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Sizing For Boiler Replacement?

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This question has two answers. First you need to know if you are working with a steam or a hot water system. These two systems have many of the same components, yet they cannot be treated the same when determining the correct replacement boiler size for the home.

Hot Water

No matter what the heating elements consist of (i.e. Cast iron radiators, fintube baseboard, radiant flooring, etc); hot water heating boilers are sized based on the traditional Manual J method. Over the years many older homes have been updated with insulation, new windows, and doors. These improvements have decreased the overall heat loss of the home and benefit the homeowner by being able to downsize the boiler capacity. A hot water boiler heats water, circulates that water through a piping system to the heating elements where it gives up some of its heat, and then travels back to the boiler to be reheated. The water is giving up some of this heat in the piping system, too. For this reason you want to use the NET I=B=R rating when choosing the proper boiler for your application. This rating gives you a 15 percent allowance for this piping loss.

Another benefit from these improvements is the ability to lower the operating water temperature of the heating system. Many of the older homes with cast iron radiators were designed to maintain 70 degrees in the home with 180 degree water at 0 degrees outside. With the insulating improvements to the home we are now able to lower our design water temperature and improve the overall comfort and operating efficiency of the systems. High efficiency hot water boilers are a great option for this type of application. A standard used in the industry is for every 3 degrees you can lower your operating water temperature you will save 1 percent on your energy bill!


A steam heating system requires a different method of calculation. Unfortunately, all the improvements people make to a home do not affect the actual size of a steam boiler. This is not to say that people should not improve their homes. Improvements will still reduce the amount of heat that is lost to the outside over a given time and improve energy usage. However performing a Manual J heat loss calculation on a home with a steam system will almost always lead to an undersized boiler and poor performing system.

A steam boiler must be sized based on the connected radiation (i.e., cast iron radiators, convectors, etc.). This measurement is also known as Equivalent Direct Radiation (EDR). To come up with the total EDR you must measure each radiator in the system. Using a chart, like the one on this page, we are able to calculate the EDR for a given radiator. If we have a five tube radiator that is 37-inches high we know that each section of that radiator is worth 5 EDR. If that radiator has 10 sections we have a total of 50 EDR.

You must get this measurement for every radiator in the home to determine the proper size steam boiler for the job. A steam boiler must be able to provide enough steam capacity to not only fill the radiators, but also the piping in the system. When you look at the NET I=B=R rating for a steam boiler you will see the Btu rating and

also the EDR rating. The EDR may also be stated as “Square Feet of Steam.” You need to size the replacement boiler to meet or exceed the EDR rating of your system. The NET I=B=R of a steam boiler allows for a 33 percent piping loss factor.

Piping Insulation Reminder

An important part to remember about the “piping loss factor,” is that you are assuming that the supply and return piping system is insulated! You may not notice the effect of un-insulated piping as much with a hot water

system, but with a steam system you will notice a significant change in system operation. Many of the older systems have asbestos insulation covering their steam pipes. When this insulation is removed the piping system becomes a giant radiator sitting in the basement. This giant radiator will take away from the steam

capacity required to fill the rest of the radiators in the home. You will have uneven heating, hear steam vents panting, banging sounds in the main piping, and a host of other issues. Remember if you do remove asbestos insulation you MUST re-insulate that piping!

Posted In: ACCA Now, Hydronics

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