Descale or Powerflush? – This Is The Difference Between Them

Published: 31st January 2011
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This article will explain the difference between a descale and a powerflush. I’ll also attempt to explain some common problems with combination boilers and whether the solution is to powerflush, de-scale, do both, or do something else.

There is no binding definition of a powerflush actually between individual plumbers/heating engineers. Not one that you can rely on to compare quotes with anyway.

Here is my working definition of the difference between a powerflush and a de-scale.

Powerflush – the removal of rust, sludge and other debris from the water that flows through your radiators, pipework and the internal boiler waterways that the pipes you can see are connected to.

Descale – the removal of limescale from the same pipework as above.

Here’s the thing, and this is important.

When a boiler engineer talks about your boiler needing a de-scale he/she is most often talking about a build up of limescale within the Domestic Hot Water (DHW) plate heat exchanger on a combination boiler.

Sometimes called the ‘secondary heat exchanger’, this unit in your boiler is basically a ‘sandwich’ of stainless-steel plates that allow the transfer of heat from the boiler to the water coming out of your hot water taps.

Some Common Hot Water Problems With A Combination Boiler

1 Poor water flow/low pressure water from the hot taps.

In hard water areas, combination boilers installed without or with faulty protection on the mains feed to the boiler will gradually build up a skin of limescale on the ‘clean’ side of the heat exchanger. If it’s left, like a callous, it will get thicker and thicker. Eventually, it gets so thick that the water cannot flow through quickly enough.

Assuming that the incoming water pressure is good to start with, to solve this problem you will need to either change the hot water heat exchanger or have it de-scaled.

Either way, make sure you get some scale protection installed. I prefer to use the combi-care unit by Aquadial which is available from most plumbers merchants.

2 Reduced hot water temperature.

This problem is also a common one and is not always cured with a powerflush or a de-scale. Although there are common causes that would suggest a powerflush or a de-scale are in order….

a/ Limescale build-up. See above.

b/ A fault on the sensor that measures the hot water temperature.

c/ Rust particles in the DHW plate heat exchanger, although not enough to trigger the boiler overheat thermostat.

d/ A electronic problem. Often results in a pcb board change. You used to be able to repair them, but these days it’s quicker and cheaper to put a new one.

3 Boiler overheats or shuts off when hot water is needed.

If there is a blockage or restriction due to rust particles in the DHW heat exchanger you can get reduced water temperature and/or the boiler cutting out. (Due to overheating because of reduced water flow.)

If you have either of those symptoms there is a strong possibility that you need a powerflush. At the least, a clean-out or replacement of the heat exchanger.

Warning: If you do not have the system properly cleaned any work in the DHW heat exchanger for these problems is short-term. It will happen again.

Another word of warning/advice before you call anyone.

The situation is complicated by the use of a diverter valve in a combination boiler. There are dozens and dozens of them to choose from. Here are a couple of them.

This valve swaps the heat from your boiler between the hot water supply and the radiators. The paddle inside the valve can get stuck and lead to a mis-diagnosis that a powerflush is needed when it isn’t.

For example, if the paddle gets stuck between hot water and radiator output, the heating might come on when you run the hot tap, the hot water temperature may drop and or the boiler may overheat and cut out. In any combination.

If you have any of the above three symptoms, make sure you talk to a heating engineer who does powerflushing. Not a company that ONLY does powerflushing - they may not have the experience to give you full advice.

Tips for Choosing A Powerflush Firm

When you book a powerflush, make sure that you get a quote that includes a de-scale. Some companies do not include this in their definition of ‘powerflush’.

The extra chemicals are only about £20 and it’s worth every penny and the surest way of making sure your system is as clean as it can be.

If you have any doubts at all, trust your instinct. Ask for references and check them. Get a good a guarantee. Ask to see their public liability insurance – before they turn up at the door.

If you need some advice about a problem you are having, take a look at Or, feel free to call me on 01225-869036. Ask for Ian and fire away.

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