I have been told by a central heating engineer that when a boiler has been turned off and back on again the main control PCB often will not power up. He also said that heating the relays with a hairdryer can make the PCB work again. Is this possible and if so what is the hot air doing to make the PCB / relays spring back to life?
I've seen this with a relay where the grease on the pivots has dried out because it was in a hot boiler cupboard.
When the boiler was on, the cupboard (and hence the grease) was fluid enough that the relay would operate, so the cupboard stayed warm.
If the boiler had been off for a prolonged period, the whole cupboard cooled, the grease became inflexible, and the relay would not operate. As a result of not closing, the boiler would not operate, the grease stayed cold, and you were stuck.
Heating the relay with a hair dryer, would soften the grease, allow the relay to close, get the boiler going again, and things would then be okay until the next long outage period.
However, at best a short term hack for a very limited set of conditions, and the relay needs to be replaced.