Boiler boiling, tank not heating

zagmund

We have a Vitodens 100-W condensing gas boiler which has worked fine for the last 18 months or so. Today it quit working.

The first thing I noticed was that the pressure in the system was showing as something like 0.2 bar, so I asked Google what to do and it recommended increasing this to about 1.5 bar. I did this but still no joy.

The boiler feeds the hot water (normally) to a 500l tank. This feeds the under floor heating & radiators directly. The DHW is also supplied from this tank indirectly via a coil.

The return to the boiler is mixed with the flow via a mechanical valve which opens/closes to bring the return up to a suitable temperature.

When I power on the boiler I can hear it going through various checks. I've verified that there's hot exhaust coming out the back, so I know it's firing, but then it just does nothing. It doesn't seem to realise that the tank is cold and needs heating.

The boiler does briefly heat water and I can feel that the flow pipe is hot, as is the return (possibly via the mixing valve). But the tank remains cold. It's almost like there's an airlock somewhere in the system stopping the hot water flowing from the boiler, but I don't know how/where that might happen. I've checked the radiators at the top of the system and they had no airlock. The thermostat that controls the boiler is about half way up the tank. It's normally set to something like 50 or 55 degrees, but right now the water there is maybe 25 degrees, so it should be calling the boiler.

I'm going to wait for everything to cool down overnight and then fire it all up again in the morning with a view to seeing how far the heat gets from the boiler. Does anyone have any ideas as to what sort of blockages/issues might exist between the boiler and the tank?

z

scudo2

Check to see if theres a manual air vent on the coil of the cylinder, like a vent on top of radiator, it might need venting with the boiler off at the time.

Do the rad's work?

zagmund

Thanks - I had a look and there's what looks like a manual vent on top of the tank, but when I opened the valve nothing happened, not even a tiny bit of a hiss or anything to indicate anything had opened. Having let the system cool down overnight, I fired it up this morning and the boiler is now heating water and passing it to the tank. Water is flowing to the radiators and to the underfloor heating.

The weird thing is that the pressure was reading something like 0.7 bar this morning, but I know I had brought that up to 1.5 last night. I thought I might have a leak as a result, but now (with hot water circulating) the pressure is back up about 2 bar. Assume that the manual says it's supposed to be around 1.5 bar, do I measure that when cold or when hot?

Next up - Christmas lights. Fingers crossed.

z

DGOBS

Measure when cold, at 1-1.5 bar (0.5 per story)
Shouldn't rise hugely (as in approaching 3 bar when hot)