A radiator that is piping hot across the top but stone cold along the bottom edge is one of the most common calls we get in Cardiff once the heating goes back on in October. It is almost never an airlock, and bleeding it will not help. Here is what is really going on, how to check it yourself, and when it is worth calling someone out.
The quick rule of thumb: cold at the top usually means trapped air, cold at the bottom usually means sludge. Sludge is magnetite, a black iron oxide that forms when water, oxygen and the steel inside your radiators react over the years. It is heavier than water, so it settles along the bottom of each radiator and forms a layer that hot water simply flows over.
You can often feel the line where it starts. Run your hand down the panel while the heating has been on for twenty minutes or so: if there is a fairly sharp horizontal band where hot turns cold, perhaps a third or halfway up, that band is the top of the sludge bed. Older properties around Canton, Grangetown and Roath with original steel panel radiators are the usual suspects, particularly where the system has been drained and refilled for building work without fresh inhibitor going back in.
A radiator with a sludge bed might be giving out half the heat it was designed to. Your boiler runs longer to reach the same room temperature, which shows up on the gas bill and shortens the life of the pump and the heat exchanger. Sludge also circulates, so it clogs the narrow waterways in the boiler itself, and a blocked heat exchanger is a far more expensive repair than a clean system.
One thing worth knowing locally: mains water across most of Cardiff and the Valleys comes from upland Welsh reservoirs and is soft, so limescale is rarely the problem here the way it is in the south east of England. If your radiators are underperforming, corrosion sludge is the far more likely cause.
Before assuming sludge, check a few things that cost nothing.
If only one radiator is affected and the valves are behaving normally, sludge is the likely answer. If several radiators upstairs are cold at the bottom and the downstairs ones are fine, you may have a circulation or pump issue instead, and that is a different job.
For a single badly affected radiator, taking it off the wall and flushing it outside with a hosepipe is often enough, and it is the cheapest route. Expect a couple of hours of labour. It is messy work, and the black water will stain a driveway, so it is done outdoors for a reason.
If most of the system is affected, a power flush is the proper answer. A machine is connected to the pipework and pushes water and cleaning chemicals through at high velocity, radiator by radiator, usually taking most of a day for a typical three bedroom house. Costs vary with the number of radiators and how bad the system is, but for a standard Cardiff terrace or semi you are generally looking at the low to mid hundreds rather than a four figure job. Anyone quoting without asking how many radiators you have is guessing.
Whatever route you take, two things should happen afterwards. Corrosion inhibitor goes back into the system, and it needs topping up every time the system is drained. Second, if there is no magnetic filter on the return pipe near the boiler, have one fitted. It catches magnetite before it settles and it is checked at each annual service. Most boiler manufacturers now expect one for the warranty to stand anyway.
Call Simon for an initial chat, or send a couple of photos of the space and Sycamore will come back with a quote the same week.
A few photos and a sketch of the room is usually enough for a first quote. For anything structural or tricky, we come and look.