Boiler running despite timer and thermostat being off

[ms1012]

Hi all,

Our newly purchased house (modern construction, 5 years old) has an odd problem. Potterton Performa 28E system boiler, fitted with Myson room thermostat and 2-channel timer, feeding rads and a megaflo hot water tank. Timer controls 2x 2-port valves installed below the megaflo (I understand this is a Honeywell S-plan configuration?)

I have observed, on many occasions, the boiler firing up and running despite the timer being switched off. Even more annoying, with the timer off and the valves closed the radiators aren't getting hot and all generated heat seems to be pretty much wasted.

Is this normal behaviour? It's consuming significant gas, and I can't imagine the cycling is doing the boiler any favours? The system is fitted with a frost thermostat but it's been doing it since the end of summer when temps shouldn't have been an issue.

 

[seco services]

heating 2 port valve is jammed open if you say rads are getting hot.
if the rads wasn't getting hot then i would say only the micro switch is jammed on.

check the lever on the end has it got resistance or is it floppy ?

 

[spraggo]

Hi all,

The system is fitted with a frost thermostat but it's been doing it since the end of summer when temps shouldn't have been an issue.

Have you checked the frost stat setting, it should be set to 5 degrees.

 

[ms1012]

I had the boiler serviced recently and the frost stat setting was changed from -5 to +5C. I believe at the time the valves were also prodded and they looked ok, but I'll double-check.

If a frost stat kicks in, should it also open the CH valve or just make sure that the pipes are hot?

 

[Heating one2one LIVE]

If all of the above prove good,then the only other thing that could send power to the boiler and make it fire is if you have a Honeywell Sundial wiring center?
They can power the boiler(by-passing the zone valves)
This would be the electrical box that all cables from the 2 or 3 port valves wire into.

 

[seco services]

it will open the heating valve but only for a short time as it should be combined with a pipe stat to shut it back down when the return temp gets to about 20c not enough to heat your rads.

 

[spraggo]

I had the boiler serviced recently and the frost stat setting was changed from -5 to +5C. I believe at the time the valves were also prodded and they looked ok, but I'll double-check.

If a frost stat kicks in, should it also open the CH valve or just make sure that the pipes are hot?

That depends on how the frost stat is wired, to check this try turning the frost stat up .until the boiler fires and see what happens

 

[ms1012]

if you have a Honeywell Sundial wiring center?

I was looking for something like that, but haven't found anything like it. There is a control block (a many-way sugar cube block) next to the boiler that the frost thermostat (and a number of other wires) are connected to, with the output going to the boiler.

I'll see if I can get my hands on a multimeter and if I can trace any activity in there. I'll also have a run around and see what the valves do when the boiler is firing of its own.

Silly question I guess but can frost thermostats fail / stick open? I guess in this case it should be outputting a permanent 230v? I'll also try adjusting it and see what happens.

 

[ms1012]

I had a poke about with the multimeter today. External controls voltage is at 0 when the valves are shut. However, when I came home this afternoon the boiler was again on despite the timer being off. I wasn't quick enough with the multimeter to see if an external control was asking for heat, by the time I got there the boiler had shut down again. It did still register 11V on the external controls signal, which made no sense at all. A while later it was back down to 0V.

I looked at the valves, 1x Myson and 1x Honeywell, they both look ok. It looks like the megaflo was added at a later time, going by the mix of valves.

 

[ms1012]

try turning the frost stat up .until the boiler fires and see what happens

Excellent call, I think I have found my gremlin in the system. I can't even begin to think how this is supposed to work.

Tested frost stat (it's labelled Potterton) using multi-meter. Ambient temp around +5C or so. Set the frost stat to -10, it's calling for heat (verified, 220V on output). Set the frost stat to +10, not calling for heat, 110V-ish on the output line (no idea why still reading 110V, note that frost stat wired together with output from DHW/CH valves so maybe false positive?).

I can't begin to understand why the stat is calling for heat when set to -10 and current temp is well above this. It's like it's working in reverse - call for heat when ambient temp > set temp? Is this how this thing is supposed to work?

The boiler (Potterton Performa) has a built-in frost stat anyway, so I've disconnected the external one for now. Hopefully that'll stop the system from running constantly.

As an aside, I don't think the frost stat is wired to the CH valve - the cable from the stat has a dangling brown (or dark grey, hard to tell) wire not connected to anything.

I'm inclined to leave the frost stat disconnected, set the timer to permanent CH on, and get a programmable room stat to control various temps at various times. Could I replace the current room stat (Myson MRT1) with something like a Siemens Rev 24 or Honeywell CM907?