How To: Powerflush a Central Heating System

I want to know how to powerflush a central heating system properly

About the problem

Cleaning a central heating system is integral to its health and powerflushing is a popular cleaning method due to the rapid but comprehensive clean it delivers. The benefits of a clean heating system are well-documented:

  • more even heat distribution throughout the property
  • improved overall energy savings
  • lower household bills
  • increased system reliability
  • extended system life

So how do you know if a system needs a powerflush? Here are some symptoms to look out for.

  • Are certain rooms much colder than others?
  • Does it take a long time for system to warm up?
  • Are one or more radiators cold at the bottom?
  • Do radiators need frequent bleeding?
  • Is the boiler noisy?

Householder or landlord?

Some common questions:

How long does a central heating powerflush take? Done properly, the process can take several hours – and this time will be extended if the system is large or particularly dirty.

Which cleaning chemical should I use? This question is examined in more detail in our How To guide to choosing a cleaning chemical.

More information for householders

UK Building Regulations

In addition to the benefits mentioned above, the Compliance Guide to Part L of the UK Building Regulations for England and Wales requires a central heating system to be cleaned and flushed before a corrosion and scale inhibitor chemical is added during commissioning. It’s also prudent to do this when an old central heating boiler is being replaced by a new boiler. Many boiler manufacturers now make the warranty dependent on flushing and cleaning in accordance with BS7593 and Part L.

The solution

Here is a step by step guide for how to effectively powerflush a central heating system. You will need:

  • A powerflushing machine such as the Sentinel JetFlush 4
  • An appropriate Sentinel cleaning chemical – find out more about choosing the right product here
  • A Sentinel JetFlush Survey Kit to measure effectiveness of the powerflush for the customer (optional)


If you dont have a JetFlush you can also complete a manual mains pressure clean or a manual gravity clean.

Skip to: Machine set up Primary flush Secondary flush After the clean

Before the flush

  • Go around the system and record:
    • the temperatures of all the radiators – identifying those with substantial cold spots. This will enable you to demonstrate the success of the powerflush to your customer when completed.
    • the settings of any TRVs. This will enable you to restore system operation more quickly once the powerflush is completed.
  • Take a sample of the system water for reference.
  • Establish whether the system being powerflushed is open or sealed.
  • Turn off all electrical controls and electrically isolate the system and then isolate the cold water supply to the central heating system.
  • Manually close any automatic air vents.
  • For open-vented systems, cap-off or temporarily join together, the open vent and cold feed to the feed and expansion cistern.
  • Mark the position or note the setting of the lockshield valves, then fully open all valves.
  • Remove any thermostatic radiator valve (TRV) heads to ensure maximum flow through each valve.
  • Set any diverter or zone valves to their manual, open position.
  • Where practical, anti-gravity and non-return valves should be bridged, by-passed or temporarily removed as failure to do so will prevent flow reversal.
  • Connect the powerflushing machine to the central heating system following the manufacturer’s instructions.
  • If a new boiler is being fitted, to prevent damage or contamination, powerflushing should either beundertaken before the boiler is installed or with the new boiler isolated from the rest of the system.

You should now be ready to connect the powerflushing machine.

Situating and connecting the powerflushing machine

  • Place the unit somewhere within reaching distance of cold feed water and an appropriate drain point.
  • Give consideration to the customer’s property: are you protecting customer’s carpets from any splashes, stains and spillages?
    • Many powerflushing units can be ordered with a spill containment bund, but plastic sheeting will provide similar protection.
    • Wherever you are trailing pipes, check pipes are clean or put down dustsheets to protect flooring.
  • Wherever possible, the powerflushing machine should be connected in the place of the heating circulator pump for the optimum clean. If it isn’t possible to isolate and remove the circulator; the powerflushing machine can be connected across a radiator, however, this will impair the efficiency of the clean.
  • Follow the powerflushing machine manufacturer’s instructions for hose connections.

The Primary Flush

  1. Fill the unit with water from the mains and switch on the pump. Let the water circulate for 2-3 minutes.
  2. Add your cleaning chemical into the powerflushing machine and continue circulating for a further 15 minutes approximately, reversing the flow every 5 minutes or so to improve the initial cleaning process.
  3. After approximately 15 minutes of cleaning, the hot water circuit and all but one of the radiators should be isolated. Begin with the radiators identified as having cold spots in your initial assessment of the system.
  4. Circulate the water and cleaning solution through the first radiator, reversing the flow periodically. Vibrating the radiator by banging it with a rubber mallet of the flat of your hand will help to dislodge stubborn debris.
  5. Once the entire surface of the radiator is clear, move on to the next radiator. Work your way around the system in this way, repeating this process one radiator at a time.
  6. Once this process has been completed, the secondary flush can begin.

The Secondary Flush

  1. With a single radiator open, dump the contaminated water, reversing the flow periodically until the water leaving the waste pipe is clear. Isolate this radiator and move onto the next.
  2. Work your way around the system, repeating this process on each radiator.
  3. Re-open all radiator valves and flush the system through until the water being dumped to the drain is totally clear.
  4. Test the water now draining from the system using one or more of the following methods:
    • Turbidity Tube – Continue flushing until a reading of less than 100ppm is achieved.
    • Conductivity Meter – Continue flushing until the reading is within 10% of the value for the mains water to the property.
  5. If you are using a Sentinel cleaning chemical, no neutralisation will be required. If you are not using a Sentinel chemical, please consult the manufacturer’s instructions.

Following the clean

  1. Add a high quality inhibitor to the powerflushing unit and re-circulate for 10 minutes. Take a sample of the system water and test using a Sentinel X100 Test Kit or Sentinel X100 Quick Test Kit.
  2. If necessary, add more inhibitor, re-circulate and test again.
  3. Once you have confirmed system protection, the powerflush is complete.
  4. Disconnect the powerflushing unit and replace the circulator pump.
  5. The system configuration and all components should be returned to their original settings. Remove caps placed on the F&E tank if you were powerflushing an open system.
  6. Return TRVs to their original positions as recorded at the start of your visit.
  7. Switch on the boiler and allow the system to come up to temperature. Bleed radiators as necessary and record their temperatures. Compare with the original readings to demonstrate the success of the powerflush to your customer.
  8. Return the boiler and room thermostats to their normal operating temperatures.

Products related to this guide: