Potterton Prima turning on and off

[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.