Heat — the gas side
What comes out of a heating system
The photograph below is two filter magnets out of one house. Every bit of that black was going round and round through the pump and the heat exchanger, and it is the single most common reason a heating system stops behaving.
Where it comes from
Steel radiators, water and a bit of oxygen make magnetite — a fine black iron oxide sludge. It builds up slowly in every system that is not properly protected, settles in the bottom of the radiators where the flow is slowest, and gets dragged round through the most expensive parts of the boiler.
How you know
- Radiators cold across the bottom while the top is hot
- The boiler banging, kettling or making a noise like a kettle coming to the boil
- The pump going, then the next one going too
- The water running out black when you bleed or drain a radiator
- Repeated breakdowns on a boiler that is not actually old
What the job involves
A power flush puts a pump across the system and drives water through it at a much higher rate than the boiler ever does, radiator by radiator, with a cleaner in it and a magnet on the return catching what comes loose. Stubborn radiators get isolated and worked on individually. It carries on until what comes out runs clear.
Then it gets refilled with inhibitor and a magnetic filter goes on the return, which is the part that stops you being here again in three years. The filter wants emptying once a year — normally done at the same visit as the boiler service, since the casing is off anyway.
Why it matters
Sludge is not a cosmetic problem. It costs efficiency on every hour the system runs, it kills pumps and diverter valves, and it will eventually block a heat exchanger, which is most of the price of a boiler. Several manufacturers now make a filter a condition of the warranty on a new installation, which tells you what they think of it.
Ring it, or write it down
Say what it is doing and roughly where you are. That is usually enough to know whether it is an hour or a morning.