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Vaillant EcoTec Plus 938 temperamental in heating mode

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skvortsov

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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
 
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.
 

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Last edited:
Remove the hive trv heads and see if it will go into high
 
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
 
skvortsov said:
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?
Click to expand...

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.
 
Last edited:
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?
 
The internal one isn’t rated for external loads it’s only for the boiler when it’s doing it’s thing
 
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).
 
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skvortsov said:
OK, looks like need to talk to our plumber (if I can find him - it was installed 2 years ago)
Click to expand...
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.
 
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
 
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.
 
EvilDrPorkChop said:
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.
Click to expand...
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
 
skvortsov said:
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
Click to expand...
No problem 😎
 
skvortsov said:
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
Click to expand...

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 said:
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.
Click to expand...

Can't log in on this PC for some reason or other.
 
Please define reset ? You have to change it every time you try and log on ?
 
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).
 
Last edited:
John.g said:
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.
Click to expand...
Pump was always at 100%. Will post some charts on temps later
 
skvortsov said:
Pump was always at 100%. Will post some charts on temps later
Click to expand...
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 said:
Why do I have to reset my password each time I log out since this morning?.
Click to expand...
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.
 
Dan said:
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.
Click to expand...
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.
 
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