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Sizing an expansion tank for steamback
Hi all, First we want to thank you guys for all of the comments on our last posts. We are slowly but surely getting this thing together and really appreciate all of the suggestions. (Ground mounted Solar thermal system for DHW and also radiant floors in our house).
Now we are trying to size our expansion tank for steamback. We read the article "Expansion tank sizing for solar collector circuits" from PMEngineer.com and ran all the numbers but something doesn't seem right.
We're pretty sure our total expansion volume to be accommodated is about 12.35 gallons. (we have 320 sq feet of collector). Using their formulas, we came up with a minimum required expansion tank of 51 gallons!! This seems huge! How does that make sense? Are we completely off track?
If we are not completely off track, do they even make expansion tanks that big? If so, do you know where to buy them?
Thanks so much in advance for any input.
CallaW and NathanW
Now we are trying to size our expansion tank for steamback. We read the article "Expansion tank sizing for solar collector circuits" from PMEngineer.com and ran all the numbers but something doesn't seem right.
We're pretty sure our total expansion volume to be accommodated is about 12.35 gallons. (we have 320 sq feet of collector). Using their formulas, we came up with a minimum required expansion tank of 51 gallons!! This seems huge! How does that make sense? Are we completely off track?
If we are not completely off track, do they even make expansion tanks that big? If so, do you know where to buy them?
Thanks so much in advance for any input.
CallaW and NathanW
0
Comments

A system has two volumes to calculate
12 gallons sounds like the entire SYSTEM volume, but the entire system doesn't flash to steam. The other important factor in the calculation is the pressure relief valve setting, which should be at least 75psi, and no higher than 150psi for typical components.
The system volume is expanding and contracting by a small percentage. That's the first calculation, and would obviously be a small fraction of 12 gallons.
The second volume to accommodate is just the volume of the collectors. This is the volume that turns to steam, and must be entirely absorbed by the expansion tank, plus some extra. If your collectors hold a lot of fluid, they might require a larger tank, but there are many lowvolume collectors available.Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW gridtied PV system, various solar thermal experiments0 
Some more specifics
Right, that makes sense. Thanks Kevin. Trying to break it down here...
We have a height (H) of about 16 feet from pressure relief valve to top of collectors. We have about 176 ft of 1" copper pipe in the whole system.
The volume of our collector array is 9.76 gallons (Vc). the volume
of the piping and heat exchanger is 8.6 gallons (Vp). Their formula for
"expansion volume to be accommodated" = 1.1 [(Vc + Vp).08 + Vc]= 12.35
gallons........
Not exactly sure of the pressure relief valve setting, but in this article they use around 50 psi. When I plug in 75 psi, I get a smaller tank, around 25 gallons, which seems more reasonable. When I plug in 100 psi, i get a 20 gallon tank.
What size tank is most common? For those of you who install these, what is your average size expansion tank? Just curious..
Thanks!
CallaW0 
volume
you'll need to accomodate the expansion of all the fluid in the system (because when it steams back it will be at high limit temps, and expanded) and then accomodate the volume of the collectors. remember, if plumbed correctly, the entire volume of the collector won't boil, just some of it will, and push fluid out into the expansion tank. (don't use an automatic air vent on the array or it will vent the steam out there.)
if you account for all that, I think the 12.35 Gal is correct. make sure that your tank has that acceptance volume at below your PRV pressure.
karl0
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