Boiler problem

[Bolted]

Vokera Linea Plus combi, fires up, CH gets up to mid-70s temp if set to max (never to the 78/82 which I seem to remember being its max). The pump seems to be working. Flow pipe gets hot. The boiler then modulates down briefly after hitting 74/75 and then shuts off pretty much straight away. Waits 3 minutes aby which temp reading is down in the 30s/40s, fires (after the 3 min lock times out), gets back up to

70s, modulates down and shuts off again. Repeat ad infinitum. Result cold house. Add cold grumpy wife into the equation. Help!

[Simon Stroud]

Is this pump really working properly? I had a similar problem on my (Eco Hometec branded) MAN Micromat a while ago where it kept cutting out with a "flow/return limit" error message. The pump appeared to be running but on my boiler it is a fully variable modulating pump. It was only running at about its slowest speed and never going up to full speed to circulate that super-hot water out of the boiler. Result was the flow temperature shooting up very quickly, followed by the burner (also infinitely variable) modulating down, and the flow temperature exceeding the limit fairly quickly causing a shutdown.

The 240V supply to my pump is turned on or off by a relay on the controller board, but the speed is varied by a PWM signal on a third wire. I could see the PWM signal varying but the pump was staying at around its slowest speed. Fortunately they still supply these and it has been running fine since it was replaced.

On my boiler in the setup menu you can adjust the max pump speed. While adjusting it it rather handily keeps the pump running and you can check whether the pump is actually responding to the variable speed signal. Maybe your boiler has a similar feature?

HTH.

Regards, Simon.

[Bolted]

Thanks for the response.

It's one of my main suspects - I took it off and it looked fine, and ran fine, and quietly. I didn't test its torque at all, maybe I should have. It makes a bit of a clicky rattly racket when installed though, and there isn't a great deal of sign of flow noise in the rad tails (latest test inspired by your post).

Hmm.

Nothing so fancy as your boiler, so it's either broken or not.

Another oddity I found by accident is that the boiler (water) pressure switch seems to have gone. It was happy to try and run with 0 bar in the system. I'll replace it obviously, but I'm struggling to see how that would cause the CH-only problems.