Secondary hot water returns: why some taps run hot straight away

In many homes you turn on the hot tap in a far bathroom and wait while cold water runs away down the plughole. In some larger houses, the hot water arrives almost at once. The difference is often a secondary hot water return, sometimes called a recirculation loop or a hot water return loop.

This guide explains what a return is, which homes tend to have one, the trade-offs, and why adding one is a planned job for a plumber rather than a weekend improvisation. It is about the hot water you draw at the taps, not the heating.

Primary and secondary circuits

Water Regs UK, which publishes guidance on the water fittings regulations, puts the terms simply on its hot water and heating systems page: "Hot water systems supplying heating are referred to as primary circuits, those supplying hot water secondary circuits."

So the primary side is the water that heats things, such as the coil inside a cylinder. The secondary side is the hot water that comes out of your taps and showers. A secondary return is a pipe that brings that tap water back to where it is stored, so it can keep moving round a loop instead of sitting still in a long pipe.

How a return loop works

In a typical arrangement, the hot pipe leaves the hot water cylinder and runs past the outlets as usual. Instead of stopping at the last tap, a smaller pipe carries on and comes back to the cylinder. Water circulates round that loop, so the hot pipe near each tap stays warm.

What keeps the water moving varies:

• A small pump. Most modern returns use a pump made for domestic hot water, often on a timer so it only runs when hot water is likely to be wanted.
• Gravity. Some older houses have a return that relies on hot water rising and cooler water falling, with no pump at all.
• Valves. A non-return valve usually stops water going the wrong way round the loop, and there are often isolation and balancing valves so the plumber can set it up.

The pipework on a return should be well insulated, or the loop simply becomes a radiator in the walls. Our guide to pipe lagging covers insulation in general.

Which homes have one

Returns go with stored hot water: a vented cylinder fed from a loft tank, or an unvented cylinder fed from the mains. They are most common where the cylinder is a long way from the main bathroom or kitchen, in large or extended houses, and in flats or buildings with long communal runs.

Homes with a combi boiler usually heat water as it is drawn, with no store to return to, so a return of this kind is unusual there. If one tap is simply too far from the hot water source, a small point-of-use electric water heater near that tap is sometimes the simpler answer. A plumber can say which suits your house.

A return pump is not the heating pump

It is easy to mix these up, because both are small pumps in or near the airing cupboard. A heating circulator pump moves water round the closed heating circuit. A secondary return pump moves the drinking-quality hot water that you wash in. They are built for different jobs and are not interchangeable. If you are not sure which pump you are looking at, leave both alone and ask a plumber.

Why a return can help

• Less waiting. Hot water arrives at the far taps sooner.
• Less water down the drain. You run off less cold water while you wait.
• Water keeps moving. Water that sits still in long pipes is a hygiene concern. Water Regs UK notes that "good system design, commissioning and maintenance will help to reduce and address the risk of contamination resulting from microbial growth and limit waste due to taps left to run." Our guide to little-used outlets covers stagnant water more generally.

The trade-offs

A return is not free. Water moving round a loop loses heat from the pipe, which the cylinder has to replace, and a pump uses electricity. Timers, good insulation and a sensible route reduce the losses. A poorly designed loop can also make a cylinder lose its heat faster than expected or cause noise.

Water Regs UK also points out that storage and distribution temperature requirements are set out in other legislation, codes of practice and British Standards, and refers readers to the Building Regulations among others. We do not repeat temperature figures here. GOV.UK publishes Approved Document G, which deals with hot water safety in England; your installer should work to the current guidance.

Adding a return is a planned job

Adding a return to an existing house usually means a new pipe route back to the cylinder, a suitable connection on the cylinder, a pump, valves and controls, and insulation along the way. It often means lifting floors or opening walls. Done badly it can make things worse, so plan it with a plumber, ideally while other work is happening anyway.

• Unvented cylinders stay with a competent person. Any change to the pipework or controls of an unvented cylinder is for an installer competent to work on that cylinder. See unvented cylinder basics.
• Electrics go to a qualified electrician. A return pump and its timer need an electrical supply. That is a job for a qualified electrician, and electrical work in homes in England falls under Part P of the Building Regulations; see GOV.UK's Approved Document P. We do not give wiring instructions.
• Anything at the boiler goes to a Gas Safe registered engineer. A return is a hot water job, but if a quote involves changes at a gas boiler, that part needs a Gas Safe registered engineer.

Our guide to hiring a plumber covers getting quotes and asking the right questions.

What you can safely check

If your home already has a return, a few checks are reasonable:

• Is the pump timer set to times when you actually use hot water?
• Is the pump running quietly, without rattling or humming loudly?
• Is the pipework near the cylinder lagged?
• Have the far taps started running cold for longer than they used to?

If the answers point to a fault, tell a plumber what you noticed. Do not take a pump apart, and do not change settings on an unvented cylinder.

Does a combi boiler have a secondary return?

Usually not. A combi heats water as you draw it, so there is normally no stored hot water to return to. Some systems are different, so ask a plumber about yours.

Can I just fit a pump on the hot pipe to make the water arrive faster?

Not as a quick fix. A return needs a pipe back to the cylinder, the right valves, controls and insulation, and the electrical supply needs a qualified electrician. Ask a plumber to look at the whole system first.