Pump and over run

[annc1]

I had a new wireless room thermostat fitted a few months ago. Since that time, once it reaches the set temperature it is switching on and off every few minutes.

I called out the plumber who fitted it for me and suggested that it was faulty as the last one did not do this. I told him I could hear the pump stop and start in the room underneath my airing cupboard and that's how I knew.

He told me that if the heating is timed for a period of say 6 hours the pump should run continuously for that 6 hours whether or not the thermostat is calling for heat and that the thermostat should only control the boiler and not the pump.

As this has never been the case since the boiler was installed 3 years ago, I wonder if he is right or not. What does happen is that when the thermostat shuts the boiler down the pump over runs for about 2 minutes and that shuts down as well.

The plumber is wanting to send an electrician friend round to 'fix' it. Is he right?

Thanks for your help

Ann

 

[mogget]

Make and model of boiler and thermostat would help

Usually, the pump will only run when something is calling for heat, and if a pump overrun is present, for a few minutes after the boiler has shut down.

 

[Piers1989]

I had a new wireless room thermostat fitted a few months ago. Since that time, once it reaches the set temperature it is switching on and off every few minutes.

I called out the plumber who fitted it for me and suggested that it was faulty as the last one did not do this. I told him I could hear the pump stop and start in the room underneath my airing cupboard and that's how I knew.

He told me that if the heating is timed for a period of say 6 hours the pump should run continuously for that 6 hours whether or not the thermostat is calling for heat and that the thermostat should only control the boiler and not the pump.

As this has never been the case since the boiler was installed 3 years ago, I wonder if he is right or not. What does happen is that when the thermostat shuts the boiler down the pump over runs for about 2 minutes and that shuts down as well.

The plumber is wanting to send an electrician friend round to 'fix' it. Is he right?

Thanks for your help

Ann

Hi Ann

Which boiler do you have? Most new boilers (3 years including) would usually control the power that is sent to the pump. This is what gives you the over run perion that you mentioned. It allows for the boiler to get rid of the heat that it has produced to prevent any damage. It basically allows the boiler to cool down slightly. A bit like a slow walk after a run. So its perfectly normal for your pump to run a little after the boiler has gone off.

When you mentioned its switching on and off, do you mean the room stat or the boiler itself?

If its the boiler it could well be that its getting too hot. A boiler will have a sensor inside which tells it how hot it is getting, if it reaches its set temperature or slightly above it may well shut the burner down to allow for no more heat to be produced. It will see that there is a problem and that the heat will eventually be too much.

Its certainly worth allowing the chaps to look further into it for you,

Thanks

 

[annc1]

Sorry - meant to do that

The boiler is a Worcester Bosch Greenstar Cdi conventional gas fired condensing boiler.

The thermostat is a Drayton Digistat

Thanks

Just want to know if - as the plumber says - the pump should run even if the thermostat isn't calling for heat.

 

[mogget]

Piers, I'm not a chap

I'm thinking this might be a TPI thermostat set to come on too frequently, but until we know more details, nobody can know for sure...

 

[mogget]

Sorry - meant to do that

The boiler is a Worcester Bosch Greenstar Cdi conventional gas fired condensing boiler.

The thermostat is a Drayton Digistat

Thanks

Just want to know if - as the plumber says - the pump should run even if the thermostat isn't calling for heat.

Which model of Digistat do you have, and no it is not normal for the pump to run continuously.

 

[Piers1989]

Piers, I'm not a chap

I'm thinking this might be a TPI thermostat set to come on too frequently, but until we know more details, nobody can know for sure...

Im sorry! its my local talk coming out!!

 

[mogget]

no problem.

 

[annc1]

Until it packed up my previous room thermostat switched everything off pump and boiler and they remained off until the room temperature dropped - about 30 minutes) and then it switched everything back on.

New thermostat switches everything off when it reaches set temperature but a few minutes later switches pump and boiler back on - before the boiler even fires up properly it has been switched back off again. This can go on all evening.

Plumber who fitted the new thermostat says there is a problem with all the electrics and that the pump should continue to run until switched off by the timer even when the thermostat switches the boiler off.

Not sure I believe the plumber and think it just doesn't want to do anything about it. I phoned drayton who told me the new thermostat is possibly faulty or incorrectly installed.

 

[annc1]

Hi mogget

The Drayton is a Digistat SCR wireless system

 

[mogget]

The older Digistats will turn the boiler on and off every 20 mins (TP control).

Not sure if this has been reduced to 10mins on the newer Digi's.

Unfortunately I don't think there's any option to revert to on/off control as your old stat probably used.

The older Digistats are square and the newer ones are curvy and oblong -ish.

How long does this cycle last - if it's turning the boiler on and off every 20 mins this is normal behaviour for a Digistat. The pump should stay on for a few minutes after the boiler has stopped firing.

There may still be a workaround for this so please get back to us!

 

[annc1]

The Digistat is turning the boiler on and then off again within about a 5 minute period - the boiler is quite slow to fire up and so it doesn't even begin to reach heat before it is switched off again. The result is stone cold radiators all evening.

Our work around is to switch the thermostat up to about 26C and then when it reaches about 21 degrees or so switch the boiler off manually. We then wait until we start to feel a bit cold and switch the boiler on again. Better than it cycling on and off every few minutes.

 

[mogget]

Sounds like a faulty Digistat to me, but a few more questions.

Most modern boilers such as yours will start off slow and ramp up the heat gradually, this works quite well with TP/TPI control as the radiators reach the temperature required to heat the room and the boiler stays in condensing mode for longer.

But normally TP/TPI is once every 10 or 20 mins. Any less and the boiler will never get going, as you have discovered.

Does the little flame symbol show / disappear in tandem with the cycling?

 

[annc1]

I think it does - it is a little green light which goes off.

 

[mogget]

on the thermostat itself (where you adjust the temperature) there should be a flame signal showing when it's calling for heat.

I suspect you are looking at the receiver rather than the stat itself?