Boiler not igniting when thermostat bypassed.

[Benny_the_ball-valve]

A central heating system has stopped working following a new thermostat being replaced. This is a general system boiler, vented with pumped CH and gravity HW. The boiler ignites properly for the HW, but the pump does not respond despite the thermostat being on. I have checked the stat with a MM and can confirm that the ‘common’ terminal is live, as is the ‘demand’ terminal (when switched on).

As an interim measure to ensure that the pump had not failed, I bypassed the thermostat and connected the pump directly to the programmer’s switched live supply. This, of course, caused the pump to operate, but what is really confusing is that the boiler then no longer ignites. Would anyone have any clues as to what the problem might be? Thanks in advance for any help.

 

[Agile]

What I always say in these cases is that you should not have taken on a job if you dont have the necessary skills to deal with any complications which may arise!

Doing that gives our profession a bad name and does not help you keep a good reputation.

Obviously the wiring needs to be checked against the standards for the system.

Tony

 

[jackthom]

A bit more information about the old and the replacement thermostat and the boiler might give a clue.

 

[hobhead]

I would not expect the pump to run for HW on a gravity system!