How to powerflush a microbore system

Can you powerflush a microbore system? That’s the most frequently asked question during Kamco’s training sessions. Here, the company’s Keith MacBain sheds some light on the topic.

With modern high-efficiency boilers, and particularly those fitted with ErP pumps, keeping the heating system clean has never been more important, and boiler warranties often depend on it.

It has long been accepted that the most effective way of flushing a heating system is with a powerflushing pump, and many thousands of systems are cleaned every day using this method.

However, being asked to clean microbore can cause most engineers to wince. They can be powerflushed but you need to plan ahead and expect the job to take longer than other types of system, particularly if piping to the radiators is the old 8mm copper tubing – in which case some ingenuity may be needed. And don’t think that microbore is a thing of the past: many modern systems may have main runs in 22mm pipe but with only 10mm drop feed pipes down to the radiators themselves.

The laws of physics dictate that the flow rate needed to move debris of a given size in pipework reduces with the diameter of the pipework. Before quoting for any flushing job, always fire the boiler and inspect the condition and heat output of each individual radiator. This is even more important with older microbore systems.

Check and check

A sight engineers would prefer not to see is microbore systems with twin-entry radiator valves (both valves on one end of the radiator). If the internal tube that should take the hot water to the far side of the radiator is missing or has fallen off, that particular radiator can’t be powerflushed.

How would you know if the tube is there or not? If warm water is circulating through the system, check the radiators carefully. If they are only warm immediately adjacent to the twin entry valve, and cold elsewhere, the tube is missing, and you can’t powerflush this radiator.

During your inspection visit, pre-treat the heating system with a powerful flushing chemical, and even consider double-dosing. You’ll want the debris broken up as finely as possible before carrying out a flush on microbore.

If possible, the heating system should be operated normally until you can return to carry out the powerflush.

Connecting the power flushing pump into the heating system

Connection of the powerflush pump should be on to the circulator pump fittings, a circulator pump head adapter, or on to larger diameter (22mm) pipework. It should never be on individual small diameter radiator tails or pipework otherwise the overall flow rate to other parts of the system will be so low as to be ineffective.

As with all powerflushes, it is very important that the full procedure is carried out, with the flow directed individually to each radiator, in line with BS 7593:2019* guidance.

What happens if there is no flow to a radiator on a microbore system during a powerflush?

Powerflushing pumps rely on a high flow rate but generally limit pressure to only a few bar to avoid stressing systems. If normal powerflushing doesn’t clear a radiator on a microbore system, it may be that a plug of compacted sludge is blocking the narrow diameter pipework (typically 8mm copper) leading back to the manifold where it feeds from larger diameter pipe. This often occurs where the pipe has been bent sharply to avoid joists or some other obstruction.

If the pressure from your powerflushing pump is not enough to move the sludge, a useful work-around is to pressurise just the problem radiator, using air rather than water pressure.

This can be done via either the air bleed valve, or the ½” BSP plug found at each end of the top of most radiators, using a small air compressor or hand pump to raise the pressure in the radiator.

A useful tool is a dry pressure testing device. It consists of a pressure gauge and a Schrader valve of the type used on car tyres. It can be adapted to screw into the ½” tapping to be found on the top of most radiators using PTFE tape to seal.

The technique is to close both radiator valves, remove a ½” plug from one end of the radiator, and screw in the tool. Connect a foot pump or compressor on the Schrader valve and pump up to approximately 4 bar air pressure.

Quickly open one radiator valve wide and hopefully there will be a gurgling noise as the air pressure dislodges the plug of sludge. Close the radiator valve, pump up to 4 bar pressure again and open the radiator valve at the other end of the radiator.

Check for success

Your powerflushing pump should still be connected into the system, and now is the time to direct the full flow to the problem radiator, reversing the flow regularly. If you have hot water in the system, a hand on the radiator or an infra-red thermometer will tell you how successful you’ve been.

Once you’ve overcome the specific problems likely to be found with a microbore system, the powerflush should be carried out in the full sequence of events as detailed in BS 7593:2019 or according to the manufacturer’s instructions.

For further information and techniques on successful powerflushing, it’s a good idea to sign up to a manufacturer’s training course, where you can not only learn what to do but also what not to do. ■