Worcester Bosch HeatSlave 20/25 HW but no CH

[JonoJono]

Hi, hoping there's someone here who can help!

Background:
I have recently moved and inherited a Worcester Bosch HeatSlave 20/25 oil combi. It played up within 2 weeks of moving in, and I've had an engineer out to fix it - about 6 times! Several things were replaced, and the heating system has been fully flushed out. Items replaced were:
- Expansion tank
- Diverter head X 2
- Motherboard

Current situation (2 months later):
1 - There is hot water on demand
2 - Central heating, when it does work, is fine

The problem:
3 - The diverter is not moving to the CH position when there is heating demand - it remains in the HW position.

Note:
I can move it manually using a long screwdriver on the lever and hold it in the CH position, then the radiators heat up as expected. When I let go of the diverter lever at any point it slowly (3 seconds) returns straight back to HW position.
- The CH is definitely on demand
- Happens for all these scenarios: HW on demand, HW on with no demand, HW heating the boiler tank, HW off.
- Tested over a suitable period of time to ensure HW demand/tank heating cycles aren't the issue.
- The diverter head was recently replaced with an original Honeywell part (V4044F1125)

Some peculiarities:
4 - The fan is permanently on. It used to work as a pre and post burner purge for about 5 seconds, but since the engineer changed the motherboard and looked at it the fan remains on.
5 - Having said point 4 above, the 'fan remaining' on was not always the case. Prior to this the fan would pre and post purge as normal, but the relays on the motherboard would chatter. This would happen when the HW demand ceased and it was trying to switch back to CH - the engineer told me this was a 'known thing' with the Heatslave 20/25, and was probably because the diverter valve was taking too long to go back to heating and/or the diverter head was not communicating properly with the motherboard.

QUESTION:
- Does anyone have any useful suggestions? I want to avoid having to buy a new boiler outright unnecessarily. The engineer, who is pretty experienced, doesn't have any more suggestions.

Many thanks for your help!

 

[snb]

which burner do you have , riello or bentone ? check that the slave tank and main exchanger are full of water. if they get an air pocket at the top it will continue to try and heat slave tank as it thinks it cold. double check by turning the hot water stat to min. should then change over to heating. if this doesn't solve it then check that divertor valve moves smoothly. remove the divertor head to check.

 

[oilhead]

If Riello burner, could be something as simple as a failed photocell. 10 mins with a multimeter on the pcb would confirm its integrity. Also does it have the Worcester programmer. That can fail and do silly things.

 

[JonoJono]

which burner do you have , riello or bentone ? check that the slave tank and main exchanger are full of water. if they get an air pocket at the top it will continue to try and heat slave tank as it thinks it cold. double check by turning the hot water stat to min. should then change over to heating. if this doesn't solve it then check that divertor valve moves smoothly. remove the divertor head to check.

Hi snb

Thanks for your response. I have checked and the fan is a Bentone.

With regard to removing an air pocket, can I assume this would be either a and/or b bleeders in the photo I have attached? And if so, if I turn them is there a chance they will not leak/close properly after?!

 

[JonoJono]

If Riello burner, could be something as simple as a failed photocell. 10 mins with a multimeter on the pcb would confirm its integrity. Also does it have the Worcester programmer. That can fail and do silly things.

Hi oilhead

Thanks for your help!

As above, the fan is a Bentone.

Also, yes it does have a Worcester programmer (assuming you mean the digital clock/buttons on the front).

It may be helpful to know the engineer fitted a Hive on his first visit - this is when the strange behaviour started, though I’m pretty sure this is a coincidence as that was the visit when he changed the other parts as above.

When he fitted the Hive he bypassed the HW function on the hive (as it’s a combi setup - and there’s no point scheduling combi HW). The heating aspect of the hive works as expected.

The controls on the programmer seem to work fine along with the display. When switching CH and HW in and out I can hear the relay(s) responding.

 

[snb]

On an oil combi it's always worth while using the hot water function. set it turn off over night and if you are out all day. it needs to be turned on 20 mins before you want hot water though. It will save you about £50 to 75 per year and it is also good for the burner ( means that it won't be cycling as much. less starts)

Air pockets are solved with those bleeds, so i guess that has been ruled out. Only other thing that can cause heating to not work is if the hot water flow switch is sticking on. That will cause a constant demand for hot water. Follow the black cable near your markB on photo back to pcb and unplug. you can test it easily with multimeter for contunuity