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.
[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
