Gas-Free HVAC

I live in Minnesota. One of my goals for our house has been to eliminate gas from our house. Normally, Minnesota and gas-free heating have been mutually-exclusive in winterized homes. But, as technology has changed, we have changed with it!

Initially, I wanted a geothermal heat pump. These are incredible for moving temperature from below the frost line to either heat or cool an area. They are extremely efficient, and have only gotten better as systems have improved. We were looking at a closed-loop, vertical system that would have required essentially digging a well in order to lay the heat exchange loop. There are a few people in Minnesota who do these systems, but the cost to dig the loop was prohibitive because of the geology of our area. We live in an old limestone quarry that still has a lot of hard limestone and shale layers: digging a 200-300 foot well would have been both incredibly expensive on our end and potentially equipment-prohibitive with our city lot.

So, we went with an air-source heat pump with electric resistance backup. A few companies make them now, including Mitsubishi and LG, and we landed on the 3-ton Daikin system. The heat pump has a lower bottom-end range than other brands, so it can provide heat to our house down to 5° F (-15° C). Once the temperature reaches near 5° F, the system switches to using the electric resistance for providing heat. Working similarly to a typical furnace and AC system, our blower moves air through our forced air system all the time. It's only on the days of extreme cold that the blower will operate with heat. The heating coil in the blower functions much like a convection oven. The blower passes the air over a super-heated coil before moving it into the ductwork. ​

​Heat Pumps

​A heat pump works by transferring heat from one location to another.  Since heat naturally moves from areas of high heat to low heat (thermodynamics), a heat pump is an assistive device that does the work of moving heat between indoors and outdoors.  What the heat pump does is transfers heat from the air outside into refrigerant-filled coils and moves it into the air handler.  The compressor also works in reverse to cool the coils outdoors and move indoor heat outside by reversing the flow of the refrigerant.  

Heat pumps have been around a long time, especially in mild climates.  These are also what is used in mini-split systems.  The outdoor unit is the same for a mini-split and a ducted system, but in a mini-split configuration, the air handler is mounted in a space instead of connected to ductwork.  Since we replaced a ducted furnace, we stayed with a ducted blower.  But we may add a mini-split to our attic if we ever do finish that space.  Since mini-splits do not require internal ductwork, they are becoming more common for use in homes with radiant heat.  Combined with radiators, these can heat and cool homes for the majority of the year, and render a gas-fired boiler to being a back up system for extreme cold.  

Electric Resistance vs. Gas-Fired Furnace

​The blower that we purchased was one step that most people aren't ready to make. And that makes sense: a gas-fired furnace is more cost-effective to run. Most heat pump manufacturers don't yet serve capacities down to 5° F. Even though Daikin can meet that and other manufacturers are approaching it, the jump still is risky where the temperature gets substantially cold. The heater in the blower will likely only run a few months of the year, and likely not the entire time since the heat pump will handle the majority of our below-freezing days. So, there are some cost-benefit things to consider.

The biggest one is energy usage: the whole unit requires 60 amps to run. 5 amps are dedicated to the blower motor: the part that moves air through the ducted system regardless of if climate control is managed through the heat pump or the blower. The heater in the blower consumes a little under 45 amps (60 covers any over-amperage). When we first bought our house, 60 amps was the entire service (not that it was adequate). An electric dryer requires 30 amps, and an electric stove requires 40 amps. Our whole service now operates on 200 amps. A lot goes into the calculation for total needs, but I do think adding the blower pushes us to the end of that 200.

A modern, efficient furnace isn't a power-hog in the same way, and it is extremely efficient for managing heating a home. Most installers around here advise pairing a gas-fired heating system for the extreme cold until heat pumps can catch up. Since we had been planning on going gas-free for a long time, upgraded our electrical system to accommodate the changes, and eliminated all other gas systems, it made sense in our case even though the blower is more expensive BTU to BTU.

Cost Breakdown Considerations

But, there are other financial costs to consider: the cost of gas went up 60% last winter, and it is not going to slow down, gas connection incurs monthly fees (around $20/month for us) regardless of usage, and electrical costs are going down or can be managed personally.  Last winter, we paid more per month to heat our home than almost any other year.  Our usage was lower than any other year, but the cost increase was a big squeeze.  The biggest culprit was gas.  We are enrolled in our local utility's wind source program, which invests every kwh we use from the power grid into wind energy.  There are also a lot of local community solar projects that are helping to bring down the overall cost of electricity, both in our wallets and for the planet.  

The other decision we made this year is to install a 12kwh solar system on our roof... once we get a new roof.  Our house only really needs a 6kwh or 7kwh system, but our utility company allows homeowners to produce up to 200% their maximum usage and sell excess energy back into the grid.  This system will work well for us, and likely cover some of the cost of our panels while helping us invest in and diversify our personal energy portfolio (I still wonder why gas companies haven't gone whole-hog into renewables: diversification of investment is smart either way).  When reviewing energy data (US Dept of Energy, US Energy Information Association, US EPA), I also found that per kwh, solar panel manufacturing accounts for around 1/20 the greenhouse gasses as a kwh from a coal-fired plant.  That "energy debt" disappears between 1 and 3 years of use, while panel degradation should stay above .5% per year and keep on producing for another 30 years or more.

Back Up Systems

​I'm also a firm believer in having a back up system for when you need it.  I've had too many furnace ignitors and switches go out to not understand that having something to back up the heating isn't also vital.  For heating, we have a few infrared electric fireplaces.  These have some of the most efficient heating elements on the market, and our models can heat up to 1000 square feet, which has kept us warm in emergencies with our last furnace.  They also have additional built-in safety features to prevent a real fire, unlike a lot of other space heaters.  Ours both have automatic shut offs if the heat vent gets blocked or heat gets trapped inside, something that our snuggling dog and always-cold kids have discovered organically long before anyone was burned, let alone a fire started.  

I'm really excited about this system integration for this year, and I definitely plan to write a follow-up when we do get the heating bill after the deep-cold of this winter.