Steam FAQ

Most commonanlly asked questions about steam boilers & steam systems

All drawings are conceptual drawings and may be incomplete. More specific drawings may be required for your application.

When a steam boiler is replaced should it be sized by a heat loss or measuring the radiation.

Steam boilers, unlike hot water boilers should never be sized by a heat loss unless you are going to change the size of the radiation. Changing radiator size is not a common practice due to cost. To size the steam boiler measure the height of the radiator from the floor to the top, count the number of columns or tubes, and count the number of cast iron sections to make up the length of the radiator. All the radiators will have to be converted to sq. ft. of steam. One you have calculates the sq. ft. of steam for all radiators add them all together. This is the total sq. ft. of steam that is required to heat the radiation. Look at the boiler brochure and match the square feet of radiation to the boiler square feet rating in the manufacturer's literature. Never add anything to the total square feet measurement of the radiation for piping. The manufacturer already deducted 33% of the square feet of steam available for system piping losses.

What's the difference between a one pipe and two pipe steam system?

It is the number of pipes connected to a radiator. One pipe steam system will have one pipe connected to the radiator and two pipe systems will have two pipes connected to a radiator. The two pipe systems will normally have steam traps connected to the return side of the radiator.

One Pipe Steam System

Two Pipe Steam System

I've been told to drain the boiler over the summer or flush the boiler every summer, is this correct?

No! You really never want to completely drain or flush the boiler annually. This would only be done when there is service to be performed that would require draining. Every time you add water to the boiler to add oxygen, minerals, and chemicals all of which will shorten the boiler life.

I have a one pipe steam system and some of my radiator shut-off valves are wide open and some are partially open. Is this OK?

No, all radiator shut-off valves in a one pipe steam system should be closed all the way or wide open. Do not have the valve partially open.

What is the difference between a wet return and dry return?

The return piping is the pipe that carries the condensate back to the boiler from the system. The location of the piping determines if it is a wet return. If you look at the steam boiler you will see a sight glass, which gives the water level in the boiler. All the system piping below that water level will also have water in it. We call the return piping below the water level a wet return, as it is full of water. Any return piping above the water level is called a dry return, as it is not full of water all the time.

One Pipe Dry Return

Two pipe wet return

I heard a lot of terms for steam boiler near boiler piping. What are they?

Below is a drawing of near boiler piping and a list of names and functions.

Boiler Riser(s) - Delivers the steam from the boiler to the boiler header. There may be more than one boiler riser.
Boiler Header - Collects all the steam fome one or multiple boiler risers.
System Riser - Delivers the steam from the boiler header to the system piping.
Equalizer - Equalizes the pressure between the top and the bottom of the boiler.
Hartford Loop - Was invented before Low Water Cut-off controls. If the return would leak all the water would not drain all the water out of the boiler. The boiler would still have water up the Hartford loop tee. The Hartford loop would be installed 2" below the normal water level(NWL).
Condensate Return - Return water back to the boiler from the system as the steam condenses.

What is the purpose of a Hartford loop?

A Hartford loop was first designed to avoid all the water draining out of the boiler if a leak occurred in the wet return line. The old systems long before low water cut offs were invented, were piped at floor level and straight into the boiler. If a leak occurred the water would completely drain from the boiler and the coal fire would keep burning. The cast iron boiler sections proceeded to get overheated, with no heat in the building. Someone would go to the basement to add water. The cold water hit the overheated iron and there was an explosion. The Hartford loop will pipe from the boiler end of the wet return and go vertical to 2" below the normal water level. If there is a wet return leak, the water would only drain as low as the Hartford loop and keep a substantial amount of water in the boiler.

What is the purpose of an equalizing loop?

An equalizer or equalizing loop is to have the same pressure on the supply side of the boiler as the return side of the boiler. As pressure builds on the supply side of the boiler the pressure goes through the header and some goes to the system and some goes into the equalizer. In the equalizer there is water as high as the water level in the boiler. The pressure pushes down on the water. With the same pressure on the supply as return sides of the boiler the water stays in the boiler.

What is meant by an "A" dimension?

The "A" dimension is factored in when we have a one pipe steam system and is the vertical distance from the Normal Water Level (NWL) listed by the manufacturer to the lowest horizontal carrying steam main. The steam main will run around the basement and all your radiation will be piped off the steam main. Normally the "A" dimension should be 28" minimum. This 28" measurement is important for getting return water back into the boiler and avoiding a noisy operation. It is important to know that 28" of water in a vertical pipe equals 1 psi. When measuring the "A" dimension, measure down from the ceiling not up from the floor. The ceiling will be more level than the basement floor in most cases.

The boiler will normally operate no more than 2 psi. Some boilers are operating in ounces of pressure, but the majority is in pounds of pressure. If the boiler is operating at 1.5 lbs. which means there is 1.5 pounds of pressure on the supply and 1.5 pounds of pressure on the return sides of the boiler the water from the system will not enter the boiler. The water in the wet return must exceed the pressure in the boiler to allow the return condensate to enter the boiler. This is where the importance of the "A" dimension comes in. Remember the "A" dimension is the vertical distance from the NWL to the lowest point of the horizontal steam carrying main. This is normally the other end of the basement. The wet return will run around the basement and turn up to connect to the steam main. The "A' dimension will occur in the vertical pipe connecting the wet return to the steam main. When the boiler is not running the water level in that vertical pipe should be the same level as the boiler. When we build steam the steam in the radiation and steam main will condense back into water. It will run along the main and collect in the vertical pipe ant the end of the steam main. As the water stacks it will increase the pressure in the return due to water weight.

So how does the condensate get back into the boiler? As the boiler builds pressure the pressure at the boiler is going to be greater than the pressure at the other end of the system, keeping the return water out of the boiler. As the water stacks and the pressure in the main is pushing down on the stacked water, the pressure where the return enters the equalizer becomes higher than the boiler pressure and allows the return water to enter the boiler.

What is a "B" dimension?

The "B" dimension is like the "A" dimension except it is used when we have two pipe steam systems. The "B" dimension as also calculated differently. We are still interested in the distance from the NWL to the lowest steam carrying main. Instead of 28", it is now 30" per pound of steam in a system. If the system were operating on 2 psi we would need to have a vertical distance from the NWL to the lowest point on the steam carrying main of 60" or 5'. Since most basements do not allow that kind of height, we use traps with gravity return or pumped returns.

How important is venting a steam system?

Steam system efficiency is affected by venting. All the vents must work. On a one pipe steam system you should have a main line steam vent about 6" from the end of the steam main. The vent should not be located at the very end of the main. The purpose of a main line steam vent is to vent the main as rapidly as possible. This will aid in balancing the steam system. The idea of a main steam vent is to get steam to the end of the main as quickly as possible. This may require more than one main steam vent. After the steam gets to the end of the main the vent closes, and the radiator vents now have to vent the radiator and vertical piping. Any radiator larger than 25 to 30 sq. ft. should have 2 automatic air vents installed. The slower the system vents the slower the system heats. Good operating vents saves fuel dollars.

What is the purpose of a main line vent?

The main line vent is to vent the main piping on a one pipe system (one pipe to the radiator). It is important as far as fuel efficiency and even heating of the steam system. The main line vent must vent the main rapidly to allow the radiator vents to vent the radiators evenly. If all the air is removed from the main piping and the radiator vent has to vent only the vertical piping and the radiator it is easier to balance the system. I have installed 2 or even 3 main line vents on a single main.

When I have a one pipe steam boiler replaced should the main line vents be replaced and where should they be located?

Absolutely, the main line vent must work well for an efficient steam system. It is wise to add more than one if the main pipe is venting poorly. They should be a minimum of 12"-15" from the end of the main, never in a tee at the end of the main. The vent could even be placed after the last riser to the last radiator off the horizontal main.

My radiators air vents are only halfway up the radiator. Is this correct?

Yes, the air vent should be located half way up the steam radiator. The vent on a hot water radiator should be at the top. Since the steam vent will close when steam get into the vent, it would not vent as much air if it were at the top. When steam enters a radiator it wants to go to the top of the radiator immediately, thus closing the valve prematurely.

I have a few air vents that will release steam during the boiler operation. Is this normal?

No, the airvent is supposed to close when steam hits the vent. If the vent is venting steam you are losing water from the boiler. This would mean that when you need to add more water that you are adding oxygen, minerals and chemicals all which are detramental for the boiler and steam system. The more water you add to the boiler the shorter the boiler life.

I am a firm believer that all the vents be replaced at the time of the installation. I realize it is expensive to do but, it does make the system work more efficiently. The vent on a steam radiator should be about 1/2 the distance from top to bottom of the radiator. I would double vent that radiator if the radiator is larger than 25-30 sq ft of steam. The second vent should be lower than the original vent. You may hear a radiator vent hissing somewhat but not real loud. If you can hear it 5 or 6 feet away I would wonder if the main line vent was working properly or is large enough.

If the venting isn't good can that cause a problem long term?

Yes, simply put when your boiler is boiling the water this creates carbon dioxide gas. If this does not get vented out of the system as quickly as possible and it mixes with condensate it will turn into carbonic acid. The acid creates condensate grooving in rerun lines and you will also see deterioration at the treaded end of wet return lines when disassembled. It is important that all air vents are working and properly sized.

Does the main horizontal steam line need to be insulated?

The entire horizontal steam main should be insulated. We need to keep steam in a steam state as long as possible. Many years ago, homeowners removed asbestoes insulation from the steam main for health reasons, others removed partial insulation to get soem heat to certain areas of the basement. We need to insulate all of the main steam pipes and it is also a good practice to insulate the near boiler piping and the takeoffs upto the floors. I also believe we should insulate all steam lines in unconditioned spaces as well as around the boiler.

We pay to create steam. We must keep it in a vapor state as long as possible. The steam must go up to the radiators to heat the home. If the steam main is not insulated the black pipe gives off the heat generated by the steam. A first this may not sound like a bad idea until we look a little deeper. We create the steam and the steam leaves the boiler and goes into the steam main. If there is no insulation the piping gives off about 212 btu's per foot of 2" black pipe. The steam cools down and condenses into water. The condensate works its way back into the boiler. We must re-heat the water to 212f. We than have to change state from a liquid back into a vapor. It takes 970 btu's to convert 212f water to 212f steam. That is about 5 times more energy than it would take to heat water from 32f to 212f. Someone has had the main pipe insulation removed to heat a certain part of the basement. All this steam is condensing back to water, which needs to be brought back to 212f water, and then converted back to steam. Insulate those pipes and keep the steam as a vapor as long as we can.

Another problem that can happen with removing pipe insulation from steam mains is a condition we call water hammer. This sounds as if there is actually someone beating the pipes with a hammer. It occurs when there is too much water in the horizontal mains and steam glides across it. Insulate those horizontal main steam pipes for fuel savings and quieter operation.

How important is insulation on the steam pipes in the basement?

Very important for multiple reasons.

1. The steam produced should remain in a steam state as long as possible
2. A steam main with no insulation the steam produced will condense back into water quickly
3. This increases operation costs $$$
4. More condensate in steam mains will increase the chances of increasing system noises (water hammer)
5. Creates slower heating due to steam collapsing to condensate
6. Makes balancing a 1 pipe steam system very hard
Theres a lot of talk about steam going to condensate, what negative affects does this have. When the boiler gets a demand for heat it must raise the water from whatever the water temperature is in the boiler to 212F. But, you do not have steam yet, you have 212F hot water. To change the state from 212F water to 212F steam the boiler must add 340 btu"s per pound of water. To put this into perspective that would be 5 time the amount of fuel it would take to raise water temperature from 32F to 212F.

If I should not remove pipe insulation how should I heat that basement?

Heat the room with condensate or boiler water. Have the contractor add a section of baseboard in the area you want heated. He can put it on a thermostat so you can use it as needed or maintain a room temperature. This will cost more money to install but you have the control. It will also operate much cheaper than removing insulation. Basement areas are very easy to heat as they are normally below grade. This drawing shows heat and hot water being made by the boiler water.

How does a two pipe system vent?

A two pipe steam system uses traps which open when cooler and closes when steam hits them. The air in the radiators are vented very rapidly right through the radiator trap. There is normally a larger trap at the end of the main to vent the main and a large vent on a dry return or an open pipe at the condensate pump.

How do you balance a one pipe steam system?

Many people believe a one pipe steam system is balanced by adjusting the vents on the radiators closest to the boiler vent slower than the vents on the further radiators from the boiler. This could not be further from the truth. The location in the system has very little to do with balancing of the system. Balancing is done by the radiator size. First verify the main has good venting. Next adjust the larger radiators to vent faster than the smaller radiators. This makes more sense as the larger radiators have more air to vent. Remember, if the radiator is too large, usually larger than 30 EDR, double venting may be required.

After a steam boiler is installed does the boiler water require cleaning even if the water looks clean?

Yes, even though the boiler water looks clean enough to drink, we are not concerned with how clear the water is. We are concerned with oils that have been added to the water by manufacturing, pipe oil, flux, etc. The oil floats on top of the oil and must be removed. This requires skiming.

Why is the water level in the gage glass bouncing a lot and even disappears at times?

This is called surging. After the installation of a steam boiler there will be plenty of oil in the water between manufacturing the boiler, pipes and installation. This will cause surging of the water which is a violent bouncing of the water while steaming. This causes the water to leave the boiler out of the supply tapings. This is resolved by skimming the boiler. Skimming will remove the oil from the surface of the water. The oil is lighter than the water, so it floats on top of the water. The steam has a difficult time breaking through the oil but when it does it leaves at a rapid speed. The velocity increases as it finds the supply tapping and heads up the pipe. This creates a suction and pulls boiler water up with it. Some water goes to the header and a lot of it falls back into the boiler thus the rise and falling of the water level. Usually what you see in the gage glass is not as bad as what is actually happening in the boiler.

Skimming is getting the water almost steaming than draining the surface of the water off to a bucket. A small amount of water will need to be added back in while skimming. Do not skim too fast. I suggest the skim stream on a residential product to be no larger than a wooden pencil. The slower the better. Normally this does not require any chemicals. If the use of chemicals is required see manufacturer's suggestions.

The water glass on the side of the boiler has two valves, should they both be open?

Yes, they both need to be all the way open to see the water level as it changes. It is a good idea to verify both the tapping's are clear of debris as they will block occasionally. The level of the water should always be kept at the normal water level (NWL) suggested by the manufacturer.

I have an automatic water feeder on my steam boiler. Do I still need to check the water level?

Yes, the automatic water feeder does not maintain the water level at the suggested Normal water level (NWL). The water level should be maintained by you and the automatic water feeder is there in case you are not home to check the water level. The automatic water feeder will than feed enough water to keep the boiler running until you can check it and bring the water level to the proper fill amount.

For more information on automatic feeders on a steam boiler go here

Do the newer electronic low water cut-offs need maintained as the old float type did?

They do not require draining as the older float type did. The older float type needed to have water drained from them about every two weeks to keep the float chamber clean. This verified the float would be able to move and shut down the boiler if the water level dropped too much. The water should be drained until it ran clear. Do not drain any more water than needed. The newer electronic probe type still requires annual maintenance. They should be pulled out every year and the probe, the part that is submersed in the boiler water, should be cleaned, reinstalled, and tested.

What should the pressure be on my steam boiler?

Most residential cast iron radiator systems will be operated at a cut-out pressure of 2 psi and a cut-in pressure of 0.5 psi. This means the boiler will run until it meets the cut-out pressure of 2 psi and the burners shut down. As the steam condenses the pressure drops in the system to the cut-in pressure of 0.5 psi the burners will fire back on. When adjusting steam pressure, you must check the control. Some of the differential pressures are added differentials and others are subtractive differentials.

It is becoming more common today to work with lower pressures. Some systems will work on ounces of pressure. The higher the pressure the more fuel it takes. Residential systems with cast iron radiation should never need to be operated above 2 psi. The higher the pressure the greater the fuel bill. The fuel bill will grow in exponential growth. If more pressure is required to heat the radiators check the venting.

I hear you should always replace wet returns when the steam boiler is replaced. Why?

The wet return is always full of water to the height of the water level in the boiler, so there is a lot of rust buildup. This slows down the returning condensate to the boiler. If the boiler has an automatic water feeder it could cause the boiler to overfill. When the old boiler is disconnected have the installing contractor check the return lines. If the water can't get back to the boiler in the proper time frame the auto feeder will add more water and may flood the boiler. This could also end up with water backing up into the horizontal mains which will cause water hammer. The worst-case scenario is the water backs up into the system and leaks out of your radiator vents and onto your beautiful floor.

If you do not have an automatic water feeder the boiler will shut down on a low water condition. As the water slowly works its way back to the boiler through all the rust in the return piping it will come back on. This will create very uneven heating.

What about dry returns? Do they need to be replaced with the boiler?

Normally not. They are not full of water and normally are exceptionally clean many years after they were originally installed. They do not need to be insulated.

I have heard that steam systems are supposed to be noisy. Is that true?

No, steam systems should be quiet. The pounding noise is called water hammer which is normally caused by steam finding puddled water in a horizontal pipe. This can be remedied. It just takes proper near boiler piping to get dry steam (98% dry), insulation on all the basement steam carrying mains, proper use of pipe hangers to avoid sagging mains and proper near boiler piping.

I hear the term dry steam. What does that mean and why is it important?

Dry steam just means that most of the excess moisture has been removed by the steam. This happens when the steam goes vertical out of the boiler at least 24" to drop out the heavy moisture. As the steam travels through the near boiler piping elbows and piping it removes as much moisture as possible.

What is the importance of dry steam?

When we talk about dry steam it usually is 98+% dry. We have removed most of the excess moisture. Dry steam moves much faster in the system than wet steam. Wet steam also gives up the latent heat quicker which means it turns back into condensate sooner. This will cost you more to reheat it and turn it back into steam again. Wet steam uses more fuel to heat the home, takes longer to satisfy the thermostat, causes water hammer (noises) in the system and the boiler to short cycle.

Can I connect an indirect water heater to my steam boiler?

Yes, although this is simpler on newer equipment than older equipment. Most manufacturers’ offer the tapings for the indirect water heater supply and control. The tank return will normally just return back to the boiler return taping. I would suggest the contractor follows the manufacturer’s instructions when this is done. Their installation manual will normally show how to do this. Here is a general piping diagram showing baseboard and an Indirect water heater

The control would utilize the taping "L" and the indirect supply pipe would utilize the taping.

Disclaimer: The information found on this website is for informational purposes only. Any and all preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website as it should be reviewed by a professional.