is "semi pressurized" an option?

[pybyr]

hello all--

as I ponder and plan for effective storage that'll fit through my basement access hatch and also not be crushingly expensive, here's a question that crossed my mind--

I'm familiar with the distinctions between pressurized and un-pressurized, both in terms of pressurized's ability to skip the intervening heat exchangers, and also the need to not have the system open to the atmosphere, so as to keep the oxygen levels low to avoid rusting the boiler

my question is this- how much pressure does a pressurized system need to actually run and maintain in order to have that closed direct loop from steel boiler jacket to steel storage tank, and also avoid corrosion

Would it be close enough to pressurized if I had my boiler and main storage in the cellar, with a modest sized auxiliary expansion tank, open to atmosphere at its top, on the first floor-- by having the top of that tank open, it would never build up any significant pressure, but also (as long as it's monitored) would never allow air down into tha "main loop" of the boiler-tank-boiler path?

such an arrangement might allow me to use certain types of storage tanks that, while not rated for pressure in the sense of any significant PSI level, could probably hold just fine with the mere gravity pressure from that upper tank.

I'm just not clear on whether there's a certain PSI threshold that one has to hit in a pressurized system in order to get the oxygen out and keep it out

as always, thoughts and suggestions much appreciated!

Trevor

 

[Nofossil]

Can't believe our plumbing guys haven't jumped on this. I think you're on to something, and the only compromise that I can see is that lower system pressure means a lower boiling point. Boiling would cause a spectacular overflow, I suspect. In marginal situations, it might cause flash boiling at the circulator impeller as well, especially if you had a big circulator and were close to boiling anyway.

 

[pybyr]

Nofo- as far as your point about boiling, from what I gather, all the wood gasifiers tend to max out at 180 or so, (maybe 190 if you're really puttin' the wood to 'em) so I figure a full "boil" is not a factor under normal operation, and I'd have to make some provision for a safe blow-off if it veered into conditions in which it were happening

I agree that I hope that the qualified plumbing types jump in

 

[Redox]

There are a lot of older houses here that have an expansion tank on the top floor vented out the side of the house. Apparently it works well enough that the tanks and systems last. It's not truly a closed system, maybe we'll call it a "hybrid" expansion tank. I think the boilers still have a normal makeup valve and relief, but the tank just vents out the side of the house without any valves.

Chris

 

[Eric Johnson]

My dad pipes his nonpressurized Heatmor OWB directly into a propane boiler in his cellar. I forget what the system pressure reading is on the gas boiler, but I think it's around 10 psi. Not quite enough to get water into a cast iron radiator on the second floor. You have to run the pump to fill it, and then close off the vent. But it works. For some reason.

 

[steam man]

I see a number of head/expansion tank systems at my job. No matter what if the upper tank is exposed to atmosphere you always will have to consider the fact that some oxygen is getting in. Obviously the smaller surface area will minimize O2. The expansion tank will still have to be big enough to allow for expansion and it can be susbstantial. The height of the expansion tank will also dictate the pressure in the main tank. At app .434 lbs/ft of height the pressure in the lower tank is going to build up fast. The tank will have to be built heavy duty.

 

[mpilihp]

Interesting thought, how about if your house was a single story ranch and the heating system was all on one floor and you placed the semi pressurized tank above say in the attic and it was open vented. THen the system would be pressurized by the water pressure in the tank and it would not have issues pressurizing the system below it. doesnt help me as I have a two story cape and the tank would be in the basement or outside the house at the basement level....

 

[pybyr]

how about if the main storage (volume-wise) is in the basement, the "expansion/ excess" tank is in a closet on the first floor, with an emergency-only overflow (if something mistakenly boils in the system)- blow-off headed up and outdoors out of second floor somewhere. this is what I am thinking of

 

[deerefanatic]

I'm going to use two hot water heaters as expansion tanks.... Just a few feet above my main tank.... Floating wax on top to negate evaporation (and significantly hamper O2 entrance to system)

 

[mpilihp]

Please keep us posted deerefanatic on how your semi pressurized system works with the two hot water heaters for expansion.

~ Phil

 

[deerefanatic]

Will do.........

Hope to have fire within a few months......

 

[DenaliChuck]

Any updates to operational semi-pressurized systems?

Matt?

 

[Nofossil]

There was another thread that mentioned this. If I remember right, it's common practice in Europe.

 

[deerefanatic]

I'm actually running mine in a "low-pressure" setup. Tank charge is 3 psi. That way, when the boiler is up to temp, I get about 6 psi of pressure. Works good and doesn't stress my home-made boiler heat exchanger.

 

[in hot water]

Circulators need a certain minimum pressure to prevent cavitation. It's known as NPSH net pump suction head. You want to assure you have NPSH at the maximum flow and maximum temperature anticipated. At 190F Grundfos suggests a 4 psi (about 9 feet of head) inlet pressure on the 15-58. 15.6 psi, (36 feet of head) at 230F.

Open type OWF tend to go through a lot of circs when you ignore this rule

hr

 

[nt3041]

Most people running OWB (open system) reach temperatures of 190'F. Where would they get 9 Ft. of head (for pump inlet pressure) when the boiler tappings are usually no more than 4 Ft. from the bottom of the boiler where the pump is usually located?

 

[Redox]

That's one of the fundamental flaws of an OWB; they can't generate enough pressure to keep the pumps from tearing themselves up. From what I have read, it is a common problem and results in the circulator needing to be replaced every year or so.

Chris