How Do You Bleed a Radiator?
To bleed a hot water radiator, turn off the heating system, hold a cloth or small bowl under the bleed valve at the top of one end, and open the valve slowly with a radiator key or flat screwdriver until a steady stream of water replaces the hissing air. Then close it tightly. This Old House recommends doing it once a year before the heating season, starting with the radiator farthest from the boiler. Steam radiators are different: they release air through an automatic vent and are not bled by hand.
American hot water systems measure boiler pressure in psi, not the bar readings used in British guides. Taco ships its Model 329 boiler feed valve set to 12 psi on the system side, adjustable from 10 to 25 psi, and pairs it with a relief valve that releases at 30 psi. Steam systems run at far lower pressure: Hoffman rates its Model 1A and 1B radiator steam vents for a maximum operating pressure of 1 1/2 psi and says to replace them as needed or every 10 years. This guide covers how to tell which system you have, how to bleed a hot water radiator, what the gauge should read afterward, and when cold radiators point to a bigger problem.
Is Your Radiator Hot Water or Steam?
A radiator bleed valve is a small valve near the top of a hot water radiator that lets trapped air out of the system. This Old House notes it sits at the top of the radiator on one end, and you open it with a radiator key or a screwdriver.
Steam radiators work on a different principle. Hoffman explains that its radiator steam vent allows air to escape from the radiator as the steam pushes in, so the vent does the bleeding on its own every time the system runs.
If you are not sure which system you have, ask a technician to identify it before you open any valve. The bleeding steps below apply only to hot water radiators.
How Do You Know a Radiator Needs Bleeding?
This Old House lists these signs of trapped air in a hot water radiator:
Cold spots at the top of the radiator while the bottom is warm
Radiators that take longer than usual to heat up
Gurgling or bubbling noises from the radiators
Higher energy bills from inefficient heating
Air rises, so it collects at the top of the radiator and keeps hot water from filling it. Bleeding lets that air out so water can reach the whole radiator again.
Older homes can have comfort problems beyond the radiators, from outdated thermostats to hot and cold spots. The guide to common HVAC problems in older homes covers the bigger picture.
How Do You Bleed a Hot Water Radiator Step by Step?
This Old House gives this sequence:
Turn off the heating system. The water in a running system is hot and can burn you.
Find the bleed valve at the top of the radiator on one end.
Place a bowl or rag under the valve.
Insert the radiator key or screwdriver and turn it slowly counterclockwise.
Keep the valve open until a steady stream of water appears, which shows the air is out.
Close the valve by turning it clockwise, and make sure it is tight so it does not leak.
Wipe up any water and move to the next radiator.
Start with the radiator farthest from the boiler and work your way back, as This Old House recommends.
What Should the Boiler Pressure Read After Bleeding?
Each radiator you bleed releases a little water along with the air, so check the boiler's pressure gauge when you finish. Taco describes its boiler feed valves as automatically maintaining system pressure, adding water as needed. Its Model 329 comes set to 12 psi and can be adjusted from 10 to 25 psi, so your system's correct pressure depends on how the installer set it.
Taco's dual control pairs that feed valve with a relief valve set to release at 30 psi. If the gauge climbs toward 30 psi or water drips from the relief valve, call a technician. If the gauge reads far below where it normally sits and does not recover, the system may not be refilling, which also needs a professional. Taco states its feed valves should only be installed by qualified heating professionals.
Weil-McLain lists system pressure among the items an annual boiler service can check. A fall HVAC checklist is a good reminder to schedule that visit before the first cold snap.
What If a Steam Radiator Stays Cold?
Do not try to bleed it. Hoffman says a cold upper part of a steam radiator may mean the vent is plugged with scale or malfunctioning. It also says a little vapor or moisture from the vent is normal, while steam continuously passing through the vent points to a malfunction.
Hoffman's guidance for its 1A and 1B vents:
Inspect the vents every year.
Replace a vent as needed, or every 10 years.
On adjustable vents, use faster venting settings for radiators farthest from the boiler so they heat more evenly with closer ones.
A technician can replace a failed vent. If several steam radiators heat unevenly, the system may need balancing, and a guide to zoning in older homes explains other ways to even out room temperatures.
When Does a Radiator Problem Need a Technician?
Bleeding fixes trapped air. It does not fix a failing boiler, pump or control. Weil-McLain lists these warning signs in its guide to boiler replacement:
New noises such as banging, knocking, rumbling or gurgling
Uneven comfort, which can trace back to pumps, controls, zoning or radiators
Leaks or corrosion
Frequent repairs and rising heating costs
Weil-McLain also notes that excessive make-up water adds corrosion to a system. If you find yourself bleeding the same radiators every few weeks, or the boiler keeps adding water, have it inspected. The signs your heating system is about to fail include other symptoms to watch.
FAQ
How often should you bleed radiators?
This Old House recommends once a year before the heating season, and more often if you notice signs of trapped air.
Should the heat be on or off when bleeding radiators?
Off. This Old House says to turn off the heating system first, because the water in a running system is hot and can cause burns.
Which radiator should you bleed first?
Start with the radiator farthest from the boiler and work back toward it, as This Old House recommends.
Can you bleed a steam radiator?
No. A steam radiator vents air automatically through its steam vent. If it stays cold at the top, Hoffman says the vent may be plugged and need replacement.
What pressure should a hot water boiler read?
It depends on the system. Taco's Model 329 feed valve comes set at 12 psi and adjusts from 10 to 25 psi, with a relief valve at 30 psi. Check your boiler's manual or ask your technician.
Why are my radiators gurgling?
This Old House lists gurgling and bubbling as signs of trapped air. If bleeding does not stop it, Weil-McLain lists new gurgling noises among the signs a boiler needs attention.
Van Bremen's Heat & Air Services Boilers and Radiator Heat
Van Bremen's Heat & Air installs and maintains gas and oil boilers from its office at 214 Stoystown Rd in Somerset, serving Somerset, Fayette, Cambria and Westmoreland Counties and Morgantown, WV. Its heating maintenance covers boilers along with furnaces and heat pumps. A technician can check system pressure, the feed and relief valves, and radiators that will not heat. See the heating services page and maintenance services page.
Visit Van Bremen's Heat & Air or call (814) 483-1735 today. Contact us for more information.
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