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

 

[Deleted member 120897]

Is the thermostat wired up as simple on/off 240vac?

 

[skvortsov]

Is the thermostat wired up as simple on/off 240vac?

Yes. There are two zone valves - one for the radiators (controlled by Hive) and another one for UFH. The issue with low feed temperature is present in any combination (rads only, UFH only, both ON)

 

[Deleted member 120897]

When was it last serviced? Might be an issue with the gas valve, flow rate, pump to name a few.

 

[John.g]

Unlikely but possibly set up for return temperature control?, d.017 set to 1 instead of 0, flow control.

 

[skvortsov]

It was serviced 2 weeks ago and no issues reported by the engineer.

Will check D.017.

Any other ideas how I can check the flow rate through the boiler?

 

[John.g]

Its d.036 (page 52 of the manual).

Sorry, that's the DHW flow rate but d.029 may give it in M3/hr so X this by 16.67 to give you LPM.

 

[skvortsov]

Its d.036 (page 52 of the manual).

Sorry, that's the DHW flow rate but d.029 may give it in M3/hr so X this by 16.67 to give you LPM.

Thanks, will check it

 

[skvortsov]

Well, there is no d.029. After d.028 it goes straight to d.033. Why?

Could it be that the diverter valve gets stuck in DHW mode?

 

[ShaunCorbs]

Remove the hive trv heads and see if it will go into high