What are the tanks in my loft for?
The big cold water cistern, the small feed and expansion cistern, and what every pipe going in and out of each of them actually does.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 5 min
- Safety
- caution
Short answer
The big one is the cold water storage cistern: it stores mains water and feeds your cold taps and your hot water cylinder by gravity. The small one is the feed and expansion cistern and belongs only to the heating circuit — it tops the system up and gives the water somewhere to expand when it heats. Both have a ball valve and an overflow, and both should be lidded and insulated.
Do this first
If your house has tanks in the loft, it is a vented system, and those tanks explain most of its behaviour: why the upstairs shower is weak, why the hot water can run out, why you get airlocks, and why a stuck ball valve announces itself as water pouring down an outside wall. Knowing which tank is which, and which pipe is which, turns a mysterious noise or drip into an obvious diagnosis.
Safety
Step by step
- Identify the big cistern.Typically a plastic or older galvanised box holding a couple of hundred litres, sitting on boards across the joists. This is the cold water storage cistern. It is filled from the rising main through a float-operated ball valve, which shuts off as the water reaches its set level.
- Trace its outlets.Two large pipes leave near the bottom. One feeds the cold taps, the bath and the toilets. The other drops to the bottom of the hot water cylinder. That second one is why the tank running dry means no hot water as well as no cold.
- Find its overflow.A larger pipe leaving near the top and running out through the eaves or the wall, so a failed ball valve dumps water visibly outside instead of into the ceiling. If yours discharges into the loft or nowhere at all, that is a fault worth fixing.
- Find the vent pipe.A pipe rising from the top of the hot water cylinder, curling over and hanging above the water in the cistern. That open end is what makes the system vented — it is how hot water expands and how any steam would escape. It must never be capped, blocked or plumbed into anything.
- Identify the small cistern.Much smaller, often no bigger than a washing-up bowl, usually higher than the big one. This is the feed and expansion cistern and it serves the heating circuit only. It has its own small ball valve, its own overflow and its own vent pipe curling over the top.
- Check its water level.It should sit only about a third full when the heating is cold. That empty space is deliberate — the heating water expands into it as the system warms. A feed and expansion cistern filled to the brim will overflow every time the heating runs.
- Check both lids and the insulation.Close-fitting lids keep dust, insects and birds out of stored drinking water. Insulate the sides and the top of each cistern and lag the pipes, but never insulate underneath — the heat rising from the house is what keeps them from freezing.
Tips
- Old galvanised cisterns rust from the inside and eventually leak. If yours is metal and you can see rust scale in the water, plan a replacement rather than waiting.
- A cistern with no lid is storing drinking water open to loft dust and rodents. A lid is cheap.
- Take a photo of the pipework before you insulate everything, so you can see later what runs where.
- If the tank hisses or sings while filling, the ball valve is partly closed or scaled — that noise usually gets worse before the valve fails.
If it doesn't work
Water running out of a pipe under the eaves
Cause: A ball valve has failed to shut off and the cistern is overflowing through its warning pipe — Fix: Tie the ball valve arm up to stop the flow, then replace the float valve or its washer. Work out which tank it came from: the higher, smaller overflow is the heating cistern and points at a different problem.
The heating cistern keeps overflowing
Cause: Either the ball valve is passing, or — more seriously — a failed cylinder coil is letting mains-pressure water into the heating circuit — Fix: If tying up the ball valve stops it, replace the valve. If the level still rises with the valve tied, the cylinder coil has perforated and needs an engineer.
The cold taps upstairs run slowly and the tank sounds like it is filling constantly
Cause: A partly closed or scaled ball valve restricting the fill, so the cistern cannot keep up with demand — Fix: Check the mains stopcock is fully open, then check the float valve for scale. On old valves, replacing the whole valve is easier than servicing it.
Water in the loft with no visible overflow
Cause: The overflow pipe is blocked, has fallen off, or discharges into the loft — Fix: Trace it out to the wall and clear it. A blocked warning pipe removes the only safe route for a failed ball valve and is how ceilings come down.
Questions people ask
Can I remove the loft tanks?
Only by changing the system — converting to a combi or an unvented cylinder removes the need for both. On the existing system the tanks are load-bearing parts of it and removing them is not an option.
Why is my hot water cylinder fed from the loft rather than the main?
Because the cylinder is vented and cannot take mains pressure. The loft tank's height provides just enough head to push the hot water to the taps, which is exactly why upstairs showers are weak on this system.
Is the water in the loft tank safe to drink?
The kitchen cold tap is always fed directly from the main and is the drinking water tap. Other cold taps on a vented system come from the loft cistern, which is stored water — fine for washing, but the kitchen tap is the one to drink from.