Expansion tank rust's


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Old 08-19-06, 04:20 AM
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Expansion tank rust's

I have a radient floor heating sys,about 5 years old. It uses a hot water tank for the boiler,has an pump. It is an closed loop and I have to add water manually,pressure is usually 15-20 lbs. The expansion tank rusts out every 9 momths-15 months. I was advised by someone in the heating and AC business to add some anti-frezee so it would coat the inside of the sys.Now if I have to fix the sys the water doesnt come out black. Even with that I just replaced another tank.Any ideas why these tanks are rusting out.I use an #15 tank and have tried different makes.
 
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Old 08-19-06, 01:40 PM
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Is this a DIY radiant installation? Did you, or whoever did the installation use tubing with an oxygen barrier?

Your rust problems with the expansion tank are in reality oxygen corrosion and are caused by oxygen in the water and a lack of circulation in the expansion tank.

Anti-freeze is NOT the answer and if you do not have a "reduced pressure backflow preventer" (RPBP) installed in your make-up water line you are in violation of public health laws (if you are connected to a municipal water system) or at least placing your family in danger. Any time that you chemically treat the water in a heating system you must either use a chemical that is approved for human consumption OR you MUST HAVE a RPBP to prevent any backflow of the chemically treated water into potable water sources.

I am unaware of any water heater that is listed and approved for use as a boiler in radiant heating systems. The manufacturers do not honor any guarantees when used in this manner. Local plumbing / mechanical codes usually prohibit such use UNLESS the heating system is an integral part of the domestic hot water system of the residence.


Back to your problem. I assume that you have the diaphragm expansion tank piped into your system some distance from the main area of water flow and that the tank is mounted with the pipe connection at the top and the tank below the point of connection. I would suggest that you first install an approved RPBP in the make-up water line and then re-pipe the expansion tank so that it is mounted with the connection at the bottom (tank above the connection) and this point of connection is near the suction side of the circulating pump. Then you need to obtain some oxygen scavenging chemical and corrosion preventive chemical (you really need a source inside the water treatment industry) to use in your system.
 

Last edited by Furd; 08-19-06 at 05:52 PM.
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Old 08-19-06, 05:35 PM
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Rusting tanks

I agree the problem is likely oxygen & am surprised the water heater has lasted this long. Hope this radiant is not in the floor. The tubing is the most likely source of the oxygen. You should not have to add water. If you do, the water in the system is going somewhere. Read leak.
 

Last edited by Grady; 08-20-06 at 07:53 PM.
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Old 08-20-06, 05:52 PM
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Dislike chiming in with potential bad news, but it really sounds like you have an oxygen barrier problem, likely in the tubing. I have read about the potential for this, but never heard about an actual install. Sorry.
 
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Old 08-21-06, 03:14 AM
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expansion tank

As a temporary bandaid, couldn't he switch to a potable expansion tank to avoid the rust problem? I know they cost more, but I've seen them cheap on eBay.
Would have to lower the charge to what his system calls for..

Adding antifreeze, other than the stuff that is designed for campers, (propylyne? SP?)etc. is dangerous--very toxic!

Peter
 
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Old 08-21-06, 03:43 AM
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Exp. Tank & Anti-freeze

The domestic expansion tank may hold up beter but the problem would likely just move from the exp. tank to the water heater or pipes, if any are copper. It would seem like the tank is acting as a sacrificial component.
If the anti-freeze being used is boiler anti-freeze, it is basically the same as that used in RV applications.
 
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Old 08-21-06, 01:36 PM
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Ooops

Originally Posted by Grady
The domestic expansion tank may hold up beter but the problem would likely just move from the exp. tank to the water heater or pipes, if any are copper. It would seem like the tank is acting as a sacrificial component.
If the anti-freeze being used is boiler anti-freeze, it is basically the same as that used in RV applications.
After thinking about it, dumb suggestion. If he has an O2 problem, the boiler rusting out is the bigger issue in the long run. How would you fix a problem like this if indeed the wrong tubing was used?

Pete
 
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Old 08-21-06, 01:54 PM
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If non-oxygen-barrier tubing was installed the ideal solution would be to replace the tubing with one that does have an oxygen barrier. That is probably not practical.

The original poster mentioned that he has to add water manually but he didn't state how much water or how often. Make-up water almost always contains a significant amount of dissolved ogygen and a leaking system, even one that had oxygen-barrier tubing, would gain a large amount of free oxygen under these conditions.

So, the first thing to do is fix all the leaks, no matter how insignificant they may seem. The next thing would be to install the RPBP if not already installed and begin chemical treatment of the water.
 

Last edited by Furd; 08-21-06 at 07:10 PM.
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Old 08-21-06, 07:54 PM
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fixed expansion tank

As I have said I did fix the tank and that solved the small leaks.The sys has ben holding at 25 lbs for the past week (just runing high to find any leaks).I only run at about 15 when heat turned on. I usually only add very small amounts of water and have to manual connect to sys to add water (if pressure drops ). What chemicals can you add to slow down or stop the problem with oxygen. Would a plastic linned expansion tank help or hurt the sys.
 
 

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