Potterton Prima turning on and off

[West Country]

Hi everyone

I have an issue with my potterton prima boiler and would love some help. It works fine when either CH or HW are on but when they are off the boiler will fire up briefly and cut out again, it appears to be random and for 1\2 sec or severely seconds, it will continue doing this. I am turning off the power over night to stop it doing this.

My system is and old gravity-fed one, so I have HW or HW and CH. It had been working fine until last week, because the boiler seemed fine when CH or HW are on then I assumed (wrongly) that it was a control problem. Which is why I changed the old rotary dial switchmaster for a hive mini, but I still have the issue. This makes me think it's a problem with the boiler making it fire up without a signal from the controller.

 

[West Country]

I've attached a video so you can hear the noises.

 

[Ric2013]

I've attached a video so you can hear the noises.

Where?

 

[Deleted member 120897]

So you have an issue with noise and random firing? Noise could be lots of things, random firing sounds like motorised valve failure

 

[West Country]

Video so you can hear the noise

 

[West Country]

So you have an issue with noise and random firing? Noise could be lots of things, random firing sounds like motorised valve failure

So the motorised valve is for CH if I select HW only will this remove the input from the motorised valve and prove the fault?

Excuse my ignorance but I thought the motorised valve shut off the CH side of the system and didn't have an input to the boiler. I can hear when it closes and I certainly only remember hearing it operate when the heating is set to turn off.

 

[Deleted member 120897]

Video so you can hear the noise
View attachment 80397

Sounds like something is struggling like the pump or motorised valve.

 

[Deleted member 120897]

Might be best posting some photos of what you have valve wise and pump, etc.

 

[West Country]

So here you go. Top of boiler pipe work. I have a junction box, the controls (formally the switchmaster now hive) are wired into it. Live and neutral wired to the junction box via the controller Motorised valve to the junction box, junction box to the boiler. There is also a wire from the junction box going up stairs which a assume goes to the hot water tank. The pump is on the hot water and the motorised valve on the heating. I checked and I could hear the motorised valve close when I selected hot water without heating.View attachment 290356View attachment 290357View attachment 290358.

 

[West Country]

So here you go. Top of boiler pipe work. I have a junction box, the controls (formally the switchmaster now hive) are wired into it. Live and neutral wired to the junction box via the controller Motorised valve to the junction box, junction box to the boiler. There is also a wire from the junction box going up stairs which a assume goes to the hot water tank. The pump is on the hot water and the motorised valve on the heating. I checked and I could hear the motorised valve close when I selected hot water without heating.View attachment 290356View attachment 290357View attachment 290358.

 

[West Country]

Photos of how it's wired in case it helps

 

[West Country]

So some explanation may help:
Hive left cable permanent live and neutral, right cable HW on and CH on

Junction box:
Bottom left furthest left HW on and CH on, inner left permanent live and neutral.

Top left hot water tank

Top right mechanical valve

Bottom right go to the boiler.

 

[Ric2013]

So you have an issue with noise and random firing? Noise could be lots of things, random firing sounds like motorised valve failure

Funnily enough I had some very odd noises from my Potterton (Netaheat) "Profile" boiler starting Christmas morning lol. Sounded like a fan failure (fan sounding like it was fighting heavy wind, though sounded normal from outside), but turned out was the microswitch in one of my motorised valve intermittently making poor contact and only providing 200V to the boiler's switched live input (or that's how my analogue meter read it: possibly the connections were arcing and not a constant voltage at all?). Not the same as your noises, but who knows if the causes are related. If you don't have a multimeter, a low wattage lightbulb might do as a crude voltage tester - see if it goes flickery, that kind of thing.

There's something odd about the controls. Normally a gravity system (semi-gravity) uses the pump only for heating and relies on gravity for hot water only, hence why you can't have a water off, heating constant mode. Not saying this is the cause, but what I would want to know is what is providing the switched live to the boiler, as if there is a valve, the valve microswitch could be wired to switch the boiler on, rather than the boiler being switched direct from the Hive. I really can't see the boiler firing if the switched live isn't being fed, and, looking at a Prima manual, seems that it would be virtually impossible.

I'm not qualified, competent, or willing to discuss boiler faultfinding. However, from listening to your video, it sounded like the boiler was making fairly typical noises for a boiler of this type between the 2 and 4 second mark, and running the fan. Fans don't run without power, so I'm thinking an external control issue rather than a boiler one. If I'm correct, can we twin our boilers and both put little signs on them saying twinned with Potterton Prima / Netaheat Profile serial number xxxxxxxxx?

 

[West Country]

Hi everyone, so I've tried something for fault diagnostic purposes. I electrically disconnected the motorised valve and used the manual override. Still have the same fault, so I'm thinking (happy to be wrong) that with the valve disconnected and the controller replaced, it rules out external inputs and it means it's a boiler problem.

 

[Ric2013]

You disconnected all 4 cable cores (ignoring the earth, if present) on the valve? If so, I'm leaning towards your point of view.

Looking at your junction box, the boiler has two cables going to it. Is one the cable where the boiler feeds the pump? Only you have switched lives from the programmer to 5 and 3. I'm confused as to which of these goes direct to the boiler if the boiler isn't switched by a further external control.

Also, if your system is semi-gravity, the Prima manual I'm looking at (may not be your exact model) suggests it's suited to FULLY PUMPED systems ONLY so if it's installed differently (wrongly), then we need to assume the wiring will be unusual. Please define what you mean by 'gravity-fed' system. Do you mean it's 'open vented'?

As for the Switchmaster (an 800, perhaps?), may be worth flogging on ebay if it works!

 

[West Country]

Hi yes I now believe I have a open vent system. The pump is fed from a 3rd cable not visible in the photos direct from the boiler the 2 cables from the boiler are:

It's a 3 core cable with only brown used and connected to HW on from the Hive number 5 in the terminal block

The other cable is connected :
8 earth
9 permanent neutral
10 permanent live

Now I've looked the wiring seems very odd the cylinder thermostat is connected to the terminal block but then nothing.
So top to bottom

1 brown/live from cylinder stat
2 earth from cylinder stat
3 CH on from hive on the left, brown to the motorised valve on the right.
4 empty
5 HM on from hive, brown single cable to boiler
6 nothing left, blue with red tag from cylinder stat on right.
7 empty
8 earth
9 permanent neural from hive on left to the boiler on right
10 permanent live from hive on left to the boiler on right.

So this is what I think is happening. The system has been set up to run HW or HW and CH. So the controller provided a signal via terminal block 5 to the boiler to fire up, this happens for both HW
and CH. The CH element is controlled by a live feed from the controller to open the motorised valve.

This all seems wrong but has been working. So I now think I have 2 problems. First the random firing of the boiler and second the wiring of the system.

I'm thinking I should have a second motorised valve controlling HW.

Oh and only 3 of the 5 wires from be motorised valve are used.



 

 

[Ric2013]

Oh and only 3 of the 5 wires from be motorised valve are used.
 

My understanding of the system you have is the same as your understanding.

The grey and orange on the motorised valve won't be used if they aren't controlling the boiler. Usually, when the programmer controls the heating and the hot water via separate valves, the grey goes to a permanent live and the oranges go to the switched live feed into the boiler. Inside the valve is a microswitch connecting the two. This would be a standard 'S Plan' setup. Hence programmer makes the valve open, the valve pushes the microswitch as it opens and this completes the circuit to the boiler which then fires up. In your case the valve is simply opened and closed but the microswitch isn't being used as you've set up your Hive to only provide CH when HW is on and CH cannot come on on its own. All as you say. Probably a legacy from your Switchmaster (600 model, I'm assuming, not 800 as I incorrectly wrote above) which in turn is probably a legacy from an earlier boiler that was probably running as a semi-gravity system.

It's not full 'boiler interlock' (i.e. boiler switches off when both thermostats are satisfied, so saves a bit of energy) but this isn't really something I'd suggest is DIY to change as there are minor differences in the design of the plumbing that means it wouldn't really be right to modify the control side without also modifying the water side of things. Primarily the need for a system bypass so the boiler can pump water around the system to cool itself if the cast-iron(?) heat exchanger is hot when the thermostats are satisfied (that's why the boiler has control over the pump).

If the only thing that can be switching the boiler on is the terminal 5 and it only comes from the Hive programmer, then I agree there is probably an internal boiler fault, though it's a strange fault and please let us know when your Gas-Safe installer diagnoses it as I remain curious.

Regarding that cyclinder stat. Cylinder stats generally have a double throw switch so may be a live in and two switched lives out: one says the cylinder is cold (contacts Normally Closed) and one says the cylinder is hot (contacts NO). I'm seeing 3 cores on that cable to the cylinder stat all of which are just parked on spare terminals. If so, it's essentially not connected at all and can be ignored. 2 probably isn't earth, else it'd be connected to 8. Green/yellow should never be used for anything but earthing, but you'll see it used for other purposes often.

 

[West Country]

You where nearly right the old controller was a switchmaster sm400, which for reasons that baffle me are worth £50 on ebay. I'll report back once I find the fault.

 

[Ric2013]

You where nearly right the old controller was a switchmaster sm400, which for reasons that baffle me are worth £50 on ebay. I'll report back once I find the fault.

Worse still. At least with the 600 you could have 'water constant, heating timed'. Worth £50? Nostalgia, I expect.

I grew up with a similar programmer. Eventually it used to stop turning if the room got cold overnight before failing altogether some months later.

 

[West Country]

So the boiler chat has been, he has seen the fault and is convinced it's theme PCB and is getting back to me once he has found a replacement. I was content with this until curiosity got the better of me and I decided to try and get a part number off the PCB. So I opened up the box that the boiler chap had indicated contained the PCB. See photos below. Is it me because I can't see a PCB?

 

[Ric2013]

Sorry, old chap. I had mistakenly thought you had a fanned Prima model!

You'll find a lot of old manuals on the Plumbase website. Potterton Prima Range - https://www.plumbase.co.uk/potterton-prima-range-10000851-0000 and scroll down.

You can probably help your RGI out by printing him out the manual for the boiler and leaving it to hand. Scroll down on this page and you'll see there are Prima B,C, and F ranges. I think only the F range is fanned and that it is the only one to have a circuit board and I (now) don't think that is what you have.

I don't really have the time to do more than scan through them quickly, but I'm guessing yours has no fan and has a permanent pilot light which you have to light manually and so probably doesn't have an electronic control (circuit board)? The only way to be sure is to find the GC number for your boiler (normally on a kind of aluminium label) as this will tell you the exact model and you can compare that to page 2 on the manuals.

Be aware the front casing forms a part of the combustion air circuit. You're in dubious legal territory (though I can't see the Health and Safety at Work Act being applied to you). If you're going to let curiosity get the better of you, I hope you'll be taking care not to damage the seal and to make sure that casing goes back on correctly and that the rubber seal is in good order. I trust you have a carbon monoxide alarm in the room?

 

[Gasinspect]

sounds like faulty pump overrun thermostat

 

[Ric2013]

sounds like faulty pump overrun thermostat

Would that cause the boiler to fire?

 

[Gasinspect]

Would that cause the boiler to fire?

Ah! good point, sorry missed that symptom. and does not have frost stat, and assume Hive doesnt either. Boilers with frost stats, guy at Potterton technical helpline once just said to disconnect it to stop random firing.

 

[West Country]

Thank you guys and thank you Ric2013 for you concern for my welfare . I'm very careful when ever I do anything with the boiler and I do have a carbon monoxide alarm, if it makes you feel better I'm an aircraft engineer.

So I've had a look at the manual and I can see why the pump overrun thermostat was mentioned it's the only thing with a permanent live. See wiring diagram below. I might be clutching at straws but could the overrun stat breakdown and provide a live back down the brown wire?

 

[Ric2013]

To be fair, you seemed to know what you are doing, but I'd rather state the obvious than have you gas yourself... before we find out what the problem is anyway But then I'm assuming you also have a volt meter then, which means you could connect it to the switched live (you might be able to use the 'test socket' terminals on the boiler) and will make a diagnosis easier.

I'm clutching at straws too, but we're thinking along similar lines. The pump is switched by the boiler and the over-run stat could power it with the system controls not feeding the switched live. But that wouldn't explain it if the gas valve is also opening. Unless somehow we have 1,2, and 3 all connected together somehow.

I suppose at this point I'd want to confirm whether, during this weird erratic behaviour, the pump is receiving power, whether the boiler is firing, and whether the gas valve is receiving power as certain hypotheses could then be ruled out. I find that sometimes in these situations, careful observation reveals it all should have been obvious all along.

 

[West Country]

So I'm away this week so can't do anything with the boiler. Here is what is happening the Hive has lights on it showing when either HW or CH is on, I'm happy that there isn't a spurious signal from the controller as it was the same problem with the old switch master. The problem occurs when the heating is off and no lights on the Hive.

The pump runs when j gets the problem and continues to run for a short period. I assume that is normal behavior for the pump to run on.

The boiler sometimes fires up so I'm assuming the gas valve opens. Sometimes it's momentarily sometimes for 30 seconds or so.

Now I think about it, I think it only does it after the boiler has been running (intentionality). I'm not 100% sure but it would fit with the pump overrun stat.

I was thinking of disconnecting the inputs from the hive and leaving only the permanent live to prove 100% it's an internal fault. I'm also thinking of getting my multimeter out and doing as you suggest. I am tempted to just change the overrun stat, I don't think they are much.

Please carry on stating the obvious

 

[West Country]

I've had someone round and I have the answer. It was the pump overrun stat, it's broken down internally (you can hear it rattle) and was providing a feed to fire the boiler.