HVAC – Heating, Ventilating and Air Conditioning

In most climates, during at least some part of the year, we choose to modify our indoor climate to make it more comfortable.  In recent years, with the advent of "sick house syndrome" due to poor choice of building materials and tighter building envelopes, attention has also been drawn to indoor air quality.

There many options for heating and cooling. In "natural" or "passive" systems, buildings are designed in such a way as to take advantage of the local climate and conditions to provide for occupant comfort.  For instance, in the southern United States, plantation homes typically featured a "dog trot" hall down the center of the building to allow for easy cross ventilation.  In the southwest, thick adobe walls were used to insulate against the daytime heat and nighttime cold.  At it's most basic level, natural conditioning can simply involve placing windows in such a way as to allow cross ventilation and insulating your home appropriately for your area.  We discuss how we implemented these strategies in our home on the "Passive Heating and Cooling" page.

"Mechanical" or "forced" heating, cooling and ventilation is the default option in most American homes built today.  While we do utilize passive techniques in our home, there are times when it is simply not enough.  By first looking at passive strategies and then complimenting them with mechanical solutions, you can significantly reduce your active heating and cooling loads (and correspondingly, your utility bill).

Consider your HVAC systems carefully.  No matter what type of mechanical heating and cooling you choose, examine your occupancy patterns and consider heating/cooling zones and programmable thermostats.  For instance, our computer room/office gets very warm, so we could have skipped actively heating that area (that zone rarely turns on).  We have been considering putting in additional wall vents to transfer the heat from this room to other areas, but for the most part we simply leave the doors open and that works quite well.  Most people prefer bathrooms a little warmer, bedrooms a little cooler.  If you utilize passive solar heating, you may wish to divide the home into direct sun/no direct sun areas.  Setting your thermostats up or down according to season when you are not home or when you are sleeping can produce significant savings, as can simply tolerating a wider range of temperatures.

Forced air systems are the cheapest and most commonly installed option.  The can easily provide both heating and cooling, and if your duct work should leak it doesn't cause obvious problems like a liquid system.  The down side of forced air is that air is a less efficient heat transfer medium, meaning your operating costs over the lifetime of the system can be higher.  Forced air systems can also cause temperature swings as the system cycles on and off, reducing occupant comfort.  Air ducts can leak, further reducing system efficiency.  Uninsulated duct work running through unconditioned areas is a common cause of poor system performance.  Air ducts can also present a ready breeding ground for dust, mold, mildew and other allergens.

Radiant in floor heating is comfortable, quiet and potentially less allergenic, but has a much higher initial cost.  It also does not provide cooling.  After living with radiant heating for the last two years, however, I would have a difficult time living with forced air again.  The radiant heat provided to our home via passive solar, the masonry stove and the in floor radiant system simply feel so much better than any heating system I have lived with in the past.

Wood heating of any kind requires time and wood.   Time is not free and unless you have the trees, wood is not either, also you will pay for gas or electricity most likely when cutting the wood. For urban folks, seriously consider geothermal before you jump into wood, pellet or other systems, as they require regular homeowner interaction.  Even “passive” solar requires that you open the blinds when its sunny and close them when its dark (if you don’t close the insulating blinds you can loose more heat than you gain).  Geothermal systems can be used with forced air and in floor radiant, although in floor applications are much more expensive.  Geothermal systems can also potentially be used to provide water heating, but usually not all water heating.

Electric resistance heating should be used as an option of last resort, at least by my standards, because you're using high grade electricity to produce low grade heat.  It is usually relatively cheap to install but has a very high operating cost.  Maybe it would be worthwhile in a cooling dominated climate as emergency heating, but in locations where you have a real heating load I wouldn't advise it.

For ventilation, again you can utilize natural or mechanical options.  When building or remodeling, materials should be chosen to minimize outgasing and potential sources of indoor air pollution, but the activities of daily life produce heat, humidity, carbon dioxide and other particulates and gases that may need to be ventilated from the home. Mechanical ventilation can either be point source, such as dryer vents or exhaust fans, or whole house, such as Heat Recovery Ventilators, Energy Recovery Ventilators or whole house fans.

We chose to use Panasonic ventilation fans with timers in all bathrooms.  These fans are extremely quiet and provide excellent ventilation.  The timer allows the user to choose adequate time for ventilation while preventing over ventilation due to fans being left on and forgotten.  (You don't want to leave the fan on indefinitely blowing out heated or cooled air.)  The dryer and the range are located on outside walls so they were easily direct vented to the exterior.  Long ductwork runs on the dryer or range can lead to the build up of dust and grease in the ducts, posing a potential fire hazard over time.

We also have a Fantech Heat Recovery Ventilation system, or HRV.  This unit is used to vent stale indoor air and bring in fresh outdoor air while reclaiming heat from the indoor air and use it to preheat the incoming outdoor air.  We use this in the middle of winter when the house is tightly sealed to maintain proper humidity levels and bring in fresh air as needed.  The HRV transfers heat only; an Energy Recovery Ventilation system also transfers water vapor.  Heathandenergy.com provides an excellent overview of the benefits of HRVs and ERVs and a map of North America showing recommended ventilation strategy for your area.

In general you want to have a supply and return duct for each bedroom and one for each common area. Do not locate returns within 10 feet (3.05 meters) of an oven or cooktop, as vaporized grease could clog the heat exchanger core and contaminate the ducts. Locate the HRV or ERV itself in an easily accessible area and as centrally as possible to all of the rooms. A sewer drain or condensate pump may be necessary for removal of water that the core extracts from the air during cold weather. Locate the outside air intake as far from driveways, clothes dryer and furnace vents as possible. Keep all supply and exhaust vents at least 10 feet (3.05 meters) from each other. 

Links

HVAC - Detailed information from Wikipedia.

9 Tips to Reducing Your Heating and Cooling Bill - From Energy Efficient Homes.com

Whole House Ventilation Systems from Toolbase.org