Finding Your Water Flow Rate
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: Flow rate is "Dynamic pressure". You need to find your flow rate not your static water pressure).
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.
(Back to nerd corner: they would calculate velocity - from the Bar / pressure - by the tube size).
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 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.
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 several feet is running at about 17 L / min.