Hot water flow rate from Vaillant 937
- Thread starter buggle52
- Start date
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Lol! We're away this weekend so we'll let them off. 
But to be honest we do have a challenge... as far as the plumbers are concerned the pipes to and from the boiler are all fine (it's all new plumbing so I expect them to have confidence in their work) and so the issue is, in their opinion, with the boiler; either it's faulty or (and this is their view) simply incapable of working as advertised. Either way it's up to us to have it out with Vaillant.
One of the Vaillant technicians we had round said we should expect some flow/pressure loss through the boiler with all the bends and valves inside, but going from 25 to 10l/min out of the boiler's internal tanks seems like a very significant loss. I was interested to see if other installations of this boiler had similar results.
But to be honest we do have a challenge... as far as the plumbers are concerned the pipes to and from the boiler are all fine (it's all new plumbing so I expect them to have confidence in their work) and so the issue is, in their opinion, with the boiler; either it's faulty or (and this is their view) simply incapable of working as advertised. Either way it's up to us to have it out with Vaillant.
One of the Vaillant technicians we had round said we should expect some flow/pressure loss through the boiler with all the bends and valves inside, but going from 25 to 10l/min out of the boiler's internal tanks seems like a very significant loss. I was interested to see if other installations of this boiler had similar results.
A
ALEC1
Its amazing when I did the course for the boiler they said about the blender, and only one piece of sales material actually says it. I can't understand why vaillant don't say more about this. (actually I can, it would make the sale of the boiler uncompetitive...by £120...)
The boiler does have a flow restrictor, which renders the store pointless, you have to blend the water down after the output of the boiler...
to get an idea of the likely flow from the boiler if you have a mixer tap, set the boiler water temp to max, then open the mix tap mixing to slightly warmer than you want temperature...
as long as you have the water and flow rate going in to the boiler then you will get the high performance for bath filling etc....
The boiler does have a flow restrictor, which renders the store pointless, you have to blend the water down after the output of the boiler...
to get an idea of the likely flow from the boiler if you have a mixer tap, set the boiler water temp to max, then open the mix tap mixing to slightly warmer than you want temperature...
as long as you have the water and flow rate going in to the boiler then you will get the high performance for bath filling etc....
Interesting info about the flow restrictor, Alec. I'm sure they must have their reason for doing this... At least that explains why the storage tanks don't provide full flow.
Presumably the flow restrictor only caps at the level the combi can provide at. If that's true, I don't understand why I can't get 15l/min from the combi side if I'm meeting their requirements for "Required connection pressure for max. throughflow quantity" (1.3 bar)... that's assuming I am; it doesn't say if this is dynamic or static pressure, and I only know that I'm getting 2.5 bar, which I presume is static. It doesn't matter if I turn the temperature to minimum or maximum, or even if I have the storage tanks switched on or not - it's always the same 10-11 l/min. Surely the flow restrictor wouldn't be set below the advertised capacity of the boiler?!
Anyway, good suggestion on the 70c mixing test. A quick timed-bucket test using your suggestion has showed I can mix 70c water down to an above-normal temperature at 16l/min from the bath mixer upstairs.
Presumably the flow restrictor only caps at the level the combi can provide at. If that's true, I don't understand why I can't get 15l/min from the combi side if I'm meeting their requirements for "Required connection pressure for max. throughflow quantity" (1.3 bar)... that's assuming I am; it doesn't say if this is dynamic or static pressure, and I only know that I'm getting 2.5 bar, which I presume is static. It doesn't matter if I turn the temperature to minimum or maximum, or even if I have the storage tanks switched on or not - it's always the same 10-11 l/min. Surely the flow restrictor wouldn't be set below the advertised capacity of the boiler?!
Anyway, good suggestion on the 70c mixing test. A quick timed-bucket test using your suggestion has showed I can mix 70c water down to an above-normal temperature at 16l/min from the bath mixer upstairs.
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Buggle - the test I propose will determine unequivocally, whether it is the boiler or the plumbing.
Heaven forfend a plumber wants to have the weekend off to spend with his family
Heaven forfend a plumber wants to have the weekend off to spend with his family
I get that (the test). Just trying to keep theories and options open. The blender valve seems like a good option if there isn't any way to improve the flow, but at the same time I want to know if it's a boiler fault that can be fixed.Buggle - the test I propose will determine unequivocally, whether it is the boiler or the plumbing.
Heaven forfend a plumber wants to have the weekend off to spend with his family
I would never dream of calling out the plumber at the weekend for something like this anyway - the boiler works, it just simply doesn't work to the level I was expecting it to.
A
ALEC1
then thats what you would with a blender valve...
the taps could have a huge pressure drop, many do especially if they are fancy designer stuff imported from europe.
My vaillant book says that the flow rate is 15l/m at a temp rise of 35c and then says 20l/m when blended.
the taps could have a huge pressure drop, many do especially if they are fancy designer stuff imported from europe.
My vaillant book says that the flow rate is 15l/m at a temp rise of 35c and then says 20l/m when blended.
I saw those figures, which is why I thought if I reduce the HW temperature to 35c and switch off the storage tanks I would see an improved flow rate. I was surprised to see the same 10-11 l/min.
No worries Buggle - i was having a little did at Tony, who has some rather esoteric working hours.
A
ALEC1
well one is a target temperature and the other is a temperature rise given to the water..
so thats why you get 15l/m on the outlet...
I would install the blender for reasons outlined in other posts, but please note that your installer has not failed in anyway.
so thats why you get 15l/m on the outlet...
I would install the blender for reasons outlined in other posts, but please note that your installer has not failed in anyway.
Sorry, perhaps I'm not explaining my thinking very well, or maybe I'm missing it completely. If the cold water is coming in at, say, 5c and I set the target temperature to <40c I would have thought I'd get the full advertised flow rate of 15l/min @ 35c rise. Is that right?
I'm only getting out 10l/min of hot water regardless of the temperature I set, which is what's confusing me.
A
ALEC1
flow and temperature are not related in this boiler...the flow restrictor is between the store and the heat exchanger, its the rate that the water will reheat at, and logically as thats what it reheats at if you put into the boiler thats what you would get out of it...
so 10l/m is what must be going into the boiler on the cold feed and 10/m is what is coming out...
With out site survey the question as to why the water company got 25 l/m somewhere else in the property cannot be answered.
As the blending valve can be located under the boiler it will need more than 10 l/m below the boiler...so with out further investigation you can't be sure that the blender will solve the problem.
so 10l/m is what must be going into the boiler on the cold feed and 10/m is what is coming out...
With out site survey the question as to why the water company got 25 l/m somewhere else in the property cannot be answered.
As the blending valve can be located under the boiler it will need more than 10 l/m below the boiler...so with out further investigation you can't be sure that the blender will solve the problem.
Are you sure the flow restrictor is between the store and the heat exchanger? The schematic in the Spare Parts document (page 17) seems to show the "water quantity limitator" (presumably the same thing?) on the cold feed input just after the filter.
After reading some other related topics on this forum and seeing some of your informative comments on those, Alec and Tony, I'm starting to wonder if the flow restrictor is key. Is it possible that it might be restricting too much? Maybe on less-than-super pressure and dynamic flow* it could actually be detrimental? We're not getting anywhere near the 14l/min cap.
I saw another post where a Vaillant engineer removed the flow restrictor (not sure if temporarily or permanently) and saw an improved flow rate on a similar setup.
Is removing the restrictor an option or would it invalidate the warranty?
*At Dan's suggestion, a boiler bypass test has been done and we can get the full 25l/min (open pipe) if we bypass the boiler. We've also tested the dynamic pressure at 1.7 bar (not perfect but within the acceptable pressure to achieve maximum flow according to the boiler specs). Of course it is possible that the dynamic flow rate could be the limiting factor but I want to explore all options.
After reading some other related topics on this forum and seeing some of your informative comments on those, Alec and Tony, I'm starting to wonder if the flow restrictor is key. Is it possible that it might be restricting too much? Maybe on less-than-super pressure and dynamic flow* it could actually be detrimental? We're not getting anywhere near the 14l/min cap.
I saw another post where a Vaillant engineer removed the flow restrictor (not sure if temporarily or permanently) and saw an improved flow rate on a similar setup.
Is removing the restrictor an option or would it invalidate the warranty?
*At Dan's suggestion, a boiler bypass test has been done and we can get the full 25l/min (open pipe) if we bypass the boiler. We've also tested the dynamic pressure at 1.7 bar (not perfect but within the acceptable pressure to achieve maximum flow according to the boiler specs). Of course it is possible that the dynamic flow rate could be the limiting factor but I want to explore all options.
So what flow rate did you get with 1.7 Bar pressure???
Tonight I had to call at a local architect at 2130 to fix his boiler. Unless I work late sometimes I will not get to people the same day! I had told him two years ago that it was time to replace it.
Removing the flow restrictor can only help the flow rate. I dont see why Vaillant would ever be bothered even if they did know about it.
Tony
Tonight I had to call at a local architect at 2130 to fix his boiler. Unless I work late sometimes I will not get to people the same day! I had told him two years ago that it was time to replace it.
Removing the flow restrictor can only help the flow rate. I dont see why Vaillant would ever be bothered even if they did know about it.
Tony
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