heating stuck on in heatwave!

[gregch]

So we’ve woken up this morning thinking it’s really hot (obvs it’s been 30 deg plus outside every day lately) but then noticed that all the rads are all on full blast! The (wireless) thermostat is set to 17 deg and reading 30 deg (indoors at 9am!).

Programmer was on (timed, as normal) so have taken it off timed and set both CH and water to off. This has stopped the boiler running so the rads are starting to cool down now...

No idea what’s going on. The hot water also seems to be much hotter than normal.

The system is an unvented megaflo type cylinder, there’s a pressure gauge in there that was reading a little higher than normal (has gone back to normal now). Both motorised valves are in the same position so I guess (unless they’ve both chosen to conk out at exactly the same time) it’s not a stuck valve problem?

The green light on the thermostat receiver wasn’t on so it didn’t seem to be calling for heat (so thermostat not at fault?) but boiler was running regardless. Boiler shut off when I set the programmer to off, so programmer OK?

For now, I’m leaving it switched off (unlikely to need heating any time soon!) and will try and get someone to look at it Monday, but any ideas what the fault might be?

 

[Crofton Gas]

Its more than likely one of the motorised valves internal switches

 

[gregch]

But it seems weird that they're both currently in the same position.. unless they're sticking rather than stuck and maybe turning it off it freed the sticky one up?? Is it worth turning the CH on and off at the programmer and see what happens to the levers on the motorised valves?? Seems likely it is the motorised valves, since they seem to conk out every year or two... do they all do that or is it worth replacing them with a better type? These are Honeywell ones.

 

[Jajodu]

i used to get through Honeywell motorised valves on a regular basis, last time i also changed the rubber ball valve assy, not had another one go for 7 years now. A plumber did say to me that if you can hear the valve motor moving from a few feet away then it's struggling to move the ball valve. Since I replaced the ball valve it's nearly completely silent.

 

[gregch]

Well I've done a bit of troubleshooting now and I think it's probably the valve sticking intermittently? Is that a thing?

Valves are 1 x Honeywell 2-port (HW), 1 x Honeywell-compatible (CORGI brand?) 2-port (CH). Programmer is a Horstmann which seem pretty common.

When this problem happened (rads hot even though current temp displayed on thermostat much higher than set target temp on thermostat) both CH and HW were ON at the programmer (usual daily timed ON period) and boiler firing. I switched both CH and HW permanently OFF at the programmer and boiler switched off and rads cooled down so all OK there.

Today, I tried switching just the CH permanently ON at the programmer. Boiler didn't fire and rads cold until I notched up the thermostat to above current temp, then boiler kicked in and rads heated up. Notched the thermostat down again and boiler switched off and rads cooled down, so all OK there.

Then (after everything cooled down) I switched the CH to permanently OFF at the programmer and HW permanently ON. Boiler remained off. Ran some hot water for a while, still nothing. Switched OFF and ON again at the programmer and shortly afterwards the boiler fired up to heat the water (possibly I didn't need to switch programmer off and on, maybe just took a while for the hot water to cool down enough to ask for heat). Water heated up (HW valve and pipe to coil hot, CH pipes and valve stayed cool). Rads stayed cold. So again, all OK there.

Finally, switched both CH and HW to permanently ON at the programmer. Again, everything worked as normal; rads stayed cold. So it looks like I can't replicate the fault, unless it was that the CH(?) valve had stuck on but has subsequently freed itself on the next cycle? Does that happen? Should I still change the valve and/or actuator?

We seem to have had one of these motorised valves fail every couple of years so Jajodu your reply is interesting!

 

[flameport]

I think it's probably the valve sticking intermittently? Is that a thing?

It is - however:

Programmer was on (timed, as normal) so have taken it off timed and set both CH and water to off. This has stopped the boiler running

Unless the system has been wired in a very unusual way, that is not what happens with a sticking valve.

With the programmer and thermostat on, power is supplied to the valve motor which opens the valve, when the valve is open a switch inside activates the boiler.
If the valve sticks open, switching the programmer or thermostat or both off will remove power from the valve motor, but not from the switch inside the valve, so the boiler will stay on regardless of programmer and thermostat settings until the valve is closed.

The green light on the thermostat receiver wasn’t on so it didn’t seem to be calling for heat (so thermostat not at fault?) but boiler was running regardless. Boiler shut off when I set the programmer to off, so programmer OK?

Green lights mean nothing - a faulty thermostat could have anything on it yet still be requesting heat.
Does the green light actually mean heat required?
Programmer most likely fine, as switching off does what it's supposed to do.

Most likely would be an intermittent fault with the thermostat.
If you leave the programmer to CH Off and the problem does not occur again, then it's the thermostat.
If the problem occurs even with the programmer off, then there is another problem.

 

[stem]

The (wireless) thermostat is set to 17 deg and reading 30 deg (indoors at 9am!).

You don't mention the make / model of thermostat. The Salus RT500RF for example, is a popular model that leaves the factory with the same RF address code settings. I have seen several occasions where the heating in one home was being controlled by the thermostat in the house next door. Changing the RF address code manually resolves it. This is unlikely to be the case here though, unless your neighbours have their heating on at the moment.

Other wireless thermostats that pair themselves during on site commissioning don't suffer the same problem.

Next time it happens, check to see if the wireless thermostat receiver shows the heating to be switched 'on'. If it does, then the thermostat is at fault, and is telling the motorised valve to open when it shouldn't.

 

[gregch]

Thanks Flameport (very nice clear explanation) and stem - I guess it does look like the thermostat misbehaving. It's a Honeywell DT92E and today I noticed it showing flashing "radio waves" which according to the user guide indicates a loss of RF communication. Now apparently the receiver box (that was, I think in the same box and pre-paired with the wireless stat IIRC) is supposed to show a red light if it loses RF communication but it wasn't.

And later, after the programmer had come on (on its usual afternoon timed session) I noticed the thermostat had stopped flashing radio waves - is it possible that there's no power to the receiver box when the programmer's off, so the wireless stat couldn't see it? And now there is and it can? Or is this another symptom of intermittently faulty thermostat?? Sorry for the endless stream of idiot questions, just hoping to understand how it all works.

Anyway yes Stem will check if (when!) it happens again and I guess that might provide the answer.