Main 24 HE combi fault

[dwlecs]

Hi all

Just had the boiler 'engineer' out to fix a fault on my combi boiler and he's gone away scratching his head saying call the manufacturer out to get it fixed as he hasn't got a clue.

The fault is this....when central heating is switched on the boiler continues to fire and heat the system until the radiators are red hot to the touch. DHW works fine and the gas valve modulates so that the required temperature is held. There are no external controls such as room stats. I have replaced the PCB twice and replaced the thermistor twice. All cables and connectors are securely fitted and in good condition. All micro switches are operating correctly as is the diverter valve.

It seems the radiators just keep calling for heat and ignore the thermistor while the hot water switches off when desired temp is reached. All mechanical components are working fine.

Any idea as to what is happening would be very much appreciated before I call out Baxi and get myself a whopping bill.

Thanks

 

[Sharpend]

I'd suggest calling baxi tech line tell them you've been called to the fault as plumber is out of ideas tell them what you've tried and see where they lead you from there? 

Save a large bill possibly?

 

[Andy™]

have you read the manual? there is often a fault finding part in it

 

[Rob_the_rich]

Baxi tech line are usually very helpful, if you are gas safe registered, I am surprised the "engineer" didn't ring them while on site. A phone call to someone who knows the boiler inside out often tells you things that aren't in the book.

OP, not sure what is wrong with your boiler. Has it had problems in the past, hence new PCBs, or did you replace them to fix it now?

a "temporary" fix might be to install a room thermostat

 

[Kurt]

If you have no heating controls you are wasting energy anyway, you need radiator TRVs - these stop the radiators getting too hot. One rad needs to be left without, can be a bathroom so it gets nice and hot. You need a room stat to switch off the boiler so it can have a rest. The boiler should have a temp dial on the boiler itself which allows the boiler to have a variable target for heating temp leaving the boiler. What should happen is when you reduce the temp say put it to minimum it should keep the temp down. Of course if it is broken then the boiler could run at max temp. with your symptoms I would have checked the voltage across the temp dial to see if it is tracking correctly.

Anyway, whenever I have had some random fault on a combi, I ring the manufacturer there and then whilst I am working on boiler, its a lot easier than coming back.

 

[Kurt]

If you have no heating controls you are wasting energy anyway, you need radiator TRVs - these stop the radiators getting too hot. One rad needs to be left without, can be a bathroom so it gets nice and hot. You need a room stat to switch off the boiler so it can have a rest. The boiler should have a temp dial on the boiler itself which allows the boiler to have a variable target for heating temp leaving the boiler. What should happen is when you reduce the temp say put it to minimum it should keep the temp down. Of course if it is broken then the boiler could run at max temp. with your symptoms I would have checked the voltage across the temp dial to see if it is tracking correctly. Also its possible the gas valve is not modulating which would need to be changed.

Anyway, whenever I have had some random fault on a combi, I ring the manufacturer there and then whilst I am working on boiler, its a lot easier than coming back.

 

[ProDave]

I think the point is the boiler's internal thermostat is not working in heating mode, so is a boiler fault.  Adding a room thermostat will indeed save money on the heating, but won't stop the radiators getting scalding hot until the room temperature is reached.

 

[Rob_the_rich]

If you have no heating controls you are wasting energy anyway, you need radiator TRVs - these stop the radiators getting too hot. One rad needs to be left without, can be a bathroom so it gets nice and hot. You need a room stat to switch off the boiler so it can have a rest. The boiler should have a temp dial on the boiler itself which allows the boiler to have a variable target for heating temp leaving the boiler. What should happen is when you reduce the temp say put it to minimum it should keep the temp down. Of course if it is broken then the boiler could run at max temp. with your symptoms I would have checked the voltage across the temp dial to see if it is tracking correctly.

Anyway, whenever I have had some random fault on a combi, I ring the manufacturer there and then whilst I am working on boiler, its a lot easier than coming back.

TRVs would help too, but they sense the temperature of the room, not the water in the radiator (to be more precise they sense the temperature of the air around the TRV). You shouldn't have a TRV on the radiator which is in the same room as the roomstat. Most modern combis have a bypass fitted which negates the need for a non-TRVed bathroom rad, and the temp dials are usually part of the PCB.

Agree with ProDave, something is wrong with the boiler, dunno what though.

 

[ProDave]

My guess is there will be two thermostats inside the boiler. One on the hot water out pipe, which is working, and one on the heating flow pipe, which is not working.

Find it and replace it would be a good start.

 

[Rob_the_rich]

My guess is there will be two thermostats inside the boiler. One on the hot water out pipe, which is working, and one on the heating flow pipe, which is not working.

Find it and replace it would be a good start.

according to the manual i posted earlier, there is only one temperature sensor, on the primary flow from the main heat exchanger near the overheat stat. It doesn't seem to directly measure the HW outlet temp.

OP are you sure it is modulating ok on the HW? Turning down the HW temp dial on the boiler reduces the temp at a tap?

 

[Sharpend]

Which manual? 

 

[Rob_the_rich]

Which manual? 

http://www.mainheating.co.uk/docs/Main_Combi_24_HE_Installation_and_Servicing_Guide_2011.pdf

Sorry, i nearly posted the link then re-read the thread, and deleted the post and posted something else.

 

[Sharpend]

So if I reading that right isn't the fault as shown on the fault finding section refer to section G ? Either gas valve or Pcb - seeing as pcb has been changed twice doesn't that only leave gas valve? 

 

[Rob_the_rich]

So if I reading that right isn't the fault as shown on the fault finding section refer to section G ? Either gas valve or Pcb - seeing as pcb has been changed twice doesn't that only leave gas valve? 

yes but why would it modulate ok on dhw?

odd one.

Edit- note that G is also in the dhw fault section

it is probably something simple like a broken spindle of the heating knob, swapping the knobs might change things around.

 

[Sharpend]

If you read the fault section for DHW it does give alternative option of same occurs on DHW prior to referring to sect G? 

 

[Rob_the_rich]

If you read the fault section for DHW it does give alternative option of same occurs on DHW prior to referring to sect G? 

new thermistors fitted, and can't see how a blocked plate heat exchanger would affect radiator temps.

So, back to dwlecs for input. Welcome to the forum. We love a conundrum.

 

[dwlecs]

Hi all

Thanks for all the replies.

I don't have any external controls on my boiler or TRV's on my rads as I live in a small 2 bed mid terrace and the temp control on the boiler is sufficient to regulate my house temp, I've had it like that for the 9 years I've lived here and it works well.

I also have the manual and followed section G and therefore changed the PCB as the gas valve was still modulating on DHW but no luck there.

The temp control knob was replaced when I changed the board (twice) as it is mounted on the PCB.

The DHW controls work fine and modulate the gas valve. This works of off the same thermistor (changed twice) as the CH so that's working fine.

Heat exchanger was changed last year due to scale build up and I had a water softener installed to stop this problem and also stop scale in the rest of the house.

All cables are in good condition so voltages are going where their supposed to be going.

Could it be a diverter valve stuck in mid-position? That's the only thing I've got left.

Thanks all

 

[Rob_the_rich]

oops, missed your reply. I don't think it would be the diverter valve

Sounds like you have had a very expensive experience trying to save money, PCBs aren't cheap.

I still think it is something simple like the control knob, perhaps it was inserted 180degrees out of whack into the receptacle on the pcb. I know it shouldn't turn, but worth checking.