expansion tank

[mark123]

I have a woodgun 180 which has an 80 gallon water jacket plus approx. 6000' of 1/2" pex in floor heating (17 gallons). My expansion tank is a bladderless type suspended up in the floor joists, I think it is 15 gallons (approx. 36" X 12") and I am having the odd blow off on mild days, not really a blow off just a few cups of water leaking out. Will adding another 15 gallon tank "T"ed into the other tanks piping solve this problem. Woodguns do not use dump zones.
Also when draining the tank is it better to do it when the system if hot or cold? What are the pros/cons or bladder vs bladderless tanks.

 

[jebatty]

...the odd blow off on mild days, not really a blow off ...

Has your bladder-less tank water-logged. That could explain the water "bow-off." If memory serve me, should be about 2/3 water and 2/3 air at the top. Did it do this when the weather was colder? If it didn't do it before, seems like it works OK, but now may not have sufficient air space in the top to tank the expansion.

 

[mark123]

That is what has happened, I am just womdering if adding another tank which will double the air space will help. When it was colder it did it a few times. I have drained it a few times over the winter. Do these tank need periodic draining or is something wrong if they get water logged?

 

[jebatty]

By tapping the tank you often can locate the water level. An air fitting on the top of the tank to blow in additional air from time to time is useful. The tank loses air because the air in the top of the tank gradually gets absorbed into the water, and the water in your system then continues to fill the tank until it "blows-off." Adding a bladder tank will not change this. Replacing the air tank with a properly sized bladder tank will eliminate the water logging issue because the bladder separates the air from the water. Bladder tanks can fail over time due to deterioration in the bladder and/or tank. My only failure experience is the bladder on the pressure tank on our well failed after 18 years.

 

[jebatty]

My first post should have said 2/3 water and 1/3 air.

 

[mark123]

So adding air periodically is a good idea, how much, if I turn off the water supply and drain the tank the boiler pressure should go to near 0, then what should I bring it back up to with the air compressor?

 

[jebatty]

Usual target is 12 psi with a cool system. This covers a normal 2-story house.

 

[mark123]

I isolated the expansion tank and drained it and then put air into it, not having a gauge I must have put over 30 psi into it because when I opened the valve that leads back to the system the blow off on the boiler let go and some water blew out, everything seems to be working great so far, not a drop has blown off even with temps up to 220 F.

 

[Willman]

I would be investing in a gauge before trying that out again. Accurate digital preferred.

Will

 

[mark123]

Is the size of my wxpansion tank OK, There must be some formula, I have approx. 97 gallons of water and 15 gallon expansion tank?

 

[pwschiller]

Is the size of my wxpansion tank OK, There must be some formula, I have approx. 97 gallons of water and 15 gallon expansion tank?

http://www.engineeringtoolbox.com/expansion-tanks-d_885.html

 

[mark123]

According to one of the graphs there, the tank should be 17% of the system at 225F, so I am a bit undersized.

 

[pwschiller]

According to one of the graphs there, the tank should be 17% of the system at 225F, so I am a bit undersized.

Unless you have a large amount of additives in your water, you're not storing it at 225 F. Pure water boils around 212 F (depending on atmospheric pressure). Your expansion tank seems like the right size, as long as it has the appropriate charge of air. Even bladder style tanks lose their charge over time, or at least that's been my experience.

On a non-bladder style expansion tank, I would think that if you tapped a couple holes in the tank, one near the top and one near the bottom, screwed in a couple of NPT to PEX elbows with a section of clear O2 PEX between them, that would allow you to monitor your air cushion. I've only had bladder style tanks myself, so I've never actually done that. Maybe someone else will chime in on how they handle monitoring how much air is in the tank, or whether the PEX/sightglass idea is a good one or not.

 

[ewdudley]

According to one of the graphs there, the tank should be 17% of the system at 225F, so I am a bit undersized.

Unless you have a large amount of additives in your water, you're not storing it at 225 F. Pure water boils around 212 F (depending on atmospheric pressure).

Except it doesn't depend on atmospheric pressure, unless it's a cistern-type open expansion vessel, and the OP has a closed tank. The limiting factor is the blow-off pressure, typically 30 psisg at the boiler elevation. Water boils at a temperature above 300 degF / 150 degC at 30 psisg / 3000 millibar.

It looks like the Engineering Toolbox charts have a generous safety factor 'k' built into them. Seventeen percent is likely plenty for the OP's system.

Cheers --ewd

 

[pwschiller]

Except it doesn't depend on atmospheric pressure, unless it's a cistern-type open expansion vessel, and the OP has a closed tank. The limiting factor is the blow-off pressure, typically 30 psisg at the boiler elevation. Water boils at a temperature above 300 degF / 150 degC at 30 psisg / 3000 millibar.

Cheers --ewd

You're right, except that I think that water boils at around 275 F at 30 psisg (~ 44.7 psia):
http://www.engineeringtoolbox.com/boiling-point-water-d_926.html

I was thinking in terms of systems that have a large storage capacity and boilers that shut down when the boiler temp gets to about 210 F. Even if the temperature spikes above that at the boiler before the fire starts to slow down, the average water temp for the system is still well below 210 F. In the case of the OP, his system does not fall into that category.

 

[ewdudley]

Water boils at a temperature above 300 degF / 150 degC at 30 psisg / 3000 millibar.

You're right, except that I think that water boils at around 275 F at 30 psisg (~ 44.7 psia):
http://www.engineeringtoolbox.com/boiling-point-water-d_926.html

Doh! That's what I get for believing the first JavaScript boiling temperature calculator I find on the InterWebs.

Although higher pressure will allow operation above 212 degF, I've always assumed that unless there's a compelling need it would be wisest to keep temperatures comfortably below atmospheric boiling point so in case something lets loose the water is not being propelled by expanding steam. Is this correct?

--ewd

 

[pwschiller]

Although higher pressure will allow operation above 212 degF, I've always assumed that unless there's a compelling need it would be wisest to keep temperatures comfortably below atmospheric boiling point so in case something lets loose the water is not being propelled by expanding steam. Is this correct?

--ewd

I'm no expert, but I would agree with that 100%.

In the case of the OP, if his system doesn't have safeguards to keep the average system water temp below 212 F, then it probably may not be a bad idea to have more expansion capacity than he currently has. Also, my idea for a PEX sightglass on a non-bladder expansion tank may be a good idea for someone who has a system with a large water volume, where expansion occurs very slowly and the water temp in the expansion tank stays well below the recommended maximum temp for PEX, but it may not be for a system where pressure can build quite rapidly and the temp in the expansion tank can go over 200 F.

Does anyone out there have a sightglass on their non-bladder expansion tank so that they can monitor the air volume? Or is there another way to to do that?

 

[woodsmaster]

Although higher pressure will allow operation above 212 degF, I've always assumed that unless there's a compelling need it would be wisest to keep temperatures comfortably below atmospheric boiling point so in case something lets loose the water is not being propelled by expanding steam. Is this correct?

--ewd

I'm no expert, but I would agree with that 100%.

In the case of the OP, if his system doesn't have safeguards to keep the average system water temp below 212 F, then it probably may not be a bad idea to have more expansion capacity than he currently has. Also, my idea for a PEX sightglass on a non-bladder expansion tank may be a good idea for someone who has a system with a large water volume, where expansion occurs very slowly and the water temp in the expansion tank stays well below the recommended maximum temp for PEX, but it may not be for a system where pressure can build quite rapidly and the temp in the expansion tank can go over 200 F.

Does anyone out there have a sightglass on their non-bladder expansion tank so that they can monitor the air volume? Or is there another way to to do that?

I monitor mine with the original float gauge on my 124 propane tank. So far so good! you can buy sight glass kits on e-bay.