Steam Boiler Pressure

Residential steam heating system Pressure

are almost always designed to operate at very low pressures, typically around 0.2 psi to a maximum of 0.5 psi - that' s 1/2 of one psi. Click to enlarge and you can see the actual pressure settings on the steam boiler control shown at left.

• 0.2 psi = 0.014 Bar or about 1378 Pascal of pressure - typical low-end of residential steam boiler pressure settings.
• 0.5 psi = 0.034 Bar or about 3447 Pascal of pressure - typical high-end of residential steam boiler pressure settings

Watch out: If your residential steam boiler is operating at higher pressures that may be an indication that a service technician or owner was having trouble getting heat distributed through the building. Rather than finding and fixing the problem, someone is trying to "force" the steam around the system.

High rise buildings 

and larger facilities using steam for heating may employ a high-pressure steam heating system. According to a University of Idaho engineering text we cite below, a 30 psi high pressure steam system might provide sufficient pressure to deliver steam heat to a building 40 stories in height. So we might see 15 psi for a 20 story building steam system.

• 20 psi = 1.4 Bar or about 137900 Pascal of pressure - typical low-end of high-rise building steam boiler pressure settings.
• 40 psi = 2.8 Bar or about 275790 Pascal of pressure - typical high-end of high-rise building steam boiler pressure settings

Above: the pressure in a steam boiler might indeed fall to zero or very close to zero when the boiler's burner is OFF and the system has cooled down.

The steam boiler pressure on the gauge and pressure setting on the pressure control (gray box at upper right) are both normal at this installation observed in Poughkeepsie, New York in January 2023.

Why Might Steam Pressure Be Set Too High?

An experienced steam heat service technician will look at the operating pressure of your steam heating boiler and if it is not set to a normal level, the technician will look for the reason.

Examples of problems that can affect the flow of steam heat through the system, leading to attempts to over pressurized the system include

• Blocked condensate returns at individual steam radiators
• Cold steam heat radiators
  
  See STEAM HEAT RADIATOR REPAIR
• Steam radiators tipped the wrong way
• Improperly relocated steam piping that has the incorrect slope
• Radiator valves that are not operating,
• Radiator steam vents that are not operating properly (most-common)
  
  see STEAM VENTS
• Steam traps clogged by rust, blocking condensate from leaving the radiator
  
  see STEAM TRAPS

For details about radiator problems see RADIATORS for ROOM HEAT or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Watch out: do not modify a steam boiler pressure control switch to try to force steam heat to cold radiators or to higher floors. Doing so makes the system less efficient, fails to find and correct the underlying problem, and in some situations it could make the heating system unsafe.

Steam Boiler Pressure Control Safety Switch Operation

The pressure control switch on a steam heating boiler is designed to shut the heating system down should unsafe high pressures develop.

As we emphasize at our description of pressure gauges on a steam boiler, residential steam heating systems are almost always designed to operate at very low pressures, perhaps around .5 psi - that' s 1/2 of one psi.

You should see similar settings on the pressure gauge and on the steam pressure control switch (the gray box our photos of steam pressure switches shown here) on your boiler.

Because the operating temperature, pressure, and form of heat distribution are different from hot water heat (typically 20-30 psi of hot water), steam boilers (typically less than 0.5 psi steam) use use controls that monitor steam pressure operated controls like the Honeywell steam pressure control shown below.

Reader Question: the gray box on my steam boiler is set under 2 psi but the boiler pressure is 7-8 psi. Is this normal and safe?

The gray box on my steam boiler is set low (less than 2psi) but the glass pressure gauge is reading 7 or8 at times. Is this normal /safe? - Brian 11/21/12

Reply:

Brian,

One wouldn't expect a boiler to blow up at 14 psi, but I agree that the system is not set nor operating at safe normal pressures. I suggest calling a service tech promptly.

Usually residential systems operate at lower pressures, less than 1 psi, often around 0.2 to 0.4 psi.

Since your steam boiler pressure control valve is set under 2 psi and the boiler pressure is way over that, and assuming that your steam boiler gauge is working and not "stuck" at an abnormal reading , the problem may be the pressure control valve itself.

See STEAM PRESSURE GAUGE
or

See PRESSURE SWITCH, STEAM BOILER.

We discuss how to set heating system pressures for both hydronic boilers and steam boilers over

at PRESSURE & TEMPERATURE SETTINGS, CONTROLS

Also see RELIEF VALVE, TP VALVE, STEAM BOILER.

Reader follow-up: changing out the steam pressure control valve did the trick

got the tech. he took off the gray box on my newly installed williamson boiler (2 weeks ago) and replaced it with the one from my older rotted out 12 year old Burnham! both gray boxes are honeywell and seem to be identical. works ok now. go figure!

New Honeywell Vaporstat L408J - but No matter the setting on the main, the Gauge attached reads .5 psi.

Our reader asked:

I installed a Honeywell vaporstat L408J. Wired across the R and B terminals. No matter the setting on the main, the Gauge I attached reads .5 psi.

Switched to a brand new ounce gauge and with 1.5 psi set on the main get 7 oz reading on the gauge. Is this a defective vaporstat or am I missing something? On 2018-11-08 by Bob Kosiba

Reply by (mod) -

Bob

IF you mean that setting the gauge to a higher or lower pressure is not changing the boiler pressure, even after a heat-on cycle (that would drive the steam vents and thus allow pressure to escape the system)
and

IF the control does not turn off the burner ever,

THEN

yes I agree something's wrong either with the control or with how it's mounted and wired.

In that case let's review the installation details in the IO manual that I give below.

OR otherwise

The control may be working perfectly correctly. It would be quite unusual for a residential steam heating system to want to operate at pressures above 0.5 psi, in fact that's a bit high for many.

Here is what Honeywell says

In all models, the differential is subtractive from the main scale setpoint.

The upper operating point is determined by the main scale setpoint, while the lower operating point is determined by the main scale setting less the differential setting. Operating points are shown in Fig. 7.

Adjust the main scale setpoint for the desired operating pressure by turning the main scale adjusting screw (see Fig. 8) on the top of the case until the main scale setting indicator is at the desired value. Adjust the differential setting by turning the differential adjusting screw until the differential setting indicator is at the desired value.

The scales are marked in psi and kPa.

Consult the operating manual for your steam boiler to confirm the desired or default operating pressure. If you think you need to set pressure higher than 0.5 psi, most likely there is something else wrong with your system such as a condensate return problem.

Below I illustrate this Honeywell L408J1009 control for other readers, and in case you don't have the instructions for your Honeywell vaporstat, you can obtain a copy from Honeywell at any supplier, from Honeywell, or at this link:

Honeywell VAPORSTAT L408J1009 IO MANUAL [PDF] at InspectApedia.com

by Bob Kosiba

Vaporstat turns out to be working properly, but pressure will not go above 7 oz. Water level does not go below LWCO. Burner keeps working, but will not increase the pressure above the 7 oz.
No water loss indicated in the sight glass over a three week period.
Any ideas?

by (mod) - normal residential steam boiler pressure levels

Bob:

At the risk of being irritating (sorry) I intended to make clear that most residential steam boilers operate at around 0.5 psi.

So your system may be operating perfectly normally - check the setting on the vaporstat.

More details are in the reply I posted on Nov. 14 (below).

If I've misunderstood or missed something let me know.

Also ssee our article dedicated to steam boiler pressure controls beginning at

PRESSURE CONTROL, STEAM BOILERS

and this article on normal steam boiler pressures

STEAM BOILER PRESSURE

Steam Boiler Pressure Gauge and Normal Pressure Ranges

Pressure and Temperature gauge on steam heating boilers: 

Residential steam heating systems are almost always designed to operate at very low pressures, perhaps around .5 psi - that' s 1/2 of one psi.

You should see similar settings on the pressure gauge (at left in our photograph) and on the steam pressure control switch (the gray box at right in our photo) on your boiler.

Details about gauges on steam heating equipment can be read

at GAUGES ON STEAM BOILERS

If your heating system uses forced or gravity circulated hot water rather than steam, controls and gauges are different:

see GAUGES on HOT WATER BOILERS

PRESSURE GAUGE, BOILER for more details about pressure gauges on hydronic (hot water) heating boilers. These are not steam systems.

What's the proper setting for a Honeywell PA-404-A 1099 Presstrol on a steam boiler

Does anyone know the proper setting for a honeywell pa-404-a 1009 prestrol control for a weil-mclane steam boiler - Rick 9/11/11

Reply:

Rick the exact pressure needed varies by installation but in a low pressure residential steam system the pressure range is usually between 0.2 and 0.4 psi.

Reader Comment: High steam pressure traced to a blocked sensor on steam pressure control

Pressure on gas steam boiler was rising far too high, so high that the pressure relief valve released twice when it hit 15 psi (as it was supposed to do).

Good buddy who really knows his stuff checked everything from pressure valve to pressuretrol. Nothing .

He then checked "pigtail" and found blockage .Problem solved!! - Ken 11/24/12

How do I lower 28 psi pressure and water coming out of my return on my American Standard steam boiler

II have a pressure of 28 psi and water is coming out of my return how can i lower the water pressure from my Steam American Standard boiler - Ronald 1/13/2013

Reply: first clarify: hot water heat vs steam heat then see this safety warning - turn off your steam boiler

Ronald,

Watch out: If your heater really is a residential steam heating system - as you said - then it is operating at an abnormally high temperature and is unsafe. Turn the system off immediately and call your heating service company.

Of course it could be that you're mis-reading the gauge or that your gauge is stuck and not working properly - something your heating service technician will check.

If your heater were a residential hot water heating boiler (hydronic heat) its pressure might be close to the upper end of normal operating pressure - and the leak at the valve could be due to a valve problem or to any of quite a few other causes.

See RELIEF VALVE LEAKS for procedures to diagnose and fix leaky TP relief valves. See the links just below for information on how the steam heating system pressure is controlled and set.

Steam Boiler Accidents & Explosions

Steam boiler explosions early in the history of steam boilers were often related to leaks, loss of water in the boiler, absence of safety controls like the HARTFORD LOOP and LOW WATER CUTOFF VALVE. In modern steam boiler explosions we are of the opinion that other forces might be at work, such as improper maintenance, tampering or work performed by inadequately-trained personnel.

• Two steam boiler explosions occurring in September 1867 were reported in the New York TimesOn 15 September 1867. Paraphrasing and excerpting the original news article from the Times archives:
  
  "At Springfield IL a boiler 24 feet long and 3 1/2 feet in diameter, was blown to pieces as if it had been paper; the flour mill in which it was situated was almost entirely destroyed. ... All the information that the public are likely to get regarding the cause of the explosion is the reporter's statement that the engineer was not in the room when the "accident" occurred, and that the iron of the boiler was probably defective ... "
• Schweber, Nate and William Neuman, "They Moved to Find a Better Life. It Got Worse", The New York Times, 21 December 2016. Excerpting:
  
  "... on Dec. 7. It was a “freak accident,” according to Mayor Bill de Blasio: A valve had come off a radiator in the room where the girls slept, filling the room with intense heat and steam. It was not clear how long the steam had been pouring out, or whether the girls’ cries had gone undetected. A neighbor, hearing the parents’ screams, called 911 at 12:08 p.m. Fifteen minutes later, the girls were declared dead.
  ...
  Mr. Ambrose said that he and his wife were awakened at about 6 a.m. when a valve popped off the radiator in the living room, where they slept. They replaced it, he said, but did not check on the girls.
  
  Ms. McGuire later went out to run errands, her husband said, and he fell back asleep — apparently never realizing that the radiator in the adjoining room where the girls slept had also malfunctioned. At about 11:45 a.m. Ms. McGuire returned to the apartment and soon went into the girls’ room to find them turning purple and blistering from the intense heat and scalding steam.
  ...
  Police and city officials are investigating the episode but have yet to provide a detailed explanation of what occurred, other than to say that a valve came off the radiator in the girls’ room, allowing steam to escape."

References for Steam Heating System Pressures & Temperatures

• Holohan, Dan, "A Steam Heating Primer", HeatingHelp.com, (2014), retrieved 2016/12/21, original source: https://heatinghelp.com/systems-help-center/a-steam-heating-primer/
• "BUILDING HEATING SYSTEM DESIGN" [PDF], University of Idaho, lectures on thermodynamics, retrieved 2016/12/21, original source: http://www.webpages.uidaho.edu/mindworks/Thermodynamics/Lectures/L14/Buildling%20Heating%20System.pdf
• STEAM BOILER PRESSURE SWITCH - details about the steam boiler pressure control device used on residential steam heating boilers
• STEAM BOILER TEMPERATURE - Residential Steam Boiler Normal Temperature & Pressure Ranges
• STEAM HEATING SYSTEMS & CONTROLS - home
• STEAM PRESSURE GAUGE - how to read actual pressure in a steam boiler used for building heating
• STEAM VENTS - installed on radiators must vent air to permit the radiator to get hot.
• Additional research Citations and References are at the end of the this page following key article links given at
  
  CONTINUE READING or RECOMMENDED ARTICLES.

All of the steam heating system controls described in our introduction are illustrated below and described in greater detail in individual articles found in this steam heat series.