Compression tank Vs. Expansion tank

MarkMEPFP · December 2017

What are the reasons one would use one over the other? I was corrected on a project when referring to a compression tank vs. Expansion tank.

hot_rod · December 2017

The Amtrol Handbook does a good job of explaining the two.

The terminology sometimes gets mixed. Expansion tank, is that a plain steel tank, bladder, diaphragm, compression?

I usually refer to either a plain steel tank, as an air management system
Or expansion tank which could be a bladder, diaphragm or bag type tank, air elimination system.

Basically with a plain steel tank, called air management, any air removed needs to be placed up in the tank. That is the expansion space. With Extrol™ style tanks air elimination is outside the tank, the expansion room is sealed inside the tank via the diaphragm.

Cost, size, weight, heat loss, proper installation and maintenance are some of the factors to consider when comparing the two.

Gordy · December 2017

Technically both "compress"an air cushion, and both absorb expansion of system volume changes. Main difference is a membrane isolating the air, and fluid. One has it one doesn't.

JUGHNE · December 2017

Another difference is that a plain steel compression tank has to be located above the air collector/separator which would typically be on the top of the boiler. This puts these hanging from the ceiling as high as possible. I have one customer with the compression tank in the attic above the boiler room.

Your membrane/bladder/expansion tank can sit or hang anywhere and be piped into the system.
These will be only about 1/3 the physical size of the type above.

leonz · December 2017

Gordy is correct in what he is telling you.

I have a 15 gallon B+G steel compression tank for my dual fuel boiler system and it works very well allowing me to run my coal stoker with very low operating pressures circulating hot water through 225 feet of 3/4" baseboard-I wish I has cast Iron radiators instead of baseboard for the additional water volume though.

The use of a steel compression requires the use of an B+G airtrol fitting to allow the air bubbles to leave the system and then allows the cooler water to return to the system via the second part of the casting in the airtrol fitting.
It also requires that the water feed valve remain closed after the system is filled and heated to operating temperature to eliminate any excess air bubbles with the airtrol valve.

In a closed system with a steel compression tank there is less large diameter plumbing to deal with as a 3/4" tapping is used for everything up to 100 gallons in capacity in a single tank OR a series of smaller tanks can be used to create as much compression volume as needed hanging them in the ceiling joists in a basement and Teeing each tank and airtrol valve in a parallel header pipe to control air pressure in a closed system
(having extra volume does not hurt either as it allows you to have a lower operating pressure).
I hope to add another 15 gallon or larger tank when I am able to do so and have even more volume for my system as it has been performing very well with no noise and very low pressures with no issues.
I especially like not having to crawl around on my hands and knees bleeding the air out of the baseboard heating heating system in this place.

If I had known what I know now about them I would have never let the plumbers rip out the original horizontal open to air steel expansion tank that came with the original heating system in this house 35 years ago when I had a wood and coal hand fed boiler installed.

An air scoop can be used with a steel compression tank or a B+G air separator(which is what I use) and the need for automatic air vents is eliminated

Once you have the boiler drains installed on the return to boiler lines you can push the air out of the system when it is empty while filling it and then you can open the valve to the steel compression tank and let the airtrol valve handle the rest of the air remaining in the system.

Its just like Dan says in his well written books the tank just sits there in the ceiling doing its job silently with no moving parts.

Our living with a drilled water well at my place makes a steel expansion tank a good idea as the system is completely closed and a water feed valve is always closed and only needed if a leak occurs in my system.

There is a plumber with a video on youtube that talks about the B+G airtrol valve that this same gentleman filmed and was not well done in my opinion as he should have read the literature about the airtrol valve to present it correctly to the viewer.

I am not disparaging him by any means but he needs to make a new video with Person that represents B+G in talking about the airtrol valve in depth for the viewers benefit

EBEBRATT-Ed · December 2017

To me not being able to use automatic air vents with a plain steel tank is a disadvantage.

Too many jobs I go to have waterlogged expansion tanks (plain steel), water blowing out of relief valves and ruined boilers from excessive MU water because the uninformed in our business have installed automatic air vents and allowed the tanks to water log. Also piping a steel compression tank means having the near boiler piping right.

Seen any steam jobs lately. They don't read so they can't pipe steam or water

I am not in the least condeming the use of a plain steel tank installed properly, just that if I have a choice there more trouble than they are worth

Solid_Fuel_Man · December 2017

My official position is that if there is an existing plain steel tank with the proper fittings and all looks good then I'll consider reusing it. If it looks to be at all compromised at the seams or has obvious leaking at some point then it goes away, and a properly sized diaphragm/blabber tank goes in.

leonz · December 2017

I can tell you the smallest 15 gallon steel compression tanks that B+G USA uses from Wessels are a high quality steel compression tank with domed top and bottom weldments and 3/4" gauge glass tappings and they are worth every penny spent on them as they do not compromise on quality.

The added benefit for these tanks is the base ring that is fully welded to the tank as it would allow it to be positioned upright for a gravity hot water system.

I am not completely sure; but the lower one inch tapping on the side of the 15 gallon tank could be used for the water loop using a short nipple then a Tee which would have 2 short nipples that would then have two 45 degree elbows to connect to the riser pipe and the return to boiler piping and a 3/4" vent line pushed through the roof or back to the basement to a laundry sink or a floor drain from the upper 1" tapping.