Domestic Hot Water Recirculation System Installation

Recirculation is most useful where the water heater is a long way from the taps. A typical case is a house with the boiler and hot water cylinder in a utility room or plant room on the ground floor and the bathroom upstairs. Larger flats, maisonettes and homes with several bathrooms are similar. In a small flat where the water heater sits right next to the bathroom, recirculation is rarely worth it. The pipe run is short, so hot water arrives quickly anyway.

The type of water heater matters too. Recirculation needs a storage cylinder for the water to return to. A combi boiler without a cylinder heats water as it flows through, so it usually can't run a recirculation loop. The exception is a boiler the manufacturer has designed for it. If you live in a block of flats with central hot water, the building management or owners' association looks after recirculation in the risers. Don't touch shared pipework without their consent.

A good installation starts with planning. The plumber first looks at where the cylinder is, how the hot water pipes run and how far the return pipe needs to go. The loop usually runs to the furthest outlet or to the end of the main branch, not to every single tap.

1. Shut off the water supply and drain the affected part of the hot water system.
2. Mark out the return pipe route as short as possible and without dead legs, ideally alongside the hot water pipe.
3. Lay the recirculation pipe, which is usually smaller in diameter than the hot water supply, and connect it to the end of the hot water branch.
4. Insulate both pipes, flow and return, along their full length, including bends and fittings.
5. Fit the recirculation pump near the cylinder with isolating valves on both sides and a non-return valve, with the arrow pointing in the direction of flow towards the cylinder.
6. Where there are several branches, fit balancing or thermostatic circulation valves so water flows evenly through all of them.
7. Connect the return pipe to the cylinder's secondary return connection, if it has one.
8. Fill the system, bleed the air out and carry out a pressure test for leaks.
9. Connect the pump to the power supply and set the controls, meaning run times or temperature.

Connecting the pump to the electrical supply, especially a fixed connection without a plug, is a job for a qualified electrician. In some countries, work on unvented (pressurised) hot water cylinders is also restricted to suitably qualified installers, so check what applies where you live. Ordinary changes to pipework inside a house or flat don't usually need building permission. If the work would affect load-bearing structures or the common parts of a block of flats, check with your local building authority before you start.

Domestic hot water needs a pump designed for drinking water, with a body made of bronze, brass or stainless steel. A standard heating circulator with a cast iron body doesn't belong here, because fresh, oxygen-rich water would make it corrode. A small pump is enough for an ordinary family home. Larger pumps are only needed for long pipe runs and big buildings.

Why do the controls matter so much? A pump that runs non-stop keeps pushing hot water round the pipes, and that water loses heat on the way. Most households need hot water in the morning and evening, so these options are common:

• Timer: the pump only runs at set times, for example in the morning before you leave and in the evening
• Thermostat: the pump switches on when the water in the return pipe cools below a set temperature
• Self-learning pump: according to the manufacturer, it remembers when the household uses hot water and adapts to that pattern
• On-demand start: a button or sensor near the tap starts the pump for a short time only

Hygiene matters as well. Water in the loop shouldn't sit at a low temperature for long periods, because bacteria such as legionella multiply more easily in lukewarm water. For this reason the cylinder outlet temperature is usually kept at roughly 55 to 60 °C, and many systems allow an occasional heat cycle at a higher temperature. The pipework itself uses the usual materials for water pipes, such as plastic, multilayer pipe or copper, with insulation of a suitable thickness.

A low loss header sometimes comes up in connection with recirculation. It belongs in heating circuits, where it separates the boiler from the heating zones, and has no place in a domestic hot water loop. If the plant room and heating are being reworked at the same time, that is a separate job.

The easiest case is a new build or full renovation. The walls are open, and the return pipe goes in alongside the other pipework. A cylinder with a ready-made secondary return connection and a short, simple route also help.

Fitting a loop into a finished house takes more work. The plumber needs a route that avoids a lot of chasing into walls, such as a service duct, a suspended ceiling, the space under the stairs or the plant room. Other things add time: old pipework in poor condition that needs partly replacing, missing isolating valves, a cylinder without a secondary return connection, or several branches that need balancing. The more of these obstacles there are, the more labour the job takes, and the more it costs.

• Pump without controls. It runs all day and all night, uses electricity for nothing and cools the water in the cylinder.
• Missing or thin insulation. The water in the pipes loses heat quickly, the cylinder has to reheat more often and the benefit disappears.
• No non-return valve. Water can flow the wrong way, and cold water can be pushed into the return pipe.
• Pump fitted the wrong way round. The arrow on the pump must point towards the cylinder.
• Unbalanced branches. Water takes the shortest route, and the distant bathroom still waits for hot water.
• Wrong pump. A heating pump with a cast iron body corrodes, and an oversized pump is noisy and uses more energy.

In practice it often happens that the recirculation works, but the bill for heating water goes up. The cause is almost always a pump without a timer and uninsulated pipes.

When the job is finished, ask the installer to show you how the system works, and check these points:

• The pressure test has been done and all joints are dry, with no drips
• Both the return pipe and the hot water pipe are insulated along their full length
• The pump is designed for drinking water and the arrow points towards the cylinder
• Isolating valves and a non-return valve are fitted at the pump
• The controls are set and you know how to change the times or temperature
• Hot water reaches the furthest tap within a few seconds
• The pump runs quietly, without humming or gurgling
• The electrical connection was made by a qualified electrician
• You have the instructions for the pump and controls, and ideally a sketch of the pipe route