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Service or replace a Grundfoss central heating boiler pump

Servicing a Grundfoss Central Heating Pump

Wet central heating depends on a steady cycle of hot water pumped from the boiler to the radiators then back to the boiler for reheating. If the central heating pump is not working properly, the result is poor circulation or none at all. Adjusting or bleeding the central heating pump may be the answer; otherwise, it may need replacing.

Bleeding The Central Heating Pump

If an airlock forms in the circulation pump, the impeller spins ineffectually and your radiators fail to warm up properly the cure is to bleed the air from the central heating pump, a procedure similar to have a jar ready to catch any spilled water.

Look for a screw-in bleed valve in the central heating pump’s outer casing. Then switch off the central heating pump and open the bleed valve slightly with a screwdriver or vent key until you hear air hissing out. When the hissing stops and a drop of water appears, close the bleed valve.

Adjusting the Pump

Basically, there are two types of central heating pump: fixed-head and variable head. Fixed-head pumps run at a single speed, forcing the heated water round the system at a fixed rate. The speed of variable-head pumps is adjustable.

When fitting a variable-head pump, the installer balances the radiators, then adjusts the central heating pump’s speed to achieve an optimum temperature for every room. If you can’t boost a room’s temperature by opening the radiator’s hand wheel valve, try adjusting the pump speed. However, before adjusting the central heating pump, you should check that all your radiators show the same temperature drop between their inlets and outlets. To test your radiators, you can f obtain a pair of clip-on thermometers from a plumbers’ merchant.

Clip one of the thermometers to the feed pipe just below the radiator valve; and the other one to the return pipe, also below its valve (1). The difference between the temperatures registered by the thermometers should be about 11°C (20°F). If it’s not, close the lock shield valve slightly to increase the difference in temperature; or open the valve to reduce it.

Having balanced all the radiators, you can now adjust the pump’s speed by one increment at a time (2) until the radiators are giving the overall temperature you require. Depending on the make and model of pump, you may need to use a special tool, such as an Allen key, to make the adjustments. Switch off the pump before making each adjustment.

Replacing a Worn or Failed Pump

If you have to replace a faulty pump, make sure you buy a new one that is equivalent in performance. If in doubt, consult a professional installer.

First, turn off the boiler and close the isolating valves situated on each side of the pump. If the pump lacks isolating valves, you will have to drain down the whole system.

At your consumer unit, identify the electrical circuit that supplies the pump and remove the relevant circuit fuse or MCB. Then take the cover plate off the pump (1) and disconnect its wiring. With a bowl or bucket ready to catch the water from the pump, undo the nuts that hold the pump to the valves or t pipe work (2). Having removed the old pump, install the new one (3), taking care to fit correctly any sealing washers that are provided. Tighten the connecting nuts. Remove the cover plate from the new pump and feed in the flex. Connect the wires to the pump’s terminals (4), and then replace the cover plate. If the pump is of the variable-head type (see above), set the speed control to match the speed indicated on the old pump.

Open both isolating valves – or refill the system, if you had to drain it – then check the pump connections for leaks. Open the pump’s bleed valve to release any trapped air. Finally, replace the fuse or MCB in the consumer unit and test the pump.