Expansion Tank Advice Needed

[rickh1001]

I know this subject has come up before, so I hope this is not too redundant, but I needed some hand-holding on expansion tanks as I start to design the layout in preparation for an EKO 60 that is on-order and due in within a month or so. Just for background, I am a non-plumber, and have no prior experience with hydronic systems. However, between this website, and reading all of Dan Holohan's books, the IBR manual and every book with boiler information I can get my hands on, I am starting to get the hang of things a little bit, on paper at least! The installation is in the bottom of a barn that we converted to a house, along with a second small home about 50 ft away. There will be 3 zones, one for each half of the barn, and one for the house via a buried PEX line. I did various heat loss calculations, and compared our prior fuel usage, all approaches confirmed the EKO 60 as being about the correct size boiler.

I am planning about 1600 gal of storage using 2 LP tanks. However, sizing up a suitable expansion tank exactly on the various websites, or using the rule of thumb of about 5%, gives a very large expansion tank volume on the order of 80 gal.

At this large volume, all I am finding are larger commercial units, with prices running $1,000 or more (for diaphram tanks). I am still just sketching out the system, so these are just order of magnitude estimates at the moment - however, the total price for the re-furbished, welded up LP tanks, plus the necessary very large expansion tank, is starting to add up to quit a lot of spare change.

I guess my question is, am I missing something here? The expansion tank required for such large water volumes is starting to emerge as a fairly major issue in the total system design and price. I was wonding what people are recommending as good sources for such large expansion tanks, and if there are any issues with these expansion tanks that I should be aware of before sizing it more exactly and actually ordering one? The total price is approaching that of an open storage tank system with heat exchangers, such as the one Tarm sells. I had kind of assumed it would be always be cheaper and better to go the pressurized storage route, but I am starting to wonder if simply buying the pre-fab Tarm storage system with hx's might be a cost-competetive alternative?

 

[mpilihp]

I thought I read another thread where someone is using used electric hot water heater tanks as expansion tanks.... Think it may have been under the thread about semi-pressuried.....

 

[MrEd]

I am not a plumbing expert, but in my own research for a tank for a 1000 gallon pressurized system I roughly calculated that 86 gallons would be big enough. As an extra margin of safety the "standard" size ex. tank already installed is also on-line.

You can get this size for about $499, for a 500 gallon you could probably go one size smaller, but please do your own calcs or check with a competent professional before making a decision on size.


http://www.pexsupply.com/categories.asp?cID=277&brandid;=


Too big of a expansion tank wastes a bit money but there is no other downside I know of...too small a tank on the hand could cause big problems.....

 

[rickh1001]

Mr Ed,

Thanks for the link. That is not a bad price at all.

When I was looking around, I couldn't find residential type tanks with enough volume, and once I got into the more commercial stuff, all the prices were over $1,000.

If I use the Extrol sizing chart, and use the maximum differentials of 40/240 deg differential, and an acceptance factor of 0.4, that would size the expansion tank at 208 gal (for 1600 gal of storage). So that would still require at the very minimum, 2 X 86 gal expansion tanks, at $499 each, for about $1,000 or so for the expansion tank expense. Even this would be a minimum, and if I go by the sizing chart exactly, it almost would require 3 tanks.

Not as bad as I was thinking, but still not trivial.

It looks like as a rule of thumb, every 1,000 gal of storage will require an expansion tank of about 120-130 gal (using worst case temp differentials and a low acceptance factor). These are darn big expansion tanks. Am I doing the calculations wrong?

 

[Nofossil]

You should live by local codes, but in my opinion the actual temperature range is something more like 70 degrees to 170 degrees. Mine over the last two years has never gone below 100 degrees or above 165 (average top-to-bottom temps).