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

 

[skvortsov]

Took Hive TRVs off 2 large column rads + 3 towel rails without valves. Still the same problem...

 

[king of pipes]

The 938 is a huge output boiler have you worked out your heating load ? It maybe that it needs to be down rated, Valliants don't like being restricted it's trying to produce heat and the hive trvs and ufh controls are holding it back ? is there an auto bypass fitted a good way from the boiler ? I would advise having one fitted before the ufh Zone valve ? you need a flow rate though the boiler for pump over run this can't be restricted a good way of doing this is with a low loss header, try down rating it to 20 kw on the heating , can you post a picture of your install ? . Kop

 

[John.g]

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?

There is no mention of d.029 in the 418 manual so more than likely not available in your model.

Did you check d.017?
Check Target flow temperature d.005 and note d.071 setting.
Also check flow and return temperatures, d.040 & d.041

Also ensure that the boiler is actually running at low power and not firing up and then burner cut out with pump overrun and anti cycle time, the anti cycle time is shown in d.002 and factory setting is 20 minutes but the actual anticycle time is also dependent on the target temperature so if the target flow temp is 60C then the anticycle time is 6 minutes, you can read it (setting) off the table.

As KOP pointed out you can also de rate the boiler output (range rating) (in CH mode) so see what you come up with, with above readings first., just take d.000 off auto and insert your own rating maybe 10kw to start with??. it will not affect the DHW performance of 38kw.

 

[skvortsov]

D.017 is set to 0: Flow control. Target flow is set to 61. I changed partial load from Auto to 9KW for now and will monitor it.

There is no external bypass valve installed. I thought there is an internal one. Is it not good enough?

 

[ShaunCorbs]

The internal one isn’t rated for external loads it’s only for the boiler when it’s doing it’s thing

 

[skvortsov]

D.071 is set to 75

 

[skvortsov]

The internal one isn’t rated for external loads it’s only for the boiler when it’s doing it’s thing

OK, looks like need to talk to our plumber (if I can find him - it was installed 2 years ago)

 

[John.g]

You can set a towel rad to full open as long as it gives bypass when any zone valves close.

You can also calculate the flow rate, set the output to say 7 kw (as low as you can without burner cut out) with steady conditions then read off d.040&d.041. your flowrate LPM = 7*860/60/(d.040-d.041).

 

[EvilDrPorkChop]

OK, looks like need to talk to our plumber (if I can find him - it was installed 2 years ago)

Internal bypass is fine, you won't need an external one. That's what it's there for.

I'd set your pump to 100% on d.014 and it maybe that you bypass wants adjusting internally on the boiler, which your plumber will need to do.

And put Partial load back to Auto.

 

[king of pipes]

Mr Evil is correct in his assumption the manufactures instructions states another auto bypass is not needed and the internal one is adjustable.
But although the boiler can handle the flow rate the system in my opinion needs One, there could be a lot of restriction on that system when rads or the ufh circuits begin to shut down it just makes it quieter and less harsh on the boiler and it's internal components, I've experienced this to a point where you have the same temperature going out as you have coming back the boiler gets confused starts cycling on and off that's no good for efficiency the boiler needs setting to its required kw output, flow rate, and cycling times in the installer menu this really isn't a DIY job has the temperature across the system flow and return been measured and recorded in the benchmark? every job requires commissioning and time spent fine tuning this is just my experience and some on here will agree others may not we're all have our ways of working . Regards Kop