Vaillant EcoTec Plus 938 temperamental in heating mode

[skvortsov]

It's still a strange one though because if the diverter valve was working originally its hard to understand why not enough water flowed even if the bypass was fully closed, the diverter valve should have directed the primary water to the zone(s) valves which presumably opened at the same time and even one rad/towelrad should provide the minimum flow necessary.

Well, I think I spoke too soon. It's doing the same thing again... Adding more turns on the internal bypass did not help - put it back to the original position now. Does this boiler not have a flow rate sensor for the heating circuit? I'm sure the flow rate is good, because I have 3 towel rails all fully open and they heat up very quickly when the boiler outputs 60C.

So the situation now is that it would sit at minimum flame for a while with return temperature about 38 and flow about 42... If I turn the hot water it heats it up perfectly fine - flow goes above 60 immediately... Then after DHW pump overrun it goes back to heating mode and temperature drop to 40 again. What else could be wrong?

 

[John.g]

In heating mode your boiler has a minimum output of 6kw so assuming that this is correct then a deltaT of 4C (flow/return) = a flowrate of > 20 LPM so no problem there it would seem.

Not sure what you mean by DHW pump overrun, if its a combi?? then the primary circ pump (the only one?)
may go in to overrun when finished with DHW and depending on the pump overrun time could reduce the temperature from 60C to 40C but should then fire up on CH/UFH and very quickly go to 60c again. Are you sure that there is nothing (remotely) setting the boiler flow temp to a very low value when CH/UFH requested?.

 

[skvortsov]

DHW pump overrun is what it shows in the Status menu after I turn the hot water off. After it's done with overrun and back to heating mode - flow temperature doesn't want to go to 60 and stays about 40.
Both zone vales are just simple 240V, no fancy smart controls connected to the boiler itself. So I'm pretty sure there is nothing setting Flow temp to low value

 

[skvortsov]

Just used your formula to calculate the flow rate assuming it is firing at min 6kW. If I have 4 degrees flow/return difference it gives me 21.5 l/min. Is this reasonable flow for 3 towel rails?

 

[John.g]

Yes, probably, especially with the pump on 100% output. I can't see you having a flow problem.

 

[John.g]

Can you also check d.001, d.002 & d.018

 

[skvortsov]

Can you also check d.001, d.002 & d.018

D.001 - 5 min
D.002 - 20 min
D.018 - Comfort

 

[John.g]

all default settings so OK.

May have asked before but presume you are sure that boiler stays on low firing after CH request because one of the Vaillant's big problems was/is that it fires up at ~ 75% output for up to 1 minute before modulating and sometimes exceeds the SP and cuts out and goes to overrun and anti-cycling before firing up again.

 

[king of pipes]

Have you down rated your boiler as advised by myself and John g in earlier posts ?? 3 towel rails may only be 1 kw a piece your system flow and return temperatures are to close to each other ideally you need between 10° - 20° differential across your system, at present you have 4° you can go into the diagnostic menu of your boiler and find out your flow rates , flow and return temperatures, pump speed. In my opinion the boiler boiler output is set to high ? reduce to 10 kw with a 60° c flow temperature and monitor can you open up the ufh zones or more rads ? What is the system rated at in kw ufh + rads + towel rails ? Can you post pictures of your install ?

 

[skvortsov]

Sorry, I was a bit busy with other things.

It seems to work fine now. What I did compared to where I started:
1) set pump speed to Auto (was 100%)
2) set partial heating load to 12kW (was Auto)
3) returned internal bypass to it's original setting (mid-point)

Now my towel rails heat up as expected and I have not experienced boiler running at min flame with flow temperature below 40 over the last few days.

Thanks everyone for your help with this.

 

[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.