What the job involves
A power flush is a deep clean of a central heating system. Over the years, rust, scale and black magnetite sludge build up inside radiators and pipework. That debris makes radiators cold at the bottom, makes the boiler work harder, causes noisy running, and can clog the boiler's heat exchanger or damage the pump.
The engineer connects a powerful pump to the system, usually at the pump position or in place of the circulator. Water is pushed through at high flow, and the direction of flow is reversed. Cleaning chemicals are circulated first to loosen the deposits. Each radiator is then worked on in turn, sometimes tapped or knocked to shift stubborn sludge, until the water coming out runs clear. The system is then flushed through with clean water and treated with inhibitor to slow future corrosion. A good engineer will usually fit a magnetic filter if there isn't one already, so the system stays cleaner afterwards.
A power flush is not always the right answer. Very old systems with tired joints, thin-walled pipework or radiators already close to leaking can spring leaks once the sludge that was plugging the holes is gone. A sensible engineer will check the system first and tell you honestly if a gentler clean, or a partial replacement, would suit better.
What affects the price
No two systems are the same, and the cost follows the amount of work involved. The main factors are:
- Number of radiators. Each one has to be connected, flushed and checked, so a larger system takes more time and more chemicals.
- How bad the sludge is. A lightly dirty system clears quickly. A heavily blocked one needs longer circulation, more cleaning chemical and more manual effort on individual radiators.
- Access. Radiators fixed tight to walls, pipework under fitted floors, or a boiler in a cramped cupboard make connection slower. Furniture and carpets that need moving add time too.
- Type of system. Open-vented systems with a loft tank, sealed systems, and those with underfloor heating loops or microbore pipework are each handled differently. Some need extra care or extra equipment.
- Valves and fittings. If radiator valves are seized or leak when disturbed, they need replacing. Isolating valves may need fitting so each radiator can be dealt with properly.
- Extras fitted at the same time. A magnetic filter, fresh inhibitor, a new pump or replacement TRVs all add to the bill, though some are worth doing while the system is drained.
- Pre-existing faults. Leaks, failed pumps or blocked pipes found along the way can mean extra repair work.
- Location and the engineer. Local labour rates, travel and how busy the engineer is all play a part, as does whether the firm is a one-person outfit or a larger company.
The prices on this page show what other homeowners locally actually paid, which is a better guide than a national average.
Ways to keep the cost down
- Get several written quotes for the same scope, so you can compare like with like.
- Do the simple prep yourself. Clear the space around radiators and the boiler, and lift rugs or move furniture before the engineer arrives.
- Ask whether a flush is really needed. If the problem is one cold radiator, balancing or bleeding it, or a targeted clean, may be enough.
- Have the filter and inhibitor done in the same visit. Doing this work separately later usually means paying for a second call-out and another drain-down.
- Book at a quieter time of year. Heating engineers are busiest in autumn and during cold snaps, so a spring or summer slot is often easier to arrange.
- Look after the system afterwards. Keeping inhibitor levels right and cleaning the magnetic filter when asked stops you paying for another flush too soon.
- Don't skip the diagnosis. Paying a little to find out why the system is sludging can save paying for repeat flushes. Air getting in, a leak or a poor repair is often the real culprit.
What should be included in a quote
A proper quote should be in writing and clear about what you are getting. Check it covers:
- [ ] The number of radiators and any other emitters being flushed
- [ ] The type of system and boiler it applies to
- [ ] Cleaning chemicals, and neutraliser or final flush
- [ ] Inhibitor added at the end
- [ ] Whether a magnetic filter is supplied and fitted, and which type
- [ ] Any valves, TRVs or fittings that may need replacing, and how they would be priced
- [ ] A pre-flush check of the system and a note of its condition
- [ ] Protection for floors and furnishings, and clearing up afterwards
- [ ] Whether VAT is included
- [ ] How long the work should take and the proposed date
- [ ] What happens if a leak or fault shows up during the job
- [ ] Any guarantee on the work, and the engineer's company and insurance details
- [ ] Gas Safe registration, if any work on the boiler or gas pipework is part of the job
Questions to ask a heating engineer
- Do you think my system needs a power flush, or would something less aggressive do the job?
- What condition are my pipes and radiators in, and is there a risk of leaks after flushing?
- How will you protect the boiler and the pump during the work?
- Will you fit a magnetic filter, and will you show me how to check and clean it?
- Which inhibitor will you use, and how often should it be tested or topped up?
- What would you do if a radiator or valve fails during the flush?
- Does the boiler manufacturer's warranty or service record require a flush, a filter or particular products?
- Are you Gas Safe registered, and can I see your card?
- Will the system be balanced afterwards so every radiator heats evenly?
- Can I have the findings and what you used written down for my records?
How long it takes and what to expect
For a typical home the job is normally done in a day, though a large or badly clogged system can take longer. The heating and hot water, if the boiler supplies it, will be off for much of that time, so plan around it.
Expect the engineer to look over the system first, then drain or isolate parts of it and connect the machine. You will hear the pump running and see the engineer moving from radiator to radiator. The water that comes out is often black or brown and can look alarming, but that is the point. Good engineers take care with hoses, towels and drainage to keep mess down.
After the flush, the system is refilled, inhibitor is added, air is bled out, and the boiler is brought back up to temperature. Radiators should heat evenly from top to bottom. Some noise and air in the system for a day or two is not unusual, and a decent engineer will tell you what to watch for. If a radiator is still cold at the bottom, or a new leak appears, tell them promptly.
Signs a quote is too cheap or too expensive
Possibly too cheap:
- It is far below the other quotes, with no explanation
- There is no mention of inhibitor, a filter or a final clean water flush
- The engineer wants to start without looking at the system
- Nothing is in writing, or the description is vague
- They promise to fix every heating problem you have
- They can't show proof of registration or insurance where it applies
A very low price often means the job will be rushed, with chemicals left in the system or sludge left in the radiators.
Possibly too expensive:
- It is well above the other quotes for a similar system, with no reason such as difficult access or heavy contamination
- There is a long list of extras that were never discussed
- A full flush is pushed when your problem looks local
- The quote lumps everything into one figure with no breakdown
- You are pressured to book on the day
If one quote is out of line with the rest, ask the engineer to explain it. A good one will happily walk you through the reasons, and compare what you have been offered with the prices on this page.