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

 

[John.g]

This boiler even with a big call for CH Iwhere the bypass would/should be shut in any case if installed only occasionally gets up to 60C so maybe some other problem. One of the main reasons for a external bypass is to dissipate some heat with boiler shutdown and pump overrun, another is to keep the flow/return < Say 30C with very low circulation flows. Some boilers, probably with SS HXs don't require them. ABVs don,t work with modulating circ pumps IN PP mode and need very careful adjustment with pump in CP mode hence the recommendation above for a manual bypass.

 

[skvortsov]

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.

Thanks a lot! Looks like this solved the issue - added 1 full turn on the internal bypass, set output to Auto and now it goes to 60 degrees flow straight away! And now it's wired in a way that radiators zone valves open when UFH is ON to make sure that there is some flow through towel rads (we have 3 of them). So I believe it will be ok without additional bypass

 

[EvilDrPorkChop]

Thanks a lot! Looks like this solved the issue - added 1 full turn on the internal bypass, set output to Auto and now it goes to 60 degrees flow straight away! And now it's wired in a way that radiators zone valves open when UFH is ON to make sure that there is some flow through towel rads (we have 3 of them). So I believe it will be ok without additional bypass

No problem

 

[John.g]

Thanks a lot! Looks like this solved the issue - added 1 full turn on the internal bypass, set output to Auto and now it goes to 60 degrees flow straight away! And now it's wired in a way that radiators zone valves open when UFH is ON to make sure that there is some flow through towel rads (we have 3 of them). So I believe it will be ok without additional bypass

That's great to hear, you suspected the flow rate originally,
It would be interesting to see if you now can on UFH only or rads only

Did you also increase the pump speed to 100%?,
Also if you get a chance you might post the flow and return temps.

Thanks.

 

[john G]

That's great to hear, you suspected the flow rate originally,
It would be interesting to see if you now can on UFH only or rads only

Did you also increase the pump speed to 100%?,
Also if you get a chance you might post the flow and return temps.

Thanks.

Can't log in on this PC for some reason or other.

 

[John.g]

Why do I have to reset my password each time I log out since this morning?.

 

[king of pipes]

Why do I have to reset my password each time I log out since this morning?.

One for Shaun John ????

 

[ShaunCorbs]

Please define reset ? You have to change it every time you try and log on ?

 

[John.g]

If I log off or just exit the forum without logging off and go back in it asks me to log on, it then displays my user name and saved(correct) password but when i select log in nothing happens so I then select "forgot password" it then directs me to my given email and asks me for new password + confirmation, I then enter my (saved )+confirmed) password and press save and I am then logged in, the same happens in the central heating forum but not in the Electrical forum (different user name).

 

[skvortsov]

That's great to hear, you suspected the flow rate originally,
It would be interesting to see if you now can on UFH only or rads only

Did you also increase the pump speed to 100%?,
Also if you get a chance you might post the flow and return temps.

Thanks.

Pump was always at 100%. Will post some charts on temps later

 

[Deleted member 120897]

One for Shaun John ????

Yep or Dan/Lou?

 

[John.g]

Pump was always at 100%. Will post some charts on temps later

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.

 

[John.g]

Yep or Dan/Lou?

I actuall registerd under a new name and Lou welcomed me on board so I replied explaining my problem.

 

[Dan]

Why do I have to reset my password each time I log out since this morning?.

Hi John,

We're toying around with a new advertising partner and that might have something to do with it.

Although it does appear as though your computer isn't remembering your logins, and you're simply resetting your password to login, rather than using the last password you chose?

(Don't ever write passwords in here, I'm sure you wouldn't have done already but I thought I'd let you know and say this)

If it happens again could you please try to clear the browser cookies, close the browser down, then reopen it, login, and check the box to remember you each time you visit (only providing you're using a computer only you have access to.

 

[John.g]

Hi John,

We're toying around with a new advertising partner and that might have something to do with it.

Although it does appear as though your computer isn't remembering your logins, and you're simply resetting your password to login, rather than using the last password you chose?

(Don't ever write passwords in here, I'm sure you wouldn't have done already but I thought I'd let you know and say this)

If it happens again could you please try to clear the browser cookies, close the browser down, then reopen it, login, and check the box to remember you each time you visit (only providing you're using a computer only you have access to.

Thanks Dan, Yes, I'm using my saved password each time in resetting and clicking the remember me box and also the stay logged in box.

 

[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.