Header Tank Overflowing or Full of Hot Water? What It Means

Which tank is the header tank?

Most older London houses with a hot water cylinder have two tanks in the loft. The big one is the cold water storage tank, which feeds the cylinder and often the bathroom cold taps. The small one, usually a plastic tank a fraction of the size, is the feed and expansion tank. That is the one people mean by the header tank. It feeds the boiler, radiators and the coil inside the cylinder, and it gives the heating water somewhere to expand as it warms up.

If your boiler has a pressure gauge on the front and there are no tanks in the loft, you have a sealed heating system and no header tank. The rest of this page is about open vented systems, which is what most homes with a tank fed cylinder have.

If it is the big tank that is overflowing with hot or steaming water, that is a different fault: the cylinder is overheating and venting. Switch off the boiler and the immersion heater, and read our page on the hot water cylinder overheating.

Is the water cold, warm or hot? That tells you most of it

Put your hand in the tank, carefully, or dip a clean glass in. The temperature of the water is the quickest clue.

• Cold, clean water, with the float valve dribbling or hissing: the float valve is not shutting off. The most common cause and the cheapest fix.
• Cold or lukewarm, level creeping up even with the float valve tied up: water is coming through from the hot water side, almost always a leaking cylinder coil.
• Hot water, steam or water spurting from the bent pipe over the tank: the pump is pushing water up the open vent. That is pumping over.
• The tank overflows only when the heating is on and settles when it is off: the float valve is set too high and there is no room for the water to expand.

Cause one: the float valve has stuck or worn

The float valve works like the one in a toilet cistern. As the level drops, the float drops and lets mains water in. As the level rises, the float lifts and shuts the valve. When the washer wears or scale builds up on the valve, which happens fast in hard London water, it stops shutting fully and the tank slowly fills until it runs out of the overflow pipe.

Lift the float arm gently. If the flow stops, the valve is at fault. It can sometimes be cleaned or have a new washer, but a new float valve is the proper repair and it is a short job. While you wait, you can tie the arm up with string to a batten across the tank. Do not leave it like that for weeks, because the heating does need to top itself up now and then.

The float should be set so the tank only holds a few inches of water when the heating is cold. If it is set high, the water that expands when the heating warms up has nowhere to go but the overflow. Bending the arm down slightly or adjusting the screw on the valve fixes that.

Cause two: a leaking coil inside the hot water cylinder

Inside a tank fed cylinder is a copper coil. Hot water from the boiler runs through the coil and heats the water around it, but the two waters are meant to stay completely separate. When the coil corrodes through, they meet. Water always moves from the side with more pressure to the side with less.

On most houses the big cold water tank sits higher, or holds more water above the cylinder, than the small header tank. So when the coil leaks, water from the cylinder is pushed into the heating circuit, and the header tank fills up and overflows even though its own float valve is shut. The test is simple: tie the float arm up or turn off the small valve on the pipe that feeds it. If the level still rises over a few hours, the water is coming from the cylinder.

Where the header tank is the higher of the two, it happens the other way round. Dirty heating water gets into your hot water, and you see brown water from the hot tap, black bits in the hot water, or a header tank that never stops topping itself up. Our guides to brown water from the hot tap and black bits in hot water cover that side.

A coil cannot be patched. On an older copper cylinder with a failed coil, replacement is the repair, and our page on hot water cylinder replacement explains what that involves. If you are weighing it up, is it worth repairing an old cylinder goes through the decision.

Cause three: pumping over

On an open vented system, a pipe called the open vent rises from the heating pipework and hooks over the header tank. It is there as a safety outlet. It should never have water coming out of it in normal use. When the pump pushes water up that pipe and into the tank, the tank gets hot, splashes, and often overflows. At the same time the circulating water picks up air every time it falls back into the tank, which rusts the radiators from the inside and makes sludge.

The usual reasons are a pump set to a higher speed than the system needs, a partly blocked pipe or valve that makes the water take the easy route up the vent, sludge building up where the cold feed and vent join the system, or pipework changed at some point without thinking about where the pump sits. It needs an engineer to trace, because the fix depends on which one it is.

Hot water in the header tank can also mean the boiler or its controls are overheating the system. If the tank is steaming, the boiler is banging or kettling, or radiators are too hot to touch with the thermostat turned down, turn the boiler off at its switch and leave it off until it has been looked at.

Is an overflowing header tank an emergency?

Usually not on the day. The overflow pipe takes the water outside, and a dripping overflow is a nuisance rather than a danger. It becomes urgent in three cases: the overflow pipe is blocked or missing and water is going into the loft, the water is hot or steaming, or you can see the ceiling below the tank staining. If water is already coming through, our guide on what to do if water is coming through the ceiling covers the first steps.

Leaving a coil leak or pumping over for months is a false economy. Both keep adding fresh, oxygenated water to the heating, which is what rusts radiators and clogs boilers.

What we do when we come out

We start by working out which of the three causes it is, using the same tests described above plus a look at the pump setting and pipework. A worn float valve is normally replaced there and then. A coil leak is confirmed before anyone talks about a new cylinder, and we will tell you plainly whether a like for like tank fed replacement or a change of system makes more sense for your home. Pumping over is traced to its cause rather than hidden by turning the pump down and hoping.

If the heating itself is also misbehaving, the boiler diagnosis visit covers the boiler and controls in the same trip.

When to call us

Call us on 020 4652 5111 or send an enquiry if the float valve is not the answer, the water in the tank is warm or hot, or you have tied the float up and the level is still rising. A photo of the loft tanks and the cylinder label helps us arrive with the right parts. We have 20 engineers across London, and our rate is on the pricing page and agreed before anyone attends.