Pump running 24/7, and leaky bypass gate valve...

28 Mar 2011
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United Kingdom
Hi there

I'd really appreciate any advice regarding this problem, before I call an engineer out.

We moved into a new house last year. CH and HW all seemed to work fine. However the pump is on 24/7 - even when the room thermostat and programmer are all off. Initially the problem was intermittent, clearing itself after a few days (whereupon the normal pump overrun would work fine, shutting the boiler off after an hour or so), but now the pump just runs and runs unless I flick the main power to the boiler off.

I believe we've got a Fully Pumped Open Vented "S-Plan" system. The boiler is a Potterton Profile 60 and is in the garage, connected to a frost stat. The electronic timing programmer is a Potterton EP2001 and there is a single room thermostat (Sunvic) in the hall.

In the upstairs airing cupboard next to the HW cylinder, the Grundfoss pump feeds into two separate 2-pipe Sunvic zone valves, one each for the CH and HW. There is a bypass circuit which runs from just above the pump (& before the values) and connects to the hot water cylinder return pipe. It has a red gate valve on it which APPEARS to be fully closed. I'm not sure though, because when I try and turn the valve anticlockwise to open it, it starts leaking! Also when I turn it it feels like the tap isn't actually turning the valve inside but is instead just unscrewing itself from the valve unit (I'm 100% not sure this is the case though)

So, right now the electronic programmer is off, the room thermostat is set to low, and the pump is whirring away as usual. The boiler will fire up every now and then, burning a short while each time before switching off. The pipes above and below the pump, above both zone valves (which I think should both be closed?), and the pipes going into & out of the HW cylinder are ALL very hot. No radiator pipes are hot though.

Any ideas on what's going wrong here?

Reading about on the net it seems to me likely suspects might be:
1) One or both zone valves are faulty?
2) Some effect of the bypass gate valve being closed? (if it IS closed!)
3) Dodgy frost stat in the garage, keeping the boiler on despite the ambient temperature being high?

One additional bit of info, is that although the CH and HW do appear to work fine, I can sometimes hear a water flushing noise, which I suspect might be the HW venting into the cold water storage tank (but I haven't 100% verified this yet).

Many thanks for your help,
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The first thing is to check the frost stat.

Maybe turning boiler on when not required. Its 8.7° here.

Hi Tony, thanks for your reply - is there anything I can easily do to check if it's faulty? I'll admit to trying pointing a hairdryer at it for a short time, to artifically warm it up but had no effect...
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Thanks lightning, I dont have a multimeter, but if I did get one, is this easy to test (I'm confident DIYer but novice with CH and electrics)?
Programmer turned off.
Pump still running.
Boiler come on sometime.

Sound like the frost stat playing up.

The bypass should be open a little so that heat from boiler can run around better, when frost stat in used.

Ok I've just realised I can flip the case of the Honeywell frost stat off to reveal the stat dial. It's set fairly low (5 deg C). Setting it lower doesn't stop the pump or stop the boiler. It clicks at about 17 deg C which is about the right temperature in the garage at the moment.

It looks like a fairly easy and cheap thing to replace, so I might as well wire a new one in to replace it and see if that sorts the problem.

What happens to the frost stat flow if the bypass valve is stuck fully shut? If the water has nowhere to go, could that over time have damaged the zone valves, keeping them open? I'm puzzled as to why the pipes get hot beyond the two valves, as they're supposed to be closed when CH is all off.
I have re-read your thread.

The 2 port valve on hot water side is seeping by which mean the valve is jammed, causing the pump and boiler to run 24 hours.

Remove the motorhead and check that the spindle on the valve body is turning freely.

Also check that the frost stat is off when turned up, use a multi-meter, just to rule this out or not.

Hi Dan

I removed the motorhead and the screwdriver-like head underneath (is this the spindle?) is perpendicular to direction of flow. I can turn it fairly easily with my fingers.


Both CH and HW values are Sunvic SZ 2301 units. There's a flushing lever on each one: when I move the lever on the CH valve and let it spring back, I can hear it turning the actuator motor inside; but with the HW valve (pictured) I can't hear it turning the actuator motor.

So does this mean that the actuator motor has burnt out? Can I just buy a replacement actuator?

If so, in the meantime should I leave the valve open or closed?
Thanks lightning, I dont have a multimeter, but if I did get one, is this easy to test (I'm confident DIYer but novice with CH and electrics)?
I would not use this particular opportunity to learn how to use a multi meter; get a pro in.
Hi all

A quick update. I've had both CH and HW actuators off their valves and I've tested that they both do turn on and off properly as the CH/HW/cylinder stats and programmer all require. So I'm satisfied that both valves turn freely and that the motors in both actuators are working properly.

However - although the actuators do turn on and off when they should, could they still be faulty? e.g could faulty circuitry mean one of them is still asking for the boiler & pump to be on, even when it's closed?

If not, I guess the frost stat is the most likely cause of the pump staying on 24/7...
Just a quick note to say.... Fixed!

I tested the frost stat with a multimeter and it was fine. So I tested the zone valves and turned out the microswitch inside the CH zone valve had stuck on. I replaced the actuator with a new one. Wired it back up carefully and all works great! It's been quite good fun working out how our CH system all works.

Thanks for the advice all - this forum is a great resource.


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