Part 2 of Julio’s Series on Clean Energy at Sol Homestead
Have you ever felt the metal fins on the back of your refrigerator? They can be very warm. That’s because your refrigerator transfers heat from the air to a refrigerant that absorbs the heat, making the air inside your fridge cold enough for milk, fruit, vegetables, etc. The refrigerant then evaporates, is compressed, and passes through a condenser where a fan releases the heat out of the back of the appliance. This is also how air conditioners work. Air is sucked into the unit, heat is transferred to a refrigerant, cool air blows through your home, and hot air blows outside. Heat pumps are just air conditioners that work in both directions (blowing hot or cold air into your home and cold or hot air outside).
It turns out that heat transfer via air circulation as I’ve described can be much more efficient than combustion-based temperature regulation at sustaining a home at room temperature. Internationally, this has been understood for decades. Air source heat pumps are all over Japan and Europe; ground source heat pumps (same idea, but pulling the air into the unit from a hole in the ground rather than the ambient air near the outdoor unit) have become popular in higher latitude areas (Canada, Norway, Greenland, etc). Consider that other heating methods still need a blower fan to circulate the heated air through your home; heat pumps are systems where the air circulation is most of the work done to maintain a set temperature.
There are limitations to the commercially available units. Many are only more cost-effective than a high-efficiency gas furnace down to 30 to 40 degrees Fahrenheit. But, with a 20-year-old central air unit that we wanted to replace on our own terms, an oversized short-cycling gas furnace, and a desire to not let the pursuit of perfection prevent us from catching excellence, we installed a dual fuel system: a heat pump matched to a 96% efficient gas furnace that work together to sustain a comfortable temperature in our home. Over time, we will monitor the performance of the heat pump at lower temperatures to find the point where we are using all of our solar panel-generated energy and will let the gas furnace heat the house during frigid days and winter nights.
The early indications suggest that this was a great purchase for us for a few reasons. Our month-to-month energy costs continue to plummet even as energy prices rise. The heat pump inverter system works best when the fan is running at a low speed continuously, so air is circulating around the house nonstop, evening out the temperature. The overall air quality in our home is better as we are no longer burning fossil fuels to heat it in the vast majority of cases, and in the instances where the gas furnace does kick on, fresh air is ducted into the furnace to fuel the flame, which means we are not burning the oxygen already in our home to power the appliance. Plus, since the furnace is appropriately sized for the home (and relevant ductwork), it is much more effective at heating the entire house. Since the blower motor and the furnace are both variable, we have several power levels to select from depending on how hot or cold it is outside. This configurability further reduces our energy usage – no more loud roar every time the temperature needs to be adjusted.
Like with solar panels, discussing renewable energy and related home changes can flare emotions and even become political. And like shifting the focus to the freedom of abundant electricity – more electricity than you can possibly use – with solar panels, understanding the comfort, improvement to health via improved indoor air quality, and stability that can occur with a heat pump or a dual fuel system might cut through the stubborn, knee-jerk reactions and emphasize the logical tradeoffs for this type of consumer decision. Admittedly, dual fuel systems won’t be the sensible option for everyone, today. But for us and for many, it’s the best economic decision when you are looking to buy a new air conditioning unit or furnace. And for new construction, a heat pump solution makes even more sense, as you could avoid running a gas line into your home entirely. One less thing to worry about!
Technology only marches forward. In the 3 weeks since we had our heat pump installed, a US HVAC manufacturer built a heat pump that functions continuously down to -23 degrees F. Commercialization of that system will make heat pumps alone viable for most of the United States (without a gas furnace backup). The cost of building and installing these machines will get cheaper and cheaper, the efficiency will continue to improve, and soon enough, our kids will consider lighting gas to heat your home the same way we would think about a coal-fired furnace as a home heat source today.