Circulation Pump Installation

A heating engineer fits a new pump when a house is built, when the heating is refurbished or when the heat source changes, for example when a wood burner with a back boiler or a heat pump is added alongside a gas boiler. Think about replacing an older pump if:

• It hums, buzzes or vibrates noticeably
• Some radiators stay cold even though the boiler is firing
• It switches itself off or will not start after the summer
• It is an old model with a manual speed switch that runs flat out all the time

Why replace a pump that still works? Older types run at the same output all season, even when the house needs much less heat. Newer pumps with electronic control adjust their output on their own. They are quieter and use far less electricity. Under EU ecodesign rules, standalone circulation pumps have had to meet efficiency requirements for a number of years, and the UK applies similar rules.

If you have a gas boiler with a built-in pump, as most combi boilers do, the pump must be replaced by a qualified, registered gas engineer who works on that boiler. In a block of flats with a central plant room, the pumps belong to the shared equipment. The building management or owners' association decides on replacing them, not an individual flat owner.

Even a careful installation cannot make up for a badly chosen pump. The heating engineer looks at several things when choosing one.

Heating or hot water

A heating pump works in a sealed circuit where the same water goes round and round. A secondary hot water pump moves drinking water, which keeps bringing in fresh oxygen. That is why its body must be bronze or stainless steel. A cast iron heating pump would quickly rust in a hot water loop. A timer or thermostat on a hot water pump is also worth having, so it does not run all night for nothing.

Output and head

The pump has to overcome the resistance of the whole system: the length of the pipework, the number of radiators, the valves and the bends. This is called the head. A pump that is too powerful causes rushing noises in the valves and wastes electricity. One that is too weak cannot push water to the furthest rooms. So a stronger pump does not automatically mean better heating.

Control modes and fit

Modern pumps usually offer several modes. Proportional pressure tends to suit radiators with thermostatic valves. Constant pressure is often chosen for underfloor heating. In auto mode, the pump finds its own setting. When replacing a pump, the engineer checks that the new one has the same length between the connections and the same threads, so it fits between the existing valves. If the boiler controls the pump with a signal, the new pump must be able to receive that signal.

The process depends on whether it is a new system or a replacement in existing heating. A typical replacement goes like this:

1. The engineer checks the pipework, isolating valves, expansion vessel and system pressure.
2. They disconnect the pump from the power and close the valves on both sides of it. If there are no valves or they let by, they drain part or all of the circuit. For a local repair, they sometimes freeze the pipe so the whole system does not need draining.
3. They remove the old pump and clean the sealing faces.
4. They fit the new pump with new washers. The arrow on the body must point in the direction of water flow, the motor shaft must be horizontal and the terminal box must not face downwards, so water cannot drip into it.
5. The electrical connection is made by a qualified electrician. If the pump is wired to the boiler, it is connected according to the boiler manufacturer's instructions.
6. They open the valves, top up the system to the specified pressure and bleed both the radiators and the pump itself. Many new pumps have their own venting programme.
7. They set the pump mode to suit the type of heating and check that every radiator warms up.

Whether the pump goes on the flow or the return depends on how the system is laid out and on the boiler manufacturer's advice. There is no universal rule.

The quickest job is a like-for-like swap: the new pump is the same length, the valves seal properly and the pump is easy to reach. The work grows when:

• There are no isolating valves on either side of the pump and the whole system has to be drained
• The new pump has a different length and the pipework needs altering
• Old pump unions are seized and crack when undone
• The system is full of sludge and needs flushing, with a filter added, before the new pump goes in
• The wiring or the link to the boiler controls needs changing
• A combined system with radiators and underfloor heating has several pumps that must work together

In practice it often happens that a new pump in an old system starts humming or seizes after a while. The cause is usually sludge from pipework that was never flushed, not the pump itself.

• Pump fitted against the direction of flow
• Motor with a vertical shaft, or terminal box facing downwards
• Old washers reused, so the joints start dripping after a few days
• Air left in the system, so the pump rushes and radiators are cold at the top
• Pump left on its highest setting "just in case"
• Cast iron pump used on a drinking hot water loop
• New pump fitted in a sludged system without flushing or a filter

When the engineer hands over, ask them to show you how the pump is running and how it is set. Go through these points:

• The arrow on the pump body matches the direction of flow
• The joints are still dry after several hours of running
• The pump runs quietly, without buzzing or vibration
• The pressure gauge shows the value the engineer gave you
• All radiators or underfloor loops warm up
• Working isolating valves are fitted on both sides of the pump
• You know which mode is set and why
• You have the pump instructions and warranty documents