Domakihub How to Bleed a Hot Water Heating System the Right Way
Stop cold spots in their tracks by purging trapped air from your radiators and hydronic pipes.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
When a radiator is warm at the bottom but cold at the top, the culprit is almost always trapped air sitting in the circuit — and bleeding the system is the straightforward fix. The video above walks you through the full procedure visually, while this guide gives you the ordered steps, safety checks, and the moments when you should call a licensed plumber instead of pressing on alone. Whether you manage a home, a holiday rental on a Greek island, or a small hotel with hydronic heating, this skill will save you a service call every winter.
Why Air Gets Trapped in a Hot Water System
A wet central-heating circuit is a closed loop: a boiler (or heat pump) heats water, a pump pushes it through pipes into radiators, and cooled water returns to be reheated. Air enters the system whenever a joint is opened for repair, when pressure drops below the minimum fill point, or simply over time as dissolved oxygen gradually comes out of solution. Once airlock pockets form, hot water cannot fill the full volume of the radiator — leaving the top section cold and making the boiler work harder.
In Greece, many properties rely on a wall-mounted combi boiler (kombothermiko) or a freestanding boiler with a vented or sealed expansion vessel. Sealed systems have a pressure gauge on the boiler; vented systems have a header tank in the loft. The bleeding process is similar for both, but the pressure-top-up step differs, as explained in Step 5 below.
When to Call a Licensed Plumber Instead
Bleeding itself is a safe DIY task — you are releasing a small amount of trapped air and a few drops of water through a valve the size of a coin. However, there are situations where a licensed plumber (ydraylikos) must take over: if the system pressure keeps dropping below 1 bar within days of you re-pressurising it (there is a leak somewhere); if the bleed valve is corroded or seized and you cannot turn it without forcing it; if you have a combi boiler under warranty that requires a professional for any internal work; or if the water coming out is rust-red or smells of sewage, indicating serious corrosion or contamination. Gas boiler faults — ignition errors, burner problems, flue issues — are always a job for a certified gas technician (technitis aerio), never DIY.
How to Bleed Your Hot Water Radiators: 5 Steps
- 1
Turn the heating on and identify which radiators need bleeding
Set the thermostat to call for heat and wait 15 to 20 minutes for the system to reach operating temperature. Walk through every room and feel each radiator carefully — wear a glove if needed, as surfaces can reach 60–70 °C. A radiator that is noticeably cooler at the top than at the bottom, or cold in patches across its face, has trapped air. Make a note of each affected radiator and the order you will bleed them in: start with the radiator on the ground floor that is farthest from the boiler, then work toward the boiler, and finish with any radiators on upper floors. This sequence pushes air progressively toward the nearest vent point.
- 2
Turn the heating OFF and let the system cool slightly
Switch the boiler or thermostat off — do not bleed a radiator while the pump is running, as this can draw more air into the circuit. Wait 10 to 15 minutes. The radiators should be warm but not scalding; you want enough residual pressure in the system to push air out but not so much heat that escaping water burns you. While waiting, gather what you need: a radiator bleed key (klidi radiator — a small four-sided key, available at any hardware shop in Greece for under €2), a dry cloth or small bowl to catch drips, and optionally a pair of rubber gloves.
- 3
Open the bleed valve and release the trapped air
The bleed valve is a small square-headed nipple at the top corner of the radiator, usually on the side opposite the inlet. Place the cloth or bowl directly below it. Insert the bleed key onto the square section and turn it counter-clockwise — only a quarter to half a turn is enough; you do not need to remove the valve entirely. You will hear a hiss as air escapes. Hold the key in place and keep the cloth ready. When the hissing stops and a steady trickle of water appears — not just drops, but a small continuous stream — the air is out. Turn the key clockwise immediately to close the valve firmly. Wipe away any water and check that the valve face is dry; a small persistent drip means the valve seat is worn and needs a plumber.
- 4
Check and restore system pressure
After bleeding one or more radiators, water has left the circuit and pressure will have dropped. On a sealed system (most modern combi boilers and heat-pump units), check the pressure gauge on the boiler: the normal cold-fill target is 1.0 to 1.5 bar — consult your boiler manual for the exact value. If it has dropped below 1 bar, use the filling loop (a braided hose with two valves, usually located beneath the boiler) to add mains water slowly until the gauge reads the correct cold pressure. Open both filling-loop valves gently, watch the gauge, and close them as soon as you reach the target. On a vented (open) system with a header tank, the tank refills automatically from the mains ballcock — check that the float valve is not stuck. Avoid over-pressurising: above 2.5 bar the pressure-relief valve will discharge water to prevent damage.
- 5
Restart the heating and verify the fix
Turn the boiler back on and bring the system up to full operating temperature. After 20 minutes, repeat your walkthrough: touch each radiator you bled and confirm it is now uniformly warm from bottom to top. If a radiator is still partially cold, it may need a second bleed — repeat Steps 3 and 4. Once all radiators heat evenly, the job is done. Note the date in a maintenance log for the property; bleeding once a year at the start of the heating season (typically October in northern Greece, November in the islands) is good practice and keeps energy bills lower.
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FAQ
- How often should I bleed the radiators in a Greek holiday rental?
- Once a year, at the start of the heating season, is the standard recommendation. Holiday rentals that sit unused over summer are particularly prone to air accumulation because the system is static for months. A quick bleed each October or November before the first guests of the winter season arrive takes under 30 minutes and prevents cold-spot complaints.
- My boiler pressure keeps dropping after I bleed the radiators. What does that mean?
- A one-time pressure drop after bleeding is completely normal — you released water along with the air. But if the pressure falls repeatedly within a few days of topping it up, you almost certainly have a leak somewhere in the circuit: a dripping valve, a corroded pipe joint, or a failing pressure-relief valve. Stop topping up the pressure and call a licensed plumber (ydraylikos) to locate and fix the leak before it causes water damage.
- Can I bleed a radiator if I have a combi boiler?
- Yes — the bleeding procedure at the radiator itself is identical regardless of boiler type. The only combi-specific step is the pressure top-up: combi boilers use sealed systems, so after bleeding you must use the filling loop to restore pressure to the 1.0–1.5 bar range shown on the boiler gauge. Never open the boiler casing or touch the burner, heat exchanger, or gas connections — those parts require a certified gas technician.