In short
When a new boiler is fitted, Approved Document L expects the heating system to be cleaned and flushed and treated with inhibitor, following BS 7593, and an in-line filter is part of that standard. Whether you need a full power flush or a chemical clean depends on how dirty the system water is.
Why does system water matter?
Over the years, heating water picks up rust and debris from radiators and pipes. It settles as black sludge, blocks the narrow passages in a modern boiler's heat exchanger and wears out the pump. Dirty water is one of the main reasons new boilers fail early, and it is one of the things manufacturer warranties usually exclude.
What do the regulations say?
Approved Document L says that before a new heating appliance is installed, the central heating and primary hot water circuits should be thoroughly cleaned and flushed, and a suitable inhibitor added. It says domestic systems should be prepared and commissioned to BS 7593. Baxi explains that since Part L changed in June 2022, the installer should clean and flush the system, fit an in-line filter, add inhibitor, and then check the filter and inhibitor at every annual service.
Chemical clean or power flush?
- Chemical clean: a cleaning chemical is circulated through the system, often for a set period, then flushed out. It suits systems that are in reasonable condition.
- Power flush: a pump pushes water and chemicals through each radiator at high flow to lift heavy sludge. It suits systems with cold-bottomed radiators, very dark water or a history of blockages.
Your engineer should look at the water and the radiators before recommending one. A power flush is not automatically needed on every job.
What does a magnetic filter do?
It sits on the return pipe to the boiler and catches iron debris before it reaches the boiler. It does not clean a dirty system on its own, but it keeps a clean system clean. It should be emptied and checked at each annual service.
Signs your system needs cleaning
- Radiators cold at the bottom and hot at the top.
- Black or brown water when a radiator is bled.
- A boiler that kettles or bangs.
- Some radiators slow to heat even after bleeding and balancing.
What does it cost?
Flushes and filters are paid extras on top of our new boiler price, which starts from £1,750 inc VAT for a basic like-for-like installation, subject to a site inspection. Ask every installer what cleaning and filter are included so you can compare quotes like-for-like.
What CombiCert would check
We test or inspect the system water where we can, look at radiator cold spots and the existing filter (if any), and follow BS 7593 / Approved Document L practice for a new boiler: clean, flush, inhibit and fit an in-line filter. Whether that is a chemical clean or a fuller power flush depends on how dirty the system is.
When to call a Gas Safe engineer
Cleaning a heating system involves draining, chemicals and refilling a pressurised system, and on a new boiler it is part of commissioning. Leave it to your installer. If your radiators are cold at the bottom on an existing system, see why radiators go cold or ask a heating engineer.
Need a hand? CombiCert in Telford
CombiCert Ltd is a Gas Safe registered (621146) gas and heating company based at The Old School House, Park Lane, Telford TF3 4TG, working across Telford and Shropshire for homes and businesses. We are open Monday to Saturday, 08:00 to 19:45 (closed Sunday).
New boilers from £1,750 inc VAT for a basic like-for-like installation, subject to a site inspection. Most quotes are higher; filters, flushes, thermostats and moving the boiler are paid extras.
See New boiler installation in Telford, or call or WhatsApp 07393 220694, or email hello@combicert.co.uk.
We cover Telford and nearby, including Wellington, Madeley, Dawley, Oakengates, Shifnal, Newport, Ironbridge and Shrewsbury.
Sources
Frequently asked questions
Is a magnetic filter a legal requirement?
Approved Document L says systems should be prepared and commissioned to BS 7593 when a new boiler is installed, and Baxi explains that this means the installer should fit an in-line filter as well as cleaning, flushing and adding inhibitor. Manufacturers often ask for a filter for their warranty too, so expect one on any new installation.
Do I need a power flush with a new boiler?
Not always. The system must be cleaned and flushed, but a chemical clean may be enough if the water and radiators are in reasonable condition. A power flush suits systems with heavy sludge, cold-bottomed radiators or a history of blockages. Your engineer should check the water and radiators before recommending one.
How often should a magnetic filter be cleaned?
At every annual boiler service. The engineer isolates the filter, empties the collected debris and checks the inhibitor level in the system water. If the filter is filling up quickly, that is a sign the system may need a more thorough clean. Some filters have indicators or apps that show when they need attention.
What is inhibitor and why does it matter?
Inhibitor is a chemical added to heating water to slow corrosion and scale inside radiators, pipes and the boiler. Approved Document L expects it to be added when a new boiler is fitted, and its level should be checked at each annual service and topped up if it has dropped.
This answer is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.