Solar-Powered HVAC is possible

One of the dreams here at Roy Creek Ranch has been to add Solar powered heating and cooling (HVAC). We’re now producing substantial power from our solar array, and it would be nice to use that power as much as possible. Also, for those times when the grid fails, some heating and cooling will keep us more comfortable and maybe even a little safer in extreme weather.

HVAC FAIL silver lining

Last year (2022), a failing HVAC system forced us to make some upgrades. Originally, there were two HVAC systems here, one for each floor of our two-story home. The upstairs unit, then 26 years old, failed and was beyond repair.

The SEERing question

HVAC systems are rated by their efficiency or “Seasonal Energy Efficiency Rating (SEER). Simply put, the rating reflects how much heating and cooling are produced for a given amount of energy. Higher numbers are better.

The original equipment for the house was 12 SEER, and although considered good in 1996, it couldn’t match newer, more efficient models. After much consideration, I purchased a 16 SEER Bryant system — a considerable improvement. With the addition of a more sophisticated thermostat, it could even reach 18 SEER.

Improved efficiency

The efficiency results from several new technologies including more efficient and ozone-friendly refrigerant and a variable-speed compressor and air handler. For comparison, the original unit required a 30-amp circuit breaker, while the newer unit requires only a 20 amp circuit.

The Bryant system’s Rated Load Amps (RLA) is 10.5, meaning worst case, the unit should use about 2500 watts. At the time of installation, I measured it at around 9 amps — just over 2100 watts. This was clearly within the capability of our 6 kilowatt solar power system. With some modifications to our house wiring, the possibility of substantial heating and cooling during a power outage existed. Amazing.

Summer bedroom cooling

The new Bryant system and repairs to the other HVAC unit solved our most immediate HVAC problems. I hadn’t thought much about it since. Then, as summer arrived, I was reminded that we couldn’t cool the master bedroom of our home without running the downstairs HVAC. This meant we were cooling the entire main floor with the older and less efficient unit during the night. Worse, the master bedroom was at the far end of the system, meaning it had never really cooled that bedroom well in the hottest part of the summer.

Mini Split to the rescue

I remembered a conversation with our friend, the HVAC tech. He had mentioned something called a “Mini Split.” As he explained it, they were meant for cooling a room or two, were very efficient and could be easily added to an existing home. Intrigued, I asked him if it would be helpful in this situation.

He said yes and made a proposal. The unit he recommended was a heat pump made by Midea and sold by Carrier in the U.S. It was rated at 12,000 BTU, ran on 120 VAC and was reasonably priced. Since is was a heat pump it could both heat and cool the bedroom. This similar unit is available on Amazon.

Not too complicated

The Mini Split (as the name suggests) comes in two pieces; the “head” or “cassette” and the condenser unit. The head is mounted inside the room to be heated and cooled, while the condenser is placed outside. The condenser unit can be set on a concrete pad or other flat surface, or it can be mounted to a wall with an optional mounting kit. Power is supplied to the condenser and it, in turn, powers the head. The two units are connected by a multi core wire and two refrigerant lines.

There are many mini split installation videos on YouTube, so I won’t repeat the process here. Suffice it to say, we installed it in a few hours. I wanted to gain some experience and he was eager to try out this model. He had a use for one in his cabin. We took our time, but the process wasn’t difficult.

 

Results!

The performance is impressive. Based on the spec sheet, I was expecting the mini split to need 1000-1200 watts for cooling. Using the Kill-a-watt meter, I could see it was much better than that. Running full out, it used around 5 kilowatt hours in 8 hours. That’s an average load of 600 watts. Besides that, the indoor unit is very quiet. Not like those HVAC units you find in hotel rooms. Nothing but cool air.

As an experiment, I ran a 100-foot heavy-duty extension cord from the mini split out to the shop. This way, I could test power the mini split with solar power. The verdict? It works just fine. As a further test, I ran the shop A/C unit at the same time. Together, they required just over 1600 watts.

We’ve been directly under a “heat dome” since mid-July, and have had 100+ degree temperature daily. I powered the mini split with solar power for at least 8 hours each day during August. I run the system full blast to dump as much cool air as possible into the house. The hope is I can minimize use of the bigger but less efficient HVAC system during the peak (10 AM to Noon and 6 PM to 8 PM) and “super” peak (Noon to 6 PM) times of day. That should have a positive effect on our monthly power bill.

Next steps

In order to run both the mini split and Bryant systems with solar power, I need to add a 50-amp circuit between the shop and the house. It will power a transfer panel in the house. With these in place, I can route individual loads to either the grid or the solar generator. I’ll power as much as I can with solar power as I add capacity, and use the grid as backup. We won’t get to that project until next spring, so stay tuned!