Is Pump Overrun Doing Any Good?

[JohnD71]

Hello all.

I’ve recently had a new boiler fitted and the fitter has explained the concept of the pump overrun facility.
It all seemed to make sense until I looked at the position of the components involved. The boiler, pump and bypass valve are all very close together, within about 2mts of pipe for the complete circuit. Basically, the over run doesn’t remove any heat from the boiler (or very little) once the boiler has stopped firing as the heat hasn’t got anywhere to go.

Looking at the situation from a layman’s point of view, would it not be far more beneficial to hold the 2 port zone valve open for the length of time that the pump overruns? I’m certain I can draw up a circuit to make this happen, and also to keep the switched live to the boiler open (to stop it firing back up) while the pump is overrunning.

Just how important is it to remove the residual heat from the boiler once it has stopped firing? Am I wasting my time / money with this modification?

Any thoughts or comments would be much appreciated, thanks.

 

[baxpoti]

boiler is?

 

[JohnD71]

Sorry, it's a Worcester Greenstar 24 Ri (Natural gas)

 

[mogget]

The pump overrun is there to make sure that the residual heat is removed from the heat exchanger in the boiler. When the burner is alight, parts of the heat exchanger are hotter than the boiling point of water.

Modern boilers have low water content heat exchangers and if the water flow was to stop the instant the burner was turned off, the water in the heat exchanger could boil, causing scale, damage and noise.

For this reason most modern boilers have a pump overrun to prevent this water boiling. The bypass circuit does not have to be huge, just enough for the excess heat to be absorbed into the water.

The manufacturer will have designed the boiler to work properly provided it has been installed in accordance with the instructions. There's no need to modify it if there's no banging and crashing when the heating demand ends.

 

[Agile]

Very well explained but I can add a small extra to help the OP understand!

The maximum temperature of the boiler flow is usually no higher than 80°C.

The additional latent heat in the heat exchanger is disapated NOT by heat loss from the pipework which you mentioned but instead by INCREASING the temperature of that water to say 85°C.

So its not a requirement to have heat dissipated from the pipework.

Tony

 

[JohnD71]

Mogget & Agile.

Thank you very much for such swift and comprehensive replies.

That makes things much more clear.

Cheers, John.

 

[htgeng]

A 'w' or 'Y' plan may have been better as there is always one port open at all times. That said, what you've got is easier to wire up and understand!

 

[mickyg]

I think some people don't understand the reason for a pump overrun!
The idea is not to get rid of the heat as such, although that serves the purpose. The motive is to keep the water moving.
Why does the sea not freeze when its -10?

 

[Slugbabydotcom]

Why does the sea not freeze when its -10?

What I think you meant to say was that flowing water wont freeze.

The sea wont usually freeze at -10ºC mostly because of the salt in it.