Heating, Ventilation and Air Conditioning

We describe here the heating, ventilation and air conditioning systems in the Florida Solar Cracker House.

In the Florida climate, heating is not really a problem, but keeping cool and dry are BIG problems during the summer. Therefore, most of this chapter will focus on ventilation and air conditioning. Also, most existing publications on passive solar home design discuss only heating and don't even mention passive cooling techniques.


This said, north Florida can have some pretty cold weather in the winter, and some heating is required (if nothing else, for hot water). Space heating is achieved by passive solar, a wood stove, and backup propane heaters. Our original goal was to use no fossil fuels at all. For the sake of convenience, we have given up on that ideal, and now have propane gas available for backup space heating and cooking. We have a 60 gallon tank which is topped off once a year. Our typical annual usage is about 30-35 gallons for all purposes, which costs about $75, including delivery.

Passive solar heating is significant in the winter due to sunlight entering through the south-facing windows and doors.

This figure shows a view of the south side of the house taken on Dec. 21, 2003. A typical winter day is clear and cool with abundant sunshine. Our windows are designed to reflect heat, and they work well, however, direct sunlight (visible light) that passes through the glazing lets heat in. Sometimes, depending on the air temperature and wind, we open the south-facing windows to let in more radiant heat. We also make much use of our wood cookstove in the winter, for space heat, water heating and cooking. For space heating, we open the oven door. This is usually enough to keep the house very comfortable even on the coldest days. The stove burns very clean (we usually can't see any sign of smoke coming out of the chimney once it is burning hot), and we have ample wood for fuel on our property.

Hot water for household use is mainly provided by a solar collector. The solar collector is shown in the bottom right-hand corner of the picture above. Hot water is also provided by our wood cookstove, which is outfitted with a hot water reservoir in the firebox. The hot water reservoir is plumbed into our household hot water storage tanks. A more expensive in-floor radiant heating system is probably not cost effective in this climate. The wood cookstove, a Stanley Waterford, pictured below, can also be used for cooking in the summer when it is not convenient to use the solar cooker or other alternate means of cooking. The stove is well-insulated and a small fire for cooking can be maintained that will add a negligable load to the room. Note the solar cooker on the porch in the picture above.

Here is a picture showing the side of the woodstove in our kitchen. The plumbing for the hot water reservoir can be seen behind the stove.


Good ventilation is extremely important for hot, humid climates. Mildew does not generally grow in well-ventilated areas, even in moist, warm conditions. The Florida Solar Cracker House has been designed with good airflow in mind. There are no flat ceilings to allow air to stagnate. The cathedral-style ceiling follows the hip roof profile on the inside of the house, and at the peak, a cupola with operable windows can be used to vent hot moist air to the outside. By opening the cupola window on the leeward side of a breeze, the reduced pressure created by the breeze outside the leeward window helps draw the air out. Also, five ceiling fans are mounted inside the house, and one on each porch, which greatly aids ventilation. The picture below shows a view from inside the breezeway area looking up into the cupola.

Note that both parts of the house are very open, with the only interior wall being the bathroom wall, which is required by law. The cupolas also aid in keeping the house cool and comfortable by reducing the need for electric lights. With their vertical windows and roof overhang, a cupola provides a very efficient way to bring natural light inside without allowing direct radiation to enter the house. Although popular, skylights are difficult to shade effectively in this way.

During the hottest weather, all windows and doors are kept shut during the day and opened at night. In milder weather (about 6 months out of the year in north Florida), the windows or doors can be opened. There are windows or doors on each wall for good cross-ventilation.

Air Conditioning

We had originally planned to build our own air conditioner, based on thermoelectric modules. Not really needing another major project, we looked around at commercial units to see what was available. We found that the highest efficiency air conditioners in the size we needed were made by Friedrich. We ended up buying a Friedrich Quietmaster Electronic, model # SS10J10, 10,200 BTU/hr (British Thermal Units per hour) room air conditioner with an EER of 11.7.

EER stands for Energy Efficiency Ratio. It is calculated by dividing the BTU rating by the wattage consumed. For our AC, the wattage consumed is 871 Watts, so that 10,200/871=11.7. The EER is a little hard to understand in terms of efficiency because the two terms that you divide have different units. Let's convert to all Watts and see what the efficiency really is.

First, if you are used to thinking of air conditioners in terms of "tons", one "ton" is equal to 12,000 BTU/hr, so that our unit is 0.85 tons. Now, one BTU is equal to 1,055 Joules and 1 Joule/sec is equal to 1 Watt. So, one BTU/hr = 1,055 Joules/3600sec since there are 3600 sec in one hour. Therefore, one BTU/hr = 0.293 Watts, and our AC has a COOLING power of 2,989 Watts. The real measure of efficiency is the cooling power divided by the consumed power, so our AC efficiency in this measure is 2,989 Watts/871 Watts = 3.43. This number is called the coefficient of performance (COP) by engineers. In other words, for every 1 Watt of power we put in, we get 3.43 Watts of cooling out. This is pretty good for a room unit. Of course, you can do much better with a full house unit, but that is not what we wanted.

The thermo electric air conditioner (TEAC) that we had planned to build has a theoretical COP of between 1 and 1.5. This is the real reason that we ended up buying a commercial unit. We still have all of the parts for the TEAC and plan to build it eventually for our sailboat, to use when we have "shore power" available.

So, how does our Friedrich AC work? We really haven't had it long enough to know yet. It is sized for a maximum room size of about 700 square feet. It is in a room of 625 square feet floor area, but with high ceilings. Also, we usually have it open to the whole house when it runs, which is 1400 square feet with high ceilings. On a typical day, after about an hour of running, the humidity will have dropped about 4 points and the temperature about 1 degree (F) in the side of the house farthest from the AC. Unfortunately, most days when you really need it, it is fairly cloudy and hazy out, so we can't afford the power to use it. More data will be provided as it becomes available. This picture shows the Friedrich air conditioner mounted in a solid door, which replaced one of the glass doors looking from the bedroom out onto the west porch.