensors after all!

Moving on to the potentiometers, I think I've discovered the problem... assuming they are supposed to be identical:

I unplugged the HW and CH potentiometers from the PCB, and measured their resistance range as I turned the knobs. The HW pot was 0 to 10.15 kOhms, and the CH 0 to 8.08 kOhms. So one must be wrong! And funnily enough, moving the HW knob to the 'magic' position where it works (just before the blob at 1 o'clock), yields a resistance of 8.08 kOhms! So, I assume the CH pot is OK and the HW one is broken.

Gareth: if you're reading, I would be really interested in knowing if you can see the same pattern (if not the absolute numbers).

I may order myself a new HW potentiometer. Then again, as it works, I might just save myself the thirty quid. I still can't believe a bloomin' potentiometer is broken, though!

If you are happy with how it is running then just do the thermal paste thing, remember a thin smear and leave it at that until the day when you are again unhappy with its performance.

I think I might do that. I'll certainly put your name/number on the boiler manual, though! Many thanks.

I am indeed reading Although stuck in the office at the moment...

I'll dig my multi meter out tonight and see what happens - let you know later/tomorrow...

I agree that this can't be the correct setup with us not being able to go any higher than the black blob!

 

[richw]

I may order myself a new HW potentiometer. Then again, as it works, I might just save myself the thirty quid. I still can't believe a bloomin' potentiometer is broken, though!

DISASTER!

I couldn't cope with not knowing, so I ordered a new pot set. After fitting them, the hot/cold cycling problem seemed much worse, even if I backed off the HW pot setting! I decided to put the original pots back in, and just accept that it didn't make sense.

Unfortunately, I managed to misplace the CH thermistor wire (putting it onto an unused terminal on the PCB, rather than the correct one). I tried the HW before I realised, and the boiler didn't light. Then I noticed the error and corrected it. But the boiler still refuses to light! There we are, my own needless meddling has resulted in the boiler now not lighting at all. No HW or CH.

The pump will start as soon as I power up the boiler. Switching the heating on does nothing, but you can hear a relay click. Opening a HW tap will switch the diverter valve, but the boiler never lights.

My multimeter tells me that the HW and CH sensors (which usually have some voltage across them, depending on their temperatures, of course) are always 0V.

Have I blown up the PCB?!

(Slugbaby, I think I may need to call you first thing on Monday!)

 

[Slugbabydotcom]

You might be lucky and have just popped a fuse. Is there one fuse on the pcb or two?

 

[richw]

There are two fuses on the main PCB (right-hand side) but they both check out OK.

Within reason, an incorrect wiring connection really shouldn't kill anything - so say my electronic engineer colleagues.