Vaillant EcoTec Plus 938 temperamental in heating mode

[skvortsov]

Hello,

we have a Vaillant EcoTec Plus 938 boiler and it started behaving when running in heating mode (hot water is fine - no problems with that).

The problem is that quite often when there is demand for heat (either from one of Hive TRVs or from under floor heating thermostat) boiler starts up but feed water temperature stays below 40 degrees (typically 35-38). Pump seems to be running, burner is on at minimum level. Sporadically it can go up to 60 for a short period of time then back to 40. Sometime however it works fine. What could be the issue? I'm suspecting low water flow rate, but I have no way to check this. Looked at diagnostics menu on the boiler and it doesn't seem to have water flow displayed in there.

Appreciate any help with diagnosing this issue.

Thanks,
Sergey

 

[John.g]

Good to hear you are back in business, so the changes you have made permanent is: pump speed from 100% to Auto and changed partial heating load from Auto to 12kw. The 3 towel rails were always in the picture even with problems?.

 

[skvortsov]

Towel rails were initially not ON when UFH was running. Now the rads zone valve opens together with UFH zone to provide additional bypass. However, when I did this first I was still having that problem with low flow temperature, until I changed pump speed and partial load settings. I have not tried running UFH only without rads zone yet - but will do this later.

 

[John.g]

When you get a chance, it would be interesting to see how the auto pump speed works, ie flow/return temp differential or whatever. you might take the following readings on Rads only, UFH only and Rads+UFH, Flow/return temps d.040/d.041 and Target pump speed/Actual pump speed values d.014/d.015.
the UFH on its own can be a challenge for some boilers as the boiler circ flow might only be a third of the UFH circulating flow but the towel rads by pass should help this if they are circulating with UFH only required.