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I have just fitted an digital room thermostat to replace an older identical model that was becoming unreliable. The heating was not always coming on in the morning or while we are away from home. The new thermostat works well most of the time but is still unrealiable in that sometimes the heating still fails to come on in the morning and occaisionally during the daytime even though the thermostat shows that it is calling for heat. Also it sometimes switches off the heating before the room is up to temperature.

The boiler shows no fault codes and does not respond to the resetting button when the heating fails to come on. The only way to get the heating to work when this occurs is to temporarily reset the room thermostat to a higher setting.

The thermostat is a two wire Sunvic Digital TLX 7501 model

Any suggestions as to what is wrong would be appreciated.

This is a bit obvious, but did you switch off all power to fit the new thermostat? Have you reset all the timings on the boiler timeswitch? If the boiler is "OFF", the new thermostst will have no effect,

Grandad

Thank you for the reply 'grandad '

Good point yes I did switch off the supply in order to fit the new thermostat and I swithced it on again. I have just checked the clock and it seems to have corrected the time setting outomatically.

The morning after posting my request for help it happended again- the boiler clicked on at about 6.30am took the temperature up to about 16 degrees C and then swithed off although the thermostat was set for 17degrees C. After resetting the thermostat to 18 degrees C temporarily then back down to 17 degrees C it kicked in again and brought the temperature up as required.

I went out and bought a replacement simle mechanical type thermostat thinking that maybe the system did not like digital devices. However I have not fitted it yet as the boiler has come on for the past 2 mornings and cut out as set at 17 degrees C and continuing to work perfectly well during the last two days. Fingers crossed.

To have exactly the same problem with 2 different programmers is unusual or unlucky.

Could the fault be with the boiler ?

When the thermostat is calling for heat, is the demand light on the boiler lit ?

You say the thermostat has two wires ? the Sunvic TLX 7501 is a wireless room stat. Therefore if you have changed the room stat have you changed the receiver unit ?

htg

Hello Htg Engineer,
Thank you for your reply to my querie. The boiler is on standby until the thermostat calls for heat, then it fires up. The problem is that it occasionally goes back onto standby before the room temperature as selected on the thermostat has been reached and then stays dormant until the thermostat is reset. When we are away from home this can mean we come back to a cold house.

The problem could be the boiler as you say but I doubt it as it does not respond to the reset button as I would expect it to if the problem lay here.

I think it is definitely something to do with the signal it is getting from the thermostat. See my response to 'grandad' re -my next option to try a simle mechanical thermostat to see if the problem is cured. Also see my response to 'grandad' re it has worked without fault now for two days so I am leaving things alone until it does a repeat perfomance.

I have looked at the instructions that came with the new digital thermostat and it says nothing about being a wireless thermostat. It refers to the need to make access in the wall plate for the cable and to connect up as per wiring diagram.

Link the room stat out - if all works well it proves stat was at fault, still having problems - boiler at fault.


htg

Many Thanks Htg Engineer,

I will try that the next time it fails to repond to the call for heat like it did again this morning after two trouble free days-ie fired up and brought the temperature up to 15 degrees C then shut down, although the thermostat was set to 17 degrees C and still calling for heat. It fired up again only when I reset the thermostat to 14 degrees C which effectively cancelled the call for heat and then reset it at 17 degrees C. It has worked fine since then.

By 'link the room stat out' I take it you mean link the two wire terminals to the thermostat (live and neutral) to issolate it and create a call for heat situation.

Room stat's don't directly control the boiler on all heating systems. In a "Y" plan, the boiler is switched on by the mid position valve. You mention the type of new stat, but what type of boiler and heating system do you have?

Hi plumbbob,

The boiler is an Ideal 'icos system' condensing boiler m3080. The heating system is a fully pumped system with a hot water cylinder.

Unfortunately things have deteriorated since my last posting and this morning the boiler fired up at 7.00am but went off at about 7.15 am and would not fire up again with the existing thermostat, with the existing thermostat linked out or with a new mechanical replacement thermostat fitted. However after about an hour it it did fire up for about 15 minutes and went off again. It is almost as if it is overheating and refuses to fire up again until it has cooled off.

I think the time has come to call in an engineer to sort the problem out. But I dont expect I will get anyone out until next week now. Brrrrr

Many thanks for your interest

Well all is in working order now with the boiler responding to calls for heat from the room thermostat and the radiators getting hotter than they have for a long time. How come? Well the engineer called and diagnosed sludge in the system and recommended me to inject a litre of 'sentinel x 300'. It did the tric in about an hour and things are improving all the time. I will have to flush and drain the system and refill adding an inhbitor to complete the maintenance. Thanks to all who tried to help me.