Sudden Cold Water From Combi

[fouxdafafa]

I have a Vaillant ecoTEC Plus 832 combi boiler that was installed a few years ago. I'm wondering if there's a fault with the boiler. I've experienced this and had "complaints" from my wife that's she's experienced the same. It's happened with the bath and the shower. Today I was running a bath and I had the hot water tap on only. After a while the water started running cold and it stayed that way for about 10 seconds. It then recovered and ran hot again. There were no other taps open in the house at the time. It's happened before when I've been in the shower - cold (or at least tepid) water for some seconds. Long enough to be annoying in the shower. Not such a big deal in the bath.

What could cause this? Does this sound like a fault?

 

[Madrab]

Yes - typically that behaviour is due to the boiler is getting too hot and then it shuts off, cools down a little and then fires up again and the cycle repeats

Watch the boiler as you do this, does the boiler shut off and then back on again?

If so then invariably it's down to the plate heat exchanger being restricted and the boiler can't get rid of it's heat quickly enough and gets too hot. When was the boiler last serviced?

 

[fouxdafafa]

Yes - typically that behaviour is due to the boiler is getting too hot and then it shuts off, cools down a little and then fires up again and the cycle repeats

Watch the boiler as you do this, does the boiler shut off and then back on again?

If so then invariably it's down to the plate heat exchanger being restricted and the boiler can't get rid of it's heat quickly enough and gets too hot. When was the boiler last serviced?

That's interesting. The only thing I'd say is that, when I've experienced this in the shower (and it doesn't happen every time) it'll go tepid once but after that it stays hot. It doesn't cycle hot/tepid/hot/tepid. The other think I'm not sure is, whether this only happens when the central heating is on at the same time. I want to say that it does but I haven't got enough evidence - it's just a hunch.

TBH, I don't really know what's happening at the boiler. Again, because it doesn't happen very often, I can't really ask someone to watch the boiler every time someone showers. And the boiler's on a different floor to the bath/shower. Would there be a fault logged at the boiler when this happens?

Boiler was serviced this year - as per the warranty requirement. Either way, as far as I remember this has happened since it's been installed. It really doesn't happen often but when it does it's annoying. And if there is a fault, I'd like to get it fixed while it's under warranty.

 

[Johntheo5]

That's interesting. The only thing I'd say is that, when I've experienced this in the shower (and it doesn't happen every time) it'll go tepid once but after that it stays hot. It doesn't cycle hot/tepid/hot/tepid. The other think I'm not sure is, whether this only happens when the central heating is on at the same time. I want to say that it does but I haven't got enough evidence - it's just a hunch.

TBH, I don't really know what's happening at the boiler. Again, because it doesn't happen very often, I can't really ask someone to watch the boiler every time someone showers. And the boiler's on a different floor to the bath/shower. Would there be a fault logged at the boiler when this happens?

Boiler was serviced this year - as per the warranty requirement. Either way, as far as I remember this has happened since it's been installed. It really doesn't happen often but when it does it's annoying. And if there is a fault, I'd like to get it fixed while it's under warranty.

The boiler is a 32kw combi (832), the DHW takes priority, so the CH can't be on at the same time except there is a problem with the diverter valve or something like that.

What temperature is the DHW set to at the boiler?, can you run the hot tap fully open and then note.............
d.036 DHW Flow Rate
d.039 cold water temperature
d.040 Flow temperature
d.041 Return temperature

 

[fouxdafafa]

The boiler is a 32kw combi (832), the DHW takes priority, so the CH can't be on at the same time except there is a problem with the diverter valve or something like that.

What temperature is the DHW set to at the boiler?, can you run the hot tap fully open and then note.............
d.036 DHW Flow Rate
d.039 cold water temperature
d.040 Flow temperature
d.041 Return temperature

This is the first time I've monitored these things inside the installer menu, so bear with ...
d.036 = 5 l/min up to 5.13 l/min
d.039 = in my menu this is some solar thing so not cold water temp
d.040 = 68
d.041 = 54

This wasn't the bath tap, btw, but the nearest tap to the boiler. I ran it for a couple of minutes ... must say, it grieves me to see hot water going down the drain!

FWIW, there are no fault codes list on the boiler.

 

[kidgreen61]

As Madrab says, blocked plate hex.

 

[fouxdafafa]

As Madrab says, blocked plate hex.

This seems to be mentioned a lot on tinternet. I wouldn't dream of getting inside a boiler but can you explain to a layman, or point me to something that explains what this is? I'm somewhat baffled that this would happen on a fairly young boiler that's been serviced this year. Actually I think this has been happening randomly, and not very often, since it's been installed. Which I also find odd.

 

[Johntheo5]

This is the first time I've monitored these things inside the installer menu, so bear with ...
d.036 = 5 l/min up to 5.13 l/min
d.039 = in my menu this is some solar thing so not cold water temp
d.040 = 68
d.041 = 54

This wasn't the bath tap, btw, but the nearest tap to the boiler. I ran it for a couple of minutes ... must say, it grieves me to see hot water going down the drain!

FWIW, there are no fault codes list on the boiler.

A blocked/scaled PHEX can happen relatively rapidly, especially in the DHW side, this is what we are trying to see and why you might just repeat that test for a minute or two with the bath tap, for any given DHW setpoint then as the DHW flow demand increases the boiler will fire harder and harder to maintain that setpoint temp so the primary flowtemp (which I think/hope d.040 monitors with boiler firing in DHW mode) will kreep on rising and if it reaches something like ~ 80C will trip the burner but I think self resets when the temperature drops, so maybe just monitor d.036, d.040 & d.041.

Also what is the DHW setpoint set to??.

There are/should be fault codes....f.01 etc, there may not be any status codes....s.01 etc but not sure.

 

[fouxdafafa]

A blocked/scaled PHEX can happen relatively rapidly, especially in the DHW side, this is what we are trying to see and why you might just repeat that test for a minute or two with the bath tap, for any given DHW setpoint then as the DHW flow demand increases the boiler will fire harder and harder to maintain that setpoint temp so the primary flowtemp (which I think/hope d.040 monitors with boiler firing in DHW mode) will kreep on rising and if it reaches something like ~ 80C will trip the burner but I think self resets when the temperature drops, so maybe just monitor d.036, d.040 & d.041.

Also what is the DHW setpoint set to??.

There are/should be fault codes....f.01 etc, there may not be any status codes....s.01 etc but not sure.

OK thanks. That test at the bath will have to wait until tomorrow.

I can confirm that there are no faults logged in the fault list. I would have thought that if it heats enough to trip the burner then that would be logged as a fault, no?

 

[Johntheo5]

No, not all boiler trips generate a fault code which would then normally require a boiler reset, it should though have generated a status code.

For interest, some numbers below from someone who had a WB Greenstar 36kw CDI combi and the burner was tripping at d.040 (equivalent) of ~ 75C with a DHW flow of 9.2LPM at 44.5C (setpoint 55C), and a boiler output of only 16.37kw.
A new PHEX was installed and the numbers then showed d.040 (equivalent) of only 64.5C with a DHW flow of 9.9 LPM at 54.5C (setpoint 55C), but the boiler output was then 24.1kw.

 

[fouxdafafa]

Thanks for that. I'll report back when I know more.

 

[fouxdafafa]

Thanks for that. I'll report back when I know more.

Target flow temp is 65
DHW is set to 58

Need get the Mrs to have a shower/bath (!) - so that I can monitor the boiler as it's happening.

 

[RandomGrinch]

Today I was running a bath and I had the hot water tap on only. After a while the water started running cold and it stayed that way for about 10 seconds.

The other think I'm not sure is, whether this only happens when the central heating is on at the same time.

And the boiler's on a different floor to the bath/shower.

I'm very probably wrong, but would just like to clarify - does this issue occur in the first moments of a shower - i.e. some nice hot water through, and then a cool down, before heating up again?

If so there is a small possibility, that the existing water in the heat exchanger has got nice and toasty when the C/H has been running - when there is a demand from the shower, the C/H cuts off and the glug of hot water is sent through the pipes, followed by some cool water when waiting for the burner to fire back up.

 

[fouxdafafa]

I'm very probably wrong, but would just like to clarify - does this issue occur in the first moments of a shower - i.e. some nice hot water through, and then a cool down, before heating up again?

If so there is a small possibility, that the existing water in the heat exchanger has got nice and toasty when the C/H has been running - when there is a demand from the shower, the C/H cuts off and the glug of hot water is sent through the pipes, followed by some cool water when waiting for the burner to fire back up.

TBH, I wouldn't be able to say for certain because it happens so infrequently it's hard to remember. Most recently it happened to me in the bath. I ran the hot water to top up the bath - and after a while it ran tepid. I wouldn't be able to say for sure at what point it happens in the shower.

 

[Johntheo5]

No, not all boiler trips generate a fault code which would then normally require a boiler reset, it should though have generated a status code.

For interest, some numbers below from someone who had a WB Greenstar 36kw CDI combi and the burner was tripping at d.040 (equivalent) of ~ 75C with a DHW flow of 9.2LPM at 44.5C (setpoint 55C), and a boiler output of only 16.37kw.
A new PHEX was installed and the numbers then showed d.040 (equivalent) of only 64.5C with a DHW flow of 9.9 LPM at 54.5C (setpoint 55C), but the boiler output was then 24.1kw.
[/QUOTE]

Target flow temp is 65
DHW is set to 58

Target temp has no effect in DHW mode, just monitor d.036,d.040&d.041 whe you get a chance