Guidance on flow rates
Low Pressure (Gravity) Installations – Typically for a two storey house with cold water tank in loft serving hot water cylinder on the first floor.
The bathroom taps would have approximately 0.2bar pressure.
A shower head would have approximately 0.1bar pressure.
Gravity fed taps on the ground floor would have approximately 0.4-0.5bar pressure.
Normally the kitchen sink cold tap would be supplied from the incoming mains water supply and would be high pressure at anything above 1bar pressure.
Flow Rates in Litres per minute (l/m) – Approximate.
Old style ¾” bath taps = 15 l/m
Old style ½” basin tap = 10 l/m
Modern ¾” bath mixer tap = 12 l/m
Modern monobloc basin mixer with 10-12mm connections = 4 to 5 l/m
Old style ½” kitchen tap = 12 l/m
Monobloc sink mixer = 8 l/m
A gravity shower on the ground floor with 0.5bar pressure would have approximately 10 to 12 l/m depending on the actual shower valve.
High Pressure Systems – Mains fed storage systems- normally restricted to 2.5 to 3bar pressure.
Typically these would need to have a larger mains water supply coming in to the house and would give good flow rates all round in excess of 15 to 25 l/m.
If the old original ½” mains water supply was used the flow rates could drop considerably even with good pressure.
Good pressure doesn’t necessarily mean good flow.
High Pressure Systems – From a gas combi boiler.
These systems are governed for hot water delivery by the kw rating of the boiler.
Generally they are as follows:- 24kw = 9l/m
28kw = 12 l/m
35kw = 14 l/m
Oil fired combi boilers tend to have a slightly higher flow rate.
Gravity showers will only be just adequate enough at best.
If you want a good shower on a gravity system it will need to have a pump on it. Any pump producing a pressure of 1bar or above would be fine for a single headed shower.
If using body jets as well then you would be best to choose a 2.4 to 3 bar pump.
Any power shower system incorporating a pump should be plumbed in with independent hot and cold water supplies directly from the water tanks with special attention being paid to the hot water tank connection thus making them fairly expensive to install correctly.
Incorrect installation could result in premature failure of the pump.
There is no such thing as a good cheap shower.
Beware; when installing a pumped shower system consideration should be given to the size of your existing hot and cold water tanks. An average shower time is reckoned to be 8to 12 minutes.
A 12minute shower with a flow rate of 15 l/m would use 180 litres of water.
With a 60/40% split of hot to cold water the shower would use about 108 litres of hot water. The average hot water cylinder for a three bedroomed house would have a capacity of around 117 litres therefore your 12 minute shower would have used approximately 92% of your hot water.
Always get your plumber to survey your plumbing installation and advise you before purchasing.
Flow rates stated by manufacturers are typically what their fitments will pass but will be subject to specific site conditions.
Bath – Hot Water demands facts:-
NHS maximum safe water temperature for baths – 44 to 46’C
Average bathing temperature – 40’ to 42’C
BS6700 allows for standard (1700 x 700mm) bath of 100 litres at 40’C.
This is split into 60% hot and 40% cold. Or 60 litres of hot and 40 litres cold.
Standard/average bath has a capacity of 180 litres to the overflow.
Home Test: 1700 x 700mm bath with capacity of approximately 175 litres to overflow.
Hot water at 60’C and cold at 8’C
Used:- 70 litres of hot and 30 litres of cold and produced a bathing temperature of 40’C. Gave 180mm depth measured in centre for 100 litres of water. This was 100mm below the overflow.
Adult male 12 to 16 Stone (80 – 100KG) would displace approximately 0.75l/kg
Or displaces between 60 – 80 litres.
Maximum hand temperature = 47-48’C
Maximum body temperature = 42-43’C
Grampian 1800 x 800 cast iron bath has capacity of 250 litres.
1700 shower bath has capacity of 250 litres.