Pressurized vs Unpressurized

[chrishh]

Newbe here,

Question about pressurized vs unpressurized systems. I'm looking into building a new home 2742 sq ft. and I'm looking into wood gasification. From what I have read so far I'm leaning towards the BioMax40 with thermal storage. I had a heat loss/gain done on the plans and it looks like I have 47,749 btuh load and I have internal gains of 7,940btuh. Domestic hot water was estimated at 11.6 million btu per year. From what I've been able to calculate I am looking at a thermal storage tank in the area of 830 gal. Please correct me if I'm wrong on this. The question of pressurized vs. unpressurized systems comes into play here. I've been looking for storage tanks and I am having a hard time finding one that will fit. I will have crawl space so I will need something relatively flat. I do have a garage but would like to keep the tank out of the garage for space reasons. What makes pressurized and unpressurized systems so different? What are the pros and cons to the different systems. It seams to me that pressurized systems just require more expensive tanks to hold the pressure. Can't I just go with a poly tank that is rated for 200 deg F. and insulate it? Thanks for your time and I appreciate any help that I can get.

 

[pybyr]

The trouble here is that there are no commercial poly tanks "off the shelf" that will tolerate anything remotely near the sustained temps you need for storage. Believe me; I wanted to go there, and looked into it thoroughly; it's a no-go. Polypropylene rather than polyethelene might work, but from what I could find, after a lot of homework, there are no non-custom polypro tanks in this gallonage range. if you are going to consider custom polypro plastic tanks in multi-hundred gallon territory (which cost something fierce), it makes sense to look at metal tanks instead, as it's more of a sure bet anyway (Even polypro is less assured to hold up to years or decades of high heat, when compared to metal)

Pressurized allows simplicity in piping and pumping- your boiler and storage share the same water, which has the oxygen (that would rust steel and iron) driven out and kept out of it by the heating (and installation of proper air release devices to let it out). But because the system, to remain sealed, and keep the oxygen out, will need to run at a pressure equal, at least, to the vertical weight of water between the boiler (usually in the cellar) and the top of the heating system (usually assumed to be the second story of a 2 story home) you end up running _approximately_ 12 pounds per square inch in the boiler and the storage tank. 12 PSI does not sound like a whole lot when you think of tire pressures or whatever, but designing/making a multi-hundred gallon tank that'll take that kind of pressure, at or near boiling temperatures, is no trifling task. It means big steel. Failure of a large multi -hundred gallon pressure vessel at near boiling temperatures even at 12 PSI means seriously ugly (perhaps deadly, or at least expensive) consequences for anything and anyone in the near vicinity.

Some people use new or used big horizontal propane tanks for pressure systems; if you have the space and that layout/approach/ installation option works for you and floats your boat, they will very likely be up to the job, PSI and heat-wise.

Unpressurized storage allows you to build or choose a tank that merely needs to survive temperature, _not_ pressure (other than the weight and associated pressure of the water within the tank. The trade- off is that you then have to design and include various things like heat exchangers and extra circulator pumps in order to transfer heat into and out of the storage tank, without subjecting the tank to pressure. Those cost money to buy and install, and in the case of circulators, cost electricity to run on an ongoing basis to make your system work.

Short end to this long response- pressurized has a lot of simplicity in its favor _if_ ( but only if ) you can afford and install (fit) a storage tank that'll take both the temperature and pressure, and that you can sleep soundly expecting it to do so for as long as you care/ worry about. Otherwise, opt for non-pressure and "make your deal" with the additional complexity and minimially higher ongoing operating costs.

I was initially attracted to the simplicity of pressurized, but nothing that'd take the heat+pressure would fit in my basement and budget. So I am making the deal with the moderately increased complexity and cost of unpressurized.

hope that helps some

 

[DenaliChuck]

I am squarely on the fence about this decision.

I can get two 500 gallon propane tanks pretty cheap and have space in a 50* heated mudroom (next to the Tarm) to place them vertically. I also have space for a non-pressurize tank in the finished basement (12' from the Tarm). Both spaces won't be missed, tank heat loss occurs in conditioned spaces, and the cost of the tanks are very similar.

Both tank locations are close to the primary/secondary plumbing and either will work fine.

The only two differences are that the pressurized system would be water based (can't afford glycol) and in this climate this is a little risky and might limit the time away for vacations (or require the oil boiler heat the mudroom). The other difference is the additional pumps, hx and operating costs (electrical) of the non-pressurized system (this system would allow glycol in the boiler).

Is there anything I'm missing?

Given these circumstances, what would you do?

Thanks!

 

[Nofossil]

With even nominal amounts of insulation, it would take a VERY long time to freeze a 500 gallon tank. If there's no circulation the pipes to and from would be at some risk, though.

Agree with other comments. Choice boils down to installation specific issues:

1) What space do you have

2) What can you find that will fit in that space

3) What do the different avalable options cost

Pressurized requires less additional hardware (heat exchangers and pumps/valves) and delivers slightly better performance because you can get better startification and because there's no heat exchanger delta T to deal with.

 

[rickh1001]

chrishh

Since your home is still in the planning stages, I wonder if it would pay to consider another option, which is to include a partial full basement to acommodate the pressurized steel storage tanks and the boiler? You could either put the tanks in during construction, or add a 48" door to fit the tanks through.

Given the costs or lack of alternative solutions, modifying the building plans might be the most cost effective in the long run. Storage tanks in the basement have the huge advantage of basically no wasted heat. Just a suggestion.

 

[DenaliChuck]

I agree with boilerman, if you can accommodate a tank in the crawlspace it would be the best. I don't remember who, but someone here excavated a depression in the floor of his shop to place a propane tank on end and still fit under his ceiling (maximizes stratification). It would be very easy to do this at the beginning, just make one corner a few feet deeper. Slightly more excavation costs, concrete forming costs, and material cost...but the way to go.

Anyone have an idea how many KW would be used running the extra pump required for a hx and a 200,000 btu boiler?