Sudden Cold Water From Combi

[fouxdafafa]

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.

Just to update while I'm waiting for the Mrs to decide to shower/bath:

The boiler was installed 2 years to the day, pretty much. The system was power flushed as part of the installation. It was serviced after about 16 months.

I think I can safely say that this "hot water going tepid for a short period" e.g. 10 secs, has been happening from the start, although it happens so infrequently that it's difficult to say how often and what the contributing factors might be. Apart from this minor issue, I've been very pleased with the boiler, especially the power of the shower - it's miles better than the trickle (exaggeration) that we used to get from our gravity system. It works perfectly for the majority of the time.

More to follow ...

 

[Johntheo5]

Just to update while I'm waiting for the Mrs to decide to shower/bath:

The boiler was installed 2 years to the day, pretty much. The system was power flushed as part of the installation. It was serviced after about 16 months.

I think I can safely say that this "hot water going tepid for a short period" e.g. 10 secs, has been happening from the start, although it happens so infrequently that it's difficult to say how often and what the contributing factors might be. Apart from this minor issue, I've been very pleased with the boiler, especially the power of the shower - it's miles better than the trickle (exaggeration) that we used to get from our gravity system. It works perfectly for the majority of the time.

More to follow ...

Yeah, a 32kw combi at full DHW output is capable of delivering 13.1LPM in the very debths of winter at 40C, from mains at 5C and just now, with a mains temp of 15C can deliver 18.35LPM at 40C, your Mrs can fill her bath at a rate of 11.5LPM at 55C.

 

[fouxdafafa]

Yeah, a 32kw combi at full DHW output is capable of delivering 13.1LPM in the very debths of winter at 40C, from mains at 5C and just now, with a mains temp of 15C can deliver 18.35LPM at 40C, your Mrs can fill her bath at a rate of 11.5LPM at 55C.

OK Test results 1: Unfortunately they are unremarkable

This was the shower
d.36 after an initial burst settled at around 3 l/min
d.40 65 to 62
d.41 55 to 53

The heating had been on prior to the shower. No temperature fluctuation reported. The only thing I should say is that my wife doesn't like the shower too hot so she tends to turn it back from the point where you have to press the red button. I tend to have it up as far as it'll go before going into the "red zone". She also doesn't like too much power either - which explains the low-ish flow rate.

I think the bath is going to be a better test. Unfortunately, I can't be in the shower/bath and monitoring the boiler at the same time.

 

[Johntheo5]

OK Test results 1: Unfortunately they are unremarkable

This was the shower
d.36 after an initial burst settled at around 3 l/min
d.40 65 to 62
d.41 55 to 53

The heating had been on prior to the shower. No temperature fluctuation reported. The only thing I should say is that my wife doesn't like the shower too hot so she tends to turn it back from the point where you have to press the red button. I tend to have it up as far as it'll go before going into the "red zone". She also doesn't like too much power either - which explains the low-ish flow rate.

I think the bath is going to be a better test. Unfortunately, I can't be in the shower/bath and monitoring the boiler at the same time.

If the the DHW SP is still 55C then 3.0LPM at 55C will give 4.8LPM at 40C and require 8.37kw,

OK Test results 1: Unfortunately they are unremarkable

This was the shower
d.36 after an initial burst settled at around 3 l/min
d.40 65 to 62
d.41 55 to 53

The heating had been on prior to the shower. No temperature fluctuation reported. The only thing I should say is that my wife doesn't like the shower too hot so she tends to turn it back from the point where you have to press the red button. I tend to have it up as far as it'll go before going into the "red zone". She also doesn't like too much power either - which explains the low-ish flow rate.

I think the bath is going to be a better test. Unfortunately, I can't be in the shower/bath and monitoring the boiler at the same time.

Can't you just open the bath tap full for a minute or two and just look at those d parameters just once, d.040 will certainly be 65C or more, if the water is tepid then d.040 will be very low, we only want to see if d.040 is sitting aroud 70C or so or higher.?, you don't have to wait for a fault to develop.
At the lower flow like now with the shower the most likely event is the flow meter is faulty causing the tepid flow at times but either way there is no way that either of these events should have been be happening when the boiler was new/installed.
The above test means the power demand was 8.37kw assuming DHW SP at 55C which will give 4.8LPM at a showering temp of 40C.

My eFile on the EcoTEC Plus 832 does show the S parameters, S.001 to S.091 but not sure how to access them, they may not be of great value except you monitor them when a fault arrises as they disappear when the fault clears.

 

[fouxdafafa]

If the the DHW SP is still 55C then 3.0LPM at 55C will give 4.8LPM at 40C and require 8.37kw,

Can't you just open the bath tap full for a minute or two and just look at those d parameters just once, d.040 will certainly be 65C or more, if the water is tepid then d.040 will be very low, we only want to see if d.040 is sitting aroud 70C or so or higher.?, you don't have to wait for a fault to develop.
At the lower flow like now with the shower the most likely event is the flow meter is faulty causing the tepid flow at times but either way there is no way that either of these events should have been be happening when the boiler was new/installed.
The above test means the power demand was 8.37kw assuming DHW SP at 55C which will give 4.8LPM at a showering temp of 40C.

My eFile on the EcoTEC Plus 832 does show the S parameters, S.001 to S.091 but not sure how to access them, they may not be of great value except you monitor them when a fault arrises as they disappear when the fault clears.

Thanks. I will probably try the bath test this evening. I'll get the Mrs to run the bath while I monitor the boiler.

I think the S parameters are status parameters. They are shown under the Live Monitoring menu item but they only show the current status and are not stored as far as I know.

 

[fouxdafafa]

I had a bath!

I monitored the boiler while I was running the bath. Readings as follows:

d.36 initially 10 l/min settling to 9.7 ... 9.6
at intervals
d.40 55, 74, 76, 78, 77, 78, 79
d.41 45, 54, 56, 57, 56, 56, 55

Water ran hot the whole time.

One thing that dawned on me - I guess I knew this already but have never done anything about it. The water running out of the bath tap was hot and having filled the bath, it was too hot to step into so I had to add some cold. Clearly, there's no point heating the water to such a point that you have to add cold to be able to step into it - that's just a waste of energy. I don't know if this mindset comes from the days when you had to worry about legionella or what. So, what should I set the DHW temp to be?

.... Thinking about it, how can the DHW be set to 58 and yet d.40 is up to 79?

 

[Johntheo5]

I had a bath!

I monitored the boiler while I was running the bath. Readings as follows:

d.36 initially 10 l/min settling to 9.7 ... 9.6
at intervals
d.40 55, 74, 76, 78, 77, 78, 79
d.41 45, 54, 56, 57, 56, 56, 55

Water ran hot the whole time.

One thing that dawned on me - I guess I knew this already but have never done anything about it. The water running out of the bath tap was hot and having filled the bath, it was too hot to step into so I had to add some cold. Clearly, there's no point heating the water to such a point that you have to add cold to be able to step into it - that's just a waste of energy. I don't know if this mindset comes from the days when you had to worry about legionella or what. So, what should I set the DHW temp to be?

.... Thinking about it, how can the DHW be set to 58 and yet d.40 is up to 79?

You can set it slightly higher than your preffered bathing temperature, so maybe set it to 45C?. My mains is 13.5C now so your original 58C setting gives 9.65LPM at 58C with a boiler output of 29.96kw, you then just dilute it to give you the 45C, if you set the DHW temperature to 45C then the flowrate should/will be 13.63LPM and yes, it should result in a slightly reduced boiler return temp and greater efficiency but if you had that hot tap fully opened then the max flow will remain at 9.65LPM and the boiler demand will fall to 21.2kw so it will just take longer to fill the bath, over 40% longer.

Regarding, "how can the DHW be set to 58 and yet d.40 is up to 79?"

A combi boiler works in CH mode by circulating the hot water from the primary HEX (heat exchanger) through the rads and back again to the HEX, you set the flow temperature, d.040, to whatever you like, say 65C, the boiler will then just control to this temperature by controlling the flow of gas through the burner. In DHW mode the same primary water is circulated through one side of a separate heat exchanger, a plate heat exchanger, PHEX, the mains water just flows under pressure through the other side of the PHEX and is heated to whatever DHW SP temperature, 58C in your case, the boiler then controls the gas flow to maintain this temperature so the primary flow temperature will vary depending on the DHW flow required and its SP temperature. If the PHEX is fouled or scaled up then the heat transfer is affected and the primary flow temperature will keep increasing until it reaches its max allowable, possibly not a awful lot higher than the 79C you are getting now which may shut the burner down occasionally. Again though, this shouldn't have been happening with a new installation, maybe worth giving Vaillant a tinkle and ask them, giving them the info below.
Running at a lower DHW temp will result in a lower primary flowtemp assuming that the max flowrate doesn't change.

Primary Flowtemp 79C
Primary returntemp 55C
Primary Flowrate 17.89LPM (calculated)
Secondary Flowtemp 13.5C
Secondary Returntemp 58C
Secondary Flowrate 9.65LPM
Boiler Output 29.96kw (calculated)

 

[fouxdafafa]

You can set it slightly higher than your preffered bathing temperature, so maybe set it to 45C?. My mains is 13.5C now so your original 58C setting gives 9.65LPM at 58C with a boiler output of 29.96kw, you then just dilute it to give you the 45C, if you set the DHW temperature to 45C then the flowrate should/will be 13.63LPM and yes, it should result in a slightly reduced boiler return temp and greater efficiency but if you had that hot tap fully opened then the max flow will remain at 9.65LPM and the boiler demand will fall to 21.2kw so it will just take longer to fill the bath, over 40% longer.

Regarding, "how can the DHW be set to 58 and yet d.40 is up to 79?"

A combi boiler works in CH mode by circulating the hot water from the primary HEX (heat exchanger) through the rads and back again to the HEX, you set the flow temperature, d.040, to whatever you like, say 65C, the boiler will then just control to this temperature by controlling the flow of gas through the burner. In DHW mode the same primary water is circulated through one side of a separate heat exchanger, a plate heat exchanger, PHEX, the mains water just flows under pressure through the other side of the PHEX and is heated to whatever DHW SP temperature, 58C in your case, the boiler then controls the gas flow to maintain this temperature so the primary flow temperature will vary depending on the DHW flow required and its SP temperature. If the PHEX is fouled or scaled up then the heat transfer is affected and the primary flow temperature will keep increasing until it reaches its max allowable, possibly not a awful lot higher than the 79C you are getting now which may shut the burner down occasionally. Again though, this shouldn't have been happening with a new installation, maybe worth giving Vaillant a tinkle and ask them, giving them the info below.
Running at a lower DHW temp will result in a lower primary flowtemp assuming that the max flowrate doesn't change.

Primary Flowtemp 79C
Primary returntemp 55C
Primary Flowrate 17.89LPM (calculated)
Secondary Flowtemp 13.5C
Secondary Returntemp 58C
Secondary Flowrate 9.65LPM
Boiler Output 29.96kw (calculated)

Much appreciate your responses. I must confess I didn't understand everything you wrote there. But, and correct me if I'm wrong, I think you're suggesting that 79C is close to shutting off the burner due to overheating. Is that correct? I've looked through many of the menus and I can't work out where that max temp would be, or what it's called. Do you happen to know for this boiler? But it's OK for d.40 to be up 70s somewhere.

Out of interest, I read the Vaillant website this afternoon and it suggests that the DHW should be set at around 50. Anything above 45 and it could scald and would require adding cold water. So my 58 seems a little high. For CH it says anywhere between 60 and 70, so my 65 is OK.

 

[Johntheo5]

Much appreciate your responses. I must confess I didn't understand everything you wrote there. But, and correct me if I'm wrong, I think you're suggesting that 79C is close to shutting off the burner due to overheating. Is that correct? I've looked through many of the menus and I can't work out where that max temp would be, or what it's called. Do you happen to know for this boiler? But it's OK for d.40 to be up 70s somewhere.

Out of interest, I read the Vaillant website this afternoon and it suggests that the DHW should be set at around 50. Anything above 45 and it could scald and would require adding cold water. So my 58 seems a little high. For CH it says anywhere between 60 and 70, so my 65 is OK.

Yes, possibly could be close to that, I don't know what its called. I would suggest ~ 50C DHW SP because your thermostatically controlled shower will react very very rapidly to any change in conditions, whereas the combi, if set to say 40C will be a lot slower in reacting if any change in flowrate or whatever.