Central heating system destroying controllers

jmeads · 5 Nov 2020

I have a Y Plan central heating system that's been work well until recently however the issue that is occurring is the relays in the controller get stuck in the on position. This causes the heating or the hot water to be constantly on regardless of the thermostat or cylinder stat or schedule.

It seems to affect either the HW or CH or both and doesn't happen straight away, everything works fine for a couple of days and then it occurs. This issue has broken a nest heat link and another cheaper controller, currently a Wiser controller is getting a sticky CH contact - with the wiser by clicking the manual override a few times is currently resetting it.

I've confirmed this with the power off using a multi-meter to check the HW / CH contacts.

I've checked and rechecked the wiring and cannot see the problem. The heating wiring is powered from a fused 3 amp feed.

Any ideas what might be causing this issue?

Madrab · 5 Nov 2020

That doesn't make sense, even with the programmer calling for heat, either in CH or HW mode, the stats override the programmer when it comes to sending a SL to the boiler either via the MPV or directly. I'd check the wiring again.

View media item 88408

jmeads · 5 Nov 2020

Sorry I wasn't clear, your right, the cylinder stat only calls for heat when the tank is under temperature, however when this happens on the HW relay, it just means that the hot water ignores any schedule that's been set

For the CH, the room stat is "build in" to the programmer (just like the nest) so CH-on is directly connected to the white lead on the 3 port valve (5 in your pic above) so when the relay sticks CH is always calling for heat.

Madrab · 5 Nov 2020

Ok so no matter what, the programmer is outputting it's SL regardless of it's timings, which you reckon is down to a fused relay in the programmer?

If that's the case I'd take it back to basics, check your supply is ok (polarity etc) get a cheap analogue room stat and basic 2 channel programmer, remove your current programmer/stat and wire it all up again and see if it still gets an issue. If it still happens then it can only be an electrical issue (polarity or miswired) if it's still over cooking the relays and fusing them.

Silly question, it's definitely a 3A fuse? Has any other wiring in the CU etc been changed recently?

jmeads · 5 Nov 2020

Ok, will try that and see if there's still an issue (weirdly it does take time to show up).

I just double checked the fuse and it's definitely 3A and is 100% not polarity issue, the wiring matches your image above, except for CH-on going straight to 5, and the boiler and pump have live wired to 8, there's no SL on the boiler, no wiring changes other than swapping out the failed controllers.

The boiler is fairly new, was replaced about 2 years ago, the oldest items are the pump and the cylinder stat.

Could the pump be pulling too many amps when the radiators start closing down? (maybe just spiking too fast for the 3A fuse to blow?) is it worth replacing this?

oilhead · 5 Nov 2020

What boiler and pump do you have?

jmeads · 5 Nov 2020

The boiler is a Glow worm Easicom 3 25r
The pump is British Gas labelled Grunfos Multihead G

jackthom · 5 Nov 2020

Have you checked the actual voltage at the orange wire of the 3 port valve when the boiler fires with no intended demand from heating or DHW, ie is there the full 230V triggering the boiler or is it some much lower voltage?
It's not unknown for the valve to pass through a voltage sufficient to fire up some boilers, even when there is no real demand.

jmeads · 5 Nov 2020

ok, I think I have found the issue Glow worm Easicom 3 25r is wired up directly to the controller, from the manual https://www.glow-worm.co.uk/glow-wo...installation-manual-0020238431-01-1411353.pdf

Page 15
Conditions: 230 V 3-wire connection ▶ Ensure that the controller is designed for a maximum rated current of 7 A. ▶ Connect the controller to the main plug X1.

So the controllers are not rated at 7A !!!

Looks like a need an eBus controller?

picasso · 5 Nov 2020

Grundfos pumps were causing programmers to fail a while back , I would change the pump before any thing else.

Madrab · 5 Nov 2020

Took the words right out of my mouth, I'm just reading the Wiser manual and it has a max switch rating of 2.5A, whereas the 230V connection on the boiler needs a 7A switch rating on the controller according to the boiler MI.

How that works though I'm not sure given the input power supply must be @ 3A but if the current was higher then it could cause the relay's to fuse

picasso · 5 Nov 2020

Have s look at inrush current to see why your programmers are welding relay contacts.

jackthom · 5 Nov 2020

That's a bizarre current switching requirement for a room thermostat connection. Maybe a more hefty mains voltage operated relay would be a good idea to buffer the controls into the boiler.

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Central heating system destroying controllers

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