The energy savings and increased comfort far outweigh the upfront cost of an energy-efficient HVAC system.
AT THE HEART of most home heating systems is a furnace, a boiler or a heat pump. A furnace burns fossil fuel to heat air that’s forced by a blower fan through a series of ducts to the living spaces; a boiler heats water that’s then pumped to a hydronic, or water-based, distribution system. Most heat pumps run on electricity. They don’t create heat, but rather extract it from the air or the ground. Heat pumps are available for use with forced-air and hydronic distribution systems. If you want to minimize your fuel bill, an Energy Star rating is a minimum standard for these appliances.
Gas Furnace: Super Efficiency
A modulating gas furnace is the most technologically advanced fossil fuel-burning furnace you can get, with efficiencies as high as 97 percent (that’s the percentage of the fuel’s potential energy delivered as heat). It achieves this feat with a series of technical innovations. Gas is not as clean as we once believed, however, now that dirty methods of “fracking” are used in some extraction. The pollution may simply be happening at the front end of the process. No fossil fuel gets an environmental free ride. It’s a finite resource, not a renewable one.
One way that new technology squeezes more heating power from gas is with an exhaust heat exchanger. This exchanger steals back heat from the furnace’s waste byproducts. Another feature, flame modulation, adjusts the flame size, based on demand. These furnaces include variable-speed blower fans with high-efficiency electric motors. The ability to vary airflow and flame intensity also allows for nearly constant room temperatures and better air circulation.
High-Efficiency Boiler: Hot Water Wiz
A boiler burns oil, natural gas or propane to heat water. That heated water is then pumped through a system of pipes to radiators, baseboard heaters or a radiant floor.
A good boiler will offer efficiencies of 90 to 95 percent and will include many of the same technologies as a high-efficiency furnace. These include a modulating burner that matches the heat output to whatever the thermostat is calling for at the moment, advanced heat exchangers to extract more heat from the same amount of fuel, and the ability to recover heat from the exhaust gas by condensing it. The resulting exhaust is cool enough to be vented out of a plastic pipe. In the best cases, this condensing process can squeeze 10 to 12 percent more usable energy out of the fuel.
Air-Source Heat Pump: Reverse Motion
A heat pump is basically an air conditioner that works in reverse to provide heat. The heat pump captures and concentrates heat from one area, then releases it to another.
In heating mode, the heat pump takes heat from the outdoor air and delivers it to the home’s distribution system. In cooling mode, it reverses direction to work like an air conditioner, extracting heat from inside the house and blowing it outside. In cooling mode, this waste heat can also be used for water heating. The most common type is the split system, which uses separate indoor and outdoor units, but you can also get packaged systems that place everything in a self-contained outdoor unit. While air-source heat pumps can offer efficiencies of 200 to 300 percent, they’re most efficient in the southern part of the country. A backup electric or gas heater may be needed when the outside air drops below a certain temperature. Although different versions are made for forced-air and hydronic distribution, the forced-air type is the most common. Heat pumps don’t get air as hot as a furnace or boiler, so may require more airflow to maintain the same temperature.
Geothermal: Earth Energy
A geothermal heat pump (GHP) uses refrigerant-filled underground piping loops, installed horizontally or vertically, to exchange heat with the earth. These systems work well in both warm and cold climates. A good GHP is able to move three to five times more energy than it consumes. Models are available for use with forced-air or hydronic distribution systems. While the hydronic models don’t get water as hot as a conventional boiler (122 °F, compared to 150 °F or more) their low temperature output is a perfect match for radiant floor heat.
Domestic Water Heating: Multiple Options
Water heating accounts for 15 percent of your home’s energy use. Most homes have reservoir-type water heaters. In these, gas, propane or electricity heats water that is stored in a 50- or 80-gallon tank. The unit maintains the water at a set temperature, often between 120 and 135 degrees, so you have hot water when you need it. If it is time to replace your unit, and you want another tank-type model, be sure to select a high-efficiency Energy Star-certified model. Better yet, consider a “hybrid” electric heat pump water heater, such as GE’s Geospring. These water heaters extract heat from the surrounding air to pre-heat water, and are up to 65 percent more efficient than conventional tank-type heaters.
A tankless water heater is another option. As the name implies, these don’t waste energy keeping water hot in a storage tank—water that may not be used for hours; instead, water is heated on demand, when it is needed. Gas condensing heaters are even more efficient, because they capture the heat that would otherwise be lost in the exhaust gases and use it to heat water via a heat exchanger. Tankless water heaters are a natural fit with radiant floor heating.
Recirculating pumps are another way to boost efficiency. In larger homes, if the heating unit is far away from faucets and showerheads, you can waste a significant amount of water (as much as 40 gallons per day) waiting for the hot water to reach its destination. Recirculating pumps move water from the source and back again, creating a continuous loop, so no water is wasted. The pump is activated by a motion sensor and shuts off automatically.
Finally, “smart” technology is making water heaters more efficient by allowing demand response. This means that the unit can be programmed to delay water heating during periods of high demand on the electrical grid. Smart water heaters can also be managed from afar, so they can be turned down if you are going to be away for several days, for instance, and turned back up shortly before you return.
Mini-Split Heat Pumps
Ductless mini-split heat pumps, or MSHPs, are a good heating and cooling option for homes without duct systems. Mini-splits can supplement an existing wood stove or condition a new addition, and they are also ideal for super-efficient new homes. Mini-splits have two main components: an outdoor compressor/condenser and an indoor air-handling unit. A conduit, which houses the power cable, refrigerant tubing, suction tubing and a condensate drain, links the outdoor and indoor units. MSHPs are quiet, efficient and flexible, and because there are no ducts, they are easy to install. Here are a few points to consider if you are in the market for a mini-split:
- Choose the Right Mode. MSHPs work best when allowed to modulate. Using them in “on/off” mode not only compromises their efficiency, but will likely result in wide temperature swings and discomfort.
- Know Yourself. Because MSHPs don’t rely on ducting to distribute air, be aware that your habits may affect temperatures in different rooms. For example, if you tend to leave your bedroom door closed all the time, it may be warmer or cooler than the rest of the house.
- Build Efficiently. The number of MSHPs required to heat your home depends on its square footage, but also on your home’s layout. A small, very efficient two-story house might be served by just one MSHP, but be aware that there may be temperature differences between the two floors, even if you use distribution fans.
- Bigger May Be Better. Oversizing MSHPs—choosing units rated for a larger house—is okay. Oversizing can even be beneficial, since MSHPs modulate their capacity and reach their highest efficiency when running at the lower end of their capacity range.