Central Heating Pump Overrun

24 Sep 2014
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United Kingdom
Morning all,

I've been trawling this and other forums looking for the solution to my problem but never quite found it, so I thought I'd ask.

Here goes, I'll list everything I've noticed and tried...

I've recently moved into a three year old house with an Ideal ICOS HE15 boiler on what I take to be an S-plan setup (separate valves for CH and HW). I also have the Hive Active Heating controller.

My overrun problem is as follows:

HW is on a schedule and has been running nicely for the last six months. When the HW demand is satisfied, the boiler shuts off and the pump stops. No problem there.

CH works and heats the house but when it switches off, the pump will just keep going and going. Earlier today for instance, the HW was on between 5 and 6 am (no excessive overrun, checked at 6:20) but when the heating schedule finished (7 to 7:30) the pump was still running two hours later.

I think I've eliminated the valves and actuators because if I briefly cycle the heating on/off using the Hive controller, the pump stops there and then. The overrun only happens when the CH has had chance to get up to temp.

I understand the need for some overrun to dissipate heat in the exchanger etc. but this doesn't seem right. The bypass circuit is hand-hot during this, but the rads are luke warm.

While this CH overrun is happening the boiler is not doing anything. The fan eventually runs down and it shows 'C' on the display, while the burner light flashes (not really sure what this means, user manual says 'ignition'). The boiler reset button doesn't seem to work at this point but switching the power off and back on stops the pump.

That's everything I know, all of it gleaned from the internet; I have no experience with heating systems. I've got half an idea that my boiler has some sort of thermostat which monitors the bypass circuit and that this may need replacing but I'm pretty much out of my depth here and it may just be that I don't fully understand how my system works. I don't really want to have to call somebody out just for them to tell me that it's all ok and I'm fretting over nothing.

Picture of HW cylinder if it helps. It wasn't installed this way, it's the picture that is sideways...


Thanks in advance.
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Few hours now since heating was last fully up to temp. Pump still running, both valves closed, boiler fan back on, bypass feed still hot (now hotter than CH feed), all rads cold.
I would check that one of the microswitches (probably the ch) isn't sticking in one of the 2 port valve, easy to check if you have a multimeter.
I don't have one. But the builders' merchant is nearer than B&Q so I might just buy another actuator. Sounds like I may need it anyway.

I had taken the existing one apart though hoping to find something obviously broken, it all seemed fine though; microswitch clicking as you would expect.

Any sort of overrun thermostat failure would result in the pump just shoving ever cooler water round the bypass circuit wouldn't it? You shouldn't end up with the bypass being red hot while the CH feed is cold. If the bypass is hot then the boiler must be heating it, therefore something is tricking the boiler into activating even though both valves are closed. A duff microswitch on the CH side. I think you've convinced me.

Sorry to go on like this but I'm determined not to call somebody out to replace a 'plug and play' part. Thanks for answering.
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You don't really pay them out just to replace "a plug and play part" but for their knowledge in which plug and play part to replace!
Well yes, that's true. If it was the gas side or anything more complex I'd gladly call in the pros, but if a bit of reading and effort is all that is needed then I can save myself the hassle of taking a day off and also the expense.

I'm going to swap the actuators around and see if that replicates the problem on the HW side.
Icos has no pump overrun (many boilers do, but yours does not; the danger of Internet searching!). So one of your valves is definitely faulty. Probably CH one by the sounds of it since it switches off after HW demand without issue.

Also unless you have TRV's on every radiator then you can shut the bypass valve completely, it's not helping and will just cause the boiler to get to temperature prematurely causing cycling and lower efficiency.
IIRC, the C indicates the boiler has received a demand for heat. If the neon is flashing, the boiler is in ignition phase/burner has gone off because primary circuit is up to set temperature. If there is no demand on the boiler it should display 0, standby state.
Your valves are shut, but the CH microswitch in the zone valve is likely sticking, putting a demand for heat to the boiler, and forcing it to operate through the bypass line.
No overrun? Good, one less thing to worry about; has to be a valve problem then.

I swapped the actuators round to see if the problem would migrate to the HW side and suddenly everything is behaving itself. I'll just keep an eye on it for now.

System is about 50% TRVs but most of the 'normals' are shut off (utility room, hall, landing etc.) I think I'll open one and perhaps close the bypass valve.

Thanks guys.

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