DIY Heating Plumbing Tips 4 - Power Flushing Hydronic Central Heating Systems

DIY Heating Plumbing Tips 4 - Power flushing hydronic (water-based) central heating systems is basically the flushing of the 'heating circuit's' pipework, heat emitters and boiler heat exchanger with high velocity low pressure water.

When a hydronic central heating system is newly installed the system should be power flushed to remove any installation debris such as; pipe solder, wire wool or other foreign matter. This debris can interfere with the opening and closing of the system valves and later contribute or cause corrosion within the central heating system.

When you upgrade your central heating system, e.g, with a new boiler, the primary circuit (the heating pipework) and heat emitters (radiators) should be power flushed to remove any sludge, and chemically treated to prevent any future corrosion, sludge or limescale build-up occurring within the system.

Sludge and limescale build-up will reduce the performance of your system by between 5 and 15%.

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What is Power Flushing?

Power flushing is the use of a powerful self contained flushing unit designed for cleaning hydronic (water-based) central heating systems with the aid of chemicals.

There are a number of power flushing units now on the market and they generally do the same thing - they are designed to circulate high velocity water around the heating system. These power flushing units offer reverse flow circulation, water dumping to remove sludge, limescale and dirty water, water replacement and chemical treatment.

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When air gets into a hydronic central heating system - corrosion will take place:

  • The result of rusting is the build up of magnetite sludge which can cause blockages in the pipes, radiators, clogging and stopping the operation of the pump and valves leading to premature system failure.
  • Corrosion can also caused by the electrolytic reaction between two dissimilar metals when connected by an electrolyte (a liquid - in this case water).
  • Corrosion can also cause 'pinhole' leaks in radiators. Some severely corroded radiators may begin to leak after they have been power flushed. This can be a result of the rust that was clogging-up the 'micro' corrosion holes being removed when the system is power flushed and descaled.

Limescale build-up can seriously damage your boiler's heat exchanger by insulating the heat exchanger and causing localized boiling, reduced heat output and boiler failure.

Click the thumbnail below to to view the spots of accumulated sludge in a central heating system - opens in new window.

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How Does The Unit connect To The System?

A power flushing unit may be connected to the hydronic central heating system by:

  • First isolating the electrical power supply to the central heating system circulating pump.
  • Maintain the 'earth continuity bond' by attaching a temporary earth bonding wire either side of the pump's isolating valves
  • Then close off the isolating valves either side of the pump before removing the pump. The units flushing hoses can then be connected to the pump's isolating valve union.

Some power flushing unit manufacturers supply pump adaptors that connect directly onto the pump by removal of the pump's 'plate', this eliminates the complete removal of the pump to connect the flushing unit.

If you can't get access the central heating pump then you can connect the power flushing unit via the valves on the radiators tails (flow and return pipe) by first closing off the radiator valves, (user control valve and lock-shield valve) and then detach the valves from the radiator and drain.

To close off the lock-shield valve remove valve cap then untighten the top lock-nut then close off the valve spindle by turning clockwise with a flat head screwdriver.

Once the radiator has been drained and removed the power flusher's flushing hoses can then be connected to the flow and return valves on the radiator's tail pipes.

If connecting to copper or steel tail pipes then you need to be sure that the supplementary bonding the 'earth continuity bond' is maintained by attaching a temporary earth bonding wire to the tail pipes before detaching the radiator

Note: If you have an open-vented system then you will have to isolate the 'feed & expansion cistern/tank' main service valve, or tie off the ball (float) valve, and cap off the open safety vent pipe before you connect the flushing unit. The water in the 'feed & expansion cistern/tank' can then be drained into the reservoir of the flushing unit.

Note: If you have a sealed system then you will need to first drain down your system until it is no longer pressurized before you connect the flushing unit

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Preparing Your System for Power Flushing

To prepare your hydronic central heating system for power flushing you should:

  • Switch off your central heating boiler and isolate the electrical power supply to the circulating pump.
  • Record how many turns it takes to shut off the lock-shield valve on each radiator so that you can re-instate the individual settings and therefore avoid balancing the heating system all over again.
  • Fully open both radiator valves, if using Thermostatic Radiator Valve(s) (TRV) then make sure that they are set to the fully open position. You can check if the TRV is fully open by removing the head of the TRV and checking that the plunger pin moves freely.
  • Make sure that the motorized zone valve (diverter valve) is fully open on the heating circuit, you can manually set it if necessary .
  • Because power flushing units have a reverse flow action any check or anti-gravity valve must first be by-passed / bridged to enable full reverse flow flushing to take place.
  • Close off the water supply to the 'feed & expansion cistern/tank' in open vented systems by closing off it's service valve, or by tying off the ball (float) valve in the 'feed & expansion cistern/tank if you can't access the service valve, and cap off the 'open safety vent pipe'.
  • De-pressurize sealed systems to atmospheric pressure by draining off some water from the heating circuit.

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Connecting The Power Flushing Unit

Generally power flushing units are connected to the hydronic central heating circuit via the circulator pump's unions after the pump has been removed.

Some manufacturers provide pump 'adaptors' that connects directly onto the circulator pump. This device eliminates the removal of the pump to enable the connection of the power flushing unit.

The power flushing unit can be connected to the heating circuit by draining the boiler and disconnecting the boiler's primary flow and return pipes. The unit can then be connected to the primary flow and return whilst the boiler is disconnected

The power flushing unit can also be connect via the radiator tails (flow and return pipes). If you connect via the radiator tails, then be sure to remember to flush the individual radiator you have disconnected after you have completed flushing the heating system.

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Power Flushing Your Central Heating System

Once the power flushing unit has been connected to the heating circuit and the water inlet hose is attached to the unit, make sure that both the dumping & overflow hoses terminate at a suitable drain.

If your heating system is open vented, then once you have isolated the water supply to the 'feed & expansion cistern/tank' by closing the service valve or by tying off the ball (float) valve you can drain the water from the 'feed & expansion cistern/tank into the power flushing unit.

Along with the water in the heating circuit, a minimum level of water must be maintained within the unit's reservoir to enable the flushing operation.

Power flushing procedure:

  • If the power flushing descaling chemical has not already been added to the heating system it can now be introduced by pouring it into the unit's reservoir.
  • Fully open all radiator valves making sure that you record how many turns it takes to fully open each lock-shield valve so that you can reinstate the correct setting for each lock-shield valve when you have finished power flushing the system.
  • Make sure that the motorized zone valve is fully open to the heating circuit, and/or set to the 'manual' position.
  • Switch the power flushing unit to it's circulating mode and turn on.
  • Turn on the boiler and allow the water to heat to 40 - 50°C this heat will aid the chemical agent to break down any stubborn limescale and sludge within the heating circuit - Turn the boiler off once the temperature has been raised.
  • Reverse the power flushing flow every 10 - 15 minutes.
  • Shut all radiators except one, power flush this single radiator reversing the flow regularly.
  • Close radiator and move to and open then next radiator and repeat individual flushing. Repeat individual flushing of all radiators.
  • Once flushing of the last radiator has been completed switch the power flushing unit to the water 'dumping' mode. Make sure to open the unit's water inlet valve so the water level in the reservoir is maintained whilst dumping is taking place.
  • Dump the water in the radiator until the water runs clear.
  • Close radiator valve, and repeat the dumping process for each of the previous radiators.
  • When the last radiator has been dumped switch the power flushing unit to the circulating mode.
  • Fully open all radiators valves.
  • Add neutralizing crystals to the water via the unit and allow to circulate for about 15 minutes around the system to neutralize the descaling chemicals.
  • Do a final dump until the water has turned clear - test the dump water with ph paper until ph level is 7; 7 = neutral not acidic nor alkaline
  • A corrosion inhibitor can now be added to the system to safeguard and protect against future corrosion. Allow the corrosion inhibitor to circulate around the heating system.

When completed, the power flushing unit can be isolated and disconnect from the heating circuit and the heating system can then be reinstated for normal operation.

To power flush a system with ten radiators, will take roughly 4 to 8 hours depending on the severity of system scaling and rust.

Be aware, that when using descaling chemicals in older hydronic central heating systems with many years of corrosion, there is the possibility of leaks occurring from pin holes in the radiators once the corrosion and sludge has been treated and removed.

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