Why must The 12 psi fill pressure be increased for 3 floor house?

Joseph_4

I have a customer who has a tall 3 floor house.. The attic zone always gets air bound. No matter how much i bleed can't get bubbles out. The instructions on one of the feeders explains take the amount of feet say ( in my case )28 from above the feeder to the highest radiator multiply by .43 and add 3 psi and adjust accordingly. 28 x .43 plus 3 = 15.04. It seems if you dont do this the water doesnt have enough psi to overcome the height.. I dont understand.. when water is hot it goes above 18 LBS psi so why does the cold pressure matter?

Further The prv standard is 30 LBs.. It will come out anytime i have job on 3rd floor need a 35LB prv if i dont want nuisance prv opening and all residential boilers come with 30 LB prv. ( some under the boiler jacket say rated for 50psi but not sure if 35 lb prv would void warranty)

Any feedback would be appreciated
thanx
Joe
HHI Services

Steamhead

Joseph_4 said:

...... It seems if you dont do this the water doesnt have enough psi to overcome the height..

Exactly. You need 18 PSI to make third-floor radiation work properly. This also means the expansion tank must have more capacity since it doesn't have as much pressure range over which to operate.

Joseph_4

i always tell people when hot 12 psi raises to between 18 and 22. so why does is need to be 15 when cold wouldnt that make is 21 to 24 when hot?
Joe

EdTheHeaterMan

It is called Static pressure. A column of water that is 28" high will exert one pound of pressure at the . bottom of the column... If you have a column of water 28 feet high, it will exert 12 PSI pressure at the bottom of the column. (12 inches in each foot)

If you have a boiler in the basement and the third floor radiator is 31 feet above the boiler, you will never get the water up to the top radiator.

If the top radiator is 26 feet above the boiler, you will get the radiators to have water in them, bur if the circulator pump is on the return and pumping towards the expansion tank, the the pressure on the suction side will drop, causing the static pressure on the suction to lower in kind. That may cause the top radiators to actually have a negative water pressure during operation. If the room pressure in the 3rd floor is higher than the water pressure in the radiator, then any leak in a valve packing nut or automatic air vent, those leaks will be air going in... not water leaking out.

i have attached 3 slides from my Hydronics class that illustrate the static pressure needed to get the air out of the top floor radiators. If you go much higher than 30 feet, you may need to get a boiler rated at 50 PSI with a higher pressure relief valve, in order to keep the relief valve from opening from over pressure under normal operation.

Steamhead

Joseph_4 said:

i always tell people when hot 12 psi raises to between 18 and 22. so why does is need to be 15 when cold wouldnt that make is 21 to 24 when hot?
Joe

if it's going up that much, you need a bigger expansion tank.

Joseph_4

so if i raise fill pressure to 15 psi and pump up pressure in expansion tank that will fix his problem?

Steamhead

Joseph_4 said:

so if i raise fill pressure to 15 psi and pump up pressure in expansion tank that will fix his problem?

That would be borderline. As @EdTheHeaterMan says, the circulator can decrease the pressure at the top of the system. That's why we target 18 PSI in a 3-story building.

Joseph_4

so fill should be 18psi cold and use a t30 expansion tank and pump to 18.. and the prv will be fine or will i have too high operating pressure. or do i use a t60?
thx
Joe

ps its only been 17 years i guess its about time ...it should finally all come together about "pumping away" Eds explanation showed me some understanding in some things i learned from Dans bookswhich i guess was never clear

EdTheHeaterMan

Based on many old installs, with a system set up the old fashion way (circulator on the return) and the expansion tank attached to the boiler. The pump will be pushing it's outlet pressure against the point of no pressure change. if the outlet of the pump can not go up then the inlet pressure will drop down. Just say this boiler is on a 3 story home with lots of pipes going in all directions of this large mansion. and the pump head causes a 6 PSI difference from inlet to outlet. That means the water pressure on the return side will be 12 PSI minus 6 PSI. for a total of 6 PSI when operating. 6 PSI will only lift water 14 feet high. the might leave the radiators on the third floor with negative water pressure in them. Negative pressure is basically called a partial vacuum. If the top floor radiators are under that partial vacuum, any air in the system will find its way to that part of the system.

Steamhead

Joseph_4 said:

so fill should be 18psi cold and use a t30 expansion tank and pump to 18.. and the prv will be fine or will i have too high operating pressure. or do i use a t60?
thx
Joe

ps its only been 17 years i guess its about time ...it should finally all come together about "pumping away" Eds explanation showed me some understanding in some things i learned from Dans bookswhich i guess was never clear

Use the T60 at 18 PSI and reset the fill valve to 18 PSI.

Joseph_4

Thanks so much to both of you!
Be well
Joe

STEAM DOCTOR

Cold pressure definitely matters, some scenarios. As stated, you lose about .433 for every foot of elevation. Go high enough, and there will be zero pressure or close to zero, at the top. Air will obviously start to separate at that point, and whatever happens while hot, will not help to put the air back in the water. Also, be careful where you take your measurements. Are you measuring from bottom of radiator or top (assuming we are talking about cast iron radiators,the big boys). Where exactly is boiler pressure reading, being taken from? Obviously, doesn't make a difference under normal circumstances, but could make a difference, when working with a slim margin. And don't assume that the gauge is telling the truth. 

EdTheHeaterMan

@Joseph_4, I'm going to post my same PM answer here for the group... and I can correct my typos

I like to think of it this way: A man shows his son how to install a boiler, he shows him that the expansion tank always goes on a certain tapping on the boilers they install. The circulator always goes on the return and the fill valve always goes on the coldest pipes in the system so as not to shock a hot boiler with cold water. He did that for 30 years and now his son has taken over the business. The father or the son never installed a boiler in a 3 or 4 story home

One day the son gets a big job and installs the boiler in the same way in a 3 story home. As soon as the job is done and the boiler operates, the relief valve starts to leak. The son installs a new relief valve only to find the same problem. He calls the father and his father has no explanation.

Well here is the answer. They have been doing it wrong for over 30 years but it never was a problem because the incorrect design was always on a 2 story home. When they needed to fill the 3 story building, the problem showed up as a leaking relief valve because they needed more than 12 psi to start with..

See if you can see the problem in this illustration I used in my classes
When all is off and cold and All the pressure gauges are at 12 PSI. Before the flame heats the water, but as soon as the circulator pump starts, the pump will drop the pressure on the inlet side because it is pumping towards the point of no pressure change... the expansion tank. But the water pressure in the return pipe will be lower than the 12 PSI of the auto fill valve. So the fill valve will add water pressure to 12 PSI. That pump will add the pump head (let's say 6 PSI) to the 12 PSI from the inlet. That is now a total of 18 PSI. With that extra water, the expansion tank gets more water in it to compress the air to 18 PSI. There is less room for expansion now. After the system cools off everything is now 18 PSI with cold water. On the next heating cycle, the expansion tank has less room so the pressure will go up to 24 PSI. Since the relief valve will not go off until the pressure gets to 30 PSI, there is no problem. The system operates at a higher pressure for years and years.

Now apply the same problem to a home with 3 stories and you need to start at 18 PSI cold. Then add the 6 PSI to that. Now on the second cold start the system will be at 23 PSI. On the second cycle... you will be getting closer to 29 PSI when the water heats up, since the expansion tank capacity is reduced. That is only one PSI away from the relief valve blowing off.

And that is the way we have done it since forever ago! Wrong but without a problem ... until there is a problem.

I hope you all followed the chain of events here... Now that make Pumping away the greatest thing since Sliced Bread!

hot_rod

The size of the expansion tank will dictate how much the pressure increase, also. Assuming it is working and pre-charged properly.

I like that Amtrol added the "capacity" gauge to their expansion tank sizer program.

I believe the Amtrol calculator formula allows the system to go up to 27 psi.

EBEBRATT-Ed

System resistance to flow is additive to the fill pressure if pumping away from the EX-tank and water MU.

If pumping towards MU water and EX tank system resistance to flow is subtractive from fill pressure.

Sometimes you find the EX-tank on one side of the circ and the MU water on the other side which is always wrong.

MarkMurf

  And all of this is totally due to corporate America's peddling of pumps, pressure relief valves, pressure regulating valves, zone valves, etc.,etc. Give me an open system, convection flow, hand fed with the 'expansion/fill tank' in a closet on the top floor just slightly higher than the top floor radiators . Not to mention the altitude reading on the tri-dicator . What's that for again ?🤔🤔🤔?

mikespipe

The pressure reducing valve is adjustable raise it to 18 psi . the expansion tank can also be adjusted by adding air to 18 psi. doing those things should fix the problem.

HomerJSmith

The size of an Extank is based upon the volume of water in the system and the highest operating system temperature. Follow the guidelines of the tank manufacturer's formula. High system static pressure with a low air charge on the tank collapses the diaphragm and reduces the acceptance volume of the tank which is why one needs to take the water weight (static pressure) pressing down on the tank into consideration.
@Steamhead"s statement about needing a larger tank may or may not be true. If the acceptance volume of the tank is reduced because of an low air charge, the expanding hot water may not have anywhere to go, increasing the system pressure. Water on the air side of the bladder also decreases the acceptance volume.
With a properly sized and charged Extank, I expect about a 3 degree increase in the boiler gauge with 180 degree boiler water. I also add 5 psi to the air charge above the static psi.

t.p.tunstall

Dan, this is t.p. Tunstall 
without a doubt you and Marianne will always remain a solid definition of kind sincere and dedicated couple and family. 

The example you leave us with is unmatched 
thank you and Marianne and family 
the industry would not be the same without you guys 
tpt 

A1Boss

1/2 PSI per Foot Static Head of Water Go from there

EricPeterson

I can tell you from personal experience that in my house, with the static pressure set to 18, when I open the bleed valve on the third floor radiator there will be a slow trickle of water or air - just enough to bleed any accumulated air. There hasn't been much air since I installed the IAS (Inline Air Separator) and ATF-12 just before the heating system.
I've been checking it weekly and after any initial air was bled, only water has come out.

If the pressure falls much below 18, little or nothing will come out when I open the bleed valve. In that case I have to raise the pressure to 18 manually (which I do with a bike pump through a Schraeder valve connected to the tank) when the system is cold.

In my case, the gauge that was installed with the original boiler (after gravity conversion) has a line on it indicating the 18# mark.

I presume that whoever did the conversion properly sized the compression tank for the amount of water in the system, but I have not done that calculation, as well as installing that particular gauge,

Eric

PS Why don't they make gauges like this one anymore?

hot_rod

A basement with two levels above?

measure from expansion tank, fill valve to highest radiator. X .433 then add 5 psi

EricPeterson

@hot_rod - in my case a basement with three levels above. I measured 330" from the top of the 3rd floor radiator to the fill valve location below the IAS, which works out to about 27 feet. Your formula then yields 16.7, pretty close to 18.

Eric

EdTheHeaterMan

EricPeterson said:

Eric

PS Why don't they make gauges like this one anymore?

You mean like this one?

https://www.supplyhouse.com/Pasco-1451-75-PSI-260-F-Boiler-Gauge-Bottom-Mount

Or this one with rear mount?

https://www.supplyhouse.com/Pasco-1452-75-PSI-260-F-Boiler-Gauge-Back-Mount

I think "THEY" still do. You were just looking at the wrong "THEY"

EricPeterson

@EdTheHeaterMan - sort of, but the scale is not realistic for a residential boiler with PSI going to 75 and temp to 260F.

EdTheHeaterMan

Actually, the gauge in the photo has a temperature range of 80°F to 320°F which places 200° dead center of the temperature gauge. The one I posted has a more realistic range for residential use. And as for the Pressure, If you were following the original discussion, the three story residence needs to operate at a higher pressure. And 30 PSI is well documented on the new gauge. So is 50 PSI in the event you order a 50 PSI rated boiler for that 3 story home.

If it offends you to have anything over 50 PSI and a red arrow that says LIMIT between 30 and 50, I can send you some White Out® and a Red Sharpie®.

Perhaps we should just go back the living in caves, Then we kill the daughter-of-a-gun that invented that damn wheel. She started all this Progress stuff!

EricPeterson

@EdTheHeaterMan - yeah good point about the temperature range. I was wrong about that. Good point about the 50 PSI as well.

DanHolohan

@tp tunstall Thanks for your kind words. ❤️