Is my heating system the correct size for my house?
Just like athletes are fastest in perfectly sized shoes, your house performs best with the properly sized heating system. Oversized heating systems are inefficient and cost more, both upfront and in ongoing maintenance, than a heating system that fits your home.
Heating appliances are sized by the amount of heat they can provide in one hour. This heating capacity is measured in BTUs per hour (one BTU is the amount of energy produced by burning one match).
The heating capacity of your system should match the peak hourly demand of the house–or the maximum heat required in one hour on the coldest day of the year. For a rough estimate, a well-insulated, two-story house in Fairbanks might require 60,000 BTUs per hour on the coldest day of the year, so a heating appliance should be able to produce that much heat.
A properly sized system will run continuously on the coldest day in order to keep the home at the set temperature. This means the system is running at its steady-state efficiency, kind of like the mileage you get with your car when cruising on the highway as opposed to driving in stop-and-go traffic.
Oversized systems, on the other hand, tend to cycle on, produce lots of heat, and turn off, which not only wastes fuel (as the appliance repeatedly ramps up and cools down) but also causes wear and tear on the mechanical system (which ultimately requires more maintenance).
Wood- and coal-fired devices can also be oversized. When a wood stove is too large, residents tend to burn fires at a low smolder to prevent overheating the house, which not only uses more wood but also emits more particulates than a hot fire.
Sizing is also important when it comes to domestic hot water systems. Storage water systems are sized by their First Hour Rating (FHR), or how many gallons of hot water the heater can supply per hour, starting with a full tank of hot water. This number should be within a few gallons of a building’s peak hourly hot water demand; for example, a household of three who routinely shower at night while running a dishwasher may have a peak hourly demand of about 45 gallons. So the house would need a domestic hot water heater with a First Hour Rating of 40-50 gallons.
The size of a heating system is directly related to the efficiency of your home. So if you make any building envelope upgrades to your home, you’ll require a smaller heating system—a good reason to make upgrades before replacing your heating appliance.