Combi boilers and pump overrun

[Alfie25]

Still looking for a combi boiler and having come into contact with a few over the years, I know that with some, after you've stopped calling for HW, the boilers pump continues to run for a couple of minutes, but with other boilers it's more like 10-15 minutes.

I've been keen on getting the Viessmann vitodens 100-w series combi however Viessmann tell me that the pump overrun does not run for a certain time, rather it continues to run until the temperature has fallen significantly, which I imagine could be quite a long time. Does anyone know if this is standard these days? Or can anyone recommend a new combi that doesn't run and run and run for ages after you've only had the tap on for minute or so to wash your hands!

 

[ALCPlumbing]

The pump continues to run for a good reason. Whats your objection to this?

 

[Fluffster]

The heat from the primary exchanger will dissipate very quickly into the main primaries, if it cuts off when the heat is gone it may actually be more efficient than a timed unit.

Pete

 

[ianmcd]

Why should you even be concerned ? the pump over runs for a reason, it will hardly cost you a fortune in electicity

 

[Alfie25]

The heat from the primary exchanger will dissipate very quickly into the main primaries, if it cuts off when the heat is gone it may actually be more efficient than a timed unit.

Pete

Hi Pete, can you expand on this. Say I'm just finished calling for HW, does the heat then get dissipated to the CH circuit? If the heat is no longer heating up cold water, where does it dissipate to?

As for why it bothers me, just a personal thing. Sometimes you only want a minute or so of hot water so it seems crazy to me that some boilers then continue to run the pump for ages afterwards, especially as some boilers (like a Baxi 105HE) stops running and turns silent literally seconds after the HW tap has been closed.

 

[ianmcd]

as stated above, some boilers have a pump over run that is a fixed time, say 3 minutes to dissipate heat that was generated during boiler fire, some take readings from the internal thermistors and stop the pump when it deems itself cool enough, either way the boiler does this to protect its self from high temperatures and the energy used by the pump is minimal, the only problem you usually get is noise nuisance

 

[Alfie25]

Hit the nail on there Ian, noise nuisance. My current system (open vent, fully pumped) makes one heck of a racket due to inadequately secured pipework. When I get the boiler replaced, I'll be asking for all the suspect pipework to be reclipped as required but I still fear noise.

That said, where will the pump be recirculating this water? Round the boiler itself or round the system? If just round the boiler itself then that shouldn't cause any knocking from pipes elsewhere should it, but it does mean the pump may take ages to shut off.

 

[ianmcd]

What boiler do you have Alfie ? it may help if you fit a by pass between the flow and return pipes leaving the boiler

 

[Alfie25]

My boiler and existing system's days are numbered, so I'm only focusing on the new system rather than repairing what I've got.

Do you know the answer to the question i asked about where the heat disspates to on a combi after HW is no longer being called for? Is it recirclated round the CH circuit or just internally in the boiler?

 

[ianmcd]

usually to the heating circuit, but is not normally noticed as the heat dissipates very quickly, if however you have an S plan with no by-pass this can cause problems

 

[Alfie25]

Got a Y plan at the minute but the pipework will of course be getting overhauled when I get a combi fitted.

Not good to hear that the HW side of the combi boiler opens up to the CH side to dissipate the heat when the pipes are as noisy as mine.

 

[Servotech]

The H/W side of a combi' boiler is instantaneous.

Example - you open up the hot water tap and the boiler fires up you close the tap and the combi' switches off (no pump involved).
servotech,

 

[Alfie25]

The H/W side of a combi' boiler is instantaneous.

Example - you open up the hot water tap and the boiler fires up you close the tap and the combi' switches off (no pump involved).
servotech,

Hang on, you're right and I was getting confused but something does continue to run after you close the HW tap. If it's not the pump then what is it? Or does the pump then kick in after you have closed the HW tap to disspiate the heat elsewhere? All I know is with combis, when you close the HW tap, something in them keeps running, and on some boilers it runs a lot longer than others.

 

[Dean744PiN5]

Of course the pump runs when demand for hot water on a combi how else will the water heated from the main hex circulate through the plate hex?
Unless you fit an intergas which dosen't have a plate hex, but a bithermal heat exchanger and is truly instantaneous and dosen't require the pump to run.

 

[Dan Robinson]

Woofter Botch boilers spin their pumps in hot water mode.

Thought you'd know that Mr Bisby.

Not many that don't need their pumps.