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?

  • \$\begingroup\$ Welcome! "turned off and back on again the main control PCB often will not power up" is the very definition of a bad design. \$\endgroup\$
    – winny
    Commented Feb 21, 2023 at 12:44
  • \$\begingroup\$ All nonsenseless, starting from "heating engineer". \$\endgroup\$
    – user263983
    Commented Feb 21, 2023 at 13:47
  • \$\begingroup\$ I doubt it's the relays which is the culprit for not starting. Best guess without schematic and photo is out of spec capacitor, probably electrolytic, or crack in the PCB which bridges again when heated. \$\endgroup\$
    – winny
    Commented Feb 21, 2023 at 14:08
  • \$\begingroup\$ @colintd that is a very plausible explanation, are modern mini relays greased? \$\endgroup\$ Commented Feb 21, 2023 at 14:25
  • \$\begingroup\$ Many aren't for exactly this reasons, and are either dry or use a light oil. However, you will still see grease on larger power relays. \$\endgroup\$
    – colintd
    Commented Feb 21, 2023 at 15:38

1 Answer 1


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.

  • 1
    \$\begingroup\$ And this is why electrical engineers should know physics and other branches of engineering. I wonder if this question will be marked as off topic and closed, now that it has been shown it has not.to do with basic circuit theory... \$\endgroup\$ Commented Feb 21, 2023 at 15:59
  • 1
    \$\begingroup\$ This is why theory is great but needs experience. I would teach that a screwdriver tap is better to find the bad stuck relay for planned replacement. \$\endgroup\$
    – Hoagie
    Commented Feb 21, 2023 at 16:39
  • \$\begingroup\$ Absolutely agree that tapping stuck relays is invaluable, as is "Percussive maintenance" more generally. Only reason I mentioned hairdryer was the question content. \$\endgroup\$
    – colintd
    Commented Feb 21, 2023 at 16:50
  • \$\begingroup\$ On a similar subject we had a Compaq server in the mid-90s where they used a new grease for the spindle motor in the hard-drive. Great for the first year, but after that the only way to start after power-off, was to lift the whole unit, and give it a quick rotary jerk to the right. We also had a Series 1 mini, with massive hard disks which you needed to power up one at a time, else you tripped the main breaker... \$\endgroup\$
    – colintd
    Commented Feb 21, 2023 at 16:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.