Is my expansion tank bad?

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  #1  
Old 12-16-13, 10:51 AM
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Is my expansion tank bad?

I have searched the forums for an answer to my specific question so I apologize if this has already been addressed.

My expansion tank is hanging below the scoop with the schradder valve on the bottom. I have read that if I depress this valve and water comes out, the bladder has broken and then tank needs to be replaced. Simple test. But my question is, can I do this with the system hot. It seems a quick depression of the valve will quickly indicate water or not. But how much air will I loose and will this be an issue? I have no feeling as to the capacity of the tank and how much air can escape before the pressure drops significantly.

In either case, bad or not, the tank is long over due for service (or replacement). I bought, and plan to add, a Watts RBFF to help with future service work. I'd like to know if the tank needs replaced before I shut the system down so I can have a replacement tank at the ready. Trooper wrote a great expansion tank service guide as part of a sticky and I have located a pressure gauge to validate the boiler's pressure.
 
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Old 12-16-13, 11:39 AM
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Yes you can do it with the system hot.

Measure the pressure at the expansion tank Schrader valve. (If it reads zero then stop here; you can't bleed a system or subsystem with zero pressure.)

Stop any system autofill (such as by turning off the valve connecting the system to the nearest domestic cold water line)

Now you can let as little or as much air/water out.

Then immediately add air to put the pressure back to the same level it was.

Restore the autofill (if any) back to the way it was.

This gets the expansion tank back to the condition it was in but does not mean the tank had the correct amount of air or the system had the correct pressure to begin with. There are various methods of recalibrating the tank, at least one is in the sticky thread at the top of this topic.

If considerable water comes out the Schrader valve, then chances are the calibration was way off and you should follow a recalibration procedure as opposed to just putting it back the way it was.

An expansion tank with the Schrader valve end down will probably not function effectively for any length of time if the bladder has ruptured. If you get more than a cup of water out of an expansion tank Schrader valve with that end down, you might as well stop and replace the tank before continuing testing.
 

Last edited by AllanJ; 12-16-13 at 12:00 PM.
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Old 12-16-13, 11:55 AM
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If considerable water comes out the Schrader valve, then chances are the calibration was way off and you should follow a recalibration procedure as opposed to just putting it back the way it was.
If you get water, you need a new tank, period.
 
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Old 12-16-13, 12:00 PM
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Thanks for the quick replies.

Rather than measuring the current presssure etc. A quick depression of the schradder valve will yield all air or water/air. If it has water in the tank it needs to be replaced. If it doesn't have water, will the small loss of air be a concern until this weekend when I'll have the time to cool the system off and perform Trooper's maintanance steps in his sticky. If it has water I need time to order the tank, etc.
 
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Old 12-16-13, 12:21 PM
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will the small loss of air be a concern until this weekend
I don't think so... typically with most systems the tank can actually get down to like 5 PSI before it's even noticed.

Stick the tire gauge on first... just to get a baseline. I think that's what Allan was trying to say...

but no, that little 'blip' that you give the valve won't make much difference, unless you already have no air in the tank.
 
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Old 12-16-13, 12:43 PM
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Thanks Jeff,

Based on when the tank was most likely last serviced I expect the pressure in the tank is close to zero. I did the tap test ) ) and the temperture test (top half is warm, bottom half is cooler) so the tank may be good. It is close to 30 years old, however AND (red faced) it may be close to that long since the pressure was checked/set.

The system pressure, albeit via the system's gauge, is 30psi hot. I haven't seen much if anything from the pressure relief valve but it may be right on the edge.
I have another gauge to check the pressure but, clearly, it is high time for some TLC.
 
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Old 12-16-13, 04:54 PM
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Note that if the connection with the system is not clogged, the expansion tank pressure (measured at the Schrader valve) will be equal to or greater than the pressure of the rest of the system.

The expansion tank pressure during system operation equals the system pressure except when the system pressure drops below the bladder pressure after all the water exits the tank and the bladder is up against the tank connection.
 
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Old 12-16-13, 04:58 PM
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The system pressure, albeit via the system's gauge, is 30psi hot
Yeah... you've got a little work to do.

You know I'm going to post the links to those two 'sticky' threads, right?

http://www.doityourself.com/forum/bo...ure-gauge.html

http://www.doityourself.com/forum/bo...sion-tank.html
 
  #9  
Old 12-16-13, 07:12 PM
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You are correct I have some work to do.
I checked the tank pressure at 170F. Using a 0-50psi tire gauge I got no reading. I also got no indication from the blip sound of taking the pressure that there was much pressure in the tank. I also saw no indication of water. Using the gauge that is part of my Watts RBFF (0-30psi) I measured the system pressure (@170F) to be 7psi.

Not having a broken bladder is good news but it appears I need to adjust a couple of things. My thoughts are at this point (and I'd like to get your thoughts as well) is to follow the expansion tank service procedure below as it is written. But for the final step, I may need to adjust my water supply line regulator to bring the cold system pressure up to 12 psi. I will use a more sensitive pressure gauge for setting the tank pressure.

Thoughts?
 
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Old 12-17-13, 08:28 AM
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My thoughts are at this point (and I'd like to get your thoughts as well) is to follow the expansion tank service procedure below as it is written
Yes... but right off the bat, if your system is only at 7 PSI HOT, you could certainly go ahead and add some air to the tank without even having to drain anything first.

You could probably just go ahead and pump it up to say 18 PSI.

There would probably still be some water in the tank, but as a 'hold over' you could probabaly 'get by' with that.

But for the final step, I may need to adjust my water supply line regulator to bring the cold system pressure up to 12 psi
Those regulators don't generally just go out of adjustment, at least not THAT FAR... maybe a PSI or two as the spring inside weakens, but if it was once adjusted properly and now it's not giving the correct pressure, it is FAR more likely that the valve is NFG. I would plan on replacing it, especially if it's a 'senior citizen'...

While you are at all this, did you say that your relief valve is older than 5 years? replace that too, if so.

I will use a more sensitive pressure gauge for setting the tank pressure.
I've looked at a few tire pressure gauges in the past couple weeks that START at 10 PSI, and I would absolutely NOT trust that type of gauge to measure 12 or 15 PSI accurately.
 
  #11  
Old 12-17-13, 09:35 AM
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Ok so I'm now a bit confused. I understood the cold system pressure was determined by the supply line regulator. But how I read what your suggesting is if I increase the pressure in my expansion tank my system pressure will increase.

I felt 12psi regulator pressure and the 12psi expansion tank presssure would balance each other. The expansion tank in free air set at 12psi would have the bladder biased toward the inlet end.

After then tank is installed and the system heats up, the system pressure goes up and the diaphram moves away from the inlet end thus compressing and raising the pressure in the AIR side of the tank to match the system pressure. I didn't think the system pressure could drop below the regulator pressure hence my thought the regulator was set wrong (had drifted).

Help me with my misunderstanding, please. Also please explain the impact of running the system at less than the recommended pressure. On the high end you get water on the floor, etc. But on the low end, what happens?

Doing what you suggest, it seems I could increase the tank pressure by using the system pressure gauge as a monitor?

I'm not understanding the NFG short hand unless the first and last words are no and good, respectively. Which valve are you talking about that would be NFG?

I can replace the pressure relief as well. It is a pain to get at. I was also wondering about the air bleed vents. I have two - on on the top of the air scoop and the other on top of the pressure relief. I'd expect they could stand to be retired beings they have close to 30-years of service.
 
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Old 12-17-13, 10:03 AM
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I understood the cold system pressure was determined by the supply line regulator.
Yes, that is correct.

But how I read what your suggesting is if I increase the pressure in my expansion tank my system pressure will increase.
Because there is water in your expansion tank, when you add air to the tank air side, some of that water will be pushed out into the system and will pressurize it.

This is also the reason for repeating the steps in my procedure... and you may have to run through numerous iterations of those repeated steps...

Since your system is already hot, 18 PSI would not be an unreasonable hot pressure.

By adding air to the tank when the boiler is hot, and up to 18 PSI, when the system cools again, the pressure will probably go back down around 12 PSI or there about.

On the high end you get water on the floor, etc. But on the low end, what happens?
When the system cools again, and the water contracts, if the pressure reducing valve is working properly (yours may not be), the valve will feed more water in to maintain the pressure at it's setpoint. The next time the boiler heats, the whole cycle repeats itself. Water out, Water in.

If your reducing valve is NOT working properly and feeding water to it's setpoint pressure, then no water will be added and the system pressure will go very low, often all the way down to zero, as I suspect that yours has been doing.
 
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Old 12-17-13, 10:07 AM
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unless the first and last words are no and good, respectively. Which valve are you talking about that would be NFG?
Yes, I should perhaps not use VAs (vulgar acronyms)...

The pressure reducing valve would be the one I was referring to as N*G.

wondering about the air bleed vents. I have two - on on the top of the air scoop and the other on top of the pressure relief
They do get 'crudded up' with minerals to the point that they contsantly ooze water and then the caps are screwed down tight and forgotten about. If they work still, it's up to you if you want to replace, but if they leak with the caps open, definitely replace them.
 
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Old 12-17-13, 10:41 AM
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Those regulators don't generally just go out of adjustment, at least not THAT FAR... maybe a PSI or two as the spring inside weakens, but if it was once adjusted properly and now it's not giving the correct pressure, it is FAR more likely that the valve is NFG.

Ok, more education needed. I have what I called a pressure regulator on my supply line. It has a level to toggle to full pressure which I use to purge air from a zone, etc. Upstream from that is a back flow preventor. Above you say the regulator is not bad but the pressure reducing valve is bad. I figured a pressure reducing valve and a regulator to be one in the same although I may be using the incorrect terrms.

When I install the RBFF it would an ideal time to replace the pressure reducing valve. And I might as well replace the tank as well. They are all approaching 30 years.

Again, since finding a good tire pressure gauge, I should be able to set the tank pressure using the good gauge I have for the system pressure, correct?
 
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Old 12-17-13, 11:25 AM
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I have what I called a pressure regulator on my supply line
I guess we should standardize terminology.

PRESSURE REDUCING VALVE = pressure regulator, automatic fill valve, feed valve, and a host of others.

Above you say the regulator is not bad but the pressure reducing valve is bad
I didn't mean to imply that, probably bad phrasing on my part.

If you've got low pressure in your system and the MANUAL water shut off is OPEN, there is definitely something wrong with "the valve that regulates the cold fill pressure" on your system.

since finding a good tire pressure gauge, I should be able to set the tank pressure using the good gauge I have for the system pressure, correct?
Now I'm not sure about your phrasing

Set the tank pressure per the procedure in the sticky using the good tire pressure gauge.

Set the system pressure using a known good pressure gauge on the system.
 
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Old 12-17-13, 01:00 PM
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I really appreciate your patience and your help. But I have another question for my own learning.

The pressure reducing valve establishes the initial cold system pressure - say 12psi. Hot system pressure can increase to 18psi or more due to the expansion of water. This implies that something is closing the system which I assume is the backflow preventer.

Further the system pressure and the air pressure in the expansion tank must be the same - i.e. they are in balance.

That is why I said I should be able to use the sytem pressure gauge to set the tank pressure since they should be the same - independent of water temperature.

I believe I do have something wrong with the regulator/PRV and will verify and change ASAP. 7psi means something is wrong other than the tank.

Thanks for all your help and hopefully this dialog is beneficial for others.
 
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Old 12-17-13, 03:11 PM
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7psi means something is wrong other than the tank.
Not necessarily. If you have a defective bladder or if you have a waterlogged compression tank you may very well also have low pressure along with a defective pressure reducing (fill) valve. Without the proper "cushion" the pressure skyrockets when the boiler fires, lifts the safety valve which discharges the excess water and then when the boiler water cools it leaves you with too little pressure. It is a viscous cycle that far too many people fail to understand.

I once had an otherwise competent man working with me that insisted that he was getting defective safety valves for a little boiler that he couldn't maintain the proper chemical residual. He knew it was discharging through the safety valve but he never bothered to look on the compression tank gauge glass. I went over and the first thing I noticed was that the compression tank was full up with water. I connected an air hose and blew the level down to about one-third of the tank and the problem was solved.
 
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Old 12-17-13, 04:40 PM
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Not necessarily. If you have a defective bladder or if you have a waterlogged compression tank you may very well also have low pressure along with a defective pressure reducing (fill) valve.
That's what I meant... in addition to the tank problem, if the pressure is 7 PSI it means that the fill valve isn't working properly.
 
  #19  
Old 12-24-13, 07:31 AM
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As Trooper advised earlier, I increased the pressure in the tank to 18psi (hot) as a starting point. The system pressure matched (19psi). Tank pressure was taken using a conventional 0-50psi tire gauge and the system pressure with 0-30psi bourdon tube type gauge so I expected a difference.

I'll clean this mess up when the weather warms up a bit. I plan to replace the tank even though it is not leaking (28 years old), add the Watts RBFF, replace the pressure reducer and back flow preventer and the pressure relief valve.

The RBFF has a pressure gauge that will be clearly visible after it's installed. The current temp/pressure gauge is now only visible using a mirror and flashlight. It would be nice to make at least the temp reading more visible. As discussed elsewhere the pressure gauge side is most likely worthless. Any thoughts on making a temperature reading more visible? Move the gauge I have? Add another gauge? Close circuit TV?

Thanks to all and have a good holiday.
 
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Old 12-24-13, 02:26 PM
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5240748 - Mr. PEX 5240748 - Strap-On Pipe Thermometer for 3/8"-1.5" Pipe (0-248F)

There are various 'strap on' temperature gauges available that will give 'reasonable' accuracy is installed with at least a foot of insulation either side of the pipe. There may be a 'lag time' in the display on steel pipe... copper is better. Heat conductive paste between the 'foot' and the pipe really helps.

Close circuit TV?
A friend who is a ham radio operator has a BIG RF amplifier in his garage. His operating position is in the next room. To tune the amplifier he has servo motors on the controls and a webcam on the panel meters.

But no, I don't think you need to go THAT far!
 
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