you ask a dozen people what the proper operating pressure for a
steam system is, you'll probably get a dozen different answers.
Most folks just follow "what they were taught" without giving
much thought to the results. You see most steam systems run at
ridiculously high pressures.
As early as 1900, residential
boiler manufacturers decided that no house heating steam system
should operate at a pressure higher than two psi. They could
make this statement because it's latent heat, not steam pressure
that does the actual heating work in a residential system.
Latent heat is the
energy we put into water to get it to change state from a liquid
to a gas. In the early 1800s an Englishman named Thomas Tredgold
coined the term British Thermal Unit. He defined the BTU as the
quantity of heat needed to raise the temperature of one pound of
water (about one pint) one degree Fahrenheit.
For instance, suppose
we had one pint of 32 degree water (water can exist as a solid
or a liquid at 32 degrees. Did you know that?). If we wanted to
raise that pint of water to 212 degrees we would have to add
about 180 BTUs of heat. That would give us one pint of water,
not steam, at 212 degrees (You see water can also exist as a
liquid or a gas at 212 degrees).
But how do we get that
pint of water to change state and become steam? We do it by
adding a great deal of latent heat. You know the old saying, "A
watched pot never boils?" Well, it's certainly true because to
make that pint of water turn into steam we have to add 970.3
Think of it. It only
took 180 BTUs to get that pint of water to rise from 32 degrees
to 212 degrees. But it took more than five times the heat (970.3
BTUs) to get it to move from 212 water to 212 steam. There was
no change in temperature, but there sure was a change in the
This energy is latent
heat; it's what heats the house. We get nearly all of it back
when the steam condenses in the radiators. Steam has the ability
to heat when it's at zero psi pressure. You see you don't need a
lot of pressure to heat the building. All you need is latent
To prove this is true,
consider this: If you add only 10 more BTUs of latent heat per
pound of steam to zero psi steam, you'll wind up with steam at
10 psi. That 10 additional BTUs is insignificant when it comes
to heating the building, but it can cause us many problems with
the system. As you'll see.
The job of steam
pressure is strictly to overcome the friction that steam meets
as it works its way around the system. All we have to do is
supply enough pressure back at the boiler to overcome the system
And the pressure you
need is remarkably low because years ago, fitters sized pipe to
offer very little resistance to steam flow. In fact, we measure
this pressure in ounces per 100 feet of piping. This is why
boiler manufacturers decided so many years ago that all you need
is two psi to operate any house heating system. Raising the
pressure higher than two psi will only cause you problems
because steam is a gas. When you raise the pressure of a gas,
you compress it. Just think about what happens when you put air
in your car's tires.
Steam is a gas, just
like air. When you compress it, it just naturally takes up less
space. The amazing thing is that it also begins to move more
slowly. It's not as "large," so It can afford to move more
slowly. Strange as it may seem, it takes longer for
high-pressure steam to get out to the radiators than it does for
low-pressure steam. Also, high-pressure steam, since it's more
tightly packed, will call more water out of the boiler than
low-pressure steam. This can lead to low-water problems back at
Steam travels across a
system because of a subtle difference in pressure. Besides
friction, the fire in the boiler and the condensing of the steam
in the radiators also leads to a difference in pressure
throughout the system. The fire creates the initial pressure.
Since all the air vents are open, the inside of the piping
system is at atmospheric pressure (which is 14.7 pound per
square inch at sea level and different in other parts of the
country). Steam begins to move from the higher pressure in the
boiler to the lower pressure in the system.
But as soon as it
begins to move, it also begins to condense into water. This is
because the pipes are cold and the steam is hot. When steam
condenses into water it leaves a partial vacuum in its place.
The condensing process causes this vacuum.
This is a fine point
you've probably never thought about. Steam occupies about 1,700
times the volume of water. That means that if you filled an
eight-ounce glass with water and boiled it, you would have to
have 1,700 eight-ounce glasses available to catch the steam! A
pint of water, once boiled, balloons out to fill a cubic yard!
It's like popcorn.
This also means that
when steam condenses in the radiators it will shrink to 1/1700th
of the space it occupied as steam. What we're left with (as long
as the air vents remain closed) is a partial vacuum.
This is good because
it makes the steam travel to where you need it up in the
radiators. This is why you don't need pumps to move steam. All
you need is a subtle difference in pressure.
Now think about this.
As the radiator heats, the condensing rate in that particular
radiator will slow, right? In fact, it will eventually reach a
point when very little steam is condensing. The metal will have
reached steam temperature; the room will have reached the
setting of the thermostat. Itís nature's job to equalize
temperature as well as pressure.
And this is also a
fine thing because it allows the steam to travel on to the next
radiator down the line. The boiler's job is simply to get steam
(a gas) out to the last radiator before it turns into water (a
liquid). If the boiler is too small for that task, the building
will be partially hot and partially cold youíll wind up with
You see when youíre
working with steam heat, youíre really watching a race between
the steam and the cold pipes. If the boiler is properly sized,
the steam will win that race. This is why we size replacement
steam boilers by measuring the radiators. As strange as it may
seem, the heat loss of the building is not important. Only the
"race" matters. We have to "fill that steel balloon" (the piping
system) with steam before it can condense into water. As far as
the replacement boiler is concerned, it doesn't matter if the
home owner insulated every nook and cranny and replaced all the
windows in the house. If the piping and radiators are there, you
have to fill them with steam. It's as simple as that.
Don't make the mistake
of sizing the new boiler by taking the information off the old
boiler. The person who did that sizing may have been wrong. Or,
someone may have removed or added radiators over the years.
Don't take a chance; do it right.
And keep in mind, too,
that there's a safety factor you have to add to the net
radiation load to allow for the heating of the pipes. We call
this the "pick-up" factor. Nowadays, we allow an additional 33%.
Years ago, that safety factor was much larger, but boiler
manufacturers gained experience over the years and reduced it to
where it is today.
This "pick-up" safety
factor is the difference between the Net Ratings (the actual
radiation load) and the DOE Heating Capacity Rating (the
radiators and the pipes). The firing rate of the boiler should
match the DOE Heating Capacity rating of the system (thatís
piping plus radiation.
Let's take a look
at some of the other changes manufacturers have made to boilers
in recent years.
The importance of the
Piping Around the Boiler
As boilers became
smaller, the piping around them became more and more important.
Today's replacement steam boiler contains much less water than
the boilers of yesteryear. And yet the new boiler produces just
as much steam as the old boiler! Modern oil burners and improved
boiler design make this possible. But if you want that job to be
successful you have to pay careful attention to the boiler
manufacturer's near-boiler piping specifications. Ignore them at
your own risk!
The purpose of the
piping specification is to give you a boiler that delivers dry
steam. Dry steam contains a great deal of latent heat. If you
add even a little moisture to the steam by piping the boiler
incorrectly (and letting the water leave the boiler with the
steam), the latent heat content of the steam will suffer. The
steam, in essence, will condense in the moisture before it has a
chance to reach the radiators. In short, the steam will lose the
"race" to that last radiator and parts of the building will be
And not only will the
building heat unevenly, the fuel consumption will also increase
because the pressuretrol will never reach its high limit. And to
make things worse, you'll probably also have water hammer.
Thatís the knocking in the pipes that people who donít know any
better think of as normal. Follow the boiler manufacturer's
instructions to the letter and youíll avoid most of the common
problems associated with steam. Here are a few of the things the
boiler manufacturers will tell you to do:
- Allow at least 24
inches between the center of the gauge glass and the bottom of
the steam header.
- Use full-size
risers to the header.
- Pipe the system
take-offs at a point between the last riser to the header and
- Pipe the swing
joints into the header. Use a reducing elbow to connect the
header to the equalizer.
You'll probably also
see a section on how to clean the boiler after you've worked on
it. There's really no way around this; all steam boilers must be
cleaned after they're installed. You don't necessarily have to
do it immediately, but you do have to do it. It often pays to
let the system run for a few days before you go back to give it
a good cleaning. Waiting a few days gives the oil and dirt a
chance to settle on the surface of the water.
There are many
opinions on the best way to clean a steam boiler. One of the
oldest ways is to dissolve a pound of tri-sodium phosphate (TSP)
and a pound of caustic soda (lye) in water and pour it into the
boiler. Let it cook for a few hours and then drain the boiler.
If you can't buy TSP in your town, try a commercial soap called
MEX. It works well and will not damage the rubber gaskets found
in some boilers. However, before you clean any boiler, check the
manufacturer's instructions for their recommendations.
Skimming the boiler is
the best way to remove surface oil. You'll know there's oil in
the boiler if you see any moisture at all in the gauge glass
above the water line. Many technicians are tricked into
believing the water is clean just because it appears to be clear
in the gauge glass. But they're in for a surprise because oil
can be colorless in boiler water. The part of the gauge glass
above the water line should be bone dry. It should look like
someone just ran a dishtowel through it.
If you have a surging
water line and there's moisture in the gauge glass try cold
skimming the boiler. You do this by opening a horizontal tapping
above the water line and installing a six-inch nipple. Open the
feed-water line slowly until the water level rises to the center
of the nipple and spills out. Don't be in a hurry. If you rush,
you'll be skimming from below the surface of the water and
Let the water run
slowly from the skim port for several hours. Check it
periodically by taking a sample of the water and boiling it on
the customer's stove in a small pot. If there's oil in the
water, the water will foam when it boils.
Keep skimming and
checking until your sample boils like tap water; that's when you
know you're done. Remove the nipple and start the boiler. In
most cases, your surging problems will become just a bad memory.
Skimming from the top
of the boiler doesn't work as well because the rising water will
cling to the metal before it has a chance to get out of the
boiler. Draining from the bottom of the boiler doesn't work as
well as horizontal skimming either for the same reason.
Firing a small boiler
while skimming is ineffective because the surface oil will be
emulsified in the water. Just think about what happens to the
oil you add to a pot of boiling water before you drop in that
pound of spaghetti. Oil doesn't stay on the surface when the
water is boiling. This is especially true in a highly efficient,
skimming works pretty well most of the time, if the boilerís
been up and running for a while.
Let's take a look at
several different types of steam systems.
One-pipe steam takes
its name from the single pipe that connects each radiator to the
steam main. Both steam and condensate travel in this pipe, but
in opposite directions. This is what often makes one-pipe steam
so difficult to manage. When steam and condensate travel in
opposite directions (what we call "counterflow") you have to pay
close attention to the size and pitch of the pipes. For
instance, when steam and condensate move in the same direction
(thatís "parallel flow") the pitch should be at least one inch
in twenty feet. When there is counterflow, however, the pitch
must be at least one inch in ten feet. See? It doubles.
The exception to this
is when you have a horizontal run-out to a radiator riser. Here,
the pitch should be at least one-inch inch per foot. Where you
can't get this pitch, (or when the horizontal run-out is longer
than eight feet) you have to go to the next-size pipe.
The rules are fairly
simple, but few people take the time to learn them. That's why
you wind up with so many radiators that bang and so many air
vents that spit. If you're adding or removing radiators, get
some advice from a reputable supplier of steam specialties.
They'll be able to help you out with the pipe sizing and pitch.
Let's take a look at
the basic controls on a one-pipe (and a two-pipe) system.
determines the operating range of the boiler during the heating
cycle. It's important to understand that a heating boiler
doesn't make steam all the time. It only does that when the
thermostat clicks on. During a call for heat, the boiler will
cycle up to the cut-out setting of the pressuretrol. At that
point, the pressuretrol will shut off the burner.
show the cut-out setting as "Differential." Usually, you'll add
that "Differential" to the "Cut-In" setting to get the "Cut-Out"
setting. Be careful, though, because some pressuretrols show
"Differential" as a number to be subtracted from the cut-out
setting. Take a few minutes to read the instructions and think
about what the manufacturer is telling you.
When the pressuretrol
reaches its cut-out setting, steam will be moving out into the
system and condensing in the pipes. This condensing process will
cause an overall drop in system pressure. When the system cycles
down to the cut-in pressuretrol setting, the pressuretrol will
re-start the burner, as long as the thermostat is still calling
for heat. If the thermostat isn't calling for heat, the burner
will remain off, and the steam pressure will drop to zero
Usually, you should
set the pressuretrol to turn the burner on at one-half psi and
off at the lowest possible pressure required to heat the
furthest radiator. If that pressure winds up being more than 2
psi, something is probably wrong. Most likely, the air vents
aren't working properly.
Years ago, fitters
used vaporstats to control the boiler. These are like
pressuretrols, but they're much more sensitive. A vaporstat
measures pressure in ounces. They're still available today, but
they're more expensive than pressuretrols. Nevertheless, along
with quality air vents, a vaporstat is probably the best
investment you can make. You see when it comes to steam, low
pressure is the key to success.
If you're concerned
about the burner because itís short-cycling, look to the air
vents, not the pressuretrol. Main vents are the key here. Get
rid of the air and the building should heat without short
also require a manual-reset, high-limit pressuretrol to shut off
the burner should the pressure rise too high. Make sure you
install this with the operating pressuretrol, but not on the
Speaking of which, you
pipe the pressuretrol to a steam pigtail so youíll have a water
seal between the control and the boiler. The water protects the
control from the steam temperature and extends its life.
Obviously, you should not have a valve between the boiler and
the pressuretrol. If the pigtail clogs (which it will!) replace
it with a new one. If youíre burner is short cycling, it may be
because the pigtail is clogged. Check it out.
The relief valve
protects the boiler against a runaway fire. On space-heating
steam boilers the relief valve is set to pop at 15 psi. This is
the limit for any low-pressure boiler.
The relief valve
should be rated by the American Society of Mechanical Engineers
(ASME, for short), and you should size it for the maximum load
of the boiler. For safety, pipe it to a drain, or to within a
few inches of the floor.
It's not a good idea
to pipe the relief valve to the outdoors because, should it pop
off, water will be held in the pipe by vacuum, much as water is
held in a straw when you put your finger over one end. During
the winter, the trapped water in a relief line thatís piped to
the outdoors can freeze and block the escaping steam as surely
as a pipe plug will. Thatís dangerous! If you must pipe the
relief valve to the outside, use a vacuum breaker at the
discharge of the valve. This will allow the water to drain the
water from the line after the relief valve has popped. Itís best
to avoid this all together if you can, though. And naturally,
there should never be any valves between the relief valve and
the boiler or the relief valve and the drain line.
The low-water cutoff
is required by code. Its job is to shut off the burner should
the water level fall to an unsafe point. The boiler manufacturer
determines this level, but it's usually within one-half inch of
the bottom of the gauge glass.
The low-water cutoff
can be a float-type or a probe-type. Probe-type low-water
cutoffs are becoming very common on low-water-content boilers
because these cutoffs have timing devices to prevent nuisance
shut-downs should the boiler water surge. Probe-type cutoffs
send a low-voltage charge through the water to ground on the
boilerís metal. Don't use a probe control without first getting
the boiler manufacturer's recommendations as to where they want
mount directly on the gauge glass of the boiler and sense the
movement of the water line mechanically. The low-water cutoff
manufacturer determines where the cutoff belongs. You should
never tamper with these settings.
Some installers try to
make the boiler more "automatic" by raising the low water cutoff
so that it covers the domestic water coil all year long. This,
they think, will save the homeowner the trouble of raising the
level by hand during the summer. But itís a bad idea because it
also creates a "normal" water line that's several inches too
high. It brings the boiler water too close to the steam outlet
and drives water up into the system. Before you know it, you
have more problems than you bargained for. Save yourself a
headache and have the customer cover the tankless coil by hand
once a year.
The gauge glass is
your way of knowing where the water is in the boiler. Expect to
see some minor movement in the water line. Anything between a
half- and three-quarters of an inch of up-and-down movement is
When the boiler is
off, the "normal" water line is the center of the gauge glass.
When the system is running, the "normal" water line is near the
bottom of the gauge glass. That's because the water, in the form
of steam and condensate, is out in the system. When the burner
shuts down, the level will return to the center of the gauge
glass again. Don't try to keep the water in the center of the
glass when the system is running because, obviously, this will
cause the boiler to flood when the condensate finally returns on
the down cycle. Again, this is why you shouldn't tamper with the
low-water cutoff level.
The automatic water
feeder (if youíre using one) is there to maintain a safe minimum
water line. It is not there to maintain a "normal" water line
when the boiler is off.
A water feeder will
protect the system against freeze-ups if the people are away in
the winter and, say, an underground return should spring a leak.
Without the feeder, the low-water cutoff would shut down the
burner and the house would freeze up.
So, while itís not
essential to the system's operation, you can consider an
automatic water feeder a useful back-up safety device. In
addition, a feeder will provide some convenience in an old
system that's prone to leaks. The feeder will maintain an
operating water level rather than have the burner shutting down
daily on low water.
If the customer
doesn't want his leaking, buried returns replaced, an automatic
feeder makes a lot of sense. But naturally, a great deal of
fresh feed water can also harm the boiler through oxygen
corrosion. Think about this when you're advising the customer.
Give them the facts and their options. Then, leave the decision
Let's take a minute
now to define some terms.
A wet return is any
pipe that's below the boiler water line. A dry return is any
pipe that's above the water line.
The header is the
large horizontal pipe directly above the boiler. You have to
size it to carry the entire steam load of the boiler. Nowadays,
the boiler manufacturer will often oversize the header so it
acts as a point of low velocity. That gives the steam a place
where it can slow down and dry out before it heads out into the
system piping. Always check the boiler manufacturer's
requirement on header size before you install a replacement
steam boiler. You'll often find that the old header is too small
for the new boiler.
Risers are the pipes
between the boiler and the header. They must be the full size of
the boiler tapping. Donít reduce them because youíll cause the
steam to move too fast. When that happens, the steam will pull
some of the water out of the boiler and throw it into the system
Many of the newer
boilers call for two (or three!) risers to the header. The older
boiler may not have needed as many. If you go with the old
piping and ignore the manufacturer's instructions for the new
boiler, the new boilerís water line might wind up tilting at a
severe angle. That can lead to very wet steam and, in many
cases, a broken boiler because the flame will be licking at the
boiler's exposed crown. Without water to carry off the heat, a
boiler can crack.
If the boiler has more
than one outlet, it's also important to remember to pipe the
headers with swing joints. If you don't, the boiler sections can
be split wide open like an accordion when the horizontal header
heats and expands.
If you have such a
boiler with more than one outlet (and swing joints) you
shouldnít use copper instead of steel for your header. This is
because copper expands twice as much as steel. That can cause
the soldered joints to come apart and leave your customer with
steam leaks. Consider, too, that when you use copper in a steam
system there will be more corrosion than normal because of the
dissimilar metals. Copper, steel, and iron lead to corrosion at
the places where they come together.
Take-offs are the
pipes connecting the header to the system. You probably won't be
changing these. The original installer sized them to handle the
connected load. Sometimes, someone adds radiation to the
existing take-off, and you should watch for this because it can
cause you service problems. The take-off might not be able to
carry the additional heat on a cold day. Any reputable
manufacturer of steam-heating equipment will be able to check
the size of the take-off against the connected load and advise
manufacturer determines the size of the equalizer. Its job is to
return any water that slips out of the boiler with the steam,
and also to balance the pressure between the supply and the
return sides of the boiler. Without a properly sized equalizer,
water can back out of the boiler.
Never pipe a steam
take-off over the equalizer. The steam's velocity can create a
pressure drop in the equalizer that will lift the water up,
causing a corresponding drop in the boiler's water line.
In 1919, the Hartford
Steam Boiler Insurance and Inspection Company invented the
Hartford Loop. Its job was to prevent water from leaving the
boiler, should a return line spring a leak.
The connection between
the loop and the equalizer must be made with a close nipple to
prevent water hammer. This is because steam is forming in the
Loop connection. Returning condensate can cause this steam to
rapidly condense and shrink to 1/1700th its steam volume. The
water rushes in to fill the void. As the condensate slams
against the back of the tee, you wind up with water hammer in
The dimension labeled
"A" in the diagram represents the distance you have to maintain
between the center of the gauge glass and the bottom of the
lowest dry return in the system. In one-pipe systems that have
DOE Heating Capacities greater than 100,000 BTUs, "Dimension A"
must not be less than 28 inches.
"Dimension A" provides
the force that puts the condensate back in the boiler. Without
it, water will back up into the horizontal piping and shut off
the take-offs to the radiators. The house will heat very slowly
(if at all), and certainly very unevenly. You'll probably also
have water hammer.
And this is also why
you don't see main vents on many of jobs. They're installed
improperly and get damaged on the first few cycles. That's a
shame because main vents are the key to good one-pipe steam
operation. If you're using good main vents near the ends of
every main, steam will travel very quickly to every radiator in
the building. Vent large radiators quickly and small radiators
slowly no matter where they are in the building. Focus on the
air content of the radiator rather than its location in the
building. If your main vents are working, steam will arrive at
each radiator at about the same time.
Baseboard over three
feet long has no place in a one-pipe steam system. In most
cases, you can never get the pitch or size you need to keep the
air vent from spitting water up at the ceiling. If you must use
baseboard, connect it with two pipes, vent the outlet side, and
drip the return pipe immediately into a wet return. Do not use a
steam trap; just drip it into a wet return. Pitch the baseboard
run toward the return as much as you can.
Let's take a look at
another type of system now.
two-pipe steam takes its name from the number of connections
youíll find at the radiator. As heating jobs got larger in the
old days, fitters found it made good sense to have just steam in
one pipe and just condensate in the other. That way, each pipe
could be smaller and the pitch of the pipes became less crucial
because everything was moving in the same direction.
In a two-pipe steam
system, the steam connection is usually at the top of the
radiator; the condensate connection is at the bottom on the
opposite side, but this doesnít always have to be the way. You
can also have the inlet and outlet at the bottom of the radiator
at opposite ends. Or you can have the inlet at the top, and the
outlet at the bottom, on the same side of the radiator.
At the turn of the
century there was a type of steam system called the two-pipe,
air-vent system. This system had two pipes (and two supply
valves) at opposite ends of the bottom of the radiator. Since
this system didn't use steam traps (they hadnít invented them
yet!), both pipes carried steam. It worked because of the air
vents. One pipe was always larger than the other. The larger
pipe was always on the inlet side of the radiator. When the
steam traveled through the system, it favored the larger pipe
because thatís where the least resistance to flow was.
But this was an
expensive system to install because there were twice as many
pipes as a one-pipe system, and it offered an advantage only
when the radiators were very large. With a large radiator, you
have a lot of condensate flowing backwards down the one-pipe
supply line. That can create water hammer.
The two-pipe steam
system died an early death and has been obsolete for many years.
But there are still many of them around. They were popular in
municipal-type buildings such as schools and courthouses. If you
see two supply valves at the bottom of the radiator, you
probably have one of these old systems. Be careful. This system
is easily confused with true two-pipe steam. However, it works
differently and can cause quite a few problems should you make
certain piping changes to the system.
True two-pipe steam
uses thermostatic steam traps on the radiators. The steam trap
has three jobs:
- It opens to let
air pass through the radiator and into the return piping.
Air is a great insulator. If left in the radiator, you'll
have a cold room. Also, when water boils, it releases a
great deal of carbon dioxide because of carbonates and
bicarbonates that are common to fresh water. This carbon
dioxide will travel through the system and mix with
condensate to form a mildly corrosive carbonic acid.
Naturally, this acid is harmful to the radiators and also to
the returns. This is another reason why good main air vents
are so important to steam systems. You have to get rid of
that carbon dioxide before it mixes with the condensate. So
steam traps are actually air vents as well!