Potterton Prima turning on and off

[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?