Vaillant Boiler not starting

[Algernon Goss-Custard]

My Vaillant Ecotec Plus 637 boiler wouldn't start this morning - error message saying something like "Water shortage at pump", though the pressure reading was OK.

[N_Cook]

I would try warming the controller with a hair drier, electrolytic caps that are on the way out are seriously derated in the cold

[John Rumm]

Did you get a fault code (typically F<digits>)?

Can you hear the pump spin up?

It could be the pump not working, or it could be the pressure sensor that is used to detect the abrupt change in pressure when the pump starts is not seeing what it is expecting. (could be pump, or sensor related).

You can take the big silver cap off the front of the pump (have a cloth handy it will weep a bit of water) and check the impeller spins freely when turned with a screwdriver (of pozi/phil typically).

[John J]

I've had pumps shear the impellor shaft on odd occasions. The rotor spins but the impellor doesn't.

[Algernon Goss-Custard]

Yes, I'm pretty sure it was F.74 (Pump/ water shortage). The manual advises me to check the water pressure sensor, pump, heating circuit and bypass. It suggests running the purge check programme.

It did work this morning, though.

That old one! Used to do that on a CH pump thirty years ago, every autumn.

I'll have to work out how to remove the boiler front cover. And wriggle my ageing back into the back of the cupboard.

[John Rumm]

If it is like mine, there is a single screw on the underside, in the middle, that pinches on a slot in the front panel (so you only need loosen it rather than remove it. There are also two metal clips on the bottom of the front panel either side of the screw. You push these up with your thumb to disengage them. The bottom then swings out, and you can lift the cover off the hooks at the top.

(to refit you may need to push them up a bit as you push the bottom of the front panel toward the boiler - it should click on the clip each side)

[Algernon Goss-Custard]

Yes, much the same. It is more that the thing is stuck in the corner behind some airing shelves.

I did a purge cycle. Not sure whether it did any good, there seems to be no way you can test for air in the system, but at least the boiler is running. And it's much milder today. I haven't bled the pump yet. It is running, and the relevant diagnostic says it is going at 100%, but one can't tell whether it really is running at full speed.