Low water pressure and air locks from an overhead tank: causes and fixes

In a home fed from an overhead tank — which is most homes in Nellore — the pressure at your tap comes from one thing only: how far the water surface in the tank is above that tap. A metre of height is roughly a tenth of a bar. A ground-floor shower three storeys below the tank is fine; a top-floor shower just under the tank is weak, and no motor will change that. Weak pressure everywhere means the pipes are the problem; weak pressure on one floor means height is the problem; no water at all at one fixture after the tank ran dry means an air lock.

Here is how to tell them apart and what actually helps.

Weak everywhere, all the time

• Scaled or narrow pipes. Hard borewell water leaves scale inside GI and even CPVC over the years, and a half-inch down pipe from the tank feeding a whole house was never enough. If pressure has fallen slowly over years, this is it. The fix is a wider down line (three-quarter inch or one inch from the tank, branching down to half inch at the fixtures) and replacing the worst scaled runs.
• Partly closed valve. The tank outlet valve or a gate valve on the down line left half open after a repair. Check it is fully open — gate valves turn many times.
• Blocked outlet strainer or silt in the tank bottom. Tanks collect silt from borewell water; it settles over the outlet. Cleaning the tank once or twice a year fixes a surprising number of low-pressure calls.
• Too many branches off one line. A kitchen, two bathrooms and the washing machine on a single half-inch pipe will always fight each other.

Weak on one floor or one fixture

• Height. The top floor is closest to the tank and gets the least. Options, cheapest first: choose a shower head and a geyser rated for low pressure; raise the tank on a higher stand when it is next replaced; fit a small pressure pump on the down line serving that floor. A pressure pump switches on when a tap opens and off when it closes; it needs a proper electrical point and a non-return valve, and it is a plumbing visit plus the pump at MRP.
• Aerator or shower head scaled. Unscrew the tap aerator or the shower head, soak overnight in diluted vinegar, brush, refit. Ten minutes, and often the whole "low pressure" complaint.
• Angle valve half open under a basin or toilet.
• Mixer cartridge scaled so only a trickle passes — more on that in our hard-water guide.

Air locks: what they are and how to clear one

When the tank runs empty, air goes down the pipes and collects at high points — a geyser inlet, a shower mixer, a loop of pipe that rises before it falls. When the tank refills, the weight of water in a gravity system is sometimes not enough to push that pocket through, and the fixture gives nothing or a sputter.

To clear one:

1. Make sure the tank is full.
2. Switch off the geyser at its MCB if the affected tap is on the hot side.
3. Open the affected tap fully and leave it. Open the cold tap on the same mixer too, so the cold pushes through the hot side.
4. If it still will not come, a hose from a working tap on a lower floor connected to the airlocked tap, both opened, will push the air back up to the tank. This is the plumber's trick; it works.
5. Once water runs steady and clear, switch the geyser back on.

Never switch a geyser on with air in it. The element is meant to be under water.

Stop the tank running empty in the first place

Most air locks and most "suddenly no water" calls are the tank emptying because nobody noticed. A float-switch auto controller on the motor, or a simple tank-level alarm, ends that. If you are on municipal supply with fixed hours, size the sump and tank so one fill lasts a day; a tank that is always on the edge gives low pressure every evening and an air lock every week.

Tank hygiene and pressure

A tank that has not been cleaned in years has a layer of silt and, in the humidity of the monsoon months, growth on the walls. Both reach the outlet. Drain, scrub, rinse, refill once a year — twice if you are on borewell water with visible sediment. Keep the lid on; an open tank on a Nellore terrace collects dust from the road and leaves from the neighbour's tree, and in summer heats enough to grow things.

Things that do not help

• A bigger motor. It fills faster; it does not push harder at your tap.
• Removing the flow restrictor from a shower. Gains a little, wastes a lot.
• Opening the tank outlet valve "more" when it is already fully open.

What it costs

Cleaning an aerator or shower head, opening a valve, bleeding an air lock — those are yours. A plumbing visit on our rate card is ₹299 plus labour for tracing and fixing low pressure, replacing a scaled run of pipe, or fitting a pressure pump, with pipe, fittings and the pump at MRP and shown before fitting.

When to call

Pressure that dropped suddenly rather than slowly (a blocked outlet, a valve, or a leak somewhere pulling it down); a floor that never had usable pressure; an air lock you cannot bleed with the hose trick; and any plan to re-pipe or add a pump.