Boiler expansion tank replacement: steel expansion vs bladder?

**grider** · 02-23-2011, 01:22 PM

My 48" x 12" horizontally mounted steel expansion tank has developed a pinhole leak (it's 40 years old but has otherwise served me well) and needs to be replaced as the PRV is opening due to excess pressure build-up each heat cycle. I realize that the standard tank nowadays is the bladder type, but they will never last as long as the old type, so, my question is: Should I replace the old tank with a new bladder type or one like the old one? If so, what would be the equivalent size bladder tank to the 4' x 1' steel expansion tank I now have, and is it crucial where it should be located in relation to the boiler? Thanks for any helpful comments! Greg

**jadnashua** · 02-23-2011, 02:47 PM

My personal opinion...no 'closed' system heating system likes air; the only way to keep air out is if it is truely closed. A bladder tank does that. Each time you open yours up, drain the tank, you introduce new air into the system. This slowly gets purged out, but in the process provides oxygen to rust any ferrous parts. A bladder tank can last 5-years or more, and are fairly inexpensive.

The way to size the thing is to determine how much water is in the system when it is full, then specify how big of a swing you have (i.e., from cold to max hot). That determines the volume you need to provide for expansion. Watts makes a decent tank and has a sizing calculator. Useful, even if you choose a different brand. http://www.watts.com/pages/support/sizing_ET.asp

The best place to install it is on the feed side near the circulator, but it will work anywhere in the system. this way, when the pump turns on, it has a little extra volume available.

**tk03** · 02-24-2011, 07:32 PM

I prefer the old steel tanks due to the fact the bladder is oxygen permiable and the air from the tank get into the water. The old steel tank is a maintenance free tank , no draining, if properly installed. The biggest downfall of the old steel tank is the systems have auto air vents installed. No auto air vents on the old steel tank systems. Pipe uphill all the way, no valves in the horizontal piping.
For more info check out this link.
http://www.comfort-calc.net/expansion_tank_info.html

**Tom Sawyer** · 02-25-2011, 05:39 AM

What? In 36 years of servicing boilers I have NEVER ever run across a steel tank that was maintenance free. The air cushion gets absorbed into the water over time (usually within a couple years) and requires the tank to be drained which can be time consuming and difficult to do especially for a homeowner. Nobody installs steel expansion tanks anymore. Even on very large commercial jobs we install bladder type tanks.

A direct replacement for what you have would be a extrol #60

For the record, whoever wrote that web page needs to do some serious editing for typo's and some more research for content because many parts of it are just plain wrong, (cool down period? there is no cool down period unless the boiler is cold starting) though he did do a nice job of cutting and pasting information from manufacturers.

**tk03** · 02-25-2011, 05:11 PM

The air cushion gets absorbed into the water over time (usually within a couple years) and requires the tank to be drained which can be time consuming and difficult to do especially for a homeowner

Where does the air go when it leaves the tank?

Many people leave the old steel tanks in due to never need draining.
The reason for draining is due to auto air vents venting the air that leaves the tank.
Sure the water goes in and out of solution and leaves the tank. It than goes thought the system and back into the tank instead of venting out due to some moron installing an auto vent in the system.

**jadnashua** · 02-25-2011, 06:55 PM

Oxygen will combine with any ferrous materials in the boiler. The nitrogen anc carbon dioxide will form acids. Air has NO place in a closed system.

**Tom Sawyer** · 02-26-2011, 06:23 AM

The air does'nt leave the tank it stays at the top of the tank. the loop does not circulate through the tank. There are no auto air vents on the tank. If there was an auto air vent there would be no air in the tank and no cusion for water expansion.

**tk03** · 02-26-2011, 09:36 AM

The air does leave the tank!
As you heat water you drive the air out and as the water cools off the air goes back into the water. The EXPANSION TANK is where the water expansion goes to when heated and comes out of when the water cools. The air and water does move in and out of the tank. the idea is no auto-vents anywhere in the system as the air will find the auto-vents. This is why the tanks need drained.
If the air does not come out of the tank why would you ever have to drain it? You stated it needs drained due to being absorbed into the water. If it is absorbed into the water it is still in the tank, by what you say and would still absorb expansion. The air has to leave the tank to create a pressure problems.

**Tom Sawyer** · 02-26-2011, 03:02 PM

Sorry but you are just plain wrong on all counts. Go to heatinghelp.com

**jadnashua** · 02-26-2011, 06:33 PM

Any time there is an air/water interface, some of the air will be absorbed by the water (not to say it will stay there). As a result, there's some oxygen disolved in it. This gets distrubuted by the pump, and it WILL react with the iron it finds. Same with the CO2 and N2 and any other gasses and impurities in the water. The heat helps fuel some chemical reactions...simple chemistry. You really don't want air in a closed system. An open system is designed with less reactive materials (copper, brass, SS, plastics for the most part) and can generally handle it.

**Tom Sawyer** · 02-26-2011, 09:54 PM

Any air in the system, that is outside of the expansion tank should be taken care of by the air scoop and the automatic air vent. The air that is in the expansion tank stays there.

**jadnashua** · 02-26-2011, 10:03 PM

Originally Posted by Tom Sawyer

Any air in the system, that is outside of the expansion tank should be taken care of by the air scoop and the automatic air vent. The air that is in the expansion tank stays there.

And where does that come from, and why do you need to drain the tank eventually?

**Tom Sawyer** · 02-27-2011, 06:45 AM

If the system is tight the air will be purged within a few hours of operation. If there are no leaks in the system the only oxygen comes from the initial filling. As the water is heated he oxygen comes out of saturation and the automatic air vent and the air eliminator remove it from the system. With a steel tank the air cushion will eventually re-saturate into the water and be eliminated as before by the air scoop and air vent and the lost system volume is re-filled by the auto fill which because there is no longer an air cushion for expansion means the boiler pressure climbs and the relief valve pops and then more oxygen rich water is added to the system and the cycle continues until someone drains the tank and re-establishes the air cushion. This does not happen with a bladder thank because the air cushion is isolated from the water in the boiler. 90% of hydronic systems that we install have no connection to the water system. They are filled with anti-freeze/water mix, purged and sealed.

**tk03** · 02-27-2011, 04:18 PM

I will agree that air should not leave the tank and would not with the use of airtrol fittings. But most jobs do not use airtrol fittings the air comes out and back into the tank due to heating, cooling and the use of air separators.
With a steel compression tank you should not use auto-vents any where in the system if you have no airtrol fitting. Of course of there is an airtrol fitting the tank would not require draining.
The bladder type tanks will allow oxygen to enter the system due to the bladders are oxygen permeable. Most manufacturers today charge the bladder type tanks with nitrogen for two reasons. One the nitrogen is more stable and not having oxygen to get into the system.