WHEN THE SUN HIDES
Backup Heating Systems for Passive Solar Houses
Philip S. Wenz
COPYRIGHT, Philip S Wenz, 2006
Originally Published in Ecological Home Ideas Magazine, Fall 2006 Issue
If you build a passive solar house, you can expect to get 30 to 60% of your heat directly from the sun. On a cold but sunny day, cheery rays will stream through your windows and warm not only you, but also your floors and walls. Made from dense materials such as concrete and tile that absorb and store the sun's heat during the day and release it at night, those warm floors and walls help pay your heating bills.
But what happens when the sun hides? What if there is a succession of cloudy days or you live so far north that the sun only peeks above the horizon for a few hours in the dead of winter? Your house no longer heats up, and the warmth that it stored during the last sunny period dissipates. You need your backup heating system-furnace, radiator or pellet stove-to kick in.
Of course you want your backup heater to be as energy efficient and pollution free as possible. As a long-term goal, each building should consume "zero net energy" or even produce more energy than it consumes, giving the excess back to the power grid. But as my client John Walmsley and I found out, the path to these sustainable ideals is strewn with obstacles in the form of building codes, energy regulations and the cost of environmentally beneficial heating systems.
John and I worked together to design a large passive solar addition to his San Francisco home. Placed on the south side of the house, the addition has great potential for solar heat gain. But stretches of foggy or cloudy days and cold weather go hand in hand in the City by the Bay, and we knew that installing a good, alternative-energy backup system was essential to meeting John's goal of living in a "green" house. In our search for that system we considered electric resistance radiant floor heating powered by photovoltaic (PV) panels, hot water radiators or radiant floor coils driven by a heat pump, and even a pellet stove. While all these systems had promise, however, we discovered that a combination of the codes and the price made their implementation seem impractical. Yet we were determined to find a solution that would at least provide a transition path to sustainability.
Our first obstacle was the International Building Code (IBC), which governs HVAC systems and is used by most cities and counties in North America. The code requires a residential HVAC system to heat to an even temperature of 68° Fahrenheit three feet above the floor of every room in the house. Also, that system must be mechanical, that is, you have to be able to operate it by pushing a button or adjusting a thermostat. That requirement eliminates pellet stoves and other stoves, which need the owner to carry the fuel to the stove, for the house's primary heating system.
Primary heating system? Let's clarify. The codes generally allow you to have pellet or wood burning stoves as your secondary or auxiliary system, but not as the primary system that provides heat for your whole house. (Many houses have a forced air furnace and fireplace or wood stove, for example.) Meanwhile, passive solar designers talk about your backup heating system, meaning the system that backs up the sun, the same system that the code enforcement authorities would consider your primary system because the sun is often absent. The building department's primary system is the solar designer's backup system.
Along with the IBC, we encountered California's state energy regulations, which are in some ways the most advanced in the country. However, the California law is aimed at conserving energy, not producing new energy from alternative sources. And, unfortunately, it takes a lot of electricity to heat a house. So much, in fact, that electric baseboard heaters and radiant floor heating simply will not qualify as the heat source for any room, nevertheless the whole house. (And since electric radiant heat isn't allowed, generating your own electricity with solar panels becomes a dubious investment.) As California energy expert Mike Gabel put it:
" Your client gets no credit under [California's energy regulations], nor will anyone for the foreseeable future, for putting in solar PV panels. The reason is simple: the state wants to reduce the demand side, not rely on alternative energy systems on the supply side. If the PV system fails, then the whole house is suddenly drawing a huge power demand for electric heating on the grid."
(Energy regulations vary from state to state, though many are based on California's model. Check with your building department before designing your home or addition.)
John and I were beginning to get the somewhat contradictory message that the path to the future was to continue to invest in systems that burn fossil fuels. In fact, in many solar houses the primary backup system ends up being the familiar gas fired, forced air furnace we see in most homes. Worse, while standard furnaces are moderately priced as heating systems go, they're not cheap, averaging $4,000 to 8,000 for a complete system with its ducts. If your solar house is well designed, you won't need to use your backup system much, yet the code requires that you pay for it and make space for it. At the same time the codes and regulations discourage or prohibit the use of alternative backup systems for passive solar homes.
Cost was our next hurdle. We considered installing a hot water circulating system, either with tubes imbedded in tile floors or using traditional wall radiators. For efficiency, the system would be run by an electric heat pump. Drawing about one third of the electricity of radiant heaters, the pump would meet the state's energy standards. Though the pump itself is a little more expensive than a gas furnace of the same capacity, it's operating cost is the same or lower, and will continue to go down, relatively, as gas prices rise. Further, the pump could be powered by "net metered" PV panels to dramatically reduce the operating cost and environmental impact (see accompanying article). But while that system looked good from an environmental standpoint, the estimated installation cost for the whole house, existing plus addition, was a whopping $20 to 25,000 (including the PV panels). A standard gas furnace cost about $5,000 to $6,000, installed.
What could we do?
Like most houses, John's already has an existing forced air gas furnace that sits in his basement looking like an environmental bad guy with big white duct-arms spreading all over the basement. From the beginning of the project he wanted to get rid of it, calling it a "space hog" with huge ducts that blew dust all over his house. In an email he said, "You mentioned using it as a stop gap or interim fix. If getting rid of it is a deal killer, then OK. Otherwise I'll drive it to the recycling center and find it a nice new home."
But after we priced the hot water system he wrote, "Good [expletive] grief! Suddenly my furnace seems more like an old friend than a pariah."
In the end we followed one of the most basic principles of ecological design and of nature—working with what you have. The old furnace has enough capacity to run John's entire house, including his addition, so it will qualify as his primary heating system under the building codes and energy regulations. Sealing and insulating its ducts and cleaning it annually will increase its efficiency. The dust can be filtered. Also, the furnace has embodied energy, the energy used to create it, and scrapping it for a new gas unit, which is only incrementally more efficient, would have its own environmental cost. The only real expense will be running new ducts to the addition. The old furnace stays.
But what about getting away from fossil fuels and designing a transition path to the future? The answer is that the furnace itself is the transition system. Its days are numbered. It will heat the house and satisfy the codes, and, because it's already paid for, leave room in the budget to develop, gradually, an environmentally sound solution—a heat pump powered by PV panels to replace the forced air furnace. The ducts are already in place.
John's PV array can be installed incrementally, starting with enough panels to run his lights and appliances, and adding panels when needed. As the old furnace wears out, natural gas prices climb, the price of heat pumps and PV panels decreases and electricity is produced more cleanly, the codes and the costs will demand that John's house be heated entirely by the sun—it's just a matter of time.