Water Flow Rates - Finding
Finding Your Water Flow Rate
Finding your Kitchen Tap Flow Rate
This is fairly simple. You just need a stopwatch and a bucket - or a one or two litre milk carton!
Just turn on the cold water tap and measure how much water goes through it in litres per minute.
Finding Your Whole House Flow Rate
NOTE: If you're looking to size a saltless Scale Centurion softener, check this first
To size a whole of house water filter correctly you usually need to establish the flow rate. But finding the water flow rate in a building is not always as straightforward as it could be.
People - especially plumbers! - often get misled by "Bar" readings.
Bar eg "3 Bar"... or "6 Bar" refers to static pressure. This is not your flow rate.
(Nerd corner: The flow rate would be affected by variations such as the pipe diameter... number of corners in the pipework pipe... and height to be pushed to etc. You need to find your "Dynamic pressure" not your static water pressure - which is measured in BAR).
Even a plumber or engineer would only be guessing your water flow rate. They would have to work it out on a case-by-case basis.
The Good News
Provided you are prepared to roll up your sleeves for a few minutes, it is not that hard to get an accurate idea of the flow rate in your property.
The best way of doing this is to actually get a bucket and stopwatch and time the flow from each tap or shower head. This might seem basic but it will be accurate.
The trick is to consider the maximum amount of water you might use at any one time. That will be your maximum flow rate aka "peak flow rate" or "peak demand".
The classic case here is hotels: They have to work out what their maxium demand might be - eg for showers in the morning.
The golden rule is to always assume you will have peak demand and you can't go wrong.
How to Find Your Whole House Flow Rate
Step 1: Turn on all the taps in your property. (this is important as the flow in each tap can change greatly when others are on).
Step 2: You already have a stop watch, and empty 1 or 2 litre milk bottle or similar and a note pad ready. Well done!
Step 3: Now rush around the house taking the readings for each tap / shower etc
Step 4: After you've finished doing all the readings go back around the house turning off all the taps.
"If you have a water meter a useful trick for measuring flow is to turn on all the taps on and use a stop watch to time 10L on the meter, for example, rather than filling buckets from each tap". (Thanks to Tony Matthews for this suggestion).
Flow Rate Considerations
There are various factors to bear in mind.
For example, your water heater may only allow one hot supply at a time? So, while you might have three bathrooms, actually only one can be used to run a bath or a shower at one time. (This typically applies to gas-fired combi boilers).
Your flow rate will be affected by the time of day ie when other people in your area are all having a shower in the morning. This doesn't usually make much of a difference. But if you are in a low water pressure area it can. So don't take your measurment at this busy time (becuase you want to know your peak flow rate).
Power showers can produce 40 to 50 Litres per minute.
Toilets might flush several gallons but these come from the toilet's cistern/tank which then fills up slowly.
A cold water tap turned on fully and running vigorously enough to throw small splashes out for a few feet is running at about 15 L / min.
Pump / Water Pressures and Flow Rates
Don't be tricked by the reported pressure of your mains water or your borehole or well water pump - if you have a private water supply. (See "Static Pressure" / Bar measurements above).
Just because a pump can work up to a certain rate does not mean that is the flow coming into your house. This is set by the amount of water you are using in the house at any given time.
Put simply the water only flows when you turn on your taps. The number of taps you have on at any one time is what will affect your flow rate.