Unpressurized Hydronics

NRT_Rob

who is doing this? Lots of unpressurized tanks and wood boilers and solar drainbacks out there. is anyone really running that water straight into their heating systems with good results and no intermediary heat exchange?



We've seen lots of problems, but you never know if it's the method or the means,... lot of people say radiant doesn't work too, but we know that's not true.



Can you do this well with 2 or 3 story buildings? Does it require huge pumps?



this is spinning me around quite a bit actually.

Mark Eatherton

I've done a 2-1/2 story home....

with a condensate heating system off of an atmospheric pressured steam boiler, but the emitters were cast iron radiators.



THere ARE limitations as to how high you can maintain a column of water under negative pressure tho.



I suppose one could do a "drain back" style of heating system, but boy would it be NOISY when it was filling AND draining back. Noise is not in my definition of comfort...



Here's a link to the article Dan penned on heating with steam condensate that might answer some of your questions.



http://www.heatinghelp.com/article/11/Hot-Tech-Tips/201/Hot-water-zone-off-a-steam-boiler



What's causing you to anguish over this less than preferable method? Remember, most people perception of comfort is basic heat. The noise that comes with it is WYSIWYG.



ME

NRT_Rob

simply

Dropping heat exchange, fill valve, backflow prevention and expansion tank from the equation is a significant cost/labor/maintenance/replacement reduction for those atmospheric wood boilers or even just in unpressurized tank solar systems. Also there is an efficiency improvement as you lose the differential requirement to drive heat exchange.



so it's compelling, if it can reliably be done quietly and without air problems. But how to do that is the question. It's been done in a variety of systems of course, but I can't find any good design info on such systems, just hack wood boiler diagrams that, as often as not, don't seem to work very well judging by the number that have been converted to closed systems here in Maine. So I wouldn't follow their lead.



but I do think this can be predictably and safely done in some circumstances. Sure would like to know how to define what those situations are though...

Mark Eatherton

I can tell you what you CAN'T have in the loop...

and that would be any means of automatic air removal at the top of the system. Auto vents make great vacuum breakers when negative pressure is applied to them. I learned the hard way waiting for a closed loop solar system to self purge whilst being filled with glycol. The bubbles just kept coming and coming and coming. I finally climbed to the top of the system array and closed the auto vent on top, and the air cleared immediately.



You would have to maintain certain velocities so that when air does come out of suspension in the upper regions of the system, that you have the ability to push them back down to the heat source. This is why the downcomer on most drain back solar systems is smaller than the supply riser, to establish and maintain a good siphon. The return piping has to terminate below the water level if you want to maintain fluid in the upper regions of the system, or has to NOT be immersed to allow air in and water back if it is to drain back, like a solar system.



It requires more valves to insure that the system can be filled and purged, then brought on line with the static heat source. The other thing you have to watch is NPSH, which as Siggy called it once, Not Pumping So Hot... If you can't maintain the minimum required static suction head, the output capacity of the pump is significantly affect, negatively. Remember also that you have a 30' ceiling above the standing water level of the boiler due to atmospheric pressure considerations. Exceed that and it will form a vacuum strong enough to bring air out of suspension in the down comer. Not a good nor quiet scenario.



Also, the pump is not physically having to overcome the static head (lift), so it is still just a circulator, assuming that the suction and return lines are completely immersed and purged.



Now, if the heat source is as high or higher than the building, then all the dynamics change, and so does the design. :-)



As for air separation on the top end of the system, remember, it is going to be right at or slightly below atmospheric pressure, so you could have a "recovery" tank up high to capture and hold air, but all piping would need to be pitched up to that Apex point. Tough to do with a RFH or even BBR system.



Tough call. Fun project, but tough call. And it is ALL controlled by the quality of the installation. If details are not paid attention to, it won't work right, if at all.



G'Luck!



ME

NRT_Rob

thanks mark

this is helpful.



do you know of any handy pressure guidelines to use to avoid cavitation or air out of solution? you mention 30 feet, that would seem to indicate something like -13 PSI would pull air "explosively out of solution", is that about right?



interesting stuff. thanks again.

Mark Eatherton

As with any hydronic question, the only correct answer is...

It DEPENDS! In your case, it depends upon how hot the system fluid is going to be. The hotter the water, the lower the pressure required to flash it to steam. There are vacuum charts that show at what temperature and negative pressure water will boil at. I've always used 30' as the maximum based on Dans recommendations. That allows you to provide heat to a 3 story tall building. If its over that, then the expense of the other amenities is not an option...



Not sure where you'd find the charts, probably in a steam section some place, but if you limit your system design to 30'. you shouldn't get into too much trouble, unless you are high in the Rockies, in which case the atmospheric pressure is less, and the allowable system height is also less...



You can create some back pressure on the down comer to avoid issues, using the pumps excess dynamic head.



ME

Brad White

Vapor Pressure of Water

You can Google a chart of the vapor pressure of water to determine the cavitation point, Rob. Mark has your back and true, it all depends. As Mark said, higher temperature, higher vacuum/cavitation potential, closed system or not. But in your case, you just have no where to go! The thing about most vapor pressure charts is that they often use arcane or unfamiliar units such as the Torr, not to mention the BAR, Pascal, and representing velocity in Furlongs per Fortnight. So bring your conversion tables to translate back to your brain. That is what I do.  But the point is, keep the circulator low in the system for best results, if I gather your question correctly.



As you likely know, the basic premise of unpressurized (ie: open) system height starts with a perfect vacuum, 29.921 inches Hg. at sea level. As a quick rule, one inch of mercury is two psi (2.036...) so close-enough. Thus at sea level, the atmosphere is in effect "pushing in" and holding up a column of water over 33 feet high. This is exactly how the weight of air was first determined by the way. 



As Mark also alluded, it is the defacto vacuum or rather, "a vacuum filled with water" which holds things in check. Break that and go get a mop.

NRT_Rob

thanks guys

appreciate the guideance. this was going to give me the vapors.



*ducks flying fruit*