Flow Rate Measurement... one tap or both?

[John Smith Esq.]

I am measuring this by filling a 1L jug and timing it. I have been doing this with the cold water tap only. However, if I open both hot and cold, the flow rate is stronger. Which reading represents the flow rate for my home? Online sources say to only use the cold tap, but shouldn't it be based on what is optimally possible, i.e. opening both taps?

Thanks

 

[oldbuffer]

What is the source of hot water, combi boiler or stored hot water?

 

[John Smith Esq.]

Combi

 

[oldbuffer]

Suggests you have some obstruction in the cold, reducing the flow rate. The flow rate should be measured at a mains outlet which has no restrictions between the incoming main and that outlet. Examples of restriction are isolation valves other than full bore valves, stop cosks not fully open, taps connected by restirtive flexible hoses. The best measure is "open pipe", i.e. with the pipe from the mains open ended. However, it's rarely practical!

 

[Madrab]

To obtain an accurate test you are looking to test it at a full flow tap as close to the incoming mains as possible. That tends to be an outside tap or a sink tap that is a screw down type. If none are available then usually I would fit one temporarily to provide an accurate test, say on an appliance outlet or similar.

1/4 turn ceramic valves can be restrictive and any other outlet further downstream can be affected by bends and fittings in the supply pipeline, reducing output.

 

[John Smith Esq.]

Thanks for the responses.

I have two outside taps and one is very close to where the water enters the house, although that does have a screw type isolator for shutting it off during the winter. From what you're saying, that could interfere with the results. I could use the tap at the back garden, which is further away from the stopcock but is full pipe.

In terms of the kitchen sink that I was using before though - putting to one side the flexi pipes and so on - is it more accurate to measure the flow with just the cold tap open, or both hot and cold open? It seems to me that both is more accurate because I get a higher flow rate that way, indicating that there is more flow available than what the cold tap alone can provide.

 

[Johntheo5]

Ideally you require a pressure gauge to measure the dynamic pressure, you can buy one which can be attached to the outside tap but let a tiny leak/dribble from this in order to read the correct pressure as there will probable be a NRV on this tap, I think you probably require ~ 2.5 bar at a flowrate of say 15LPM (others will advise), so take the static pressure first, then open up a few inside taps and measure the total flow via your jug, also take the static pressure immediately before and after testing.

 

[John Smith Esq.]

So is flow rate best measured by having both taps open or just cold?

 

[Johntheo5]

What exactly is your problem?, I remember your ubiquitous name, you have a 25kW combi that you run at 44C.?

 

[Madrab]

is it more accurate to measure the flow with just the cold tap open, or both hot and cold open?

Sink taps can use things like aerators that can affect flow though if, as you suggest, the flow increases when both are open then yes it would make sense to have both taps running. That being said though, I'm not sure that would give an accurate reading of the optimal flow achievable from the system.

You could change the outside tap's ISO for a full bore type, easy 1 min swap.

As suggested though, what is it that is trying to be achieved?

 

[Harry Bloomfield]

So is flow rate best measured by having both taps open or just cold?

If the flow from both taps, is substantially more than from one of the taps, then measure from both, running simultaneously. A substantial difference, suggests that one pipe has some sort of restriction in the pipework somewhere.