Need help with rather old gas CH system no starting up

[cl]

We have a CH system which has a Potterton "Netaheat" boiler running on bottled gas (big tank in the garden). It's a fully pumped system with a Honeywell ST7100 controller.

It only gets turned on occasionally, when we have visitors and, as now, when it's particularly cold.

However, this time, it's not starting up properly. The boiler fires up UK, gas lights etc. but after a while (I presume) the water overheats and the 'overheat thermostat' trips and turns everything off.

I *think* the problem may be that the pump isn't running, so, as it's a pumped system, it's not surprising that the above sequence occurs.

I did move some of it's wiring (the external wiring away from the boiler by the ST7100 controller) a while ago and I'm wondering if I may have mis-wired something. I did check quite carefully when I disconnected and reconnected but I *might* have got something wrong. I have wiring diagrams for the ST7100 and the boiler so I can ultimately check from first principles.

However I'd like a sanity check on how things are supposed to work. Am I right in thinking that the pump is powered via what the boiler wiring diagram calls the "Hot Water Thermostat" such that the pump will only run when the water has reached a certain temperature and will continue to run until it has cooled down. That's how it would appear to be (from the diagram) but that's depending on some assumptions about how a "Switch" works etc.

[Andrew Gabriel]

Pump rotor may have stuck if not used for a while. There's usually a giant bleed screw on the rear of the pump which gives you access to the rotor. I would remove this and see if the rotor will turn, and if it is turning when the pump should be on. Some water (usually with black dirt in it as it's also the bearing lubricant) will come out whilst the bleed screw is removed - be careful it doesn't run into the electrical connections, or on to anything where any resulting staining will be a problem.

[cl]

I've been there and done that, it rotates freely.

I don't think there's power getting to it, but I want to check when I should see power applied (hence the question).

[Andy Burns]

Run the pump temporarily from a 13A plug, and see if the boiler runs without cutting out?

[Roger Mills]

Unless it's an unusual system, I would expect the pump to run whenever the boiler demand is turned on by external timers and stats *and* (if the boiler has pump over-run) to continue running for a while after the boiler stops, to prevent the boiler from over-heating due to residual heat.

If you ever find that the boiler is running but the pump is *not*, you have a problem!

[cl]

I was thinking about doing that, I'll try it tomorrow.

[cl]

Thanks Roger, what you say is what I *thought* should be true but I couldn't make out from the wiring diagrams. Certainly at the moment the pump doesn't run (and there's no power to it) when the heating is turned on with the controller.

I'll do some more in depth checks of the wiring I moved and also try and work out how the pump should be powered on.

It did seem to me that "pump over-run" should simply keep the pump running for a while after the controller switches everything off.

[cl]

Well it seems it was just down to a dirty connection or a sticking switch somewhere because after opening up, waggling connectors and twiddling thermostats it has now come back to life! :-)

I *thought* I'd been pretty careful to get the connections right when I moved the wiring!

[Roger Chapman]

I have recently junked a Potterton Profile boiler of 1991 vintage that was running perfectly well apart of an increasing tendency to blow the overheat thermostat. Like you I thought I might have cocked the wiring up as I modified the control system multiple times after I installed it. Whether or not I had it wrong at some time or other I do not know but I am sure I laterly had the pump over-run connected properly.

I eventually ended up with 3 possible causes which, to complicate the matter further were not mutually exclusive, These were:

1. The automatic bypass valve was stuck so the bypass circuit did not operate at all.
2. The DHW circuit had so little resistance (the gate valve that was supposed to vary this was immovable by this time) that there wasn't a big enough temperature drop to cool the return temperature down far enough to avoid the overheat stat operating as intended.
3. The overheat stat was tired and functioned at a lower temperature than it should have.

Also on the list at one time was the function of the boiler thermostat itself but since I replaced that I discounted that being faulty.

[Andrew Gabriel]

You don't still have it, do you? I look after one in the family, and a spare would be useful for parts.

A Potterton Suprema which I also look after has a tendancy to form an air pocket in the pipework where the pump is, for first 4 hours or so after refilling, resulting in no flow. That generates similar symptoms. I've bought an auto-air vent bottle valve to fit in the pipework next time I have to drain it down for any reason.

[Roger Chapman]

Some of it but I didn't treat it very kindly as I took it out. Is your reply address valid?

[Andrew Gabriel]

Yes. It didn't used to be, hence the sig, but it now gets well spam filtered.

[Roger Chapman]

I e-mailed earlier. If received please confirm.