Only in the last hundred years or so has it become common to expect a uniform, easily controllable indoor thermal environment in a middle-class home. Heating came first. In the upper mid-west of late nineteenth century America the new technology gave rise to the open-plan spaces of Prairie School homes, by contrast to the cellular, individually heated (or not) rooms of the European tradition. Air-conditioning followed, taking root as central systems in Southern homes from the 1960's on, fueling major population growth in the sun-belt states in the latter part of the last century.
With such a relatively brief history it's no surprise we're still learning how to do it right. Traditional construction systems worked fine for homes which were only sporadically heated and never artifically cooled, but they need to be re-examined as our expectations of comfort continue to rise, and as the disastrous consequences of our haphazard global and domestic energy policies are beginning to take effect. We've written about methods to improve the thermal enclosure of our homes here and here, but now we'll give a quick overview of currently available heating and cooling systems (HVAC). We'll start with the various flavors of heat pumps as these are most common in our area.
HEAT PUMPS - CONVENTIONAL
These have been a popular option for many years as a single mechanical component (the compressor unit) supplies both heat and cooling, conveyed to the distribution system (air handler and ductwork) by a a paired loop of Freon lines. Early installations with high-velocity ductwork were problematic in the heating cycle because of the wind-chill effect but these issues have long since been remedied. The standard unit is air-to-air, meaning an air-source compressor unit using ambient exterior air as its energy sink and using ducted air as its distribution medium to all parts of the house. Conventional air-source heat pumps begin to lose their heating efficiency as the outside temperature approaches 30° or so, which means they must rely on expensive-to-run electrical resistance heat strips or gas flame supplements in cold weather - this has restricted their use in colder climates. We can expect these systems to dramatically improve in efficiency over the next decade as a result of competition from….
HEAT PUMPS - MINI-SPLIT
The new wave of mini-split heat pump systems from manufacturers in Korea and Japan have vastly improved low-temperature performance and boast cooling efficiencies of up to 26 SEER, by comparison with conventional systems which currently max out at about 17 SEER.* These are commonly ductless units - the air handler is located in the main room of the home and there is no conventional distribution system, though natural air movement can be supplemented by small separate circulation fans contained within the thickness of separating walls. These units provide a very high overall performance efficiency and go very well with large single spaces like studios and with small well-insulated homes.
HEAT PUMPS - GROUND SOURCE
Ground-source heat pumps (also commonly referred to as geothermal) have a higher efficiency than conventional air-source units but they are also much more expensive to install. They require either a deep well or a large ground loop for groundwater heat exchange so they are not suitable for all circumstances. They have experienced a recent surge of popularity because of very generous federal and state tax incentives but they are otherwise hard to justify economically.
GAS FURNACE
If piped natural gas is available to your lot a gas furnace is an economical heat source. Gas furnaces fired by propane however are not economical to run and should be avoided. Gas furnaces can be paired with an external a/c compressor to use the same internal air handler and distribution system to both heat and cool the home. We do NOT recommend external gas-pak combination units which combine furnace, a/c compressor and air handler on the outside of the home, feeding to a ducted air distribution system on the interior, those are very wasteful of energy and are hard to justify when better options are available.
RADIANT FLOORS
We've talked so far about options for heating and cooling a home with an expectation of a conventional air distribution system of some kind, the most common option in our area. But let's take a quick look at hydronic (water-borne) systems. These are most commonly implemented by circulating water through tubes buried in a concrete floor slab or by stapling the tubing up under a regular wood-framed floor. The heat source for that circulating water is often a gas boiler (note our caveat above about the high cost of propane compared with natural gas) but other options are possible. The nice thing about a radiant floor is that everyone likes warm feet on a cold morning, right? However, if you have a really well-insulated tight home the heat will be running only sporadically, just as often as required by the limited heat loss of the enclosure. In between those times it will cycle down to room temperature. With a wood floor system this will not be a particular problem, but a radiant slab at the bottom of its heating cycle will feel just like any other piece of concrete, i.e. cold to the touch. A radiant slab will also bring up the issue of thermal lag: in certain circumstances high thermal mass can be your friend, but if you're used to quickly punching up the thermostat when coming home after a winter absence you may not be happy with the slow response of the system. It should also be mentioned that if you're considering a hydronic system you'll need a completely separate system for air-conditioning, which inevitably adds to cost.
ENERGY SOURCES. We've not discussed the environmental impact of the primary energy source of these various systems. Needless to say they all come with significant environmental cost and it's not a simple matter to definitively balance out the impact of one approach against another. This is what we know: in North Carolina electricity largely means coal and nuclear, each highly problematic in its own way. Natural gas has a lower greenhouse gas impact than coal, but continued use means fracking and the strong possibility of serious groundwater contamination. PV and wind power will change the picture for electrical power over time, but they will still be minor players for the forseeable future (I hope I'm wrong). As this picture gradually becomes clearer, the least and the best we can do is to strive to use the most efficient mechanical systems we can afford in the best-designed, not so big, most superbly-insulated homes we can manage.
*SEER is the efficiency rating of the cooling unit of an a/c or heat pump system: the standard of performance has risen from about 10 SEER a dozen years ago to 14 SEER or more today (higher is better).