Central heating pump wont switch off

[Tracnation]

Hi there,

I'm having an issue with my central heating which i cant seem to solve. Its an old Ideal WLX RS60 boiler with Sunvic timer, tank stat, room stat and 2x 2 port motorised valves.

I've only been in the property just under a year and both CH and HW have always worked without issues, however when the boiler first switches on there has always been a very loud waterfall noise. The problem has been evident the last few days where the pump will continue to run long after both CH and HW have been turned off. It appears to only happen when the CH has been switched off as each time the HW is turned off automaticly by the timer the pump will stop within a few minutes. Also when CH is switched off the radiators now cool down very quickly as i guess the pump is still cycling the water round the radiators. The boiler itself is off and never fires again once CH switched off, just the pump itself.

The pump itself appears to be only 2 years old though the rest of the heating system i'm guessing is orriginal and 25-30 years old. I've also now noticed a small leak on the pipes leading into and out of the pump but unsure how long this has been going on or if it has anything todo with the problem.

The pump itself and the pipes leading into and out of the pump remain red hot while the pump is still cycling even after the radiators themselves are cold. Manually opening the two motorised valves result in the waterfalling noise again and the boiler firing up so it appears neither are stuck open and as both CH and HW work fine they dont appear stuck closed either.

I've had to resort to switching off the system at the mains once the heating is off.

The boiler is in a bedroom wardrope which is always warm so dont think the frost stat is the problem and i'm unsure if the boiler has an overrun sensor or not but cant find any mention of it in the manual or part list.

The only thing i can think of is the CH valve is sticking partially open alowing enough water through to keep pump running but not enough to fire boiler but i dont understand how the water is remaining so hot going through the pump hours after the boiler last fired.

Appologies if any of this makes little sense as i know next to nothing about central heating systems other than what i've read trying to diagnose the problem.

Any help would be appreciated.

Neil

 

[stem]

Hello Neil and welcome to the forum.

To be sure, you will need to trace the wiring and see what is actually providing the power to the pump. There are several possibilities, but I would have expected that in your case due to its age and your description, that the system is fully pumped and the boiler doesn't need a pump overrun (your instruction manual will confirm if this is the case, or possibly someone who is familiar with your particular model of boiler may follow along shortly) the pump is then usually wired across the boiler terminals and is switched on and off with the boiler. This is achieved via micro switches inside the motorised valves that switch 'on' when the valve is open.

It is possible that these micro-switches can stick in the 'on' position even when the valve is closed so the pump will continue to run. The boiler will also be powered but you won't hear the boiler operating because it will have heated up the water inside it and the burner switched off by its internal thermostat. Because the valves are closed and the water has no where to go, it remains hot and so doesn't re-ignite.

A pump continually operating when there is nowhere for the water to go will generate heat by the impeller thrashing around in stationary water, and from the pump motor because it is under an abnormally heavy load. The connected pipes may get hot by conduction from the pump, or by a tiny flow of hot water due to seepage past the closed valves which is just enough to keep the main flow pipe hot, but is dispersed once it reaches the main distribution pipe system.

Having said that, I can't explain why the pump has been continuing to run for a short time and then switch off, unless either your boiler does have an overrun facility, or that it is connected to a pipe thermostat somewhere. This takes me back to my original comment that to be sure you will need to trace the circuit back and find out what is actually powering your pump.

 

[Tracnation]

Hi Stem and thanks for the welcome and help

The pump itself, along with the valves and boiler are wired into a Sunvic control box which i havent opened as yet to see what is inside. What should i be looking for in there? If it does turn out to be the microswitch, i'm guessing it would mean a whole new valve replacement?

Thanks again for your help.

 

[stem]

The microswitch can be replaced as they are fairly common items used for lots of applications and can be obtained from industrial electrical suppliers. Usually though it's easier to replace the entire valve or just the powerhead. which is not a bad idea anyway if the valve is getting on a bit.

The switch wires from the valves are usually grey and orange, do either of them go to the pump?

Unfortunately there are lots of ways to wire up what is known as an 'S' plan that achieves the same thing, so it's not really possible to tell you what to look for. For example the valves may switch the pump and then the wire continue on the the boiler, or the boiler and the pump may be wired back to the same terminals in the wiring centre. It all depends upon the location of the various parts of of heating system and the personal preferences of the electrician that wired it up. If there is a boiler overrun facility, the pump will be wired to dedicated pump terminals at the boiler.

However, by clicking here and then on the 'S' Plan schematics you should be able to get a general understanding of how it all works.

If you are handy with a multimeter, have a basic understanding of electrical circuits, and can work safely on live components, it is possible to test the switch to see if it's stuck or not.

 

[Tracnation]

Ive just had a look in the wiring centre and the live feed for both pump and boiler are connected to same terminal fed by the two valves. I also tool the powerheads off and they both act differently. The HW one has allot of resistance when opened manually and returns back to closed automatically. You can also clearly hear the motor being pushed against. The CH one on the other hand has almost no resistance and is completely silent. Does this sound like a powerhead failure?

Thanks again

 

[stem]

The the HW one sounds as it should. There should be some resistance on the lever when the valve is not powered up and you can usually hear the motor whirring as the lever is moved. On the other hand, when the valve is powered up there should be no resistance.

If you carried out the test when there was no power to the valves, it sounds as if one may be stuck in the open position, which could be your problem.

It would probably be worth making sure there is no power on the valve when testing, or disconnecting the live from the wiring centre. Sometimes due to an electrical fault elsewhere, the valve could be powered up when it shouldn't be.

 

[Tracnation]

The test was performed with the power off, so i was pretty much convinced that was the problem and it was just stuck open however i decided to monitor them both while removed from the valves and both opened and closed properly in response to the timer after power restored, albeit it very slowly. Took around 20 to 30 seconds for them to turn the 90 degrees to open, though both closed fast enough so now i'm unsure. Neither valve is stuck and both move freely by hand and the pump cut out as normal each time so unsure if it was just coincedence or if the problem only happens when attached to the valve.

One thing i did notice was that the actuator plastic cover was hot to the point of burning me when i picked it up to put it back on the valve after having being left off for 3 hours with the CH on. The HW one did heat up but not anywhere near as much, though it was only on for 45mins.

Also appears your right though and the pump always has switched off immediently, unsure why i thought it ran on for a few minutes but at least it makes a bit more sense now.

Have a feeling the microswitch maybe over heating after a length of time and not registering when closed as it appears to be a common issue with the Sunvic SZ2301.

 

[stem]

They do open slowly. Their motors have very little power, so they have a little gearbox to slow the drive cog down. (Think cycling up a steep hill in a low gear, legs going going fast, but cycle progressing slowly)

Valves are pulled back to rest by a spring when the power is removed so this is pretty quick. If you listen carefully, you should hear the microswitch click at the far end of their travel. If your pump wires are connected to the microswitch this is what switches it on and off.

Valve motors will get warm when the valve is open, because the motor is powered continuously and 'stalled' to hold the valve at the far end of its travel, but I can't really comment on how warm, because I've never had one powered up on a bench for a long period, and if one was warm in situ, I would probably assume it's because of the hot pipe it's connected to.

Microswitches can wear and fail, or become intermittent and stick 'on' or 'off'

 

[Tracnation]

Thanks again for your help and advice. I took the actuator apart to have a look at the Microswitch which appeared to work fine but in the process the whole thing fell apart and had to rebuild the actuator from scratch. That was 2 days ago now and it has been working flawlessly ever since! I'm guessing it will eventually start acting up again so will order a replacement just incase.

Many thanks

 

[stem]

That happens sometimes, perhaps the microswitch was not seated properly. I have an S Plan heating system with 2 motorised valves. I always keep a spare powerhead just in case.