Thoughts on expansion tanks and other things.

Reply

  #1  
Old 03-27-11, 12:54 PM
Member
Thread Starter
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,489
Received 32 Votes on 24 Posts
Thoughts on expansion tanks and other things.

Trooper is fond of stating that diaphragm expansion tanks lose one or two psi a year in normal operation. My experience is mostly with larger systems (commercial and industrial) rather than residential but the system I put in my parent's home ran for more than twenty years (1973 to 1997) with no loss of pressure. I know because I was the only one doing any maintenance or servicing on that system.

On the other hand, many of the diaphragm expansion tanks I encountered in my work did indeed lose pressure on a regular basis. I wonder if the materials used in tanks of the last twenty years or so are simply inferior to what had previously been used and if so was it done strictly as a cost-saving measure for the manufacturer or was it planned obsolescence?

On the matter of non-bladder (conventional) expansion tanks it seems to be the consensus of many of the "regular" posters that the steel non-bladder tanks are really superior to the diaphragm tanks and that they last longer. In my working career I saw many conventional tanks and sooner or later they all failed and it was mostly by holes rusting through right at the maintained water line. Understand that many of these tanks were in systems that were chemically treat to resist corrosion. I also found that these tanks lost pressure (air cushion) faster than did the diaphragm expansion tanks which would cause the system pressure to take large excursions as the temperature rose and fell. These pressure excursions would sometimes be so bad as to cause the safety valve to open and dump water at the high temperature and then at the low temperature the pressure was so low that the make-up water would open and add more water and then the cycle would continue.

In the earlier years of my career these hot water systems would be virtually ignored and while the periodic (sometimes weekly,sometimes less often) chemical tests would always show low chemical content the people charged with maintaining the systems would often just add more chemical and never look for WHY the chemical residuals were always falling. I had one man that insisted the safety valve was defective and so had it replaced only to have the cycle continue. He then insisted the replacement valve was also bad and then specified a steam safety valve (rather than hot water) and when that was also "bad" I decided to look for myself. What I found (and the other man would have seen if he had lifted his head to the expansion tank) was that the gauge glass on the tank was full rather than between the one-third/two-thirds that it would have normally been. I pressurized the tank and blew out water until reaching a normal level and magically the "defective" safety valve was no just fine and after adjusting the chemical level that also stayed normal for several months.

It was amazing to me that no one else had caught this so I started asking questions. The operators were all men of many years "experience" not a one of them had a clue about what had been happening in this particular system. I then took on the task of educating these men and every new employee to the department. Amazingly, it was men with several years of stem experience that were the hardest to get to understand. They had an idea that if they could run high-pressure steam boilers then there was NOTHING about hot water systems that they didn't already know.


I also had trouble with degreed and licensed mechanical engineers who "thought they knew everything" and would all too often use expansion tanks far too small for the systems they were designing. Their most common error was designing to the "normal" variation of temperatures in an operating system while ignoring the much larger "swing" from normal to ambient temperatures. They never included water meters on the make-up systems, something that I always had to add. I could never understand their reluctance to the meters because they only cost about $50 to $75 in systems that cost thousands to install. The make-up water meter is one of the simplest means to determine leaks in a system and in my opinion every hydronic heating system should have a make-up water meter.

Since steel expansion tanks were always rusting at the water line I decided that even with chemical treatment of the system water it was oxygen corrosion at the water line that was causing the corrosion. Since then I started (and continue to advocate) using nitrogen to add the "cushion" to all expansion tanks, conventional or diaphragm. This is probably not practical for a homeowner but all service people should carry nitrogen for use in pressurizing expansion tanks. This includes the purging of any atmospheric air in the conventional tanks prior to system start up.

Just my opinions.
 
Sponsored Links
  #2  
Old 03-27-11, 01:54 PM
hvactechfw's Avatar
Member
Join Date: Jan 2011
Posts: 6,245
Received 0 Votes on 0 Posts
At my business we only use diaphragm tanks. I trust these tanks and from the newer systems that I maintain that others have installed they are also all of the diaphragm type. As far as make up water meters go. I really have never heard of them before just now. I don't know that they are needed on a system that is maintained on a regular yearly basis. Most of the time I catch the small things before they become big problems. This is of course my opinion and I believe it is all about opinion and both tanks have been proven to work and using or not using a meter has worked. Is there a best type solution to the debate? No, I don't believe so.
 
  #3  
Old 03-27-11, 04:26 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 2 Votes on 2 Posts
I really don't think there's gonna be a 'best' solution offered.

I read something somewhere recently... don't remember where... that the bladder type tanks are now charged with nitrogen from the factory?

There are many tire installers these days who offer nitrogen filling for auto/truck tires... you will know because they have GREEN valve caps on the tires.

Nitrogen molecules are bigger than oxygen, so I guess the fill doesn't permeate through the rubber...

I do believe that the older bladder tanks were far superior to the ones they are selling today. My own is over 25 years old. It does lose a couple PSI / year though.
 
  #4  
Old 03-27-11, 07:24 PM
Member
Thread Starter
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,489
Received 32 Votes on 24 Posts
I never meant to present it as a "best" method or practice, just what I have learned and worked for me over thirty-plus years of working in the field.

As stated, nitrogen may not be an option for the DIYer but any installer or service company is likely to have bulk-tank propane or air-acetylene torches and nitrogen is readily available at all welding supply stores.
 
  #5  
Old 03-27-11, 10:00 PM
Member
Join Date: Jul 2009
Location: Toronto
Posts: 1,174
Received 0 Votes on 0 Posts
If a system is maintained, and just how many are, then I would suggest that a conventional style tank would be more cost effective. It would take many a year to rust thru the tank, and fixing a "broken" air cushion tank is rather easy if water logged.

For systems that don't have a lot of maintenance, I would say that the pre charged bladder style tank is the way to go. Though I have found that tanks sized near their max capacity WILL fail sooner than later. I suspect that the expansion / contraction cycles add up to rupture the diaphragm. Certianly not well documented, but thats my take on it as I size my tanks generously. Go big or go home I guess.

My 2 cents worth.
 
  #6  
Old 03-28-11, 05:04 PM
NJT's Avatar
NJT
NJT is offline
Member
Join Date: Oct 2006
Location: USA
Posts: 23,539
Received 2 Votes on 2 Posts
TO, I think your thoughts on the root cause of failure of many bladder tanks are right on.

When the system is cold at 12 PSI, and the air charge in the tank is at 12 PSI, the bladder is in 'neutral'. There is no (or extremely little) water in the tank. System heats up, water expands and pushes into tank... bladder is pushed down...

If the tank is properly (or generously) sized, the bladder won't be pushed all that far, and when the system cools, it will return to 'neutral'.

BUT, let the pressure in the tank go down from lack of maintenance and now the bladder is held in that pushed out position for long periods of time... and it is stretched much farther than design... so sure it's gonna fail sooner! It's like runnin' yer car with a half-flat tire!

In some cases, it absolutely makes sense to change out to a bladder tank. Case in point is the recent thread with the 'trap' on the inlet to the tank... that's just plain WRONG, and rather than spend money to make it right, it makes perfect sense to change to a bladder... (although, that whole system should be in a museum! amazing condition and all original it seems!)

On the other hand, if a perfectly good, properly piped standard tank is in place, and it's worked fine for decades, there's no reason to advocate spending money to replace it. If it ain't broke...
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes
 
Ask a Question
Question Title:
Description:
Your question will be posted in: