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Hydronic heating not working? Cold panels, low pressure, noise and lockouts
Hydronic heating not working on a cold night is miserable, but plenty of the causes are quick to check. Some you can sort out yourself, like air in a radiator or a timer that lost its settings in a power cut. Others, like a stopped pump or a boiler that keeps locking out, need a licensed plumber or gasfitter. Below is how the system works, what each symptom points to, and where your part stops.
Quick answer: If you smell gas, don't touch switches or the boiler controls. Turn the gas off at the meter if it's safe to, get everyone outside and ring the gas emergency number on your gas bill (000 if there's any fire risk). If a carbon monoxide alarm goes off, or anyone feels sick or headachy while the heating runs, switch the boiler off as you leave if you can, get everyone into fresh air and call 000. Otherwise check the power, timer and thermostat, then read the pressure gauge on the boiler. Low pressure is a common reason for a boiler to stop and show a fault. One radiator cold at the top is often air, which you can bleed out. Cold at the bottom points to sludge. Pressure that keeps dropping or climbing, or water from the relief valve pipe, needs a plumber.
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How hydronic heating works
A hydronic system heats water and pumps it around the house through a closed loop of pipes. The same water goes round and round. Knowing the main parts makes the symptoms below easier to read.
- The boiler. Most are gas boilers, on an outside wall or in a laundry or cupboard. It heats the water and has its own controls, a display or lights, and safety cut-outs that shut it down (lock it out) when something isn't right.
- The pump. It pushes hot water out to the radiators or slab and back. On many newer boilers it sits inside the case.
- Radiators or in-slab pipes. Panel radiators (often just called panels) heat each room, and most have a thermostatic radiator valve (TRV) with a numbered head. In-slab systems run loops of pipe through the concrete floor, fed from a manifold with a valve for each loop or zone.
- The expansion vessel. Water expands as it heats. This small tank, sometimes inside the boiler case, has a cushion of air behind a rubber diaphragm that takes up the extra volume.
- The pressure gauge. On the front of the boiler or the pipework under it. On a sealed system it's your quickest health check.
- The fill valve and relief valve. The fill valve (some manuals call it the filling tap or filling loop) lets mains water in to top up the pressure. The relief valve lets water out, through a pipe that often ends at an outside wall, if the pressure gets too high.
Some systems run off a heat pump instead of a gas boiler. The water side works the same way, but the gas parts of this guide don't apply.
Safe checks before you call anyone
Stop and get help straight away if
- You can smell gas. Don't flick switches or touch the boiler controls, and put out any flames. If it's safe to, turn the gas off at the meter and open doors and windows on the way out. Get everyone outside and phone the emergency line printed on your gas bill. Call 000 if there's any risk of fire.
- A carbon monoxide alarm goes off, or people get headaches, dizziness or nausea that ease outside. Turn the boiler off at its controls if you can on the way out, get into fresh air, and call 000. Our guide to carbon monoxide warning signs covers the symptoms.
- You see soot or scorch marks around the boiler or flue, the flue is loose or damaged, or a visible flame looks yellow and lazy instead of blue. Turn the boiler off and leave it off until a gasfitter has checked it.
- Water has reached a power point or the boiler or pump electrics. Keep clear, and only switch off at the switchboard if you can reach it without touching anything wet.
Power, timer and thermostat
- Is the boiler's display or power light on? If not, check the power point near it and the switchboard. A circuit that trips again straight after you reset it needs an electrician.
- Is the timer set to heat right now? A power cut or a daylight saving change can scramble an older timer.
- Is the thermostat set above the room temperature, with live batteries? On a zoned system, check the thermostat for the cold zone.
- Is the gas on? If the cooktop or gas hot water works, gas is reaching the house.
Read the pressure gauge
Look at the gauge with the system cold. Your boiler manual gives the right range. On a lot of sealed systems it sits around 1 to 1.5 bar cold (100 to 150 kPa) and rises a little once hot. Write down the cold reading, the hot reading and how fast it changes. That tells a plumber a lot before they arrive.
Fault codes and resetting
If the display shows a code or a flashing light, write it down and look it up in the user manual. Most manufacturers put manuals online by model number. Many manuals allow the owner one reset. If the boiler locks out again after that, stop. Resetting a boiler that keeps failing to light or keeps overheating fixes nothing and can hide the fault the gasfitter needs to see.
Bleeding a radiator
Air collects at the top of a radiator, so one that's warm at the bottom and cold at the top often only needs bleeding. Some user manuals describe this as an owner job, so check yours first. You need a radiator key from the hardware shop and an old cloth.
- Turn the heating off and let the radiators cool. Hot water can spit and scald.
- Find the bleed valve at a top corner, a small square spindle or a slotted screw.
- Hold the cloth under it and turn the key a quarter turn anticlockwise. Air should hiss out.
- Close it as soon as water comes out in a steady dribble rather than spitting.
- Check the gauge. Bleeding lets water out, so the pressure drops.
Black or dark brown water from the bleed valve means sludge, which is covered below.
Topping up the pressure
On a sealed system the pressure is topped up through the fill valve, usually under the boiler, which lets mains cold water into the heating. Some boiler user manuals list topping up as an owner task. Many keep filling for a licensed technician. If yours does tell the owner how, follow it exactly: open slowly, watch the gauge, close the valve fully at the cold figure in the manual, and don't overfill. If your boiler uses a removable filling hose, take it off once the valves are closed, as the manual says. If the manual doesn't cover it, or the system keeps needing a top-up, leave it to a plumber. Every top-up brings in fresh water and oxygen that help sludge form.
Only do what your own boiler manual tells the owner to do, and leave everything else to a licensed plumber. That includes undoing fittings, opening the boiler case, or working on the pump, valves or expansion vessel. Our guide to what plumbing you can do yourself explains where the line sits in Victoria.
One radiator cold, or cold at the bottom
Only the top of the radiator is cold
That's air. Bleed it and check the pressure. If the same radiator needs bleeding again within weeks, air is getting in somewhere, or corrosion inside is giving off gas, and a plumber should look at it.
Completely cold while the others are hot
Turn the TRV head to its highest number, and check the valve at the other end hasn't been closed. If it's still cold, the pin inside the TRV may be stuck, which is common after the valve has sat closed all summer. On most TRVs the head unscrews by hand with no water coming out, and the pin underneath should spring up and down when pressed. If it's stuck fast, don't lever it or grab it with pliers. A plumber can free it or replace the valve.
Cold at the bottom or in the middle
This points to sludge. Steel panel radiators corrode a little on the inside over the years, and the black sludge settles low in the radiators and in the boiler. Black water when you bleed is another sign. Bleeding won't fix it. A plumber cleans the system out, refills it with the right inhibitor, and often fits a magnetic filter so it doesn't build up again.
Radiators near the boiler hot, far ones lukewarm
Each radiator has a lockshield valve, normally at the opposite end from the TRV under a plain cap, which sets how much flow it gets. Out of balance, the radiators nearest the boiler take most of the hot water. Balancing means adjusting each lockshield and checking temperatures across the house, so it's plumber work. A weak pump or sludge can cause the same pattern.
In-slab: one zone stays cold
Check that zone's thermostat first. At the manifold, each loop often has an actuator (a small electric head on the valve) and sometimes a flow meter. A failed actuator, air in a loop, or a loop turned off at the manifold can leave one area cold. A slab warms slowly, so give it a good while after any change.
Whole house cold, or the boiler has locked out
If nothing is heating and the power, timer, thermostat and gauge are fine, the boiler is either not firing or not getting its heat out to the house. The display or lights often tell you which.
Low pressure lockout
Most sealed-system boilers shut down when the pressure falls too low. If the gauge is low and a fault is showing, that's likely your answer. A top-up, if your manual allows it, may get the heat back on, but the water went somewhere, so read the pressure section below.
Ignition lockout
If your hydronic heating boiler tries to light and shows a flame or ignition fault, check other gas appliances work and the gas valve to the boiler is open. If the valve is off and you don't know why, or there's a warning tag on it or the boiler, leave it off and call a gasfitter. Past that, it's the igniter, flame sensor, gas valve or gas pressure. That's licensed gasfitting work.
Overheat lockout, or cycling on and off
If the boiler fires, gets hot fast, then cuts out, the heat isn't getting away. Likely causes are a stopped or seized pump, a blocked filter or heat exchanger, sludge, or closed valves. A pump that hums but doesn't turn, or pipes hot at the boiler and cold a metre away, point at the pump. Codes mean different things on different brands, so when you ring, give the plumber the code plus the make and model off the boiler's data plate.
What each symptom points to, and who fixes it
| Cooktop Type | Typical Layout | Best Suited For | Planning Considerations |
|---|---|---|---|
| Standard 4-Burner | One large, two medium, one small simmer burner | Everyday family cooking | Fits most standard bench cut-outs and kitchens |
| Wok Burner (High Output) | Five burners with a double or triple-ring flame | High-heat cooking and stir-frying | Requires wider cut-out and stronger ventilation |
| Fish or Griddle Burner | Elongated oval burner | Long pans and cast-iron griddles | Needs additional horizontal space |
| Modular (Domino) Cooktops | 1 or 2 burners, approx. 30cm wide | Custom kitchen layouts | Often combined with induction or other modules |
| Dual Fuel Setups | Gas cooktop with electric oven below | Flexible cooking and baking | Requires gas and electrical planning |
What you can do, and what needs a licensed plumber or gasfitter
| Cooktop Width | Common Use | Notes |
|---|---|---|
| 60cm | Standard kitchens | Common in apartments and existing homes |
| 75cm | Larger cooking zones | Popular in renovated family kitchens |
| 90cm | Five-burner layouts | Requires ample bench space and ventilation |
| 30cm (Modular) | Custom layouts | Used in modular cooking stations |
Pressure dropping or climbing, and a dripping relief valve
Pressure keeps dropping
A sealed system shouldn't lose water. If the gauge drifts down again over days or weeks after a top-up, water is getting out. Look for a crusty white or green mark on radiator valves and bleed valves, check the pipework under the boiler and the end of the relief valve pipe outside, and watch for damp patches, warm spots on a slab floor, or ceiling stains near pipe runs. A small drop after bleeding is normal, as long as it holds once topped up.
Our guide on how to find a water leak in your house covers listening for leaks and the meter test. The meter test only helps if the heating is being topped up through an open or automatic fill valve. On a sealed system, trust the gauge and get a plumber to find the leak.
Pressure high when hot, low when cold
The pressure should rise only a little as the system heats. If it swings from low when cold to near the top of the gauge when hot, and the relief valve pipe drips while it's hot, the expansion vessel has most likely lost its air charge. Recharging the vessel means taking the pressure off the system first, and on many boilers it's inside the case, so it's licensed work.
Pressure creeping up with the heating off
If the gauge climbs while the system is cold, mains water is getting in. The fill valve may not be fully closed, or it may be worn and letting water by. On a boiler that also heats your tap water, an internal fault can do the same. Check the fill valve is fully closed, and if it still climbs, call a plumber.
Relief valve pipe dripping
Water from the relief valve pipe means the pressure reached the valve's limit (3 bar on many boilers), or the valve is worn. Never cap or block that pipe, because it's a safety outlet. Note whether it drips only when hot, all the time, or after a top-up, and tell the plumber.
Noises from the radiators, pipes or boiler
A healthy hydronic system is close to silent, so note any new noise, where it is and when it happens.
- Gurgling or trickling. Air in the system, or pressure that's too low. Bleed and check the gauge. If it comes back quickly, air is getting in somewhere.
- Kettling. A rumbling, kettle-like boiling sound from the boiler. Water is boiling in spots on the heat exchanger because of scale, sludge or poor flow. Left alone it can cause overheat lockouts and damage the heat exchanger.
- Banging when the heating starts. Often air, sludge or a pump problem, sometimes a valve snapping shut. Worth a look, especially if it's new.
- Ticking or creaking as it warms or cools. Pipes expanding against joists or clips. Often harmless, though a plumber can add clips or packing where a pipe rubs.
- Humming or whining from the pump. A worn bearing, a pump speed set too high, or air in the pump.
What the plumber or gasfitter does next
Hydronic heating repairs follow roughly the same order as your checks, just further into the system:
Pumps, valves, expansion vessels, igniters and sensors can all be replaced on their own. If the heat exchanger has failed or parts are no longer made, a new boiler can make more sense, and the plumber can talk you through both. How often the boiler needs a service is set by the manufacturer in its manual, so check that first. For safety checks on the rest of your gas appliances, see how often gas appliances should be checked in Victoria. Our hydronic heating page has more on our hydronic work, and the boiler's gas side falls under gas fitting. Gasfitters are licensed by the Building and Plumbing Commission (BPC), and Energy Safe Victoria is the gas safety regulator.
Hydronic heating not working: common questions
Low pressure isn't dangerous in itself, but most boilers lock out once it drops far enough. A system that keeps losing water also takes in fresh water with every top-up, which speeds up corrosion inside. Top up if your manual allows it, and get a plumber to find where the water is going.
Air is getting in, or gas is forming inside the system. Possible causes are a small leak that draws air in, pressure that runs too low, frequent top-ups, or corrosion inside the radiators and pipes, which gives off gas. A radiator that needs bleeding every few weeks is worth a plumber's visit.
Only if your boiler's user manual tells the owner how to. Many manuals keep filling for a licensed technician, so if yours doesn't cover it, leave it to a plumber. Needing regular top-ups means there's a leak or fault to find.
Slab heating is slow by nature, because it has to warm a lot of concrete before you feel it, and it's meant to run on a steady schedule rather than be switched on when you get cold. If it's slower than it used to be, or one area never warms, have a plumber check the zone actuators, the flow at the manifold, air in the loops and the boiler.
No. Reset it once if the manual allows. If it locks out again, write down the code and leave it off. Repeated resets on a boiler that can't light or keeps overheating can do more damage, and with a gas appliance it isn't worth the risk.
Most of the parts that fail can be replaced on their own, so repair is often the sensible first step. Replacement starts to make sense when the heat exchanger has failed, parts are no longer made, or the boiler keeps breaking down. A licensed plumber or gasfitter can tell you which side of that line yours is on.
Before the plumber or gasfitter arrives
Having these ready makes the visit go faster:
- The make and model, and a photo of the data plate
- Any fault code or flashing light pattern, written down
- The pressure reading cold, and again once the heating has run
- Which radiators, rooms or zones are cold, and since when
- Any noises, drips from the relief valve pipe, or damp patches
- Clear access to the boiler, the manifold and the radiators
- The boiler left off if you smelled gas, saw soot or had a CO alarm
