Pressurized vs. non-pessurized heat storage

If you can obtain a pressurized tank and locate it in a relatively easy place to insulate (preferably indoors), then that can elimate the need for a heat exchanger (copper coil or flatplate). Some unpressurized also have liners or are made of materials with temp limits around 180*. Pressurized can usually go to 200+ sometimes. The higher the temp you charge, the fewer gallons of storage are needed (but more storage is useful for longer times between charges). If you are doing the labor yourself, find materials and plans that fit your specific skillset.

Green Mtn Boy said:

I've had my Solo 30 for over a year now, and I'm starting to think about how I would like to add some hydronic heat storage. The house is still in the construction stage so I'm just dumping all the heat into my 36' x 48' slab for now. This won't be practical once I finish and turn on the two remaining radiant zones so I'm trying to plan early. It seem that more and more people are using pressurized storage versus non pressurized. I was wondering if I could get some of you that have storage to chime in with the pros and cons of each type. I'll be performing the work with either system so labor won't be an issue. Just looking for honest info so I can make a informed decision and move forward with my plans.

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[taking a break from fitting things in the cellar] I'll take a stab as someone who looked into both pressurized and unpressurized extensively.

empirical observation seems to suggest that most people with 500 gallons or less of storage wish for more, and more than 1500 seems to get into diminishing returns, so bear this in mind in considering either pressurized or unpresurized

the [big] advantage of pressurized is that you need no heat exchangers (copper coil, flat plate, etc., any of which are moderately to considerably expensive in time and/ or labor) between your boiler and your storage- because the storage, like your boiler (assuming that they are set up correctly with air removing devices) will drive the oxygen out so that nothing will corrode fast or severely

the disadvantage of pressurized is that you need a _very_ large one-piece vessel that can truly, no doubt about it, handle some pretty huge physical pressures, while letting you let you sleep at night, go to work, and know you are not endangering your loved ones- and you need to be able to shoehorn that vessel into some sort of indoor (or highly insulated) space. in my case, I had no luck in finding anything afforable that would fit down the hatch into my existing structure. Big pressurized vessels of water at or near boiling are nothing to fool with if you are not very, very sure of their integrity-- see pre-turn-of-20th century news articles about boiler failures.... only people who have professional experience building pressure vessels from scratch should try this on a DIY basis.

Many people seem to have good luck with 500 or 100 gallon propane tanks if they can fit them in- but I will let those who have done that advise you on pros and cons of that option. If I could've fit one into my cellar, and found one at a good price, I probably would have gone that route.

the advantage of unpressurized is that your vessel only needs to be strong enough to withstand the weight/ pressure of the water from its top to its bottom (which is still surprisingly huge, and nothing to trifle with when you consider the image of somewhere around a thousand gallons of potentially near-boiling water escaping into or near your living space). because the forces on the tank are less, you have a much better chance to do some kind of DIY tank, in place, that has relatively low odds of catastrophic failure.

the disadvantage of unpressurized is that you need to have some sort of heat exchanger (gigantic copper coils or a flat plate heat exchanger) between the tank and boiler, because your storage vessel will absorb oxygen from the air, which would rust out your boiler before its time. the heat exchangers add cost in time and/ or money, and also add complexity in the piping, pumping and valving schemes.

there, as far as I can tell, is not really a "better" or "worse" -- it's all a matter of what fits your space, your needs, and your own comfort level with a variety of trade-offs.

hope that helps

back to the cellar I go

Well said Trevor!!

Couldn't add too much to that response but I ended up going with the unpressurized storage mostly because of space to accommodate a round quite large pressurized tanks and expansion tank. So many people have helped me make this decision here from Hearth.com in the Boilerroom. I am still in the plumbing stage but should be boiling in a week or so first without the storage then I'll add it later(still looking for a reasonable liner online Canada).
I built my own tank very similar to Stephen (Maine). He has posted pics of his epdm wood tank. Just search for epdm lined wooden tank.

Keep ur sticks oon the ice!!

Rob

There is another important advantage of pressurized storage. Besides avoiding the expense of a heat exchanger, you also avoid the temperature drop. With a heat exchanger between your boiler and storage, it's very hard to get the storage anywhere near your boiler outlet temperature. You have a similar problem when withdrawing heat from storage - the water going to your zones isn't as hot as the water in storage.

With unpressurized storage, coils save the expense of buying and running an additional circulator that's needed with a flat plate exchanger. On the other hand, flat plate exchangers generally provide better thermal performance (less temperature drop) per dollar.

Stratification in storage is critical. To get maximum stratification, you'll want to charge the tank with top-to-bottom flow, and discharge it with bottom-to-top flow. There are many ways to accomplish this, some quite complex and expensive. Flat plate heat exchangers work best when plumbed as counterflow - hot to cold on side A opposite from hot to cold on side B. This can be a challenge to figure out, especially if you're using the same heat exchanger for charging and discharging.

There are threads that discuss different options, including two stickies at the top of the boiler room.