How to Bleed Radiator Heating: The Order the Steps Actually Happen In

Bleed radiator heating in this order. Confirm the radiator is cold at the top and warm at the bottom, the only pattern trapped air explains. Establish whether the system is sealed (a pressure gauge and a filling loop at the boiler) or open-vent (a small tank in the loft, no gauge). Switch the heating off at the programmer and wait about an hour for the water to cool, which Vaillant's homeowner guidance puts at "approximately 1 hour (or 2 hours to be cautious)". Hold a cloth under the bleed valve and turn the key anti-clockwise a quarter to a half turn until the hiss becomes a steady dribble, then close it. Repeat in the order your manual gives. Check the gauge against the cold range in that manual, 1 to 1.5 bar for Worcester Bosch and 1.0 to 2.0 bar for a Vaillant ecoTEC plus, and top up through the filling loop with the boiler cold if it has fallen.

Read the radiator before you reach for the key

The advice that dominates every short tutorial is to bleed any radiator that has gone cold. It is wrong often enough to cost people an afternoon and, on a sealed system, a gauge reading they then have to put back.

Air rises. It gathers at the top of the panel and nowhere else, which makes the diagnosis simple. Run the heating for twenty minutes, then feel each radiator at both top and bottom corners.

| What your hand finds | Most likely cause | Does bleeding fix it? | |---|---|---| | Cold top, warm bottom | Air trapped at the top of the panel | Yes | | Cold bottom, warm top | Magnetite sludge settled along the base, or restricted flow | No. Needs a flush and a filter | | Cold all over, other radiators hot | Thermostatic valve pin stuck down, or lockshield closed too far | No. Free the valve or rebalance | | Cold all over, whole house cool | Pump, boiler, controls, or pressure below the lockout threshold | No. Check the gauge and controls first | | Warm but gurgling or clanking | Air moving in the circuit, sometimes pump speed | Often; retest afterwards |

The second row sends people up a ladder for nothing. A radiator cold along its bottom edge has sludge or a flow problem, and opening the bleed screw releases a little water and changes nothing else.

Which heating system do you have, and why it changes the job

Most bleeding tutorials assume a sealed combi. That fits a large majority of English homes and misses about a quarter of them. The English Housing Survey's 2023-24 chapter on energy efficiency records 19% of dwellings with a combination boiler and 55% with a condensing-combination in 2023. The rest run heat-only or system boilers, many open-vent, with a feed and expansion cistern in the loft.

Look for the tell. A sealed system has a pressure gauge on or near the boiler and a filling loop, either a braided hose with two valves or a built-in key. An open-vent system has neither, because the loft tank tops it up by float valve as water is lost. Bleeding one changes no reading you can see, and if the radiator does not refill, suspect a seized ball valve in that tank before anything at the boiler.

What your boiler's cold pressure should be

This is where a borrowed number does real damage. Worcester Bosch tells homeowners the gauge should read "between 1 and 1.5 bar on the pressure gauge (the indicator needle would usually be in the green section)". The Vaillant ecoTEC plus operating instructions give a filling pressure of 1.0 to 2.0 bar cold, tell you to top up below 0.8 bar, and note that the boiler shuts down and shows fault F.22 below 0.5 bar. Vaillant also brings the building into it: 0.1 bar for every metre of installation height, so a system running 8 metres from cellar to roof targets 1.8 bar.

Two manufacturers, two ranges, one of them varying with the house. The 1.2 bar someone sets confidently in a video is that person's appliance and no one else's.

I learned to quote documents rather than summarise them at an inquest, where the gap between an instruction and somebody's memory of it was the whole question. Find your boiler's manual by model number and read the pressure paragraph as written.

How long to wait before opening a valve

Vaillant's answer is "approximately 1 hour (or 2 hours to be cautious) to ensure that all radiators are cold". Glow-worm publishes the same hour for its own boilers.

The reason is the water temperature you are opening a hole into. UK boilers commonly run flow temperatures of 70 to 80°C, which is why Nesta's Money Saving Boiler Challenge spends its effort persuading people to drop condensing combis to 60°C. The scald charts the US Consumer Product Safety Commission derived from Moritz and Henriques' 1946 burn studies put water at 60°C causing a full-thickness burn in around five seconds — the low end of what sits behind the bleed screw.

Switch off at the programmer, not just the room thermostat, so the pump stops, and open every thermostatic valve fully while you wait. It has drizzled here since first light and this room is warm, which is the trap: a warm room tells you nothing, and cast iron holds heat far longer than a steel panel. Put a hand flat on the metal. Cold means cold.

How to bleed a radiator, in order

1. Heating off at the programmer, and wait the hour. Check by hand that the radiators are cold.
2. Open all thermostatic valves fully.
3. On a sealed system, read the gauge and write the number down.
4. Work in the order your manual specifies. Vaillant's published order is every ground-floor radiator before you go upstairs. Convenience is not an order.
5. Put a towel underneath and hold a jug under the valve.
6. Fit the key. Modern UK valves take a 5mm square; older imperial radiators often want a clock key around 4.8mm, and universal keys carry 8mm, 10mm and 3/8" sockets.
7. Turn anti-clockwise a quarter to a half turn, which is Vaillant's figure. Stay there. Opening it wide or wandering off is how a bleed screw comes out and a ceiling gets wet.
8. Close it as soon as water runs steadily instead of spitting. Snug, not forced. A rounded square or cracked screw becomes a real repair: replacement air vents are most commonly 1/8" BSP, though 1/4" and 1/2" exist, so measure the old one or ask the radiator manufacturer.
9. Recheck the gauge, then move on. If the reading is near the bottom of your manual's range, top up between radiators rather than at the end.

How to put the pressure back

Bleeding lets water out along with the air, so a sealed system reads lower afterwards, and the fall scales with how many valves you opened. Let the boiler go cold, open the filling loop slowly, and watch the needle rather than the clock. Stop inside the cold range your own manual gives.

Overshooting has a consequence people miss. The pressure relief valve lifts and discharges through a pipe outside, and you are topping up again a fortnight later. Worcester Bosch sets the baseline plainly: "The pressure in the system will usually require topping up once or twice a year."

Open-vent systems need nothing here; the loft cistern refills them.

How to check the bleed actually worked

Turn the heating back on, give it an hour, and feel the same four corners. A panel that warms top and bottom is finished.

Take a second gauge reading with the system hot and compare it with the cold one you wrote down. Worcester Bosch draws a line there: "If pressure rises by more than 1 bar when the heating is up to temperature then the expansion vessel may require repressurising. This will require a service engineer."

Why a radiator is still cold after bleeding

Work down this list before touching the bleed screw again. Pull the head off the thermostatic valve and look at the pin underneath; if it sits flush and will not spring back, it has stuck down and the radiator is shut whatever the dial says. Gentle taps usually free it. Check the lockshield at the other end has not been wound closed. Feel the flow and return pipes at the floor: both cold means nothing is reaching the radiator, and the fault is the pump, the boiler or the balance.

Sludge is the long game. BS 7593:2019, the code of practice for preparing, commissioning and maintaining domestic central heating systems, requires an in-line filter able to capture magnetic and non-magnetic debris on all systems, a yearly on-site test of inhibitor level and system cleanliness at the same visit as the boiler check, and re-dosing of inhibitor every five years. A radiator cold along its bottom edge in a system that has never had a filter or a water test tells you which of those was skipped.

When recurring air is a fault rather than a chore

Air that comes back is information. Topping up once or twice a year is Worcester Bosch's normal. The Vaillant manual is blunter still: if the pressure drops frequently, the cause of the heating water loss must be identified and eliminated.

Bleeding the same radiator every few weeks points at a leak drawing air in as the system cools, an expansion vessel that has lost its charge, corrosion generating hydrogen, or a failed automatic air vent. More bleeding fixes none of them.

The annual service is the lever most people already have and do not use. Worcester Bosch's guarantee terms require at clause 4.1.6 that the product is serviced annually by Bosch or another competent, officially certified engineer and recorded in the service record, and failure can reduce, limit or invalidate the guarantee. Landlords have no discretion. The Gas Safety (Installation and Use) Regulations 1998 require gas appliances and flues to be checked for safety at least every 12 months by a Gas Safe registered engineer. Owner-occupiers face no such duty and still lose the warranty by skipping it.

What to write down

Count your radiators and keep the number somewhere. The pressure drop from a bleeding session is proportional to how many valves you opened, and eight radiators explain a fall that two would not.

Then log four things each time: the date, which radiator, the gauge reading before and after, and whether air actually came out. My neighbour keeps hers on a card taped inside the airing cupboard door, which is how she noticed the same bedroom radiator appearing three times in five weeks. Three entries across a winter is a maintenance record. Three in a month is a fault report, and handing it to the engineer at the door saves him his first half hour.

Questions people ask

Should I bleed a radiator with the heating on or off?

Off. Switch it off at the programmer and wait roughly an hour so the water cools and the pump stops pushing air around the circuit. Vaillant's guidance says approximately one hour, two to be cautious. Bleeding a hot, circulating system risks a scald and keeps the air moving instead of letting it gather at the top.

Should I start bleeding radiators at the top or bottom of the house?

Vaillant's published order is every ground-floor radiator first, then upstairs, and that suits most sealed systems because the highest points collect air last. Your own boiler manual overrides any slogan. On open-vent systems the pipe layout decides, so follow the manual or ask whoever installed it rather than a general rule.

What signs show a radiator needs bleeding?

A radiator cold at the top while the bottom stays warm is the reliable sign. Gurgling, clanking or rumbling as the heating fires supports it, as does one room warming noticeably slower than the rest. A cold bottom with a warm top is the opposite diagnosis and means sludge or restricted flow, not air.

Why is my radiator still cold after bleeding it?

Bleeding only clears air from the top of the panel. If it stays cold, suspect a thermostatic valve whose pin has stuck down, a lockshield wound shut, magnetite sludge lying along the base, or a pump or boiler fault. Feel the flow and return pipes at floor level; both cold means the problem sits upstream.

How can I bleed a radiator without a key?

Many bleed valves have a slot cut across the square that takes a flat-blade screwdriver. If yours does not, buy a key rather than gripping the square with pliers, which rounds it off. Keys cost very little, while a rounded or snapped bleed screw means isolating the radiator and fitting a replacement air vent.

What happens if I bleed a radiator while the heating is on?

The pump keeps water and air circulating, so the air never settles where the valve can reach it, and you can end up drawing water out instead. On a sealed system, hot water under pressure can spray from the opening at temperatures that scald in seconds. Switch off and wait for the metal to cool.

When does recurring air mean my heating system has a fault?

Topping up once or twice a year is normal, per Worcester Bosch. Bleeding the same radiator every few weeks, or watching pressure fall repeatedly, indicates a leak drawing air in, a failed expansion vessel, internal corrosion, or a faulty automatic air vent. Book a Gas Safe registered engineer instead of repeating the bleed.