There are quite a few failsafes in modern boilers, and without knowing the details it's difficult to advise (and arguably not legal, either!).
Do you have the installation/service manual for the thing? Stuff I would check:
0. Fuses: if ANY fuses have blown, anywhere in the system, there must be a reason. They are not themselves a 'fault' they're there to protect you and to protect the electrics from further damage. If you cannot find the reason and fix that, DO NOT simply replace the fuse - you can cause more damage. Get qualified help, i.e. someone who knows that make/model of boiler.
1. system water pressure: There should be a gauge. It needs to be consistently over about 1.5bar. If it won't hold pressure you have a leak somewhere, which you need to find and fix. Depending on the boiler design, it may well 'go through the motions' and not ignite.
2. flow and/or thermostats: you can check room/tank thermostats with a meter (disconnect power first, as older ones run at 240V). Undo one of the two signal/power terminals, and check across it that it switches open/closed as you turn the control. If it's too cold to do that, at least it should be closed (short-circuit across the thermostat).
3. airlocks: If it's been filled in the past few months or you had work done on the heating, there may be an airlock inside the boiler. There are usually Shraeder valves (tyre valves) and/or air traps in various places, in mine that's the highest point in the boiler and the pump. check that you get water from these and not air - if air, purge it, and it may then light and run properly. If you use a matchstick to push the pin, don't let a splinter catch in it (DAMHIK!). While you're there look for any signs of leaks in the boiler pipework. If there are any -- crusting around fittings, pooling in the case (usually evaporates leaving tidemarks), etc. the boiler has problems. Leaks may not be the cause of failure, but are signs it needs a service soon.
4. fuses: there can be several on the boiler's control board. remove power; check with meter. If gone, they're usually 20mm glass (current rating depends on design), BUT there will be a reason for the fuse blowing, which you need to find and fix. If a fuse has gone, I'd get a plumber in straightaway unless you have found the fault. Did I mention getting help at this point?
5. there may be some sort of airflow meter too: in most condensing boiler designs, unmonitored failure of the fan would be highly dangerous, so some of them check for air movement. Again you need the service manual really to find it and check it, as it may be a pulse generator of some sort (we have something similar in our dishwasher!).
6. there are over-temp thermocouples usually, too. These should be 'fail to open-circuit'. Usually found in the pipe leaving the heat exchanger. Remove one connector gently (if that's possible, or remove entire connector) and check across it for continuity.
7. the actual ignition circuit. Most modern boilers are electronic spark ignition, as it saves the gas used by a pilot light. These circuits are similar to a car ignition - low voltage high current switched into a transformer to make a spark. They have similar problems: the HT lead to the plug gets dirty and the HT tracks to the metalwork, instead of making a spark; the LT side fails for some reason; the transformer dies. You can't really test the spark in the same way as you would on a car - it's dangerous - but you can check it's all clean, and that the lead and the plug cap fit properly (check the lead going into the coil, too). In all cases do this with the power off! If there is a viewport into the combustion chamber, try to fire up the boiler in the dark and look for a spark.
8. If all the above look OK, it could well be a failure on the control board. I'd have a careful look over the board, especially check for scorching around high power resistors and/or mechanical failure of soldered joints. Modern lead-free solder is brittle and at least partly responsible for most consumer electronics failures nowadays. Any part of the board that heats/cools regularly in use is subject to mechanical stress and the joints can/do fail. Cleaning off the joint and re-making it with leaded solder will make a repair better than new. Check carefully around where any off-board wires connect into it - connectors are a common source of trouble. A magnifying glass and strong light will be helpful, as fractures in soldered joints can be hard to spot. Aged, modern, lead-free, soldered joints look 'dry' after a while, too, so that in itself isn't necessarily a failure.
All of the above voids any warranties, is probably illegal if you're not gas certified somehow, will upset some of the more bolshy plumbers, and is the kind of bl**dy obvious fault-finding that children should be taught in school (IMHO).
Oh, and if your two-year-old boiler fails just out of warranty, document everything, write to the manufacturer and basically, give 'em hell until you get your costs back.
Unless we start making their quality standards commercially unacceptable, they'll keep on selling us the sort of carp they presently do. If the old designs, that cost less in energy and materials to make, managed thirty+ years with simple maintenance, how can any of the present designs be honestly described as 'energy efficient'?
Don't get me started on the design of heating zone valves... :twisted:
E.
