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# Heating Basics

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Member Posts: 13
Please tell me the reason why hydronic systems are run at 12 psi? Is it to prevent boiling at temps over 212? If the system pressure is reduced to 0 psi and there is no air in the system wouldn't the weight of the water in the pipes going to the highest point in the building equal the weight of the water coming back?....or maybe the weight of the water and the psi in the system have nothing to do with each other. Thanks for the help.

• Member Posts: 2,398
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The real reason?

12 PSI is typically enough to get the water to the high point of the system in a two story house plus about 4 PSI to spare. It has to do entirely with the height of the system, a taller building requiring a higher pressure.

In your example, "If the system pressure is reduced to 0 psi and there is no air in the system wouldn't the weight of the water in the pipes going to the highest point in the building equal the weight of the water coming back?"

If the system pressure was zero there would not be any water in the pipe. If there were water in the pipe to a certain height, the pressure would be equal to the height of the water column in feet divided by 2.31. (Conversely the height of the water column in feet multiplied by 0.433.)

You are grasping that the weight of water going up is cancelled by the weight of water going down, but only in a closed system. This is why pumps in closed systems are called "circulators". As with a Ferris Wheel, the water dropping essentially siphons the water rising, leaving only pipe and fitting resistance to be absorbed.

Does that make sense?
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

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Barry

usually 12 PSI(min) is used as as a "vertical stacking height", 1 PSI will lift vertically 2.31Ft, and as most homes have the boiler in the basement, and perhaps 1 or 2 stories, this is 8+8=16 Ft of VSH. That`s 6.93 PSI to get water up to that highest vent(no overspill), then if it is force pumped (circulated),this pump requires a certain inlet pressure to its volute to prevent flash steam if it falls into a negative pressure. Did I say that right? Come-on guys help me here! Where`s Brad?

Dave
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There he is,

Dave
• Member Posts: 2,398
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Right above you, Dave

You did very well!

To add to the flashing into steam part, that only becomes an issue where the water is heated above it's flash point. The hotter the water the more likely this is to happen and the lower the pressure the more likely this is to happen. Combine the two and you will cavitate your pump.

Flashing will happen frequently in systems run at or near steam temperatures and when the expansion tank is on the discharge side of the pump (circulator). The suction pressure drops below atmospheric and the water, already near boiling, boils. Goodness! Time for tea.

Flashing is really not a concern in a system run at conventional HW temperatures provided there is any gauge pressure in the system. Flash points are based relative to absolute pressure so even 1 psig above atmospheric will not allow flashing of water at 212 degrees. One would hope you would be below that temperature.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

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and I did see you, but after the fact. You always know how to say things, I did`nt know you were here. (I was hoping though!)

Dave
• Member Posts: 2,398
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You are way too kind Dave

Happy to partner with you! What a team!
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

• Member Posts: 13
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Thanks Brad, Dave, I think I pretty much understand the concept of the weight of the water causing the pressure at the boiler altitude gauge. What I still don't understand is: if I drain about a gallon of water from the system the pressure goes to zero and yet I haven't drained the whole system. So if I drain just enough water to get to 0 psi will the system still work? Or am I risking a vacuum at the top of the system that may allow air to enter??
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That`s right Barry

allow the system pressure to get too low, and you sure will "suck-in" air from various high points. Want me to get Brad after you? LOL. He is a great fellow and can put things into words much easier than me!

Dave
• Member Posts: 812
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But then, what about the size 12 boots my plumber wears?
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Ya Ya

your close Christian, but my plumber wears size 13. What`s with that? LOL

Dave
• Member Posts: 4,909
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"How Come" by Dan Holohan.

Doesn't get as much attention as "The Lost Art Of Steam Heating", but it is a FANTASTIC book!

IMHO, his best book.

Love that book!

Mark H

• Member Posts: 812
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Oh that stinks

There goes another perfectly good theory
• Member Posts: 2,398
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Barry

If you have a stack of water indicating 12 PSI and with zero pressure at the top, that stack of water would be about 27.72 feet high.

If in a closed system, say two stories or 16 feet, that would allow the top to have a residual pressure of over 4.8 PSI, enough to force out air and give a reasonable cushion against vacuum.

Now, if you had such a stack of water reading 12 PSI, you drew off a gallon and the pressure went to zero, my hypotheses would be:

1) The volume of the stack you drained was one gallon ; or

"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

• Member Posts: 873
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might not be \"broken\"

Most gauges are accurate near the middle third of the scale, less accurate towards both ends of the scale. So when your pressure starts to rise above zero, it may take cosiderably more than "zero plus a tiny bit" to see a movement.
• Member Posts: 13
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You guys are the best!! I had an epiphany about 2:00am (OK OK don't go there)....If I drain about a gallon of water from the system and the pressure goes to zip the reason more water doesn't come out...or the reason the pressure is 0 is because atmospheric pressure is holding the water in. I bet if I measured the pressure at the top of the system I'd be pulling a vacuum. Also, it takes presure to fill a system (duh) so as the system is filling the pressure to force the water up the risers is
• Member Posts: 2,440
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But, one gallon?

If the gauge is at 12 lbs.... I just cannot see the gauge going binary ("you have pressure -or not"). This is a tad beyond "2% accuracy in the middle of scale"....
• Member Posts: 13
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You guys are the best!! I had an epiphany about 2:00am (OK OK don't go there)....If I drain about a gallon of water from the system and the pressure goes to zip the reason more water doesn't come out...or the reason the pressure is 0 is because atmospheric pressure is holding the water in. I bet if I measured the pressure at the top of the system I'd be pulling a vacuum.
Also, it takes presure to fill a system (duh!) so as the system is filling the pressure to force the water up the risers is .433/' ..........Thanks again!!!
• Member Posts: 2,440
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Hallelujah Brother Barry!

Never thought of that but does it ever make sense!

I learn every day now....
You have a tight system indeed.
• Member Posts: 638
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Yes Barry you're right

To get the same effect put a straw in a glass of water put your finger over the top end and pull out the straw and magically the water stays in the straw. Now open an air vent at the top of the system and see what happens.
• Member Posts: 15
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What about the length of the system?Say you have an apartment building that is 250' long(or 1000',whatever)and is two stories high with the boiler in a mechanical room at one end.Will 12psi be sufficient to push the water to the second floor at the opposite end of the building?I can understand additional height requiring additional pressure,but I can't think of ever seeing the length of the system being mentioned when system pressure is discussed.Just curious. -jim
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I learned most of what I know about pumping, purging and pressurization from working with solar systems. Tall solar systems. One time, I discovered the hard way that an air vent on the top of the system becomes a vacuum breaker and allows air to be sucked into the loop and come down the down comer to the glycol bucket. I thought to my self (after about 2+ hours of pumping, waiting for the bubbles to clear out of the flow...) Where the heck is all this air coming from???"

I applied what I learned from that to the closed loop snow melt systems we deal with, with lots of elevation.

You can not pump/fill and fully purge a system with an autovent open at the top. Close the auto vent and you will kick a siphon in no time. The ferris wheels of fluid...

I remember filling a mountain job a LONG time ago, and all we had for a pump on the outside of the system was a little giant garden hose dewatering sump pump. We used that to pump the glycol out of the mixing bucket and into the system. Once wetted, we'd turn on the systems circulators and put them all in series, and quickly and easily fill and purge the system with perfectly premixed glycol and water. This plumber on the job kept walking over and looking at our little giant sump pump saying "It's just a little friggin giant...." and walk away shaking his head, mumbling. Funniest dang thing I ever saw. He obviously didn't get it.

Anyway, the reason I started this thread was to say that you can tell when you've got a perfect purge when you close the return. Watch the fill bucket. If it continues to take fluid, even for just a second, you've still got a pretty sizeable bubble in the system. When it's tight and completely purged, you shouldn't get ANY water out of the system. Just like the straw. Maybe a SPIT. But that's it. Now THAT is a tight, well purged system, like you have to have in a closed loop solar sytem.:-)

Little friggin' GIANT...

ME
• Member Posts: 147
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My guess on the length of the system is that pressure will move liquid an infinite length minus the resistance (the walls of the pipe). I would also guess that there are charts/mathematical formulas that you can use to figure that out. The larger the diameter of the pipe, the less the resistance, I think.
• Member Posts: 873
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who's who

Is Brother Barry and Chap Chap one and the same?
• Member Posts: 147
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No, I was thinking Mark E. was Barry since he stated "the reason I started this thread..."
• Posts: 0
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I forgot to mention...

The expansion tank would have to be OFF line in order for my previous statement to be true...

Otherwise, it would take on fluid if your charging pump had enough cajones :-)

Sorry for any cornfusion...

ME
• Member Posts: 13
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No alias's....thanks. I get enough "huh?" with just Barry.
And....as long as I'm in here already. Looked at a hot water Lochinvar system yesterday that was in the attic (third level). It was dressed out so that the two supplies branched from a 1 X 3/4 T. Two zone valves a #15 Amtrol expansion tank and a 0010 Taco were on the return line and in that order. The boiler was set to fire at 160-180. There was a ball valve bypass full open between the supply and return. My friend complained of marginal heat. I cranked the boiler up to 200 and closed the bypass. I also recommended he eliminate the zone valves and install 2 circulators on the supply side. I'm waiting to hear if he woke up to a warm house.
Any other suggestions?
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Length has nothing to do with height...

as it pertains to the FILL pressure requirements of the CLOSED LOOP system.

Length, pipe diameter and flow rate has EVERYTHING to do with the pressure drop associated with flow, and the pressure generator's (pump) ability to over come that pressure DROP to maintain a given volume of FLOW (GPM).

People get static fill pressure confused with pressure drop all the time. Cracks me up to be sitting on the parts counter at the local heating supply house and have someone come in to get a replacement pump, converstation usually goes soemthing like, "I need a pump", conterman, How BIG a pump you need? "I dunno, whattya got?" Well how many feet of head are we talking, and how many GPM? " The building is 40 feet tall, and a block long..." conterman says, "What kind of room do you have between the flanges?

"Oh, about 6-1/2", counterman grabs series 100 pump and hoists it up on the counter, plumber exclaims "THAT'S IT, the red one...

No intention of maligning any one in the above statement. We're all in this together:-)

ME
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Water is like my ex brother in law...

Wet, lazy and stupid. It must be shown the path you want it to take.

If your friends home has high mass heat emmiters (cast iron radiators/baseboard) the bypass may be necessary. Otherwise, probably not needed with low mass emmiters (copper finnned tube baseboard).

In any case, follow the manufacturers piping recommendations. They've dealt with most of this stuff before, and they ARE the experts in their own systems...

I'm betting he woke up in a sweat :-)

ME
• Member Posts: 873
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No reason to worry

The red one will pump a city block long. You know anyone afraid of Hieghts? My friend says he's afraid of widths.
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