Hotwater heater for a pressure tank?

[Paul Whipple]

I'm shocked at how expensive some of these pressure tanks are! I could easily spend as much on two of these as my boiler so I'm looking at the alternatives. A hot water heater with a shrader valve at the top and a couple of plugs and you would have a pressure tank. Is the working pressure on these not sufficient for a boiler application? I'm also looking at the propane tank method. Is there a drawback to introducing oxygen to your system each time you recharge it?

 

[Tennman]

Do you mean expansion tanks? I bought a bladder expansion tank big enough for a 500 gal storage tank for $300. Or are you talking storage tanks which I paid ~$250 for an old 500 gal propane tank for storage (then spent money foaming). Yes, removing air from the water reduces corrosion.

 

[ewdudley]

I'm shocked at how expensive some of these pressure tanks are! I could easily spend as much on two of these as my boiler so I'm looking at the alternatives. A hot water heater with a shrader valve at the top and a couple of plugs and you would have a pressure tank. Is the working pressure on these not sufficient for a boiler application? I'm also looking at the propane tank method. Is there a drawback to introducing oxygen to your system each time you recharge it?

I got lazy and bought a membrane tank, ended up being about ten percent of the out of pocket cost of the whole boiler project. So don't have any first hand experience with big expansion tanks for wood boilers with storage, but have observed a few of the old tank-above-the-boiler-in-the-floor-joist systems and can say they can work very well if set up right.

The key I believe is to have an upward path from the hottest, preferably lowest pressure point in the system where gas comes out of solution up to the tank. What you're up against is that as water in the expansion tank cools it will dissolve some small amount of gas and will transport it back into the system. If you can set it up so that when the gas comes back out of solution it ends up in the expansion tank then you're good to go. If it leaves through an air vent then you're going to be adding air periodically throughout the life of the system and causing corrosion.

I think the ideal non-bladder expansion tank would be a good pressure tank of about 12% or more of system volume situated completely above storage with one line going from the top of storage to the top of the expansion tank, and another line going from the bottom of the expansion tank down to the bottom of storage. To make it work the supply pipe from the boiler would travel continuously upward to the top of storage with no air vents. Also once the system was bled out initially, all other air vents would need to stay sealed and only cracked once every year or so to see if everything is A-OK.

 

[Fred61]

What would the result be if the non bladder expansion tank was charged with nitrogen?

 

[ewdudley]

What would the result be if the non bladder expansion tank was charged with nitrogen?

Presumably all the oxygen goes toward reducing iron to oxides of iron, so we end up with pretty much all nitrogen either way. So it's the same problem, gas going into solution in the expansion tank, coming out of solution someplace hot, and needing an upward path to get the gas back to the expansion tank.

 

[Fred61]

But doesn't that bypass the creation of oxidized iron which is what you're trying to eliminate?

 

[ewdudley]

But doesn't that bypass the creation of oxidized iron which is what you're trying to eliminate?

OK, I'm catching up slowly. Yeah, charge with nitrogen, and then if we lose some gas go ahead and add some more nitrogen. That would work, wouldn't it?

 

[Fred61]

One would hope in a perfect world that recharging incidents would be few and far between. Actually nitrogen isn't that expensive. Years ago I used purified nitrogen to spray thin film and I recall that cost was not a factor.

 

[maple1]

I'm not saying you should do what I did - but what I did was just get a 110 gallon propane tank for like $80, put extra tappings on the bottom & top of it, sit it upright beside my storage, and run a 1/2" line from the bottom into a T just upstream of my load circ. I also T'd in a pex sight glass from the bottom to the top, and a couple extra T's at top & bottom for drain & fill & pressure guage & snifter valve. Pretty simple - and after a year and a half, I can't really say I've seen evidence yet that that was a bad thing to do. Water level seems to have stayed constant (rising & falling with the temps that is), and I haven't had to add air (or water) to my system except for after I had to purge & drain an airlock out of an upstairs zone early this winter. I did add some at fall system startup, but I think I misread my pressures before I did that because I ended up letting pretty well what I had added to it, back out again after I got up to temp. I typically see a 4psi rise from cold (130/120) to hot (185/170) when operating. One thing I also have, is a second expansion tank - one of the old in-rafter type, up in my joists above my boiler. It was there from my old boiler - and since it was there, I just ran a line up to it from the top of my boiler where my dump zone comes out, on a T. I do get the odd bubbling I hear going up into it once in a while when my boiler is getting up there in temps, but I put a sight glass on it and it stays the same too - about half full. So it seems to serve as an air trap.

By boiler manual shows two expansion scenarios - one with an elevated open expansion tank that should be 8' above the highest system point, and another with a closed expansion tank at floor level. It doesn't spec that the closed tank has to be a bladder tank. So my jury is still out on whether what I did was proper or not. So far, so good - but I would feel better if my expansion was up high above the system. I just don't have a place to put it up high - and I don't think setting an expansion tank up in the master bedroom would fly too far here.

 

[Paul Whipple]

Expansion tank is what I was getting at. On plumberssurplus.com they have a great deal on a 80 gallon expansion tank, for only $5120.00! (on sale though so take a grand off that price) That must be one heck of tank!

 

[webie]

I found a cheap date standing on the side of the street . 50 gallon water heater 50 bucks . It has been working great . My storage is in the back of the garage and my expansion tank is sitting on the floor in my basement .

 

[Vizsla]

Back in the day we used air scoops, and all they were was an open tank that fit up in the joyces. Bladder tanks took up less space. Using an open tank requires more volume than a bladder style. Careful system setup and gauges to monitor.