No HotWater from Ravenheat NSF20/20ET Combi

[MikeG]

I have a problem with my rather old Ravenheat NSF20/20ET (RSF20/20ET)combi boiler. I aquired this boiler with my house back in 1999.

I have had to change the divert valve because the rubber diaphram had torn of so I ended up buying the complete valve back in 2001 I think it cost £180 plus a new pump another £60. I also changed the pump as it was sometimes not starting.

All has been fine all this time from 2001 onwards, then suddenly the boiler has been making a loud grinding noise when shutting the hot water tap.

Now suddenly I cannot get hot water, basically the burner does not light up nor does the fan start when opening the hot water tap.

I have done some diagnostics and I hope someone can point me to the problem.

If I start the heating the everything is fine, the flame fires, the fan start and the boiler starts to heat the water, I have also checked the 4 neon lamps on the electronic circuit behind the timer unit, first the 2 neons light up followed by the 3 and finally the 4 neon which indicates the HT for the pilot/burner piezo is working as the boiler lights up.

With the heating working if I open the hotwater tap, all the 4 neon indicators turn off and the burner is extinguished and the fan also stops running. I can confirm the pump is working perfectly and so is the diaphram, the divert valve pin is pushing the two micro switches when opening and shutting the hot water tap.

The fact that the heating works perfectly, can I conclude the Gas Valve etc is working properly?

If I shut the hot water tap while in heating mode, the burner ignites again and the boiler starts heating in this mode.

I have turned the hotwater thermostat fully UP but this doesn't seem to make any difference.

Is there a sensor or pressure switch somewhere in the heat exchanger which controls the hotwater start/stop signal?

What could be wrong? Is this going to be something to do with the Heat Exchanger having got scaled up inside?

Your advice and comments will be greatly appreciated.

Mike
London, Middlesex

 

[chrishutt]

Presumably you are referring to the fault finding section of the manufacturer's Instructions? These indicate that you need to determine (1) if the diverter valve is actually operating the micro switches and (2) that the micro switches are switching correctly.

From what you say I would suggest checking micro switches and leads to PCB with continuity tester. If they are working properly, check function of DHW thermostat (audible click? continuity test including leads?). If all OK try Ravenheat tech. helpline.

 

[MikeG]

Presumably you are referring to the fault finding section of the manufacturer's Instructions? These indicate that you need to determine (1) if the diverter valve is actually operating the micro switches and (2) that the micro switches are switching correctly.

From what you say I would suggest checking micro switches and leads to PCB with continuity tester. If they are working properly, check function of DHW thermostat (audible click? continuity test including leads?). If all OK try Ravenheat tech. helpline.

Thanks Chris for your comments:

I haven't been able to find any troubleshooting flow chart of this particular Ravenheat boiler, even they don't seem to list it anywhere.

I did the diagnostics using my logic being an electronics/electrical engineer and plumber.

Yes, the divert valve is working perfectly, the switch(s) can be heard clicking when closing/opening the hotwater tap. I have not checked the continuity of the connections from the micro switch or the hotwater thermostat because it was working perfectly.

I will proceed to check the wiring.

I forgot to mention earlier that this occassionally used to happen but after opening/closing the hot water tap the burner used to fire and kept on working. This time it will just not come on!..

 

[chrishutt]

You referred to the boiler as

my rather old Ravenheat NSF20/20ET (RSF20/20ET)

I am assuming that these are the same as RSF 820/20ET, but advice should be valid anyway. You should be able to get Manufacturers Instructions from Ravenheat. Have you tried?

 

[MikeG]

You referred to the boiler as

my rather old Ravenheat NSF20/20ET (RSF20/20ET)

I am assuming that these are the same as RSF 820/20ET, but advice should be valid anyway. You should be able to get Manufacturers Instructions from Ravenheat. Have you tried?

There happens to be two labels inside the boiler, one which is printed as NSF20/20ET and the other label which is stuck underneath on the side which states model no RSF20/20ET.

I have had a look at the service information for RSF820/20ET but is a different boiler although I guess the technology is very similar in terms of its functionality.

I have been doing further diagnostics and did the following:

I opened up the cover housing the electronic assy, checked for dry joints and soldered some joints that looked suspect. Removed one of the 2 relays which appeared blackend (I though that might be the culprit) cleaned the contacts and refitted it but still the problem remains.

I checked the two variable resistors for the hotwater and heating temperature control both read 10Kohms.

By removing the micro-switch from the rear of the divert valve with the boiler in heating mode if I now open the hot water tap I can get the hot water so the internal divertion within the valve is functioning properly.

This leads to the conclusion that the fault has to be of a electronic nature because in heating mode the burner fires and works perfectly, but if the divert valve micro-switch contact is pressed the burner and fan cuts out immediately but keeps the pump running!.

I will try and follow where the 3 wires from the micro-switch connect to I think they go into the main electronic control board.

Chris, I will take you advice and study the RSF820/20ET service instructions in detail to see if I can find something relevant.

 

[ChrisR]

I don't know the boiler but there may just be a HW temp sensor (thermistor) involved. If it's telling the boiler the water is hot of course it wouldn't fire. I have read that the Ravers ones are not long lasting!
On second thoughts your boiler is prob a bit old to have a thermistor.

It would be worth checking that the wiper on the DHW temp pot is actually connected - ie giving an output which follows the knob.

 

[chrishutt]

MikeG wrote

I checked the two variable resistors for the hotwater and heating temperature control both read 10Kohms.

Aren't these the thermistors?

 

[ChrisR]

No. The knobs on the front panel may be connected to a potentiometer which connects to the pcb. The "pot" would have 3 legs, the middle one being the wiper which moves along the track. The track is the 10k Ohm bit, connected between the ends of the pot. So if the pot has 24 volts across it and the knob is 1/3rd the way from the 0V end the wiper will read 8 volts.
Sometimes the tracks crack so you get very odd outputs, ot the wiper wears out so you can't change anything.
Current ones are like that generally.
The thermistor is the temperature-sensitive thing on the pipe. But you knew that..!

Older controls were more often directly on a thermostat (with bulb and capillary) though. The only Ravenheat MI I have is for the RSF 80-20, which is that type.

 

[MikeG]

No. The knobs on the front panel may be connected to a potentiometer which connects to the pcb. The "pot" would have 3 legs, the middle one being the wiper which moves along the track. The track is the 10k Ohm bit, connected between the ends of the pot. So if the pot has 24 volts across it and the knob is 1/3rd the way from the 0V end the wiper will read 8 volts.
Sometimes the tracks crack so you get very odd outputs, ot the wiper wears out so you can't change anything.
Current ones are like that generally.
The thermistor is the temperature-sensitive thing on the pipe. But you knew that..!

Older controls were more often directly on a thermostat (with bulb and capillary) though. The only Ravenheat MI I have is for the RSF 80-20, which is that type.

Hello ChrisR,

I only checked the resistance on the two potentiometers they both read 10KOhms and when checking the centre pin the resistance did vary from 0-10KOhms. Because I did not have any voltage readings I did not measure the voltage across these two pins.

I will try and swap the potentiometers to see if for any reason the HW one might be at fault.

Yes, there is a thermocouple attached to the inlet (i think) hotwater pipe going into the HW heat exchanger, although this only has one pin connection, the Heating HX has a thermocouple but with 2 pins.

I was suspecting this at first but thought this sensor might be a single pin version.

I have checked and can confirm that it is the DHW sensor, the second pin has broken inside. I quickly swapped the connection onto to CH sensor and on opening the hot water tap it fired the burner.

Many thanks for your advice which prompted me to arriving at the fault.

Would like to now where would be the best place to obtain this sensor, from Ravenheat (do they supply mail-order ??? does Plumpcentre sell this type of spare?

More power to this fantastic forum!....keep up the good work everyone.

 

[chrishutt]

Cheapest source of this spare might be BES -http://www.bes.ltd.uk/nav_graf/frames_cat.htm, but check that the temp. probe (part no. 12237) looks like yours. Otherwise Parts Centre (often attached to Plumb Centre) are major parts supplier, but will be more expensive.

You should be able to swap over DHW and CH thermistors, since they are usually identical. You can then get your hot water back on stream now and replace CH sensor when you get spare. You shouldn't need heating this week anyway.

Well done ChrisR for identifying fault. RSF 820/20 doesn't have thermistors (just liquid sensing probes from stats) so I assumed the model in question wouldn't either.

 

[MikeG]

Cheapest source of this spare might be BES -http://www.bes.ltd.uk/nav_graf/frames_cat.htm, but check that the temp. probe (part no. 12237) looks like yours. Otherwise Parts Centre (often attached to Plumb Centre) are major parts supplier, but will be more expensive.

You should be able to swap over DHW and CH thermistors, since they are usually identical. You can then get your hot water back on stream now and replace CH sensor when you get spare. You shouldn't need heating this week anyway.

Well done ChrisR for identifying fault. RSF 820/20 doesn't have thermistors (just liquid sensing probes from stats) so I assumed the model in question wouldn't either.

Thanks again for the info, it looks more like P/N 17188 and yes, Chris I have swapped over the sensors and hot water is now back!!, we haven't had a bath since Thursday . with a 3yro you can imagine how desparate I must have got...

I have had a look at the RSF820/20 version 3.1 PDF file and it turns out it is very identical just some very slight difference.

During this mission of finding the fault I have ruptured the very tiny tubing connected to the water pressure gauge so unfortunately I have had to solder the end of the tube coming from under the pressure release valve. Very bad design...I have seen in even Valiant boilers designed in 1980's it is of a electrical type instead of a fluid pressure design. I have filled the system adequetly but cannot be sure if the pressure is at 1.5Bar. I think the only way to fix this is to purchase a whole new pressure/temprature gauge withe the capilary tubes.

One more question, the previous owner only fitted the heating to sell the house so he didn't bother to fit a room thermostat. Earlier I just couldn't find where this could be connected so I decided I will leave it.

After having had a look at the service info I have found that this is quite simple and it is connected into the main control board. Although there doesn't seem to be any indication as to what type of room thermostat I can use with this boiler. From what I remember some are negative or positive resistance types?

 

[chrishutt]

Mike, you could fit a new cheap standard pressure gauge (see BES above)more or less anywhere on the CH pipework below boiler. No need for official spare part, which will be overpriced. Although you might find a generic part that will fit (Ravers use lots of generic parts).

Sounds like you'd make a first class heating engineer/boiler repair man. Only problem is getting ACS and corgied. On ACS you only have to take the assessment (training is optional) but I think getting accepted by corgi is now more difficult. You'll find lots of threads covering this if you do a search.

 

[MikeG]

Mike, you could fit a new cheap standard pressure gauge (see BES above)more or less anywhere on the CH pipework below boiler. No need for official spare part, which will be overpriced. Although you might find a generic part that will fit (Ravers use lots of generic parts).

Sounds like you'd make a first class heating engineer/boiler repair man. Only problem is getting ACS and corgied. On ACS you only have to take the assessment (training is optional) but I think getting accepted by corgi is now more difficult. You'll find lots of threads covering this if you do a search.

Thanks once again ChrisH, I will certainly look into it again as a part time study.

 

[Agile]

If you enjoy soldering you may be able to repair the capillary tube by getting a larger one. Slide the broken ones inside the larger one cut to about 35 mm long and solder the joint at the ends of the larger tube using a minimum of solder to avoid it getting sucked into the centre.

Tony