Published: August 13, 2015 - by Ray Wohlfarth

Categories: Commercial Heating

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Designing a commercial boiler replacement where you were not the original designer or installer could have some hidden dangers  that may haunt you for years.


The project is going to be a series of assumptions, educated guesses, design skills, and some good old fashioned luck. The boiler is only part of the heating system, albeit an integral part. Always assume that the existing boiler is installed incorrectly. When designing a replacement low pressure steam boiler, it should be sized according to the existing connected load. The connected load includes all piping, radiators, and heat emitters. Unlike the steam boiler replacement, a replacement hydronic boiler system will be sized according to the building heat loss. I contend that we need to think holistically when designing or replacing a hydronic system as there could be hidden liabilities inside the system. This may not affect the sizing of the boiler but the existing system can influence the design and type of boiler we use.

Irrefutable Rule of the Radiator

This rule of thumb simply contends that that if the original system was designed using 180 degrees as the supply water temperature and nothing has changed in the building, we must assume that we still need 180 degrees at outdoor design temperature. This means that the original boiler was sized to maintain 70 degrees F inside the building at the outdoor heating design temperature using a supply hot water temperature of 180 degrees F. Most hydronic systems including terminal equipment were designed using that temperature. This, again,  is still an educated guess. We cannot be sure what the original designer used as the design temperature or what the installer did when they installed the system. Can you see how the  assumptions are entering into this design already? There is a way to test our assumptions. When performing a building heat loss, calculate the heat loss of a couple rooms. You could then compare the heat emitter size. For instance, if your room heat loss is 500 Btuh, check to see the Btu rating of the radiator. A good rule of thumb is that a hot water radiator will emit about 170 Btu's for every square foot EDR or Equivalent Direct Radiation. This is based upon a room temperature of 70 degrees F and an entering water temperature of 180 degrees F. There are several sites including that will show you the Btu capacity of old cast iron radiators with various entering water temperatures.

Ask questions

When tasked with designing a replacement boiler, I ask questions of the person responsible for the existing boilers.  The first question is where the problem areas are in the building. Always assume that there is a problem area in the building. If the client does indeed have a problem area, you need to address it when you are designing the new system. You will own all the problems in the building once a new boiler is installed. One such building owner told me of an area that had never heated properly. After tracing the piping, we found that the 3 way valve was installed backwards twenty years ago. On a call for heat, the valve would bypass the radiator and not heat the room. The owner had installed extra heaters to provide heat for that section of the building. If the client has two or more boilers, ask if one boiler will heat the building. If not, at what temperature does the second or lag boiler start. Some facilities, like hospitals use 100% backup sizing for their boilers. Typically, most commercial systems with two boilers have each boiler sized between 66-75% of the heat load.

The owner will also be able to inform you if any changes have been made to the building shell. This may include new windows or if the walls were insulated. If that is the case, we may be able to heat the building using lower supply water temperatures. In some instances, we see that some of the heating load may have been removed from the boiler. A packaged rooftop unit may have been installed and since the rooftop has heat, the boiler load is reduced. Conversely, what if the building has a higher heat load than when it was designed? During the energy crunch of the 1970's, some buildings were designed using 5 CFM of outside air per person for ventilation. This is far lower than the current the ventilation requirements now in place. On another project, the gaskets surrounding the outside air dampers for the unit ventilators were in disrepair or missing all together. The dampers could never shut all the way, increasing the amount of ventilation air for the space. Building usage could also have changed since it was first installed. The building heat loss could be much different than when the building was first constructed.


One of the first tasks is to perform a heat loss on the building to see what size boiler we will require. If the original blue prints are available, these could be a great help. It may show the system design temperature that the original engineer chose. When performing your due diligence, look at the settings on the existing boiler. On one project, we saw that the boiler was set for 200 degrees F. When I asked the owner about it, he informed me that it had always been set that high. The original designer called for a system design temperature of 180 degrees but the project came in over budget so the owner asked the installer to "value engineer" the project to reduce the installation cost. The contractor chose to base his design on 200 degrees as the design temperature instead of 180 degrees F. This allowed him to use smaller pipes as well as emitters, lowering the installation cost. If the boiler was operated at 180 degrees at the design temperature, it would not have been able to keep the building warm at design conditions. Our new system had to be designed using 200 degree water at the outdoor design temperature. Condensing boilers were not a good fit for this project due to the elevated system temperatures.

Check the piping size for the building. On a school project, the existing boiler was 2,000,000 Btuh and the piping was sized for about half that. Our heat loss showed that the system piping was correct and the boiler was grossly oversized. The owner told us that the boiler cycled constantly, even in cold weather. 

Clock the fuel usage. This will allow you to know if the boiler is actually firing at the rated capacity. A boiler room with a 2.5 mil Btuh boiler was piped with a 3/4" gas pipe. We clocked the gas meter and found that the boiler was actually firing at less than 900,000 Btuh or about 36% of the firing rate. There was evidence of flue gas condensation due to the under fired boiler. The surprise was that the boiler actually heated the building adequately at the lower firing rate.  

The pump capacity should also be verified. If the existing pumps are too small, the delta T across the boiler is excessive. This could cause damage to the boiler and void the warranty. If the pumps are oversized, the velocity is too high through the boilers and piping. This could lead to noisy operation and increased heating costs.  I suggest replacing the pumps when the boiler is replaced. According to ASHRAE, the life expectancy of a pump is less than that of a boiler.

Look around

Many gas train components require venting to the outside. Unfortunately, this is something that is often overlooked. The components that require venting, such as gas pressure switches, usually contain some thin membrane that prevents the natural gas inside from escaping. On a system with low pressure gas (less than 1 psi), a 1/4" fitting can leak enough gas within an hour to fill a 10' x 10' x 10' room with a combustible mixture. The outdoor termination should contain a bug guard as spiders and bees seem to be drawn to the pipe and will build nests to block the pipe. This will lead to a "No heat" call.

Asbestos is another concern with old boilers and the piping. It was common to use asbestos on the pipe insulation. This may have to be removed or encapsulated if piping changes are done. Some old boilers also used asbestos inside the boiler. You would  commonly see it between the cast iron sections or where the flue hood attached to the boiler.  The refractory inside a boiler may also contain asbestos.

Three-way valves could be very problematic on some systems. It could lead to thermal shock in the boiler which causes extensive damage to the boiler. Thermal shock occurs when the temperature rise through the boiler is greater than the manufacturer recommends. A common rule of thumb is that most boilers are designed for a 20 degree rise. Thermal Shock causes rapid expansion and contraction in the boiler. Most manufacturers require a blend pump if three way valves are installed. A blend pump takes hot water from the boiler supply piping and injects it into the return pipe to raise the boiler return temperature and lower the Delta T of the boiler. The blend pump may require 10-20 pipe diameters upstream of the boiler return opening to allow proper mixing. This could require extensive re-piping inside the boiler room.

All fuel-fired boilers and water heaters require air for proper combustion of the fuel.  When using natural gas, it is typically 12 -15 parts of air for every part of gas. The combustion air should be verified that it is sufficient for the equipment. If the boiler uses a direct connection to the outside for the combustion air, care should be used when setting the fuel to air ratio of the fuel burning appliance. If the boiler is started in moderately warm weather, the fuel to air ratio will change as the combustion air gets cooler. Cooler air is more dense and contains a higher oxygen level in the burner, decreasing the boiler efficiency. On the other hand, if the boiler is started and adjusted in the cold weather, it could have insufficient air when the weather is warmer as that air will be less dense or contain less oxygen. Another consideration with combustion air is if there is an air conditioning unit in the boiler room. If that is the case, the boilers may require direct venting of the combustion air or a refrigerant monitoring system in the room.


Be wary of the existing flue when considering a replacement boiler. If you plan to reuse the existing flue, make sure that it is the proper type and size for the new boilers. For example, if the new boiler is to be vented with a Category 1 vent that is negative non condensing and the existing vent was a Category 3 which was positive non condensing, I have heard of the flue collapsing due to the negative conditions in the flue. Another consideration is if you are planning to abandon the existing chimney, check to see if the existing water heater vent is still inside the old chimney. This is referred to as an "orphan" and will have to be addressed. This may require something from installing a chimney liner to completely changing the flue.

When sidewall venting, extreme caution should be used. There are many regulations that have to be followed when sidewall venting. Even if you heed the requirements and comply with the codes, you may still have an issue. An local engineer designed a new boiler room for a nursing home that vented through the wall and the design met the code. On a cool, rainy Fall day, the flue gases gathered next to the nursing home and were drawn into the building, sickening some of the residents. The building was evacuated and the designer and installer had to pay for a new stainless steel flue from the boiler room up three stories. The job bankrupt both of them.

Installation of new stuff

In many facilities, the boiler room was built around the boilers. If that was the case, the old boiler would have to be cut up to be removed. The designer or installers will have to fit the new boilers into the existing room. Will they fit through the door? A designer in my area had to cover the cost of removing a block wall to install the new boiler as he missed the sizing of the access doors, an expensive lesson. Another consideration is spacing between the boilers. In my state, boilers are required to have 30" between each boiler. This should be considered when choosing a boiler. Some boiler rooms require space above the boiler as well. The spacing may range from 2 to 7 feet above the boiler, depending on the boiler type. There is a listing of these clearance requirements in the International Mechanical Code.

Teflon tape When designing or installing a new boiler, Teflon tape or pipe dope is commonly used on pipe threads.  When used on fuel oil or natural gas piping, it could void the warranty or cause a dangerous condition. Please see the following warning in the installation manual of a Webster Engineering model JB burner:




It seems clear cut that they do not want you to use products containing Teflon. It could void the warranty and all responsibility falls on the installer and designer.

Boiler startup

When the boilers are started, the fuel to air ratio is adjusted at startup. Typically, the boiler startup is in the early fall so the boiler will be ready for the heating season. To properly adjust the fuel to air ratio, a burner should operate at least 15 minutes to allow the flame to stabilize.  In most instances, the boiler will not be able to operate that long before heating the loop and shutting off on the operating or limit control. I would recommend that the boiler fuel to air ratio be re-checked and adjusted during the middle of the first heating season. It would be best to do it during the coldest part of the winter so the boiler will operate long enough to get a proper adjustment. This could be part of the job specifications.

Good luck and be careful of the hidden dangers inside the old boiler rooms. 

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