Worcester boiler pump runs for hours

[Geraint Thomas]

Hi all,

We've bought a property that has a Worcester Greenstar 37CDi boiler installed, with an old British Gas RS2 Thermostat linked to it. I'm not clued up on boilers in any way, though I'm quite handy with mechanics and always like to learn how things work.

The problem: The internal Bosch pump will run for hours, even when the heating and water are set to 'off'. If I switch the boiler off and on, it's quiet for around 60 seconds, to which the pump will fire back up. The boiler isn't heating, it's just the pump. Sometimes the boiler will go days without making a sound, other times it's on all night and all the next day before I turn it off out of concern. It's running as I type this now.

A few things to note:

• The black plastic flow manifold had a tiny leak in the seam of the plastic. Multiple layers of super glue left to dry had fixed this issue with no leaks since, although the boiler has had this behavious before the leak.
• The internal condenser pipe (I think) was full of gunk. It's the grey plastic device that has a clear flexi tube attached to it. I've taken this out, cleaned it out and re-installed it.
• I've checked the wires that control the pump speed (purple, white and black wires), but these don't look damaged.
• The boiler was installed in 2007.

Is this normal operation? If not, is there anything I can do or check? I'm happy to try and repair most things, but if gas is involved in any process I'll stay away.

Many thanks!

 

[Geraint Thomas]

Update:

I've taken a video:

It shows 24°C in the video before I stepped into the shower earlier. After the shower it was around 55°C, but now 30 minutes later it's only dropped to 53°C, which doesn't seem to be dropping that fast.

I turned the Eco mode off, then back on again, then held the reset button to reset the boiler. It then started to short cycle, and the temperature has now dropped. The boiler is now quiet and remains silent.

What on earth is causing this?

 

[ShaunCorbs]

No not normal please put the case / cover back on

Try turning the heating dial eg left hand knob to frost does the pump still run ?

Might take 5-10 mins to turn off once dial turned

 

[Geraint Thomas]

No not normal please put the case / cover back on

Try turning the heating dial eg left hand knob to frost does the pump still run ?

Might take 5-10 mins to turn off once dial turned

Well it's still silent, as mentioned earlier on... if it happens again I'll certainly give it a go. Since it short cycled we've been using the hot taps in the house and all is well at the moment.

I'll give it a go if it happens again, although I did turn the heating and water both to 'off', so I presumed that would override the dials?

 

[George4321]

I have the same problem. Every time the boiler is turned off at the mains or there is a power cut, after about a min or so, the pump runs, and it runs forever (the burners are not on in this time). During this time if i run a tap or turn on heating it works but afterwards the pump continues to run... forever.
However, I have discovered if a hot tap is running or if the heating is on (and thermostat set to on) before the boiler is turned on at the mains and then turned off a few seconds after the burners start then the pump will stop after its normal over run of a min or so and then stay off. They boiler will then function as normal.
I would still like to find a cause/proper fix to this as I am concerned if we have a power cut when out/away then it will run its pump forever until we return to "reset it".
I have tried resetting the service settings on the PCB to defaults but that makes no difference. I also tried turning off the syphon pre-fill cycle at startup (again in the service settings) that didn't help. I even had our gas engineer change the PCB at it's service and that also didn't make any difference and the engineer had no other ideas as everything else appearked ok at its service.
Anyone have any ideas?

 

[Knappers]

Hi, not on all boilers but definitely on that one.
The case is part of the gasway and by taking it off your putting the safe operation of the boiler and yourself at risk.
Your entitled to do your own gaswork but if you don't want to take the risk then it's important you know that by taking case off you are doing gas work!

 

[George4321]

Personally I haven't taken the case of mine at all, only my gas engineer has done so at it's service. But any ideas on why the boiler is running it's pump like this - mine is exactly the same as in the video above. I have worked out a work around on how to stop it (see my post above) but it's still not supposed to do this? It doesn't seem to be the main PCB but what on earth could it be? Geraint, have you worked it out? It seems to be a common probem so sombody must know the answer?

 

[George4321]

PS - does the pump itself have it's own PCB in that box/connection bit on the side of the pump or is it simply connectors etc? Is there any way it could be running independantly of the main boiler PCB control board? I believe along with a control wire from the main PCB the pump has it's own permanant live feed.

 

[John.g]

I read somewhere that weather compensation, if fitted, will cause the above, if you have weather compensation installed can you disable it to see if the pump stops?.

 

[George4321]

It doesn't have any weather compensation systems/settings set. Any other ideas anyone?

 

[John.g]

If the CH is on and you turn the roomstat down, does the pump stop after its overrun period and then restart shortly later or does it just keep running continuously?
What does it do when you turn off DHW with eco mode selected on? and is the behaviour the same with Eco mode selected off (DHW preheat on)?.

What do Worcester say?

 

[George4321]

Hi. I have tried everything you mentioned. No, after about 20seconds from power up it starts its pump (just pump) and runs forever. If CH or HW used during this running that works normally but the pump continues afterwards and continues constantly and doesn't stop after normal overrun period.
As I said in my 1st post - the only way I have worked out to stop this is if there is a "demand" any demand, that could be heating, hot water or water heating if eco button is off, as soon as boiler turned on from the mains. So long as the fan & burner run immediatly the boiler is turned on once that initial demand is stopped the pump will stop just as it should after its 3 min overrun period. But without that instant demant on power up the pump will start after about 30seconds and never ever stop. So unless by chance the room thermostat is turned up & the heating set on on when the power comes off the boiler will do its constant pump running, that means any powercut in summer for sure will send it into its constant pump running.

 

[Michael H]

Hi. I have tried everything you mentioned. No, after about 20seconds from power up it starts its pump (just pump) and runs forever. If CH or HW used during this running that works normally but the pump continues afterwards and continues constantly and doesn't stop after normal overrun period.
As I said in my 1st post - the only way I have worked out to stop this is if there is a "demand" any demand, that could be heating, hot water or water heating if eco button is off, as soon as boiler turned on from the mains. So long as the fan & burner run immediatly the boiler is turned on once that initial demand is stopped the pump will stop just as it should after its 3 min overrun period. But without that instant demant on power up the pump will start after about 30seconds and never ever stop. So unless by chance the room thermostat is turned up & the heating set on on when the power comes off the boiler will do its constant pump running, that means any powercut in summer for sure will send it into its constant pump running.

I have exactly the same problem, it’s had a new pcb, pump, diverter motor and as far as I know all the sensor’s replaced, it’s also had the heat exchanger removed and cleaned and the pump still constantly runs, occasionally if I press the reset button it will stop, start for few minutes before finally stopping only to start running again a short time later, one thing that is slightly different is that the water temperature drops from 55 degrees to between 17 and 20 by about 1 degree per second

 

[Knappers]

Worcester help page says check purple/black wire to pump

 

[John.g]

The outside temperature compensation because of its stupid requirement to keep the pump runnimg 24/7 may still be influincing this even if not connected/installed, is there any parameter/switch to select temperature comp. On/Off.?

 

[George4321]

No afraid not. I have contacted worchester bosch as well, this boiler does not have a weather/temperature compensation unless an external unit is connected (which it isn't). There is no setting to turn it on/off and is only active if fitted.

 

[John.g]

What's left then? frost protection problem, PCB?, you would think worchester should be able to track down whatever is telling the circ pump to run like this expecially since your's isn't the only one.

 

[George4321]

Thats the confusion! My gas enginner who serviced it last week put a new PCB on it and was rather lost by the problem. "the pump can only run if the PCB tells it to" was how it was left but the PCB is new. No fault codes or anything. WB are also lost and can only suggest get engineer to phone them when the boiler is opened up to test each component - maybe I will do that next year at its service. It would make sense being frost protection but if it was a sensor or something then I don't get why it only does this if unit is turned on from the mains without a "demand" but if there is a "demand" at time of turning on its all ok and functions normally and its pump turnes off as expected, i would expect the fault all the time. Looks something i will just have to live with, thanks for your help.

 

[ShaunCorbs]

Why is the mains being turned off and on ? Boiler should have constant power

 

[George4321]

Only turns off in a powercut which for some reason we have a few times a month, often just a few seconds but enough to cause issues with the boiler. Turning it off for longer makes no difference, any loss of power for any length of time causes this pump to run 24/7. Concerned could occur when away meaning pump would run for days and days.

 

[ShaunCorbs]

Yep I had the same issue which requires a few presses of the resetting etc might be best to wire the boiler into a ups to save having problems

 

[Knappers]

I know it's a bit of a cop-out but what do worcester offer for one off callout.
Do they do no fix no fee or something?
When it comes to just swapping out parts to test, then fixed price repair is definitely the way to go.

 

[ShaunCorbs]

No sorry they get paid for the call out

 

[Don Draper]

Hello, your problem is the module on the side of the motor. I had the opposite problem where my motor would stop intermittently, causing boiler to turn off due to overheat (U1-U9 etc) and motor would not run again until power was cycled. To prove it was the module, I unplugged the purple and black cable plug from module and that gave me your issue, pump runs all the time. I bought a scrap boiler from EBay for £40 for spares and swapped just the module lid, with the PCB in, and hey presto, runs perfect now. Was your replacement pump a new one or refurb? And did it come with a new module on the side? The purple and black cable stops and starts the pump and sets speed for modulation.

 

[Michael H]

The problem with turned out to be a faulty frost stat.
The boiler keeps the pump running when the frost stat activates but will not heat the water until the water temperature drops below 9 degrees.
If you have a frost stat try disconnecting the switch wire in the frost stat to the boiler.

 

[John.g]

Thats the confusion! My gas enginner who serviced it last week put a new PCB on it and was rather lost by the problem. "the pump can only run if the PCB tells it to" was how it was left but the PCB is new. No fault codes or anything. WB are also lost and can only suggest get engineer to phone them when the boiler is opened up to test each component - maybe I will do that next year at its service. It would make sense being frost protection but if it was a sensor or something then I don't get why it only does this if unit is turned on from the mains without a "demand" but if there is a "demand" at time of turning on its all ok and functions normally and its pump turnes off as expected, i would expect the fault all the time. Looks something i will just have to live with, thanks for your help.

The problem with turned out to be a faulty frost stat.
The boiler keeps the pump running when the frost stat activates but will not heat the water until the water temperature drops below 9 degrees.
If you have a frost stat try disconnecting the switch wire in the frost stat to the boiler.

Don't think there's any frost stat as such, when/if the boiler flow temp falls to 8C the pump only runs, if the temp hasn't risen to 9C in 30 minutes then the boiler fires until the
temp reaches 12C and boiler shuts down and the pump switches off.

 

[Vontwinzig]

Update:

I've taken a video:

It shows 24°C in the video before I stepped into the shower earlier. After the shower it was around 55°C, but now 30 minutes later it's only dropped to 53°C, which doesn't seem to be dropping that fast.

I turned the Eco mode off, then back on again, then held the reset button to reset the boiler. It then started to short cycle, and the temperature has now dropped. The boiler is now quiet and remains silent.

What on earth is causing this?

Did you ever resolve this problem? Ours has been doing the same thing, identical to your video.

Thanks

 

[Nick Hawkes]

As I said in my 1st post - the only way I have worked out to stop this is if there is a "demand" any demand, that could be heating, hot water or water heating if eco button is off, as soon as boiler turned on from the mains. So long as the fan & burner run immediatly the boiler is turned on once that initial demand is stopped the pump will stop just as it should after its 3 min overrun period. But without that

I've been having exactly the same problem that you describe. Reading your post about cycling the power with the 'demand' active also worked for me - brilliant!, I hadn't been able to work out why a reset sometimes worked and sometimes didn't. I was on the point of replacing the PCB but from what I read here this would have been a waste of money,

I had some correspondence with Bosch technical support, who were quite helpful. One of their suggestions was that in some installations the presence of small stray voltages on the switched live can cause the pump to run even when there is no demand. At the time I didn't think this explained things because with the switched live disconnected altogether the pump would still run. However in light of your experiences I will experiment with shorting out the switched live input and see if this also stops the continuous pump overrun. I suppose it's still possible that the PCB is sensitive even just to nearby noisy signals.

(The pump seems to be commanded by a PWM signal with a tachometer output back to the controller to regulate the speed. The relevant chip on the PCB has a Bosch logo and from the part number is something originally designed for automotive antilock braking. There is no datasheet that I can find, but the waveforms look 'kinda sensible' to me, so I don't think the pump itself is to blame.)

 

[LongJonSilver]

Hi. I have tried everything you mentioned. No, after about 20seconds from power up it starts its pump (just pump) and runs forever. If CH or HW used during this running that works normally but the pump continues afterwards and continues constantly and doesn't stop after normal overrun period.
As I said in my 1st post - the only way I have worked out to stop this is if there is a "demand" any demand, that could be heating, hot water or water heating if eco button is off, as soon as boiler turned on from the mains. So long as the fan & burner run immediatly the boiler is turned on once that initial demand is stopped the pump will stop just as it should after its 3 min overrun period. But without that instant demant on power up the pump will start after about 30seconds and never ever stop. So unless by chance the room thermostat is turned up & the heating set on on when the power comes off the boiler will do its constant pump running, that means any powercut in summer for sure will send it into its constant pump running.

Hi George4321

This is the only post I have found that describes my exact same problem and thankfully I have used your work around to get the pump to stop. I have a Greenstar 30 CDi System boiler.

Did you ever get to the bottom of this?

 

[John.g]

I remember reading somewhere that some boilers used to keep running with a Y system installation as 70 to 90V is/was remaining on the mid position valve orange wire cured I think my installing a capacitor on the switched live to the boiler or something like that, don't know if this is of any help in the above pump runnin on problem.

"Note: Honeywell and Siemens Y plan valves require a capacitor in the external wiring block or wiring centre to avoid any feedback problems, if there is no other alternative, please contact Worcester Bosch helpline"

 

[Art42cdi]

Same problem with a 42cdi caused by a neutral fault on the mains feed (northern power grid). After replacing the pump and fan, this constant pump on problem was present. Turns out to be pcb, the new pcb came with a wiring harness both were replaced and it’s good as new. Hope this helps.

 

[ShaunCorbs]

Same problem with a 42cdi caused by a neutral fault on the mains feed (northern power grid). After replacing the pump and fan, this constant pump on problem was present. Turns out to be pcb, the new pcb came with a wiring harness both were replaced and it’s good as new. Hope this helps.

Yes the new pumps (as Worcester have changed the design) require a new pcb as well deletes the transformer on the back of it etc, the pump should of had a sticker on the box advising this

 

[Kevin McPheat]

Mines the same. I’m no gas engineer but I am an engineer, I supposed I could meter the pump trigger output from the pcb to the pump to see if it’s the pcb causing it. My service guy said it was a known issue between the main pcb and the pump pcb. Anyone found a solution yet?

 

[George4321]

Hi. Thank's for all the suggestions. As it seems quite common there must be a cause & repair known by someone? I am still using my workround of turning on with a demand and then turning off the demand. Did shorting the switched live work for anyone? My boiler does this even if the controller is set for hot water only and heating is off and additionally wether the room thermostat is turned up or down - so if the heating is off on the timer and thermostat is also down I would be suprised to have any stray voltages as that is 2 switches both off on the switched live.

 

[Michael H]

A further update from me, the pump would still run constantly even after the frost stat was disconnected, I have a theory that the boiler was overheating due to the heat exchanger being blocked and that was triggering the pump to constantly run. After several flushing and cleaning of the boiler, removal of the hot water heat exchanger and cleaning with spirits of salts it now seems to be running correctly.