How many l/min for a combi?

[jase158]

Had one the other day 12L/min that was very slow, customer insisted on me fitting it even though I told him it wouldn't be great pressure. I got told on here it needs to be 7l/min on each outlet, so bath, shower, kitchen sink, basin, toilet, and downstairs basin and toilet = 48L/min??? very unlikely?? how do you work it out?

 

[northcarr]

Hi jase158 not sure what you are asking

 

[lame plumber]

combi will only run one outlet at a time!

 

[REDSAW]

12ltr is good, 9.5ltr not so good
did you measure the flow from the boiler?

 

[AWheating]

the hot water on a combi is only as good as the cold mains supply and only suitable for single use design. Measure the cold mains dynamic pressure and flow rates when deciding what system is suitable for a property.

If the boiler choice has been forced upon you, then its not really something you have control over if the flow rates are low due to poor supply.

 

[Scott_D]

12 l/min I would suggest a 30kw combi
someone won't be showering, running a bath, flushing the toilet and washing hands in the bathroom.

I try and explain how a combi works and say opening taps while someone is showering will reduce the flow to the shower. If they can't live with that a combi is no good for them.

 

[Ermintrude]

Custards always know best . "now I want a combi and it needs to be condensing. My friend has saiid I will need the same as hers. It's a HEA flexi Baxi 24? " yes but she has a one bed flat with electric shower ?

 

[stevetheplumber]

despite some wild claims by combi makers back in the day you cant change the laws of physics a kw of gas will only heat x amount of water by y amount all 24kw combi give around 9 litres and 30kws about 12 litres at a usable temp unless the water main is exceptionally poor it doesnt realy come into the equation it might be different in the country but in town we rarely see less than 2 bar direct from the mains
we only see problems in poorly converted flats with one main shared and these are getting fewer and fewer as the water board rolls out water meters for everyone

 

[jase158]

I mean, do you check the flow on the initial quote? what is the minimum l/min? I know the hot will be lower, but on that boiler I got 12l/ min on cold, 8/9 l/min on hot, but when I opened the hot and cold together the hot basically stopped. Is it because of old taps? old pipework, taps meant for low pressure etc?

 

[Ermintrude]

I mean, do you check the flow on the initial quote? what is the minimum l/min? I know the hot will be lower, but on that boiler I got 12l/ min on cold, 8/9 l/min on hot, but when I opened the hot and cold together the hot basically stopped. Is it because of old taps? old pipework, taps meant for low pressure etc?

Cold needs to be at lest 1.5 times what boilers going to deliver.... Theory; boiler delivers at 65 u use shower at 45 so need cold to cool it to 45 about 1/3 of flow. So 2/3 hot 1/3 cold.

 

[jase158]

Got a customer who wants combi but also wants powerful shower. Checked the flow tonight, 18l/min, thinking of supplying a 30cdi, would she be better off with a shower pump and open vent system?

 

[Ermintrude]

Got a customer who wants combi but also wants powerful shower. Checked the flow tonight, 18l/min, thinking of supplying a 30cdi, would she be better off with a shower pump and open vent system?

No. Be better with a 33kw duotec he boiler! More than enough... Assuming 1 bathroom?

 

[AWheating]

you can also run the cold to the combi from the first tee after the cold mains stop tap. This will help stop the hot slowing to a trickle when you turn on cold at the same time.

do this all the time on new builds with combis, seperate cold from stop tap to combi.

 

[Mrs Tara Plumbi]

combi will only run one outlet at a time!

I don't often disagree with you BUT....

Now I'm not a plumber, I don't now the KW of our Remeha Avanta plus combi and I don't know our incoming water main pressure.
And, we do not have any eco, low flow taps in our house

I wonder if I'm going mad so did a little experiment to confirm what i know.
I turn on two hot taps downstairs (boiler is downstairs) good strong flow and hot from both.
Yes there must have been a drop in temperature and in pressure but not significant.

Then I went upstairs and turned on the tap - still good pressure from all but temperature is more warm than hot now we 3 taps going.
Turned off the upstairs tap turned on the shower - you know that fancy rain head thing that must use tonnes of water - the pressure was still good enough for that - though with 3 hot taps going the temperature is warm.

My conclusion is that with strong mains pressure and combi boiler with big enough KW output then you can have 2 hot taps/ outlets going FULL - no problem.
And I could have had a cold tap on as well.

I already know this from experience as 5 people live here the water is being used constantly - thank goodness we are not on a meter!
As I so often read on this forum that what happen in my house is not possible I thought I'd just check it again. One day I might try to use 2 showers at the same time!

 

[stevetheplumber]

it obviously depends on the size of the shower heads and how good the main in is some of the campsites weve stayed at could run ten showers of a 24kw combi