If you are (and you should be!) building using tight construction methods, use your design savings on your smaller HVAC system to pay for higher-efficiency equipment and also for fresh air ventilation. Ventilation should not be an afterthought, so make certain that whatever type of HVAC system you are considering has this component included. If your area has adopted the 2012 IECC, it requires mechanical ventilation in all new homes and on any major renovation. You might think that HVAC systems (where the “V” has always stood for ventilation) include this function, but most do not. However, higher-end systems do sometimes have integrated fresh air ducting, or are integrated with separate whole-house ventilator systems.
Another method for ducting fresh air through existing HVAC systems to condition it before introducing it into the living space work by having a damper-controlled outside vent tied to a return air plenum. This simple inexpensive solution is to run a four-inch or six-inch duct from an exterior soffit to a return air plenum with a filter and damper operated by a timer that also controls the air handler. Periodically the timing device turns on the air handler, opens the damper, and voila! you have filtered and heated or cooled fresh air coming in through your furnace or air conditioner.
If you have more money to spend, consider an energy recovery ventilator (ERV) or a heat recovery ventilator (HRV). These units bring in fresh outside air and exhaust stale inside air at the same time. The two airstreams go through a heat exchanger where the incoming outside air is warmed or cooled while passing by the outgoing conditioned inside air. This recovers much of the energy used to warm or cool the inside air. The HRV only transfers heat between the two streams of air while the ERV transfers heat (or cold) while managing humidity to prepare the air for use in your home. This process of heat and humidity exchange reduces the impact of ventilation on heating and cooling loads. The HRV will greatly reduce the temperature difference between the fresh air and the conditioned air in the house by passing the two air streams over a heat exchanger. This will warm the incoming air in winter and cool it in summer. The ERV will do the same thing, but in addition it will pass the air through a device that will transfer much of the humidity in the outside summer air into the outgoing airstream. In this way, it reduces both the temperature difference and lowers the amount of humidity introduced into the home by ventilation in the summer. ERVS are appropriate for warmer, more humid climates, while HRVs are appropriate for cold climates.
Spot ventilation (kitchen and bath exhaust vents) should be vented to the outdoors in order to remove the moisture created while cooking or bathing. This is important for keeping indoor humidity levels in check, protecting indoor air quality. It is also important to note here that any large ventilation devices (exhausting 400 cubic feet per minute or more), including fireplaces and some of the larger kitchen exhaust vent models, must by code be provided with their own source of outside makeup air. There’s nothing efficient about exhausting hundreds of cubic feet per minute of air that you have used good energy to condition. The loss of this air creates negative pressure that causes the house to suck in the same amount of unconditioned air from any unsealed combustion appliances, poorly sealed building assemblies or adjacent unconditioned spaces, such as garages, attics or crawlspaces.
Hot Water Systems
The second-largest consumer of energy in most homes, and the largest in homes in mild climates, is typically water heating. Any type of storage tank water heater that uses gas or grid-supplied electric power wastes a tremendous amount of power keeping water hot 24/7 while we’re at work, at school, watching TV, playing outdoors or asleep. To determine which system is best for your family, you need to think about your family’s hot water usage patterns. As was discussed in Building & Materials Category, centrally locating the water heater within the thermal envelope and having a compact plumbing design, coupled with a high-efficiency storage tank-type system can provide very efficient operations in the average home if you try to work out a schedule where you can spread out showering, laundry and cooking activities so that you can purchase the smallest-sized system you need.
The best water heating strategies use “free” energy (such as solar or geothermal). That is, it is free once you recover the investment in the system itself. Traditional water heating appliances never offer “free” water heating. These types of systems use fuels that you must purchase (electricity or gas products) over their entire useful life. They are available in numerous sizes and efficiency ratings, so if you’re buying one you should purchase the highest-efficiency model that you can afford. High-efficiency gas-tanked water heaters still dominate this market. For these units, look for high EF (Energy Factor) ratings. Be sure to check the current ENERGY STAR rating criteria, as ratings are different for various sized equipment. Remember, it’s the energy required to keep that water tank hot all of the time that matters.
Tankless models only use fuel when the hot water is needed, so they don’t incur standby heat losses from storing hot water all the time like tanked units. Natural gas and propane are the most common. However, they usually consist of numerous elements that are fired up simultaneously to heat the volume of water demanded, so they can use a lot of fuel at once. If you have a large family, this frequent volume of energy can add up to negate any efficiency that you might have expected.
Typically, electric water heaters are not as cost efficient as gas models, since natural gas is currently the cheapest energy available. Unless you live in an area that does not have natural gas service available, and the sun does not shine enough for solar thermal, you should only consider an electric water heater as a last resort, unless you have a solar photovoltaic array and you are sizing it larger to carry the electric water heater load. This is especially true for electric tankless units, which take huge amounts of energy (often 18 kilowatts or 18,000 watts) to produce a good volume of hot water. These are only appropriate for use at remote locations that do not use very much hot water, like a handwashing sink in a workshop outbuilding. If you must use a large tanked electric water heater, make certain it has a well-insulated tank. In larger homes that require multiple water heaters of any type, efficiencies can be lost or at least significantly reduce our return on investment. Research the quality of the product that you are considering purchasing. Look for products that are constructed of durable components with a long life expectancy that will deliver years of continued efficient performance. Also, both tank and tankless gas units require frequent maintenance, which most homeowners fail to do. This can significantly reduce both the unit efficiency and life expectancy.
“Free” water heating typically comes from either the sun or from the ground. “Solar thermal” is the term used for water heating systems that include solar panels mounted on the roof strictly for the purpose of heating water for use in the home. Geothermal heating and cooling systems (ground, air or water source) can also provide water heating. The term “desuperheater” refers to the component of a geothermal system that is used to exchange the heat it absorbs with the household water heater. During the cooling season, the heat pump takes heat out of the air in your house and, instead of dumping it outside, uses it to heat the water in your water heater. This is incredibly efficient and is close to being free. It actually makes your air conditioner more efficient, too. During the winter, the heat pump uses its ability to produce heat for half the energy of electric resistance units to heat the water in your water heater at a very low cost. Neither one of these systems is literally free, but some utility rebates and tax incentives may be available to offset their initial cost. After amortizing the payback on the systems over the cost savings on the utilities necessary to provide water heating forever, the system pays for itself quickly and you do end up getting free water heating.
Solar thermal systems can also be used for hydronic space heating, such as underfloor or radiant floor heating. Or solar thermal hydronic heat exchange lines can be run through a coil at the furnace for the purpose of preheating air for space heating. Look for systems certified by the Solar Rating and Certification Corporation (SRCC) or the Florida Solar Energy Center (FSEC). These systems can be very efficient and cost effective, especially in warm or mild climates. In northern climates, however, they are much less effective in the winter, so typically they must rely on backup water heating to provide household volume requirements. Also, this type of system can be expensive, although they are also eligible for the 30 percent Federal Tax Credit through 2016.
Another viable water heating alternative may be to install a heat pump water heater (HPWH). Heat pump water heaters are air-source heat pumps that remove heat from the air to heat the water in the hot water tank. They are about three times more efficient (COP 3.0+) than electric resistance water heaters. An add-on HPWH can be used to convert an electric resistance water heater into a heat pump water heater. Be careful where you put these units, though. Since they remove heat from the surrounding air, they can serve as supplemental space cooling in the summer, but may significantly increase winter space heating loads.
