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# Multifamily Steam Pressure @ Radiator & After Radiator

Member Posts: 6
Hello there! I'm looking for some insight from experts.

I'm reviewing some calculations about steam heat in large multifamily high rise buildings. Most of these buildings are over 5 stories, some as many as 20. Most of these buildings generate steam at a minimum of 5 psig, some as high as 12 psig.

However, pressure is always measured near the boiler, and in these calculations, that pressure is used to calculate steam effects throughout the building. If a boiler is set to 10 psig, the calculations I'm reviewing use a steam pressure of 10 psig at the opposite end of the building and at a radiator.

General google searches indicate that most residential steam systems operate at less than 2 psig. Does that still hold for large multifamily buildings? I would assume that there are pretty significant pressure drops across pipe runs and such, and that space-heating radiators and in-unit steam traps are experiencing significantly lower pressures.

Can anyone comment on typical operating pressures at the radiator in these large multifamily buildings?

Also, what is the typical steam pressure at the steam trap at the outlet of one of these radiators?

Thanks! All help and insight is appreciated!

• Member Posts: 4,692
The Empire State Building operates @ 3#'s Top to bottom!
• Member Posts: 233
It's worth reading The Lost Art of Steam Heating Revisited by Dan Holohan if you want a detailed understanding of various steam systems and the calculations involved. Typical HVAC textbooks (like Mechanical & Electrical Equipment for Buildings) by Wiley & Sons don't even touch on the subject.
• Member Posts: 108
First, what led your interest in steam heating?
Second, you've come to the right place.
As mentioned above, real good books based on actual time and experience will propel you into the land of steam heating.
Amazon has the publications available for your learning curve. Check it out, but first, hang around for a bit and get to know what the Dead Men taught us.
• Member Posts: 22,912

Less than 2 psi is used for all buildings. Express risers may, in some situations, be more -- but they will have pressure reducing valves before distribution. As noted, the Empire State operates on 2 to 3 psi.

The operating pressure at the radiator inlet is typically around 0.5 to 1.2 psi.

The operating pressure at the trap inlet is 0, or very close to that, until the trap closes, and may still be close to 0 with a closed trap if the system is well balanced. It is 0 after the trap.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 9,412

Less than 2 psi is used for all buildings. Express risers may, in some situations, be more -- but they will have pressure reducing valves before distribution. As noted, the Empire State operates on 2 to 3 psi.

The operating pressure at the radiator inlet is typically around 0.5 to 1.2 psi.

The operating pressure at the trap inlet is 0, or very close to that, until the trap closes, and may still be close to 0 with a closed trap if the system is well balanced. It is 0 after the trap.

All of this is prefaced with should be. If parts of the system are broken or it is improperly designed this may not be the case.

This is all applied to low pressure steam.
• Member Posts: 6
edited January 2023

First, what led your interest in steam heating?
Second, you've come to the right place.
As mentioned above, real good books based on actual time and experience will propel you into the land of steam heating.
Amazon has the publications available for your learning curve. Check it out, but first, hang around for a bit and get to know what the Dead Men taught us.

What tickled my interest here is actually a really long answer. I'll try to be as brief as possible, but I'm really bad at that.
I'm actually an energy efficiency engineer. I review energy savings calculations for improvement measures.
Many states have what's called a "Technical Reference Manual" or TRM that provides some prescriptive guidance on how to estimate energy savings for different measures. When contractors report savings, they tend to use the TRM to estimate savings.
Replacing steam traps that have failed open obviously saves energy because an open steam trap is just blowing (wasting) steam.
Calculating savings for steam traps comes down calculating how much steam is lost, then multiplying by a few simple factors. But calculating steam loss is not as straightforward as you may think.
Calculating steam loss through a steam trap is essentially calculating vapor passing through a nozzle. Most TRM's use either the Grashof equation or a modified Napier's equation.
In reviewing some steam trap replacement projects, the TRM's algorithms were providing ludicrously high savings. They were essentially reporting steam loss on par with the entire capacity of the boilers. In some instances, they were claiming annual savings over the entire annual consumption of the building. This is obviously wrong. Additionally, when we compare pre- and post- steam trap replacement bills, we sometimes find very small or immeasurable savings.
The problem is that these nozzle equations are empirical equations that hold true within certain limits. The nozzle restriction is essentially that the equations work "as long as the absolute pressure downstream of the nozzle (steam trap) is less than 58% of the absolute pressure upstream of the nozzle (steam trap)." At sea level, where absolute pressure is about 14.7 psi, this turns out to be 14.7 psia after the steam trap (atmosphere or in condensate return) and 24.9 psia before the steam trap. ie steam psig upside of the trap needs to be at least 10.2 psig.
If the upstream pressure is less than 10 psig, the nozzle equations still work, but they require a hyperbolic correction factor multiplier that goes to 0 when the upstream and downstream absolute pressures are equal and go to 1 when the downstream absolute pressure is less than 58% of the upstream absolute pressure. For steam at 5.2 psig, the equation overestimates steam loss by 10%, and it gets worse rapidly from there. At 3.2 psig, the equation overestimates by 20%. At 2.2 psig, 30%. And so on.

Thus my interest in gauge pressures at steam traps. What I'm hearing is that, even though the boiler may produce steam at 5-8 psig, the gauge pressure at drip legs off the distribution lines is likely lower, 2-3 psig or less, and the steam pressure after radiators at the exit steam trap is going to be closer to 0.1-0.5 psig. (FYI, at 0.2 psig, the nozzle equations over estimate steam loss by 80%. At 0.1 psig, the overestimate is closer to 90%.)

Less than 2 psi is used for all buildings. Express risers may, in some situations, be more -- but they will have pressure reducing valves before distribution. As noted, the Empire State operates on 2 to 3 psi.

The operating pressure at the radiator inlet is typically around 0.5 to 1.2 psi.

The operating pressure at the trap inlet is 0, or very close to that, until the trap closes, and may still be close to 0 with a closed trap if the system is well balanced. It is 0 after the trap.

@""Jamie Hall" , That's awesome. Exactly what I needed. Thank you so much!
• Member Posts: 4,692
Where is the difference between boiler and radiator?
• Member Posts: 9,412

Replacing steam traps that have failed open obviously saves energy because an open steam trap is just blowing (wasting) steam.

There are a lot of assumptions in that which are not usually correct. Usually there is a steam trap or vent on the return so a failed radiator trap doesn't mean that the steam just passes unimpeded to the atmosphere.

Steam in the returns causes other problems that are a lot harder to measure. Typically it causes parts of the system to not heat because the steam in the returns prevents other emitters from venting by closing the steam trap on another radiator when the steam in the return meets it, by closing the vent/trap on the returns so all venting stops or by blocking the flow of air. This uneven and lack of heating will cause overheating of some parts of the building and inadequate heating of other parts so some may open windows while other will run space heaters or simply run the zone continuously because the part with the thermostat isn't getting heated.
• Member Posts: 1,188
edited January 2023
Here's a textbook that goes into the proper calculations for energy savings related to steam trap replacement:

Steam: The Perfect Fluid for Heating and Some of the Problems by Jacob (Jake) Myron

EG, P.E.
• Member Posts: 6
pecmsg said:

Where is the difference between boiler and radiator?

@pecmsg , good point. I guess I am on the verge of making other dangerous assumptions. I had assumed there would be some kind of pressuretrol or pressure regulator stepping down the higher main-line pressure to smaller distribution lines, or that smaller distribution lines (and radiators) would just experience or cause a pressure drop similar to static pressure in ducted hvac systems. I have reviewed 20+ multifamily buildings, and the lowest gauge readings near the boiler on the main steam line were 5 psig. With some running up to 8psig. I assumed 5 psig at the boiler would be stepped down to 2 psig or less before reaching radiators. But it sounds like my assumptions of pressure drop similar to those experienced by other fluids may not apply to steam? Curious...
• Member Posts: 7,554
edited January 2023
pecmsg said:

Where is the difference between boiler and radiator?

Over in the corner of the building. That's where they hide the steam pipes.
And I think that he is just making up names like the Grashof equation or a modified Napier's equation to sound smart. He probably tells his significant other to check blinker fluid and send them to the auto parts store for Kronston valves and Finnigan pins.

Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
• Member Posts: 9,412
There are 2 reasons they crank the pressure up. 1. They don't know any better. 2. The venting on the system is broken and if they put enough pressure in the system the air that can't get out of the emitters will compress and a little steam will get in the emitter and heat it some and as that steam condenses the volume it occupied pulls in more steam so cranking the pressure up will get a seriously neglected system to heat a little at the expense of more fuel usage and likely destroying whatever vent devices were still working in the system.
• Member Posts: 4,692
Where is the difference between boiler and radiator?
Over in the corner of the building. That's where they hide the steam pipes. And I think that he is just making up names like the Grashof equation or a modified Napier's equation to sound smart. He probably tells his significant other to check blinker fluid and send them to the auto parts store for Kronston valves and Finnigan pins.
You just can’t make it up!
• Member Posts: 15,312
@Forever_Student

Lacking the original design, you have to do some guestimating. @DanHolohan describes this in his books (LAOSH) I think.

You measure from the boiler to the farthest radiator in the building. Then taking into consideration the pipe size and the load connected to that pipe you find the steam pressure drop in that pipe. Lets' say you get 1 1/2 lbs then you would set the boiler to maintain 2-21/2 lbs of pressure. If the boiler can supply the run with the most pressure drop (usually the longest run) it should be able to supply any run in the building. Most buildings are designed to run at a couple of lbs. But every engineer has different design parameters.
• Member Posts: 22,912
For a further comment, the pressure loss between the boiler and the radiator is almost entirely friction loss in the piping -- that steam is moving right along in there. Ther pressure loss through the radiator is harder to visualise, but what is happening is that the steam is condensing in the radiator. Ideally if the system is perfectly done, almost all the steam condenses in the radiator, leaving very little residual pressure at the trap -- if any.

As @mattmia2 said, though, is keep in mind that all this sort of assumes that things are all working as they should...
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 1,214
edited January 2023

For a further comment, the pressure loss between the boiler and the radiator is almost entirely friction loss in the piping -- that steam is moving right along in there. Ther pressure loss through the radiator is harder to visualise, but what is happening is that the steam is condensing in the radiator. Ideally if the system is perfectly done, almost all the steam condenses in the radiator, leaving very little residual pressure at the trap -- if any.

As @mattmia2 said, though, is keep in mind that all this sort of assumes that things are all working as they should...

That condensing steam in the radiator may actually cause a vacuum to form.

Back to waste from steam traps.....As you've seen the energy savings calculations for repairing steam traps are typically ludicrous. They assume that any steam leaking through the trap is lost into some black hole. This is also ludicrous. The steam going through the traps increases the temperature of the return piping which is almost always in the envelope of the building.... so it is not lost. I suspect that most of the savings associated with trap replacements are due to better heating balance in the building. You no longer have to overheat large sections of the building to provide barely adequate heat to other areas.

I'm located in Illinois and IIRC we have efficiency programs locally using the TRM. From my experience, the programs are grossly inaccurate in predicting fuel savings due to improvements. It appears that many measures do not correctly model what actually happens within a building when the improvement is made and also not using the correct information. A couple more examples I have seen are:
1) the reductions in fuel usage due to going from atmospheric boilers to power burner boilers in space heating systems. They appear to base savings on the efficiency ratings of the boilers when firing continuously, which may only be a few percent. However, these boilers are not firing continuously and the differences in efficiency of the boilers over the heating season ( a real AFUE type number) is more like 10 to 15%.
2) Similiarly, often the manuals appear to take the AFUE numbers and use that to calculate savings. These AFUE numbers are incorrect when you take into account electrical use, which reduces the relative efficiency of forced air furnaces and inflates the output of forced air furnaces ( the electricity they use just doesn't disappear, but turns into heat in the airstream), and tends to have the opposite effect on hot water boilers and especially steam boilers ( which use almost no electricity, so lower energy usage and almost no inflating the heat output). I've discussed these very issues with the Institute of Gas Technology, the research arm the natural gas industry in the U.S., and they have been trying to get the AFUE numbers corrected for decades, to no avail.

BE VERY SKEPTICAL about what's in Energy Savings Manuals. The massive discreptancies in LEED building performance are a clear warning that there are large errors in many calculations and much that is left out of the calculations that can have a huge impact upon performance.
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I know several engineers that saved energy at the cost of the equipment!
• Member Posts: 1,214
pecmsg said:

I know several engineers that saved energy at the cost of the equipment!

That's why I like to take of an approach of using simple, basic equipment and systems, but using it so you maximize its effectiveness. This is one of the reasons I promote envelope efficiency improvements over high tech equipment....improvements in thermal envelopes multiple thier own effectiveness because they not only reduce the energy needed to keep a building comfortable at a given temperature difference, but also shorten the heating or cooling season.
• Member Posts: 887
There are several different steam system out there.
1-one pipe steam
2.- two pipe steam no steam traps with vent valves on radiators (rules are the same as one pipe steam)
3.- two pipe steam with steam traps could operate without a zone valve.
4.- two pipe steam sub atmospheric low vacuum. could operate without a zone valve.
5 - two pipe steam sub atmospheric high vacuum. Needs a zone valve.

If all these systems were engineered properly and not screwed up by morons the highest steam pressure needed will be two psig.

Systems with zone valves where properly designed may have boiler pressures set at anywhere from 5 -10 psig. But the discharge side of the zone valve at the coldest day will be 2psig. Many engineered systems may have a pressure controller at the discharge side of the zone valve that make the zone valve close if the pressure rises above 2psig.

Understanding the type of steam heating system you are looking at requires more than a basic knowledge of steam heating.

Jake
• Member Posts: 6,701
Amen.  Mad Dog 🐕
• Member Posts: 16,723
Here is one example where we added a LOT of venting and were able to drastically lower the pressure. We lowered their fuel consumption, too- by about a third!

https://heatinghelp.com/find-a-contractor/detail/all-steamed-up-inc
All Steamed Up, Inc.
Towson, MD, USA
Steam, Vapor & Hot-Water Heating Specialists
Oil & Gas Burner Service
Consulting