One cold winter, my gas furnace stopped working and the part that broke was destroyed beyond any temporary repair. Of course this happened on a Friday evening and I had to wait until the following Monday to make a service appointment. Luckily, someone was able to be dispatched that day to get things working again. But I had to make it through the weekend without a key part of my HVAC system in below-freezing temperatures. I managed to create some warm zones in the house with a mixture of a fireplace and a couple quickly bought, inexpensive space heaters, along with occasionally running hot water through a faucet to keep water from freezing in the pipes and possibly damaging them. After that experience, (and other recent cases where other appliances broke–all installed around the same time when this house was originally built), I decided I should replace the heating and cooling units for the house.
My original goal was to replace both the furnace and air conditioning units with a single heat pump. But since I live in a colder climate, the cost for something that was able to keep the house warm during the coldest winter months (a geothermal solution) was prohibitively expensive. However, there were mid-range heat pumps available that did not cost much more than a plain air conditioning unit and would allow me to use them for heating roughly four more months out of the year leaving my furnace to cover the coldest three months.
I ended up replacing the furnace with another gas model and the air conditioner with a heat pump. I did not get anything too fancy with the hope that the new furnace would last much longer. Not only would it be running much less, but the previous furnace was oversized for my house which put more stress on it. My hope was that when the heat pump (or future) technology was able to fully handle my climate fairly inexpensively, I could install one of those and leave the (hopefully still in good shape) furnace as something to only use in emergencies.
I had the heat pump installed in the spring and that summer it performed quite well. I also have a solar system on the house, and during the summer months even with the heat pump running frequently, my electricity bill was practically 90% less than what I paid over the same months of the previous year. Unfortunately, as summer turned to fall, I started to see a moderate rise in my electricity costs. The shorter and less sunny days contributed to that, but even more was the fact that the heat pump was running longer per cycle as the average temperatures dropped. The heat pump was designed to pass heating duties to the furnace once certain conditions were met, but those limits were not something that could be adjusted. By comparing the energy draw of the house while switching between the furnace and heat pump, I calculated that the heat pump consumed roughly three times as much electrical energy as the furnace. While the furnace added the cost of natural gas, electricity where I live was and still is the highest (depending on time of day due to variable pricing) in the country. While I did not dive too deep into testing and calculations, I experimented switching between the heat pump and furnace with different outside temperatures to roughly estimate when I wanted the furnace to take over heating duties. The next question was how could I force the heat pump to stop trying to heat the house when the outside temperature reached a point well before its internal systems would activate the furnace. Luckily, I already had the tools I needed to do this.
Several years ago I replaced the single thermostat that controls the home’s HVAC system with a model that I could control with my home automation software. Like all of my home automation devices it did not need any sort of Internet connection to work. The downside of that–a cost I am more than willing to pay–is I had to do a little more work on the back-end. On the other hand, I ultimately had more control over how things worked, and those things will continue to work even if the company that makes the thermostat goes out of business or stops supporting the model. There were also some possible security benefits to keeping everything “in-house”.
After some reading of the thermostat manual, I found that it allowed for the control of a primary as well as a secondary heat source. It also had the ability to switch between heating and cooling automatically based on the interior temperature. Unfortunately, switching between the two heating options was a manual process. Fortunately, this gap was overcome with some programming of the home automation software. I also ended up adding additional controls based on if power from “the grid” was out in order to extend the time the house can run on solar and batteries.
The first rule I added was to switch the thermostat to use the furnace when the outside temperature went below 50°F / 10°C. This may seem a little warm, but when placing the lower limit to 40°F, the air out of the vents did not feel much warmer than the ambient temperature and the heat pump was running noticeably longer per cycle. Likewise, I configured another rule that the furnace would not be used when it reached over 50°F outdoors.
The final rules that I created were to enable or disable the heat pump depending on if the house was disconnected from the electrical grid and the amount of energy stored in the batteries. If the house lost power and the batteries dropped below a 50% charge, then the heat pump would be turned off to conserve energy. The furnace, however, would continue to work. If the power fails in on a hot day, chances are that the sun is shining and keeping the batteries above the 50% threshold meaning that the heat pump could keep the internal temperatures low enough to not be a risk to safety. Even if the heat pump was disabled overnight, the house would probably not raise to levels where there was a potential for heat stroke until the batteries were recharged enough for the heat pump to be enabled again. Of course, the heat pump could always be manually turned on if internal temperatures reached dangerous levels. I did not automate this, though, as if I faced that situation I would just relocate to the basement which should naturally remain cool enough to not be a health concern.