Secondary Return Pump Not Working? Why Hot Water Is Slow

What a secondary return is

In a small flat the hot tap is a few metres from the cylinder, and the cold water sitting in the pipe clears in seconds. In a larger London house, with the cylinder in a basement plant room or a loft and bathrooms on three floors, that pipe can hold a lot of water. Without help you would run a bucket of cold water down the drain every time you wanted to wash your hands.

A secondary return fixes that. The hot pipe runs out from the top of the cylinder, past the taps, and then a smaller pipe comes back to a connection lower down the cylinder. A small pump on that return pipe keeps the water moving round the loop, so the hot supply is always hot right up to the branch for each tap. There is normally a non return valve so water only goes one way, isolating valves either side of the pump, and a timer or a thermostat that decides when the pump runs.

Because the loop carries drinking water, the pump is a different animal from the one on your heating. It has a bronze or stainless steel body rather than cast iron, because an ordinary heating pump would rust in fresh, oxygenated water. Grundfos and Wilo make most of the ones we see.

Signs the secondary return has stopped working

The symptom people notice is the wait. The water at the tap is cold, then lukewarm, then hot, and it takes about as long as it would in a house with no return at all. Other clues help narrow it down.

• Every tap in the house is slow to run hot, not just one: the loop has stopped, rather than one branch having a problem.
• The pump is silent and cool to the touch when it should be running: no power, a failed motor or a seized rotor.
• The pump hums but the return pipe stays cold: the rotor is stuck, usually with scale, or a valve on the loop is shut.
• It worked until the clocks changed or there was a power cut: the timer has lost its settings or is running at the wrong times.
• Water around the pump, or a crust of white scale on its body or unions: a seal is weeping and the pump is on its way out.
• Hot water coming out of the cold tap, or the cylinder running out faster than it did: a stuck non return valve can let water go the wrong way round the loop.

What you can check yourself

None of these involve opening the pump or the cylinder. They are enough, in a good number of cases, to either get the pump running again or tell an engineer exactly where the fault is.

• Find the pump. It is normally near the cylinder, on a pipe smaller than the main hot outlet, often with a small plug in timer or a fused switch beside it.
• Check the fused spur or plug that feeds it. A pump on a switched spur is easy to turn off by accident, and a blown fuse is common when a motor starts to fail.
• Look at the timer. Make sure the clock shows the right time and the on periods cover the hours you use hot water. Mechanical timers with pins lose their place after a power cut.
• Feel the return pipe where it comes back into the cylinder, once the pump has been running for ten minutes. Warm means water is going round. Cold means it is not.
• Check the isolating valves either side of the pump. The slot on a service valve should line up with the pipe. Someone working nearby may have turned one off and not turned it back.

Do not tap or strike the pump to free it, and do not undo the pump unions yourself. On an unvented cylinder the loop is at mains pressure, and on any system the water in it is hot enough to scald.

Why these pumps fail

The main reason is scale. A secondary return pump runs for hours every day in hot, hard water, and in most of London that means limescale builds up inside the rotor chamber until the rotor sticks. Our guide to hard water and hot water cylinders in London explains why hot water systems here work harder than in soft water areas. A pump that has sat idle for weeks, after a holiday or a long void period in a rental, is especially likely to seize.

Motors and capacitors fail with age, and seals start to weep, which is when you see a white crust or rust stains on the pump body.

The non return valve on the loop can stick open or shut. Stuck shut, nothing moves. Stuck open, water can take the easy route backwards, which sometimes shows up as warm water where it should not be, a problem covered in our guide to warm water from cold tap faults.

Is it a problem if the pump just stays off?

You can live with slow hot water for a while, and nothing will break. The two things to think about are waste and hygiene.

The waste is water and heat: every time you run the hot pipe cold waiting for it to warm, you pay to heat that water and then throw it away.

The hygiene point matters more in a house that is not in daily use or in a rented property. A loop full of water that sits lukewarm is the kind of place bacteria like Legionella grow. Water in a working loop should stay hot all the way round, and the water coming back to the cylinder should still be at least 50 degrees. Our guide, what temperature should a hot water cylinder be set at, covers the numbers in more detail, and landlords should read our note on landlord hot water responsibilities as well.

Running costs and the timer

A loop that runs 24 hours a day is a long radiator. It keeps hot water moving through pipes in floors and walls, and every metre of it loses heat. That is why the pump is normally on a timer, and why the loop should be insulated wherever you can get at it. Our guide to insulating hot water pipes covers which runs matter most.

A sensible setting is to run the pump for the periods you actually use hot water, with a short gap either side. If you have a timer that has been left on constant, a new pump is a good moment to set it properly.

What the repair involves

An engineer will confirm the pump has power, check it is actually turning, and test the valves on the loop before blaming the pump. If it is seized, a stuck rotor can sometimes be freed, but on an old pump with scale inside it the fix rarely lasts, and replacement is usually the right call. The new pump must be one rated for drinking water, sized for the length of the loop, and fitted with the non return valve and isolating valves in the right places.

On a tank fed, vented system this is a straightforward job. On an unvented cylinder, the loop is part of a pressurised system, and extra pipework on the hot side adds to the volume of water the expansion vessel has to cope with. Anyone working on it should check the cylinder's safety devices before they leave, which is part of our unvented cylinder repair service. If the pump is fine and the delay is at one tap only, our guide to no hot water upstairs or at one tap is the better place to start. If the loop is humming on a timer and you want to know whether the sound is normal, our page on a hot water cylinder making noise explains what each sound means.

When to call us

Call us on 020 4652 5111 or send an enquiry. Tell us what kind of cylinder you have, where the pump is, whether it hums or is silent, and whether the problem started after a power cut or after other work in the house. A photo of the pump label and the cylinder label saves time. We have 20 engineers across London, and our rate is on the pricing page and agreed before anyone attends.