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Why might you have a no water flow situation at tap or shower on a Gravity System?

Plumbing System Hydraulic Gradient Design Failure Explained


There are number of reasons why you may at any time have a no flow situation at a tap or other outlet usually;

Valve failure on a plumbing system or a boiler.

Interruption to supply, mains supply e.g. burst water main repairs.

Air lock on a gravity system.


However if you have a gravity system, with a permanent on on going intermittent issues, you may have a plumbing system design failure is you have pipes outside the hydraulic gradient.


If your outlets are above the source of your supply naturally on a gravity system there will be no flow, however if the horizontal distance is too great there will also be a no flow situation as the outlets will be outside the  hydraulic gradient.


My drawing gives a rough to a hydraulic gradient curve








It's not just the location of the tap but also the route of the pipe work that determines weather flow problems will occur. 


Friction caused by distances, pipe fittings and valve counter pressure from head height.


For accurate calculations on hydraulic gradients apply Darcy-Weisbach equation


Multiple demands, taps being open will reduce the working pressure, in effect changing the gradients, hence common problem; using the kitchen tap leads to no water in the upstairs bathroom.


This issue can occur across buildings see this report we sent to client a few years ago where a flawed design left whole flats without water during multiple demand.

The following hot water flow rates were recorded on the second floor in X House,
Flat 25; Basin 7 lts/min, Bath 10 lts/min, kitchen 4 ltrs/min
Flat 24; Basin 9 lts/min, No bath, kitchen 6 lts/min
25 is the flat furthest from the communal hot water cylinders (on the ground floor), and 24 slightly nearer, hence the slightly better flow rate.
These flow rates were recorded below on the first floor;
Flat 15; Basin 12 ltrs/min, bath 22 ltrs/min, kitchen 10 ltrs/min
A significant improvement (caused by the assistance of gravity)

The fault with the system was found to be with multiple demand on shared pipe drops (the main carcass runs along the loft space).
The opening of the bath hot water tap in flat 15 results in flow rates of 0 ltr/min in the bathroom in flat 25, it also leaves the the branch in 25 full of air as the water is syphoned off -this is why tenants are experiencing air gaps in their supply.
Multiple demands in the kitchens also produce a flow rate of 0 in flat 25. The same pipe line also supplies flat 4, (and the tenant in flat 15 also reported occasional reductions of flow.)
Likewise the opening of like outlets simultaneously in flat 14, and flat 24 results in flow rates of 1ltr/min in flat 24.

It is likely issues may exist throughout X House regarding multiple demand.
However these flats (23, 24, and 25) are furthest from the CWS tanks and hot water cylinder, they will be in existence outside the hydraulic gradient. It is likely that the flats below them, (with their good flow rates) exist within the hydraulic gradient. When there is multiple demand the outlets outside the gradient are starved of supply resulting in a no flow situation.

The introduction of new taps and pipework may have increased the flow enough on the lower floors so as to increase domination of supply over the second floor flats.

It may be possible to introduce some balance to the system. If multiple access is organised to flats on the same drop. Isolation valves could be part closed in the lower flats until a balanced system is achieved. The findings could be used to specify flow rates for flow-reducing-valves to be installed on the lower floors.
Alternatively an engineer may need to be consulted to redesign/recalibrate the system. 
(Possibly a new service could be ran to feed the second floor flats.)

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