My boiler works fine most of the time but I recently noticed that when the timer switches off, then so does both the boiler and the pump, and the internal temperature gets really hot, like 95 degrees. This is different to when only the room thermostat goes off. Then the pump continues to run to ensure the hot water in the system drops down to a satisfactory level after the boiler has stopped delivering heat. The boiler is around 20 years old - was it usual for the boiler to do this then or should there be some extra internal wiring to drive the pump?
Is this normal - Worcester Bosch R29 boiler?
- Thread starter andyv1
- Start date
- Joined
- 27 May 2011
- Messages
- 2,254
- Reaction score
- 1,204
- Country
R29 ?? Got a GC number?
Sorry it's an R40 HE and the GC is 41 311 61.
What I've since found is that if the switched live wire to the pump also connects to the boiler and turns the flame on and via some magic should also send a signal back from the boiler to the pump when the timer goes off. Hot water in the boiler isn't getting dispersed by the pump so vents through the expansion pipe and overflows the header tank. My guess is that something in the internal circuitry of the boiler needs attention - maybe only a fuse, but I think this is as far as I go with DIY unless you have any ideas what I might do next.
What I've since found is that if the switched live wire to the pump also connects to the boiler and turns the flame on and via some magic should also send a signal back from the boiler to the pump when the timer goes off. Hot water in the boiler isn't getting dispersed by the pump so vents through the expansion pipe and overflows the header tank. My guess is that something in the internal circuitry of the boiler needs attention - maybe only a fuse, but I think this is as far as I go with DIY unless you have any ideas what I might do next.
Have you got zone (motorized ) valves, if so, then, assuming CH only on, if the timer is on and calling, then it supplies 230V to the roomstat, if the roomstat is also calling, it opens the zone valve and a auxiliary (end) switch closes which then supplies 230V to the boiler, the boiler then starts and "tells" the pump to start/run, if either the timer times out or the roomstat is satisfied then the switched live to the boiler is turned off, and if no other zone valve calling should shut down the boiler but leave the pump running (overrun) for a few minutes. It seems strange that this only appears to happen only when the roostat is satisfied, if I'm properly interpreting how it works. If that switched live supplies the boiler AND the pump then both should stop, period?. Has the boiler got a automatic bypass valve (ABV)?
(If the minimum boiler output is more than the minimum heat demand then the burner will cut out at the target flow temperature + 5C but the pump will continue to run, the burner will then refire after the anticycle time has elapsed, but you are probably aware of this.)
(If the minimum boiler output is more than the minimum heat demand then the burner will cut out at the target flow temperature + 5C but the pump will continue to run, the burner will then refire after the anticycle time has elapsed, but you are probably aware of this.)
Last edited:
S Plan central heating system
S-plan central heating system operation and wiring diagram
flameport.com
I've referred to the S plan diagram above. It's pretty much the same as on mine except that the boiler has only 4 wires coming out. A black wire I've taken to be switched live connects to terminal 10. The regular live, neutral and earth wires go to terminals 1, 2 and 3 and the pump live comes from terminal 10.
The pump seems to run signalled by the motorised valves which also tell the flame on the boiler to fire up. I'm assuming this same wire tells the pump to continue running when it would otherwise be off.
I don't know if this makes sense but inside the boiler there is a plug with three female spade terminals each with lilac coloured wires. This is just hanging loose. I can't see whether this is meant to be plugged in somewhere or is to be used for servicing purposes.
It does seem to have an ABV.
Terminal 10 should supply SL (switched live) on the boiler and PL (pump live) on the boiler should supply the pump, when term10/ SL has 230v then PL (pump live) also has 230V from the boiler's L (permanent live) and the PCB, when the programmer is switched off or all the zone valves close there is no power to ter10/SL but the boiler's PCB ("delay off timer") should keep PL (pump live) live via the boiler,s L (permanent live) for a few minutes and then switch it off.
I have a S plan oil fired boiler but no pump overrun so is wired like the bottom schematic.
I have a S plan oil fired boiler but no pump overrun so is wired like the bottom schematic.
Many thanks for taking the trouble to explain this. Regrettably I don't have a separate PL wire from the boiler only the SL wire. If this boiler is normally capable of getting the pump to run after the timer and/or thermostat have tripped out, then I think I have an internal fault or the spare plug I mentioned is not plugged in as intended. This what it looks like below but I can't find where it might go or what it does.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
