If you have a radiator in your home that refuses to heat up, this can be quite annoying. So, what is causing this to happen and how do you fix it? Let’s first get into what type of boiler you own.


A steam boiler works by heating the water in the boiler heat exchanger until steam is created. As the steam travels from the boiler, it forces air out of the lines and radiators via various vents which close when the density changes. The steam flows up the main supply lines, branches off to each smaller run, and travels through a radiator. As the steam flows through the radiator, it cools and condenses the steam back to liquid water which then runs back to the boiler from a return line. 

If you have a radiator not heating in this system, you need to look at your radiator vent. (photo below) If it is older than 5 years old and doesn’t make a “hissing noise” when the boiler starts running, you may have a failed radiator vent. These are very inexpensive and just unscrew by hand. When you install your new one, be sure to add some Teflon tape to the threads to ensure it doesn’t leak and hand tighten. 

If the vent isn’t your issue, then you need to look at your Thermostatic radiator trap. (photo below and installed at top of article)They are located on the return side of the radiator near the floor. These traps have a diaphragm that expands when steam enters the trap and closes. Once the steam cools and condenses back to a liquid, it opens and allows the water to drain and fresh steam to enter the radiator. When these fail, the radiator will not heat. Radiator traps are a little more difficult to change without proper tools, and are generally a pain in the butt! 

Hydronic Boiler (Hot Water Boiler)

A hydronic boiler works by heating the water in the system to a pre-set temperature and then circulating this water throughout the home, from the radiator to the radiator with the aid of a circulating pump. These systems are used in heating floor slabs, baseboard radiators, and even swimming pools. 

If you have single or a few, hydronic boiler radiators not heating, the first thing you need to check is the boiler shut off. Be sure that it is open and not leaking. Next, check if there is air in the radiator. You can do this by finding the bleeder. This will be located on the return side of the radiator and can be opened with either a flat head screwdriver or a bleeder key (shown below). Simply open the bleeder till water comes out, then close it. If water does not come out, close the bleeder and go to the boiler and add water to the system till the pressure reaches the recommended pressure (usually 12-15psi). This will also ensure that you have the proper water pressure to get water to reach all of the radiators. Then go back to the radiator and bleed again. Repeat this as many times as necessary till you get water from the bleeder. Air is naturally going to rise in a sealed system so it is then very common to have a radiator on an upper floor or attic area become “air locked” and need to be bled. A hydronic boiler system should be periodically bled once a month during the heating season. 

If bleeding doesn’t do the trick, you might have a blockage either in the line, the shutoff, or the radiator itself. Water will always follow the path of least resistance. This means, that if it is easier for water to flow around an object than through it, it will bypass it altogether. Boiler systems are designed specifically for this purpose. This way, if one radiator is blocked, air-locked, or shut off, water will continue to flow to the next one in line. Otherwise, if one radiator failed, you would have no heat at all. If you believe that a blockage is your issue, I would recommend having an HVAC contractor company that specializes in boilers, come out and assess the issue and locate the problem area. 

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