Identifying Your Auto Vents

And How They Work

Auto Vents are used to let the air that accumulates in a hot water heating system work its way out on its own. This is where the name comes from and the principle is just that.

Most auto vents work in the same way. The basic design uses a very small tank, about three inches in height and about 2 inches around.Taco Automatic Hot Water Heating VentCredit: Dreamaker. There is a plunger, or float inside the canister. This plunger starts out on the bottom of the vent and rises as the system is pressurized and it fills with water.

As the heating system runs, water that has micro bubbles of air passes by the vents tapping or connection. The air naturally rises to the top, so it goes up into the auto vent chamber until it gets to the top.

Over time the air that accumulates inside the vent pushes the water down. When this happens enough the plunger or float falls and an air valve on the top of the unit will open as a result. When this occurs the system pressure pushes more water into the vent which pushes out the air that's at the top. This happens until the plunger or float rises enough with the water to close the valve and the process starts all over again.

Get The Air Out

Replacing the auto vents can save time and money


Preparing The System Before Changing The Auto Vents

Shut Down And Draining Procedures


A Typical Multi Zone Hot Water Heating SystemCredit: DreamakerAuto vents are imperative to keeping any hot water heating system functioning properly. Without them all systems eventually become air bound. This can cause a no heat situation and in time burn out circulating pumps and harm other vital components.

This type of heating system reaches high temperatures. The surfaces as well as the water within the system can reach 210 degrees Fahrenheit. For this reason you should only work on this type of heating system when it's completely cool.

When working on any mechanical equipment first make sure you have shut down the power source. This can be an emergency switch at the boiler or at the top of the cellar stairs. It can be done right inside your main breaker or fuse panel. Make sure you have no power by trying to make the system call. This can be done by adjusting a thermostat all the way up. If the system doesn't come on you know the power is off.

Hot water heating systems are operated under pressure. This requires the pressure to be relieved before this job can be done. To accomplish this we must have a drain valve somewhere in the piping. Once the drain valve is located. A hose should be attached and placed where it will terminate in a location that will cause no injury or property damage. This may include  a drain line or outside away from the house.

Find the water feed that fills the system. This will be a small line coming from the water supply. Either your well or a city water main provides pressure for the system. This line must be closed to prohibit water from refilling as you drain.

Once you've shut off the water and the power to your hot water heating system, next you should close all the caps on the existing auto vents. After this is done the system should be tight. Now open the drain valve until no more water comes out of the hose.

After the water stops draining. Your system will be under a vacuum condition. This means that there is a negative pressure within the entire system. Now you'll be able to remove the existing auto vents and replace them with new ones without any further draining of the system.


Replacing The Old Components

Out With The Old, In With The New

Once you've located any auto vents within the system, any proper size channel lock pliers can be used to remove the vents. Grasp the assembly with pliers or a pipe wrench and rotate the vent counterclockwise to remove it from the threaded tapping.

It's recommended that you do these vents one at a time so you don't relieve the vacuüm you created in the last step. Remove one vent and replace it. Then close the air cap on top and move on to the next one. The procedure is repeated until the system has all new vents.

Changing only one, is not recommended because of the inexpensive cost of the vents. It takes too long to prepare the system and then restart everything. It really makes sense to change them all at once.

Use teflon pipe joint tape and pipe thread compound on  the new vent threads. Then wind your replacement  auto vents into the tapping in a clockwise motion. It's best to do this by hand. After you have installed the vent hand tight you can always re-tighten it a little more if it leaks.

After all the vents are replaced within your system, you'll be ready to move on to the next step. Always inspect the entire system to look for any other leaks while you're at it. If any other leaks are found they should be repaired before you refill the boiler.


Refilling And Completing The Job

Final Check-List

After you've completed the job it's time to see how you did. Close any drain valves you may have opened and turn the water feeder valve on to begin filling the system. Watch the areas you've worked in and if there are any leaks re-tighten the vents until they stop.

Once the system is tight and you can hear the water-feed stop adding water, you can turn the power back on. turn up a thermostat to put your system back into operation. As the system starts and circulating pumps begin to do their job, your likely to hear some air flowing through the piping at first. Wait about five minutes to let this air subside. After the noise calms down you'll want to open up all the caps on the new auto-vents.

You should hear some air release with each one as they're opened. When the air stops check for any leakage at the air outlet. The valve should be closed. A small amount of water escaping as the valve shuts itself off is normal. Wipe the top of the vent and continue to watch for a leak. If the water still leaks then there could be something stuck in the outlet try tapping it with your channel lock pliers to seat the valve.

Once any leaks are resolved, your job is done. There are systems that will need the air be to be forced, or purged out of the piping and boiler. This will prove necessary if the air noise doesn't go away after ten minutes of running the zones. If you need to purge your hot water heating system, here's a link for another article I've written to cover the procedure for that.

Purging your hot water heating system

How to replace an auto vent on a hot water heating system isn't something that's very difficult. Most handy homeowners can do this themselves. Savings can reach several hundred to a thousand dollars depending on where you live and the extent of your heating system.