12 Litres Per Minute - OK for a combi?

[gerbils]

if its only reading 2.0bar then you can forget about your combi

Due to the issue with multiple appliances drawing hot water at once?

 

[Johntheo5]

If the pressure is 2.0bar in the property while flowing say 12LPM, then this 2.0bar must be sufficient to meet the combi boiler plate heat exchanger loss, the shower head loss and the elevation from the ground to the bathroom (say 2.5M, 0.25bar), you may get 8LPM or so IMO.

 

[gerbils]

If it's not in the best of condition then always a good idea to replace with a nice and shiny full bore lever valve but may not have an impact on the flow unless it isn't opening properly. What size of pipe comes off the valve, is it 22mm?

Yes 22mm

 

[gerbils]

If the pressure is 2.0bar in the property while flowing say 12LPM, then this 2.0bar must be sufficient to meet the combi boiler plate heat exchanger loss, the shower head loss and the elevation from the ground to the bathroom (say 2.5M, 0.25bar), you may get 8LPM or so IMO.

Yikes, that's a big loss.

 

[gerbils]

@Madrab I brought my own pressure gauge. It's 2.0 static and when I turn on the kitchen tap fully it's 1.4 - I guess this is the dynamic pressure?

Is that good or bad or somewhere in between

 

[Johntheo5]

It's terrible because it shows that the water board are only providing a 2bar supply at their end, you might normally expect 4 to 6bar IMO.

 

[Madrab]

@Madrab I brought my own pressure gauge. It's 2.0 static and when I turn on the kitchen tap fully it's 1.4 - I guess this is the dynamic pressure?

Is that good or bad or somewhere in between

It is a dynamic figure, yes and unfortunately it's not the best, where is you location roughly? What happens to that figure when you open up another 2 mains outlets?

 

[Johntheo5]

I think you said the kitchen tap flows 10.0LPM, the pressure drop is, 2-1.4, 0.6bar, if you wanted 20LPM then the pressure drop will be 2.4bar, if you require say 3 bar dynamic pressure d/stairs at this flowrate then the water board would have to give you a dynamic pressure (at their end) of 5.4bar.
If you settled for 2.5bar dynamic d/stairs at 20LPM then the supply needs to be 4.9bar say 5.0bar.
If you were happy with 15LPM at 3 bar d/stairs then the dynamic supply pressure needs to be 4.35bar, if you settle for 2.5bar dynamic d/stairs at 15LPM then a dynamic mains pressure of 3.85bar will suffice.

 

[gerbils]

It is a dynamic figure, yes and unfortunately it's not the best, where is you location roughly? What happens to that figure when you open up another 2 mains outlets?

OK so I have opened 3 mains outlets, 2 x outside taps and 1 kitchen tap. *The pressure gauge is on one of the outside taps.

It goes down further, to 1 Bar when all 3 are open.

One thing I did notice is that our second outside tap (closer to the boundary) is much faster than the one I have been using to test flow. This has been turned off for years as we don't use it.

This is giving us 16 Litres per Min, which is way more than the 11 reported by the water guy and 11 by the plumber who both assumedly used the kitchen tap or the other garden tap....

 

[gerbils]

I think you said the kitchen tap flows 10.0LPM, the pressure drop is, 2-1.4, 0.6bar, if you wanted 20LPM then the pressure drop will be 2.4bar, if you require say 3 bar dynamic pressure d/stairs at this flowrate then the water board would have to give you a dynamic pressure (at their end) of 5.4bar.
If you settled for 2.5bar dynamic d/stairs at 20LPM then the supply needs to be 4.9bar say 5.0bar.
If you were happy with 15LPM at 3 bar d/stairs then the dynamic supply pressure needs to be 4.35bar, if you settle for 2.5bar dynamic d/stairs at 15LPM then a dynamic mains pressure of 3.85bar will suffice.

Thanks, all bit above my head. I am a bit in the dark as to what I need in terms of bars and flow. I just want to know if getting a Combi will be feasible or a complete nightmare I should avoid at all costs. IF that meant the shower would drop from 10L/min to 6-8L/min when someone brushed their teeth or used the loo then that may be workable. But if it stopped the water this would be a problem.

Just trying to understand this all before we chose to stick with unvented or go combi. It's been an enlightening process getting my head around it all.

 

[Johntheo5]

If the PG is still connected you might just run one more test with the upstairs cold tap which flowed I think 6.0LPM?

 

[gerbils]

If the PG is still connected you might just run one more test with the upstairs cold tap which flowed I think 6.0LPM?

I believe this comes from the stored water in the loft so wouldn't have any effect?

 

[Madrab]

I believe this comes from the stored water in the loft so wouldn't have any effect?

Makes sense that the bathroom taps would be cistern fed. No that wouldn't have any effect on the mains

OK - 1 bar isn't the end of the world and will be more a 'true' dynamic figure especially given that would be closer to the high end use. Hopefully you did a flow test on one of those outlets and just didn't detail it? That would then give us the same 'true' dynamic flow rate and will give an answer to this

the shower would drop from 10L/min to 6-8L/min when someone brushed their teeth or used the loo then that may be workable.

 

[MNW67]

OK so I have opened 3 mains outlets, 2 x outside taps and 1 kitchen tap. *The pressure gauge is on one of the outside taps.

I thought Madrab had suggested opening another two outlets, in addition to the kitchen and the one with the pressure gauge? Apols if I have misunderstood the situation. Also, I was wondering if it is better to only use outlets in the house rather than the second garden tap. Or try it both ways! All information is useful, I think, when you have a borderline situation.

 

[Madrab]

Did Madrab suggest opening another two outlets, in addition to the kitchen and the one with the pressure gauge? Apols if I have misunderstood the situation. Also, I was wondering if it is better to only use outlets in the house rather than the second garden tap. Or try it both ways! All information is useful, I think, when you have a borderline situation.

@Madrab I brought my own pressure gauge. It's 2.0 static and when I turn on the kitchen tap fully it's 1.4 - I guess this is the dynamic pressure?

It is a dynamic figure, yes and unfortunately it's not the best, where is you location roughly? What happens to that figure when you open up another 2 mains outlets?

OK so I have opened 3 mains outlets, 2 x outside taps and 1 kitchen tap. *The pressure gauge is on one of the outside taps.

I don't think we quite got 3 outlets running with the gauge on the 4th but I think the 1 bar dynamic is indicative of where the mains is going as far as worst case dynamic figures are concerned. We have gone from 2bar static to 1.4 w. 1open > 1 bar w. 2 open then I would suspect there would probably be a further drop with 3 open. With the external mains @ 2 bar dynamic though then that would suggest to me the supply pipe to the property could be the bottleneck