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All you need to know about a boiler or furnace expansion tank.

Why does my furnace need an expansion tank?
When water heats it expands and water can not be compressed, so without an expansion tank installed, pressure will build and cause the relief valve to blow off.

The boiler expansion tank absorbs the expanding water by allowing the air in the expansion tank to compress, this prevents the pressure from building and allows the boiler to heat and cool without too mush pressure building up.
Removing the furnace relief valve from the boiler or plugging it so that it does not leak is a very bad idea, this can allow the pressure on the boiler to reach dangerous levels causing an explosion that can destroy your whole block. So don't do that. Now on to the expansion tank.

Amtrol Expansion tank

For the most part Amtrol expansion tank is the most commonly used expansion tank.
Click Any Image To See A Larger Image.

Expansion tanks

There are other Brands out there and they work much the same.

For most Residential Boilers the following 4 tanks are mostly used.

The Amtrol # 30. The # 15 The #109 and The #110

Some larger homes need the Amtrol #60. and Larger.

Fill Trol 110 expansion tank

And they get much larger but this page is for Home owners. So I will not bother with the larger tanks. Replacing An expansion tank can be very easy in most cases, but you are dealing with VERY hot water and you can get scalded. It may be best to have a heating contractor do it. but like the one I did on the Weil McLain boiler, turned out to be a real pain.

Boiler expansion tank

Before I get into how to replace it, I should ask, does it really need to be replaced?
If it is leaking, it's simple, it needs to be replaced. See Boiler relief valve
If it is full of water, it needs to be replaced, Tap the tank the back should sound hollow, if not, it is full. if it is full, that means it will be very heavy, So be careful not to let drop on your fingers when you remove it.

Note: the tapping on the expansion tank will tell you if it is full or not, so that you know that it will be very heavy once it is unscrewed, but if it sounds hollow, this does not mean the tank is good, the tank can be defective even if it has a hollow sound.

Furnace expansion tank

If there is no air pressure in the tank. use an air gauge you should have 12 psi. But remember you let air out every time you test it. If it does not have 12 psi you can pump it up to 12 psi with a pump. But this is just a temporary patch and it will go flat again. so it should be replaced.
Although a water logged Expansion tank will cause a relief valve on a boiler to leak and /or blow off. It is not the only reason the relief valve leaks and or blows off.
See why is my relief valve leaking? by clicking here.

Amtrol 109

Note: The #110 and the #30 are the exact same size tank. but they are not interchangeable.
The #110 is part of a Fill Trol System and only screws into a Fill Trol fill valve. There are 2 types of Fill Trol fill valves one has a valve handle to turn the water off the other does not. In the pictures below you will see the both of them.
Fill Trol With Handle and Fill Trol With Out Handle. Click on Picture for larger picture. The #110 and the # 109 will only screw into one of these valves. The #15 and the # 30 will screw in to any 1/2" iron pipe size fitting. The #109 and the #15 are also the same size as each other. but about 1/2 the size of the #30 and The #110.

Amtrol number 30

This is a # 30 it screws into the bottom of an air scoop This one will be easy to change, but if it is water logged it will come down hard once you unscrew it. It is very heavy when full of water.

Furnace expansion 30

This is also a #30 tank And this would also be easy to replace.
Ok So now we have determined if we should replace it or not and what type of tank we need. So lets replace the thing already.

Boiler expansion tank 15

First Turn the power to the boiler off. Then Turn the water to the boiler off. Follow the fill valve back till you see a valve to turn off. If you have valves on your supply and return lines. you may want to turn them all off to prevent draining all your zones.

Expansion tank test

Then hook up a hose to the lowest boiler drain and relieve the pressure.

Auto air vent

If you have to drain the whole boiler to remove the tank you may want to remove an auto vent to allow air to get into the top to allow the boiler to drain faster.

Dielectric coupling    110 Fill Trol expansion tank

 The Rest is easy unscrew the old one and put some kind of pipe dope or tape dope on the threads. (only on the15 and #30) and screw the new one back in The #109 And #110 do not need and pipe dope or tape dope they have an O ring that makes the water tight seal. ok that's it you are done put the auto vent back. turn the boiler drain off turn the water back on Get the Air out turn the power back on.

Furnace purge

Before you turn the power back on and fire up the boiler, be sure the boiler is full of water and the heat lines are also full of water.

Get air out of boiler

You may need to remove air from the heat loops please follow this link to remove air from your system Remove Air
Running a boiler without water in is will cause damage to the boiler, please be sure it is full before you turn the power back on.

Find replacement boiler expansion tanks here.