Centrally ducted forced air systems could be used for heating, cooling, or both. These systems have two parts: a) the appliance, which creates the hot or cold air using either a combustion fuel, heat pump, or A/C evaporator coil, and b) the distribution part of the system, made up of a blower, which carries treated air around the house through trunk and branch ductwork, and the return ducts, which return an equal amount of air to the heating/cooling appliance. In order for the systems to perform well, it is critically important for the feed and return air flows to be balanced. When comfort is uneven, poorly balanced airflow design is often at fault.
Ductwork can account for 20-30% of energy loss according the U.S Department of Energy. (By comparison, a three to five percent increase in furnace efficiency is a HUGE deal!) That 20-30% waste of energy isn’t just unnecessary, it’s expensive so check out these ways you can improve the performance of your ductwork.
Forced hot air
In the early part of the 20th century, forced hot air systems were state-of-the-art technology that replaced individual room heating. These systems were fueled by coal or wood and required stoking about eight times per day. It wasn’t until oil became widely available in the late 1920s that central heating became automated and people were relieved of laborious rituals to keep warm, but heating continued to be very bad for greenhouse gas pollution.
Today, these systems are often converted to gas, which is also surprisingly bad for the climate crisis when you count the greenhouse gas effects of methane and CO2 combined. Liquid propane (LP) is about as bad as heating oil or jet fuel; natural gas is as bad as coal. Replacing an oil or gas furnace with one that runs on low-cost, renewable, automated (no bags/hands free) wood pellet heating fuel saves between six and nine tons of carbon emissions per year for every home that can be converted. By comparison, adding solar panels saves about two tons and switching from a conventional internal combustion vehicle to a hybrid saves about three tons.
There are a number of advantages to running your forced hot air system with a boiler instead of a furnace.
Keep in mind that a furnace only makes hot air but a boiler can do many things. It can make hot air for your existing ductwork through the use of a hydro air handler and heat your hot water with an indirect hot water maker. (See indirect hot water makers. Special note: the linked page from energy.gov shows only an oil-fired example. Indirect hot water makers can work with any kind of boiler, including those that run on gas or wood pellet fuel.)
Boilers can transmit their heat to a wide variety of emitters. They can melt your driveway snow and provide individual heat zones for renovation additions or cold rooms. This could have an important impact on your decision-making process if you build an addition and would like the option of a radiant floor, for example.
Hot water through a 3/4 inch pipe transfers 3500 times more heat than a standard hot air duct, so it’s a much more efficient way of getting heat from the heating appliance to the rooms around the house.
When upgrading your forced hot air system, you can also add air-conditioning through an air-conditioning coil or centrally ducted heat pump. A heat pump also gives you the option of running your heating system on electricity rather than a combustion fuel during the shoulder seasons. Keep in mind when adding air conditioning to a forced hot air system that ductwork needs to be insulated and drained because condensation can form when cold air is passed through a warm, humid space. If not properly insulated and drained, mold conditions could develop.
Centrally ducted forced air heat pumps
These systems provide cooling in the summer and heating in the winter down to about 10-30°F outside temperature, depending on the heating capacity of the system and the building’s heat loss. Heat pumps can have a very low carbon footprint especially when paired with on-site photovoltaic, net-metered solar panels. If the heat pump is drawing power from the local electric grid, their carbon footprint will depend on the energy mix of the local utility. Utilities are required to publish a disclosure statement detailing the fuels and emissions used to create the electricity they send to the grid.
In geographic locations where wintertime temperatures regularly drop below freezing and the heat pump can’t keep up with the building’s heat loss, consider adding a hot air furnace in line to the existing heat pump/blower/ductwork. Even a small number of cold days can equate to a high percentage of annual heat need, cost, and pollution. When adding a furnace to an existing heat pump system, skip the gas and oil furnaces and instead add an automated wood pellet furnace. The comparative life-cycle emissions are about half, and while the initial cost to purchase is high, the comparatively low cost of wood pellet fuel will save tens of thousands of dollars over the life of the system.
Centrally ducted air conditioning
Forced air systems in warmer climates are often air conditioning only. Climate change is bringing with it temperature extremes that are both hot and cold. Heating in temperate climates can be very expensive. Consider replacing A/C-only central air with a heat pump. This will provide heat down to temperatures above 30°F at least and will lower your electric bill for cooling.
Centrally ducted air purification
One of the advantages of a forced air system is the ability to improve indoor air quality with a HEPA filter or electrostatic filtration. Keep in mind the filter should be monitored and replaced both for efficacy and system performance.