Before explaining how my house is heated, here is a quick overview of the status quo for heating system design in Big Bear, and for that matter, most of the country. First, build a house with the code minimum amount of insulation. Depending on how it is installed, this insulation may or may not be very effective at minimizing the amount of heat flowing out through the walls in the winter. But because the house is not tested for air leakage, a lot of heat will escape as the hot air simply flows around the insulation to the outside through gaps in the walls. So when a house is losing its heat so quickly, the only way to keep it comfortably warm is to pump a lot of heat directly in to each room. This is usually done using a furnace, which burns natural gas to create hot air, and a fan which blows the heated air through a series of ducts into each room. Depending on the furnace’s efficiency, it might be able to convert 80-96% of the gas’s energy into heat. But the blowers use a surprising amount of electricity, usually in the neighborhood of 800 watts to one kilowatt. Another downside to furnaces is that they usually have two speeds: full speed and off. This results in uncomfortable and sometimes noisy cycling on and off. Its like driving a car that doesn’t have a gas pedal, just a “80 MPH” button and an “off” button.
With my house, I started by creating a reasonably leak proof exterior shell, so warm air stays inside during the winter. A small ventilation fan ensures there is just enough fresh air in the house. I then included a higher level of insulation (including windows) than is required by California’s building code, which is one of the most strict codes in the country. Finally, I have a large insulated thermal mass in the house — the first floor slab — which can soak up a lot of heat during the day and radiate it at night. So because the house retains heat very well, you don’t need to blow a lot of heat quickly to the bedrooms because the don’t cool down very quickly. Instead of a furnace, I can use an alternative called a ductless mini-split heat pump to keep my house warm. Instead of generating heat, these units employ a refrigeration cycle to move heat from the exterior to the interior, or vice versa. Although its hard to believe, there is actually a lot of heat in the air even when its 0 degrees F outside. If you look at Kelvin, a temperature scale that measures absolute heat, total absence of heat is 0 K. So even when it’s freezing outside, it is still 273 Kelvin out there, and your toasty interior is only slightly warmer at 294 Kelvin. Its just a matter of harvesting that heat and moving it indoors.
A mini split has several advantages:
- Moving heat from the exterior air using a refrigeration cycle is a lot more energy efficient than making heat by burning nature gas. Even the most efficient gas furnaces are only 96% efficient, and that does not count the 800 Kw they use to move the hot air around. But a heat pump can surpass 300% efficiency. For every one unit of energy consumed, it can output 3 units of heat.
- Minisplits use a variable speed fan which adjusts the heat output dynamically. This reduces or eliminates cycling on and off, and makes for very quiet operation.
- Because the entire house is being heated as a system, you don’t need to install ducts to each room. Even if the bedroom doors are closed at night, the temperature generally won’t drop more than 3-4 degrees compared to the main living areas where the minisplit heads are located.
- Minisplits run on electricity, so with some planning you can run your system entirely on solar energy.
My main heat source are two Mitsubishi Hyper Heat Mini Split heat pumps, each rated at 12,000 BTUs. These units are Energy Star rated, and very efficient at 26 SEER cooling and 12.5 HSPF (heating efficiency) which translates to a coeficient of performance over a 3. They can operate at full capacity down to 5 degrees, and still operate at 80% capacity in temperatures of -13 degrees. The ASHRAE Manual J design temperature for Big Bear is “only” 7 degrees so these units will do well in this climate. As a bonus, they can run in reverse and provide cooling during the summer.
At the beginning of the project, I explored the possibility of an all electric house, even though natural gas was available at the site. However, after hearing some stories of winter blackouts lasting several days, I decided to install a direct vent natural gas fireplace as a back up heat source. (Plus, its nice to sit by a fire after a long day of skiing!) The model I selected used only outside air for combustion and at 18,000 BTUs, was capable of keeping the house nice and warm on its own, down to about 7 degrees outside temperature. It has a 62% fireplace efficiency ratio and a battery back up starter.
Domestic Hot Water
For hot water, I decided to use a condensing 97% efficient Energy Star rated tankless water heater from Navien. This direct vent appliance only uses outside air for combustion, and uses no standby energy when there is no hot water demand. This could come in handy for when the house is unoccupied in stretches. Also, since it has unlimited capacity (unlike a standard tank model), it will work well with the Jacuzzi tub. The house design isolates all plumbing to the southeast corner of the house. The upstairs bathrooms are adjacent, and directly above the downstairs bathroom and mechanical room. The kitchen is just outside the mechanical room. this makes for very short pipe runs, which will save water since you don’t need to wait very long for hot water to make it out of the tap. Also, all hot water lines were insulated.
Ventilation
For ventilation, I purchased a Swedish inline fan system from Resource Conservation Technologies, Inc. This fan moves 140 cfm with only 38 watts of power at 0.04″ static pressure which means 3.7 cfm/watt (easily meets Energy Star requirements). This “exhaust only” system expels a slow but constant amount of air from the house, to guarantee that there is a healthy amount of fresh air in the house at all times. This system is designed to meet ASHRAE 62.1 standards for ventilation. The air is exhausted from the bathrooms constantly, but motion and humidity sensors in the grill increase the airflow substantially when the bathrooms are being used.