6 Steps to Bleed a Sealed Heating Circuit Using a Fernox TF1 Filter

Start at the gauge, not the radiator

Before a single bleed valve turns, read the pressure gauge on the boiler face. A sealed circuit that reads 1.2 bar cold has roughly the right charge; anything under 0.5 bar and the boiler will drop into lockout the moment you release air. That is the trap. Every bleed you open lets water escape along with the air, and pressure falls. On a small combi feeding six radiators the drop per radiator can be 0.1 to 0.2 bar, so bleeding four rads in a row can push a marginal system below its cut-off threshold.

The Fernox TF1 filter matters here because it gives you a controlled refill point. On the compact TF1 the isolation valves sit either side of the brass body, and the drain plug on the base takes a hose. More useful for this job is the fact that once the filter is isolated and reopened, you have re-primed a slug of water back into the return. Note the cold reading, write it on a scrap of card taped near the boiler, and you have a baseline to chase back to after the air is gone.

Isolate and inspect the TF1 before you touch anything else

The TF1 collects magnetite. That black iron oxide is the reason a system that bled cleanly two winters ago now gurgles and runs cold at the top of the upstairs rads. Close both isolation valves on the filter, the two levers turn a quarter to shut. Position a shallow tray, the body holds maybe 300 to 400 ml. Unscrew the drain cap and let the sludge run. If what comes out looks like weak tea you are fine; if it is opaque black, the circuit is loaded with corrosion product and no amount of bleeding fixes the underlying circulation problem.

The magnet in the TF1 sits in a well. Pull it, wipe it, and you will see how much has accumulated since the last service. A heavily coated magnet on a system dosed with Fernox F1 inhibitor suggests the inhibitor has been consumed, which happens after a partial drain-down or a leak top-up that dilutes it. Reopen the isolation valves slowly. Watch the gauge climb as the filter refills. This single action often recovers 0.1 to 0.3 bar, buying you headroom before the bleeding even begins.

Work the circuit from lowest to highest

Air migrates up. In a two-storey house the trapped air ends up in the top-floor radiators and, if there is one, the towel rail in the bathroom that always stays cold at the top. So the bleed order runs ground floor first, then upstairs, finishing at the highest emitter in the building. Doing it the other way, top down, means the air you release upstairs simply gets pulled back up from the lower rads as the pump runs, and you chase your own tail.

Turn the circulator off at the isolator before bleeding. A running Grundfos Alpha2 on a low proportional-pressure setting still moves water, and bleeding against a live pump can draw air in through the valve rather than pushing it out. With the pump off, open each bleed nipple a quarter turn with the key, hold a cloth beneath it, and wait for the hiss to become a steady dribble with no spitting. Close it. Move to the next.

On a system with fifteen radiators this is a forty-minute job done properly. Rushing it is how you end up bleeding the same three rads three times across a week. After every third or fourth radiator, walk back to the boiler gauge. If it has fallen toward 0.8 bar, stop bleeding and top up through the TF1 or the filling loop before continuing. The whole point of starting at the filter was to keep this loop tight.

The underfloor manifold changes the sequence

Wet underfloor heating does not have bleed nipples on the emitter. Air comes out at the manifold, through the automatic air vent on the flow bar and the manual bleed screws on each actuator port. If the property mixes radiators upstairs with an underfloor loop on the ground floor, purge the manifold loops first using the flow and return isolation on each circuit, then bleed the radiators above. The manifold holds far more air volume than a panel radiator, and leaving it trapped starves the whole downstairs of heat while the pump strains.

Run each underfloor loop individually. Close all circuits at the manifold, open one, and let water push through until the flow indicator on the flow meter shows steady movement with no bubbles crossing the sight glass. Close it, open the next. On a six-loop manifold this takes real patience because each loop might be 80 to 100 metres of pipe. The blending valve target for underfloor sits around 35 to 45 degrees, well below the radiator flow temperature, and air pockets in these long loops are the classic reason one room in an otherwise warm house never gets comfortable.

Recharge to the cold target

Once the whole circuit is silent and the pump has run for ten minutes without new gurgling, bring the pressure back to the cold baseline you wrote down at the start. Use the filling loop, the braided silver flexible hose with a valve at each end, or top up through the TF1 drain point with a proprietary filling kit. Open slowly and watch the gauge. Overshooting past 2.5 bar risks lifting the pressure relief valve, and once that PRV weeps it often keeps weeping, turning a bleeding job into a valve replacement.

This is also the moment to redose inhibitor if the TF1 magnet told you the F1 was spent. A standard 500 ml dose treats around 100 litres of system water, which covers most domestic circuits of ten to twelve radiators. Introduce it through the filter drain or a dosing point, then reopen the isolation valves and let the pump distribute it. Skipping this after a partial drain leaves the freshly bled water more corrosive than what came out.

A worked pressure example

Take a system that started at 1.2 bar cold, feeding ten radiators. Bleeding all ten dropped it to 0.7 bar, uncomfortably close to a 0.5 bar lockout. Isolating and reopening the TF1 recovered it to 0.9 bar. Then topping up through the filling loop brought it to 1.3 bar, which will settle to roughly 1.5 bar once the boiler fires and the water expands. That expansion margin is why you do not aim for the hot target when charging cold; a system charged to 1.5 cold can hit 2.3 hot and nudge the PRV.

If that same system had refused to hold pressure, dropping steadily over the following days, the bleeding was never the problem. A falling gauge with no visible leak points to a failed expansion vessel, a weeping PRV, or a pinhole under a floor. The TF1 keeps the water clean, and bleeding clears the air, but neither addresses a circuit that cannot hold its charge. That is the question worth sitting with: if the gauge is still sinking a week after a careful bleed, where is the water actually going?