Worcester Heatslave 20/25 fan keeps running

[Avocet]

Hi All,

Our oil boiler (Worcester Heatslave 20/25) fan keeps running after the burner shuts down, rather than just running for 30 seconds or so, and then shutting down. Even with the heating and hot water switched off, it just runs continuously, as soon as there is power to the boiler.

I'm wondering if that's the Satronic controller? (TF830B)

 

[oilhead]

Disconnect the photocell and see if it stops. If you have a plug-in cell which is most likely, then it is easy, but if an old cell, then you need to remove the box and disconnect at the base.

 

[Avocet]

Disconnect the photocell and see if it stops. If you have a plug-in cell which is most likely, then it is easy, but if an old cell, then you need to remove the box and disconnect at the base.

Thanks for that. I had a quick look and unfortunately, it looks like it's not a plug-in one. Also, things have taken a more complicated turn. When I switch off the hot water, it seems to be behaving OK with just the central heating running. Not quite sure what to do now!

 

[oilhead]

You originally said with both water and heating switched off, the burner fan ran. Was it actually firing or just the fan running?
Do you have any frost stat fitted?
Is there also a fan purge timer fitted? They were standard on older models.

 

[Avocet]

You originally said with both water and heating switched off, the burner fan ran. Was it actually firing or just the fan running?
Do you have any frost stat fitted?
Is there also a fan purge timer fitted? They were standard on older models.

Yes, it as doing it with both the water and heating switched off, but now, it seems OK with only the heating switched on, so I'm wondering if it's something to do with the water side of things? The diverter valve has been a bit "iffy" for some time now, but I can't find a replacement.

Definitely just the fan running, not the burner. You can hear the controller "tick" and the burner shuts down. Normally, the fan carries on running for 20-30 seconds and then shuts down too, but it just carried on.

I don't think it has a frost stat, and I'm afraid I don't know what a purge timer is? The boiler was installed in 1995 though, if that gives any clues?

 

[oilhead]

if the burner carries on for a period then you have a postpurge timer and it has probably failed. It is easy enough to remove this and rewire the control box to convert to running without it. I think it is an obsolete part, and later models did not have it.

 

[Avocet]

OK, do you mean if the burner carries on for a while after it's supposed to shut off? If so, that doesn't happen. It's the fan that carries on running after the burner shuts off.

Today, it did it even with the hot water switched off, so it looks like that one as a red herring and it's just an intermittent fault.

 

[oilhead]

sorry, should have said if the burner motor runs on after the burner extinguishes. Do you have a wiring diagram for your boiler/burner? Is there a post purge timer/pcb?

 

[Avocet]

I've got these in the installation manual that came with it:

...if those shed any light on it? I can see they refer to something called a "Post Purge Unit"?

 

[oilhead]

Yes, you have a post purge unit. The motor feed from the control box (pink wire) feeds this unit. This unit then puts a feed back to the motor to run the burner, and this unit also keeps the burner motor running for a period after extinguising. This is what has failed, but at £120 + vat, I would by pass this altogether . Many boilers run quite happily without a post purge and they can in fact be counter productive by cooling down the boiler.
Edit to add: Any fumes left in the boiler are contained in the flue system and those not dissipated before the next firinf will be expelled by the Satronic pre purge programme in the controller.

 

[Avocet]

Brilliant! Thanks ever so much for all the help on this! I'll have a look at it at the weekend. Do I just disconnect the pink wire from Pin 3 in terminal X3?

 

[oilhead]

The difficulty with the Worcester unit is that all the connections are multi plug, and without the unit to look at physically, I can't be sure how to overide it. Basically the burner motor is driven from terminal 4 in the base, but this goes into the PPU and drives the motor from the yellow wire, so you need to disconnect terminals 4 and the yellow wire from the ppu, and put the feed from 4 into spare connect in the base, making safe the yellow wire from the ppu as I think it could still be live from an internal connection in the ppu. What confuses me is that the wiring diagram does nt show the photocell or transformer or solenoid coil, but you will be able to see these physically.

 

[Avocet]

The difficulty with the Worcester unit is that all the connections are multi plug, and without the unit to look at physically, I can't be sure how to overide it. Basically the burner motor is driven from terminal 4 in the base, but this goes into the PPU and drives the motor from the yellow wire, so you need to disconnect terminals 4 and the yellow wire from the ppu, and put the feed from 4 into spare connect in the base, making safe the yellow wire from the ppu as I think it could still be live from an internal connection in the ppu. What confuses me is that the wiring diagram does nt show the photocell or transformer or solenoid coil, but you will be able to see these physically.

Thanks, I'll try to get a good look at the weekend, but I fear it might be getting beyond my abilities! Is the "PPU" an actual physical box, or is it part of one of the other electrical assemblies in there (like the programmer or the display where I set what times I want the heating to come on and off)?

I'm actually wondering whether we should just cut our losses on this boiler. It'll be 30 years old next year, and most of the parts seem to be unobtainable anyway. The diverter valve is also playing up, so whether or not we get hot water, is a bit of a "lucky dip" situation! I don't know whether that's the diverter valve, to be honest, or whether it's something that senses when the hot tap is turned on? I'm inclined to think it's the diverter, because when you open the hot tap, there is a click, and the burner fires, you just don't always get much hot water! (You obviously get SOME, because of what's in the slave tank, but not enough to run a bath).

If I was looking for a new oil boiler, is there a brand that you'd recommend? This one is the 20/25, so it would have to be a fairly big one. I think we have about 14 radiators, but some are doubles,so I don't know if they're counted as one or two?

 

[Terrywookfit]

The easy way to isolate the post purge is to simply put the motor wire into the satronic base motor terminal and connect the wires to PPU together in the now free connector. Live feed to PPU should be made safe.

 

[oilhead]

The easy way to isolate the post purge is to simply put the motor wire into the satronic base motor terminal and connect the wires to PPU together in the now free connector. Live feed to PPU should be made safe.

which is what I said, but gave a bit more detail to the OP who would have had difficulty with the short version above.