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How do I allow boiler overrun when all valves are closed. Ideal Logic 15 boiler,

S plan Plus system (3 zone - CH, DHW and single heat zone (lounge)), I've set a boiler relay and then a DHW Bdr91 and a single zone Mot valve. On the original wiring set up, the Boiler seems to be switched via end switch wiring, so gets its signal when the valve opens and that all seems to work ok, however the boiler controls the supply to the pump has an overrun of 3 mins, so when the valves close, the pump continues to run. I could open a rad permanently, however that would mean that the rad would be hot every time anything runs (DHW even in the summer). Anybody come across this issue? Help much appreciated. I was thinking of using the orange wire from valves to power the pump, but that would stop the pump during overrun, any help much appreciated.

For an S plan system to work with boiler overrun, there should be a (pressure activated) bypass valve fitted.
The bypass valve inlet sits in the pipe between the pump output and the motorised valve inlets.
The bypass valve outlet goes to the boiler return pipe.

When the valves in your system are closed and the boiler wants overrun, pressure at the pump output will increase and this opens the bypass valve, allowing the overrun flow to go back to the boiler.

Thanks for your response, makes sense and can confirm the bypass valve is in place and i can here is working (can faintly hear it), so its working. As I'm sure you can tell, not over familiar with the overrun function, if its to help cool the boiler, this will be constantly overrunning as any of the HR92's close, just feels like an expensive solution (in terms of pump running much longer than needed and dissipating residual boiler heat that may be required minutes later if another valve opens.

At least its working as designed - so that reassures me, thanks again

Yes, it's to cool the boiler. Imagine the heat exchanger in your boiler as a big lump of metal, heated by gas on one side and cooled by water on the other. In normal operation the exchanger will be hotter than the water temperature. Stop the flow of water and gas simultaneously and the metal can't get any hotter than it already is, but it could have enough residual heat to boil the stagnant water in the exchanger. That would be bad for a number of reasons, pump overrun stops it happening.

Edit, it shouldn't make much difference to the cost of running your system. It just runs the pump for a minute or so after the gas is switched off. Most pumps are around 300W ish, so you can run them for 3 and a bit hours to a KWh oh electricity, which is currently about £0.28 UKP Sterling.

Call it 2 mins of overrun and 333W for the pump, meaning the pumnp runs for exactly 3 hours (180 mins) on a KWh...
(2 mins / 180 mins) * £0.28 = £0.003

So that's a cost of 0.3 pence to implement overrun each time your boiler shuts down. Given that overrun can increase the life of the boiler, I'd say it's a good thing.

I'm not a plumber by the way.

Hi, thanks again for your responses, all reassuring and helpful.

BrokenSkunk wrote:Yes, it's to cool the boiler. Imagine the heat exchanger in your boiler as a big lump of metal, heated by gas on one side and cooled by water on the other. In normal operation the exchanger will be hotter than the water temperature. Stop the flow of water and gas simultaneously and the metal can't get any hotter than it already is, but it could have enough residual heat to boil the stagnant water in the exchanger. That would be bad for a number of reasons, pump overrun stops it happening.

Edit, it shouldn't make much difference to the cost of running your system. It just runs the pump for a minute or so after the gas is switched off. Most pumps are around 300W ish, so you can run them for 3 and a bit hours to a KWh oh electricity, which is currently about £0.28 UKP Sterling.

Call it 2 mins of overrun and 333W for the pump, meaning the pumnp runs for exactly 3 hours (180 mins) on a KWh...
(2 mins / 180 mins) * £0.28 = £0.003

So that's a cost of 0.3 pence to implement overrun each time your boiler shuts down. Given that overrun can increase the life of the boiler, I'd say it's a good thing.

I'm not a plumber by the way.

Thanks for helping with this.

Just to say that 'most' pumps are under 100W with the latest pumps can be as low as 7W.