What Size Boiler Do I Need For My House?

If you are in the market for a boiler replacement, you have likely spent some time on Google reviewing your options. However, your web search might have left you with even more questions than when you started…

What size boiler do I need? How is boiler size measured? What’s a BTU? What’s GPH?

Before we go any further, let’s take a minute to address one common mistake that homeowners make when trying to determine how big their new boiler should be. Going to the basement and looking at the sticker on your existing boiler is not the correct way to go about it. This assumes that the existing boiler in the home was sized correctly to begin with, and that’s a very big assumption!

In a lot of cases, older boilers are oversized for the homes they serve – meaning they are designed to produce more heat than the home needs, even on the very coldest days of the year. Oversizing a boiler not only means that it’s consuming more fuel than necessary, but it can even shorten the lifecycle of the appliance. The shorted lifecycle is from what is known in the industry as “short cycling,” a process in which a boiler turns on and off every few minutes in cold weather.

On the other hand, an undersized boiler is a problem, too. A unit is undersized when it’s not capable of providing enough heat for the home during the coldest day of the year. Undersized boiler installations are pretty rare, compared to oversized boilers.

Luckily for you, the installing contractor that you choose will know how to answer any questions you may have about sizing your new boiler. Our goal in this article is to help you understand how boilers are accurately sized, and why that’s so important.

Need help choosing a heating and cooling contractor? Read our blog, 8 Tips for Choosing a Heating and Cooling Contractor.

BTUs: How heat is measured

BTU is the most commonly used value for quantifying a boiler’s capacity: how much heat can this unit make? BTU stands for British Thermal Unit, and it’s a measurement of thermal energy (heat). BTUh simply refers British Thermal Units per hour.

For instance, if you are looking at a boiler with a rating of 80,000 BTUh, you can safely assume that the unit’s gross output is 80,000 BTUs, or 80 MBH (one MBH = 1,000 BTUh).

There are other, less common ways of quantifying a boiler’s heating capacity, like GPH (gallons of fuel oil consumed per hour) and sq. ft. of steam (volume of steam that a steam boiler can produce). Generally speaking though, BTUh is the standard form of measurement.

Fun fact: One BTU is the amount of energy needed to raise one pound of water by one degree in one hour.

How boilers are sized

Properly sizing a boiler is accomplished by matching the heat output (BTUh) of the appliance with the home’s need for heat (and domestic hot water load, if an indirect-fired water heater is installed).

Click here for more information on indirect-fired water heaters.

There are no shortcuts to properly sizing a boiler. Even experienced contractors cannot simply look at a home and know what size boiler it needs. The size of the home is a big factor, but it’s not the be-all-end-all. This is because factors like the R-value of insulation and the number and size of the windows and exterior doors make a very big difference.

So how does a contractor take all of these factors into account when sizing a boiler? With a heat loss calculation.

Heat loss calculations

There are a few different methods of calculating the BTUh needed by a home. They all tally the total amount of heat required. The most common method is called the Manual J calculation. This is a room-by room heat loss calculation, and typically, a software program is used to speed up the process.

The contractor starts by measuring the footprint of the home: total square feet. Add to that the height of the walls (ceiling height), number of windows, size of windows, window value (are they new and efficient or old and leaky?), thickness of the insulation in the attic, thickness of the insulation in the walls, and a number of other things.

Obviously, this takes a little bit of time, and the contractor will need to walk through the home to make the calculation as accurate as possible. This process must be completed before any boiler replacement takes place!

The final part of the equation is to select an “outdoor design temperature.” This term simply means that when designing a heating system, the professional heating contractor must account for the coldest conditions your geographic location is likely to see. For example, outdoor design temperatures in southern Maryland and northern Alaska will be very different. The exact same home in those two places will require different size boilers.

Sizing steam boilers

Sizing steam boilers is completed differently. The nature of steam boilers, and steam itself, means that the boiler must be sized for the volume of radiation in the home. Too little steam or too much steam can cause problems, ranging from a noisy heating system to an uncomfortable home.

In order to size a steam boiler, a heating contractor must determine the square foot of radiation connected to the steam system. This process is called an equivalence of direct radiation (EDR). First, each radiator in the home must be measured: height, length and width. Once the volume of all radiators is known, a boiler can be selected accordingly.

Getting the right boiler for your home

Buying a properly sized boiler is critical for energy efficiency, comfort and return on investment. U.S. Boiler Company manufactures a wide variety of boiler sizes and models to fit virtually every home in the country. The fuel source you have available, the type of radiation in your home, venting considerations and your budget are important things to consider when purchasing a boiler. Fortunately, consulting a licensed heating contractor can make the decision much easier.