Heating kicking in whilst off

[pmcg24]

Hi all,

Long time follower of the forum; first time poster!

I noticed recently that when my central heating is OFF (in other words, turned off at the control module [where you programme days/times for auto start etc.]), that the boiler still kicks into life, when I set temperature on my thermostat. Not just a case of kicking into life in terms of noise; but is actually then providing heat to the radiators.

Have diagnosed that the opposite way is NOT an issue (i.e. when I have the heating programme ON, but disengage the thermostat to minimum temp/Off position).

Worth noting that I recently replaced the Thermostat - but just a like for like replacement (Myson). Haven't tried anything to further diagnose as of yet - open to suggestions.

Happy to provide any more information.
Thanks in advance for the advice!

 

[Deleted member 267285]

Hello and welcome to the forum. Let’s start by asking what type of boiler/system you have? Do you have zonal heating? What type of thermostat?

 

[pmcg24]

Hi CBW,

Boiler is a Potterton Performa System 12e.
Thermostat is Myson MRT1.
Timer-programmer Myson 2MEP 2c.

Hasn't been an issue before with the same combination. As mentioned, recently changed the thermostat (due to discolouration; not faulty) like-for-like. This is the first time I've noticed since installing.

Programmer heating side is OFF. When I switch thermostat to arbitrary temp (20), light comes on, boiler kicks into life and heat to radiators (all over house). No zonal heating - one thermostat for all users.

 

[Deleted member 267285]

Ok, have a look at you’re motorised valve(s) - this will be what’s likely sending w switch live (on signal) to the boiler. If you’re unsure, post a photo.

 

[pmcg24]

Sorry, bit of a rookie with plumbing/heating (hence the post!) - where would I find this? What's likely to have caused this change, from simply changing the thermostat? Faulty wiring?

 

[Deleted member 267285]

Sorry, bit of a rookie with plumbing/heating (hence the post!) - where would I find this? What's likely to have caused this change, from simply changing the thermostat? Faulty wiring?

Could just be a coincidence. It is located usually in the cylinder/airing cupboard.

Sequence is Usually on at the programmer, then providing the thermostat(s) for relevant area are calling these send a switch live to the motorised valve(s), which in turn send a switch live to fire the boiler and pump.

 

[pmcg24]

Could just be a coincidence. It is located usually in the cylinder/airing cupboard.

Sequence is Usually on at the programmer, then providing the thermostat(s) for relevant area are calling these send a switch live to the motorised valve(s), which in turn send a switch live to fire the boiler and pump.

If the sequence of the process is:
Spur live to programmer > switch live programmer (ON) to thermostat > switch live thermostat (ON) to motorised valve > switch live to MV to boiler > boiler on/heating.

The fact that I am able to turn the heating on with the thermostat switched live (programmer off, should break the circuit as per process above) - does this not indicate it is a wiring issue between programmer>thermostat as opposed to an issue thermostat>MV? In other words, I am essentially somehow getting a switched live to the thermostat, despite it not being sent from the programmer (since it is switched off)?

Edit:
As a function test of this, I tried putting the thermostat ON (neon light showing power), then disconnected the programmer from the wall (therefore, no switched live from programmer>thermostat). In doing so, the thermostat neon light switched off and the heating off too... therefore, it isn't as I theorised above.

I still can't get my head around how there is a switched live getting past the programmer when in the OFF position. Would this then not indicate a faulty programmer?

 

[Deleted member 267285]

Could be a fault with the programmer then, if that’s sending a switch live out even though off (sticking relay) - this would need testing with equipment such as a multimeter or 2 pole voltage indicator.

 

[pmcg24]

Yes, I might try replacing the programmer (see my Edit). Will pass on if this gives me any success.