Combi boilers and pump overrun

[bazdaman]

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,

Pump runs for DHW on 90% of combis for hw

 

[TCCHeating]

Woofter Botch boilers spin their pumps in hot water mode.

Thought you'd know that Mr Bisby.

Not many don't.

What about the old 24 juniors?? Admittedly pump will overun after dhw demand but only for 2 second bursts

 

[Dan Robinson]

As long as the pcb holds up

 

[gas112]

Not only can pump over run on vast majority of combis on some the fan will also continue to run to help dissipate any heat

 

[GuessMan]

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,

Got your ACS and Gassafe reg yet? KNOB!

 

[Alfie25]

Ok so a pump does run then! So where does the pump overrun circulate the water to? The CH circuit?

Dan - you're often the fountain of all knowledge on here. With the pump overruns that run based on a temperatue drop rather than time, can they take a long long while to dissipate the heat?

 

[GuessMan]

On every Combi except the storage types, yes it circulates to the heating circuit. That is why one radiator must not have TRVs and is known as the bypass radiator. Usually the bathroom or hallway rad.

 

[Dan Robinson]

They can if your rads are turned off and have no by pass.

I forgot about the fan over run.

 

[GuessMan]

The fan overrun is known as post purge and is to remove and dilute the products of combustion which can reach very high levels of CO as the flame dies away. It may help cool the combustion box and heat exchanger, but that is not its primary purpose.

 

[Alfie25]

The big living room rad will have no TRV fitted so will act as the bypass. The bathroom towel rail is also always open.

Maybe it's the fan as well as the pump I'm thinking of when I hear combis 'running' when apparently Off. The fan just blows heat into the room where the boiler is?

 

[Dan Robinson]

Some boilers had the fan running at low speed constantly.

Puma PC
Vokera Excel
Glowworm Swift flow (IIRC)
A Ravenheap did to I believe.

There are others.

 

[gas112]

guessman that not what it says in a lot of manufacturers instructions it says its to dissipate heat ,
As flame dies away ? once gas valve shuts the burners are as good as dam i immediatley extinguished and if it was to clear any products this would take seconds not the several minutes fans over run for

 

[vulcancontinental]

This is one weird thread.

I like the concept of the Intergas but with such a large block of ali that doesn't need a pump overrun to dissipate heat after hot water demand how can hot water demand be met instantaneously, the heat up time is slower I'd have thought. Has anyone ever compared it to a plate hx?

Which combis dump heat into the CH circuit after hot water is satisfied? I think the old Swiftflow might have but do others? Ours do not, the integral bypass is there for that.

 

[Steelmasons]

Potty profile/prima/netaheat had the fan purge combustion chamber before ignition hence the reason for the brass restrictor @ air pressure switch.

Plenty others used fan overrun as opposed to pump overrun to cool the heat exchanger although standing losses suffered a tad.

 

[GuessMan]

guessman that not what it says in a lot of manufacturers instructions it says its to dissipate heat ,
As flame dies away ? once gas valve shuts the burners are as good as dam i immediatley extinguished and if it was to clear any products this would take seconds not the several minutes fans over run for

Leave your analyser in the test point and turn the power off to the boiler, watch the readings and see what happens without post purge. Also have you never noticed how a lot of boilers go to max fan speed if the flame is not rectified or fails to light?