The available array of options for heating and cooling your house is significantly larger than it was just a few years ago. Heightened awareness of energy efficiency and new, related techologies, have given us options that not only provide a much greater degree of thermal comfort, but do so at less cost and most importantly provide the homeowner with monitoring tools to increase awareness of how these systems can be optimized.
The tried and true (and not so efficient): forced air systems.
These systems are really centered around their ductwork. Of course there is a furnace and air handler or fan, to push the hot or cold air around the house.
Pros: This system uses the same air handler to push both the cooling and heating requirements around the house in the same ducts. This greatly decreases the cost of the system and this lower cost is what makes these systems the default solution for most houses. And all the floor-mounted grilles are a great place for losing stuff.
Cons: maybe too many to list. There is draftiness with all the air blowing around and the noise of fans and heat exchangers cycling on and off. Multi-speed fans and zoned ductwork greatly increase the efficiency of these systems but come with often considerable cost increases. The ductwork running around the house has to be coordinated with the structure to avoid too many unsightly dropped soffits all over the house and the proper sealing of ducts is critical to maintaining the systems functionality.
The luxe (but with some complications): radiant heating
Hydronic radiant heat and fresh air systems with something for cooling (see cooling systems below) are quite nice and deliver heat to the lower half of the room via the floor, where most activity occurs. The system combines a boiler, usually gas-fired, and an extensive system of plastic tubing embedded in the floor to heat up the floor surface. This heating is greatly enhanced if the floor's mass is increased by encapsulating the tubing in a bed of concrete.
Pros: the system is quite efficient and perfectly quiet. It retains its heat well and nothing feels quite so nice on a cold day as warm floors. The system of tubing can be zoned without too much trouble.
Cons: the initial installation cost, especially with the concrete subflooring, can be considerable. Accommodation has to be made for small manifolds that usually mount in wall cavities with access panels for each zone. The overall system can be slow to heat up and cool down depending on the thermal mass of the flooring system. The performance of the system is also a bit dependent on floor surfaces and finishes. And of course this is a heating-only solution to the HVAC problem.
1. air source heat pumps - inexpensive and a bit loud, they are good for moderate climates, not so good when temperatures drop below 40 degrees F.
2. ground source heat pumps - not dependent on outside air temperature, but relatively expensive to install and often slow to respond to changing temperatures.
3. solar panels - hydronic and photovoltaic - these are really energy systems not truly HVAC equipment, but their use along with radiant flooring or other systems can greatly reduce purchased energy usage. These vary widely in costs with changing rebate programs and can have significant visual impacts. And of course to be efficient you need to have good year-round solar gain.
4. evaporative cooling - in low humidity areas, simply blowing air over water and into the house will drop interior air temperatures. They are very inexpensive and provide a steady stream of fresh air into the house and are very energy efficient. However, they need to be properly placed, usually high up on a building (for the cooler, denser air to fall in the house) and have significant visual impact. And, obviously on days when the humidity might spike up, these systems may hardly work at all. A fairly good cooling-only option in Colorado.
5. wood and pellet stoves - Clearly these have become more efficient with the recent introduction of catalytic converters to reduce pollution and quiet fans to distribute heat. However, they are usually single location devices and do not distribute their heat evenly around a house. They are also take up space and need more constant attention for fueling, including the associated mess of pellets and wood.
6. ceiling mounted electric radiant panels - this is basically electric resistant heating and works the same as the electric baseboard listed next. However, these panels are ceiling mounted and do not pose the same furniture placement issues as baseboard heaters do. Another advantage of these panels is that they can be easily zoned so that each room can be efficiently and individually heated. However, as most electricity is still produced by coal-burning plants, they efficiency of these panels is greatly compromised. And, of course, this is a heating-only solution.
7. baseboard heating - hydronic and electric - these can be relatively inexpensive and easily zoned but as they run around the room typically create problems with placing furniture. Recessing them in the wall makes for a very clean installation but may compromise the amount of wall insulation. Of course, like the radiant panels above, they are a heating-only solution.
8. passive solar heating - If you live someplace with sunny but cold winters it certainly would be dumb not to take advantage of some of that winter solar gain. How much you do so may be very dependent on your house's orientation and views. At the simplest, the use of passive solar techniques can provide summer shading of windows which allows winter sun to heat floors and walls and decreases the heating load required of other systems. Very aggressive use of passive solar means creating thermal mass inside the house to absorb heat during the day and then release it at night. These are "systems" that require some interaction with the homeowners to open and close windows and ducts and at their most efficient drive the basic form and shape of the house into a solar machine rather than a home.
9. natural cooling techniques - this is really a series of techniques and design considerations that can be combined to greatly reduce or eliminate the need for energy-input cooling for your house. They do not work for all houses in all locations and, like passive heating, can begin to determine the physical shape of the house. At a minimum, natural cooling lets in cool air at the bottom of the house and evacuates it at the top with either passive cooling towers or whole house fans. The key to all these techniques is good air circulation throughout the house and the ability to move hot air out of the house. And, like the passive solar solutions above, these techniques usually require the active, daily participation of the homeowner - opening and closing vents and window and transoms - to operate the system.
The Big Problem
The nasty secret about all these methods is that calculating precisely the potential energy use of the building for heating and cooling is notoriously difficult. Much of this stems from the base assumptions of the calculations and the use of degree days. Degree days are the historical number of days when a building will need either heating or cooling multiplied by the difference between the outdoor air temperature and the ideal indoor air temperature (usually 65 degrees F). It does not take into consideration the level of relative humidity on comfort nor the solar gain inside the house from the sun. Obviously with our conditions in Colorado these two omissions are a major problem. But what is worse is that the historic degree day data that is relied on by design professionals does not take into consideration a very common climate condition in Colorado - warm days that may need air conditioning coupled with cool nights that may need heating. The degree day takes the average high and low for the day and compares it to the ideal temperature and assumes that difference for a full 24-hour period. This greatly underestimates either potential heating or cooling loads required for a building to maintain the indoor ideal temperature.
This inherent flaw in the degree day methodology does not mean that every energy design professional's recommendations are void, but it does mean that a lot more work has to go into the design of a given building than might be otherwise needed for other areas of the country. In the past, the most common flaw associated with HVAC design for houses we have worked on has been the under-calculation of solar gain within the house. This has occasionally come from the lack of window shading - an architectural problem. However, due the desire of a client to have a certain view, predominantly western views, no amount of window shading via overhangs will suffice to cut out all late afternoon direct summer solar gain. Only adequate cooling methods will help overcome this intense solar gain. These may be any of those listed above, but they will be needed for short periods of time and the beauty and drama of large panes of glass capturing spectacular views must be offset aggressively or the heat build up over the course of the summer will drive the indoor temperature gradually and inescapably upward.
The first is to recognize that knowledge is power and that you should not skimp on purchasing good monitoring and control equipment. I don't mean that you have to have expensive computer programs with sensor modules, but certainly a 24-hour, 7-day, multi-event programmable thermostat is an absolute necessity. These are less than 50 bucks and will go a long way to help you customize your heating and cooling based on occupancy and will certainly save money on energy bills. A step up from this would be to install a real-time electrical usage monitor which will show you your energy usage of your HVAC system along with all the other sources of electrical usage in the house. Multiple studies have shown that just knowing what and how you are using energy will change behavior and save energy.
So, what to do about choosing an HVAC system? This is an impossible question to answer in general and depends very much on the type of building, its orientation, energy costs, and perceived comfort level. A good architect ought to walk you through these options and the many other systems and hybrids available. Ideally a mechanical engineer is brought in to figure out the costs/benefits of each of these systems for your application. However, as this is often prohibitively expensive, a well-qualified installer may be able to be a good team member along with the architect and a HERS rater to help you to determine the best approach for your house.
by Boulder architects M. Gerwing Architects