Three wire to two wire thermostat

[BackAche]

Hi,

I upgraded the room stat on our old fashioned boiler from a mechanical three-wire thermostat to a two wire digital thermostat.

I simply didn't use neutral, and for the most part it works but since the upgrade the radiators are not coming on unless the hot water is on.

So far I assumed it was a coincidence and have swapped out the three way valve and actuator but we are still having the issue and I can't help wondering if it could be caused by me dropping the neutral, what do we think?

 

[denso13]

Which stat is it? Was the old one correctly wired? The two wire stat is simply live in, switched live out and it pretty simple to connect. A multimeter would confirm whether the wires L and SW are indeed what you think they are.