My HVAC education

posted by Jeff | Sunday, December 10, 2017, 10:45 PM | comments: 0

When I built/bought my first house, it had an old school Honeywell thermostat with a mercury switch. That was a pretty straight forward device, closing the circuit to make the heat (or air conditioning) turn on. I had crazy high gas and electric bills, though it got better when Ohio separated transmission costs from source of energy, meaning you could choose where your energy came from. A rare, forward moment for Ohio.

Fast forward 16 years to my third house (all three have been new construction), and I had no idea what I was really walking into. Stuff has changed a bunch, but I hung out just enough with the field manager to understand a little of what was going on, though not enough to get my Nest installation right, it turns out. First off, the installation contains two systems, each with a blower and an outdoor heat pump. One set feeds the upstairs, the other downstairs. Already we've seen how remarkably efficient this is, even if the up front cost is higher. We have 57% more space than in the last house but already our November electric bill was about 15% lower. They're apparently doing this even with smaller floor plans now. That's a big change even in the four years since we started our last house.

The other new thing is that the outdoor units are now bidirectional heat pumps. This was my new discovery, as I didn't really know this was even a thing. Air conditioning isn't all that complicated. It's the process of taking heat out of the air (not putting "cold" into it). It stands to reason that if you can pull heat out of a tiny refrigerator and make the coils on the back slightly warm, you can pull heat out of the cold outside air and put it in your house. It's just the same process that the AC does, just in reverse. So the primary heat comes out of your ducts, not as hot as normal gas or electric heat, but the fan runs longer and heats over more time, ultimately using less energy. When the heat pump can't suck enough energy from the outside, it can supplement with traditional heating coils (in our case, since we don't have natural gas).

This is where the Nest actually does its best work, because over time it will learn what the most efficient combination of heat pump heat and typical electric coil heat is, factoring in the outdoor temperature and ambient heat from the other unit. A regular thermostat simply uses a cut off temperature, where it resorts to the internal heating when the outside gets below a certain temperature. We don't benefit from the thing learning when we're home (because I work from home... we're home most of the time), but for a few weeks each winter we get this algorithmic magic.

The thing I got wrong about the Nest installation was configuring it. I didn't know that it was a heat pump arrangement, and I just guessed on the wiring configuration well enough that air conditioning worked. Last night, I noticed the fan had been blowing for hours, but we were at 67 degrees and not getting any closer to 69. That's when I started learning about what the different wire conductors to the thermostat were used for, and the orange one, sure enough, went directly to the outside, where convention indicated it went to the reversing valve on the heat pump. That does what it sounds like it does... it turns it from an air conditioner to a heater by putting heat into the system instead of pulling it out. Then I remembered that my warranty registration also described the units as heat pumps, and I realized I configured the Nests wrong, without the primary and secondary heat.

After I got it right (at 4 a.m.), I ran a test to find that "auxiliary heat" was traditionally hot, while the standard heat was just warmish, as you would expect if you understood how the system works. Some hours earlier, I did not have that understanding. But by morning, the house was toasty as the outside hit 39, and to hit the programed 72 downstairs by 8 a.m., I imagine the heat pump was already running and the fan blowing.

I'm pretty excited about all of this energy efficiency, and I'm hoping that the other house sells for our target price so we have enough money to install solar and live in the inevitable future. Sustainable energy is not a goal so we can be smug, it's a goal for science and moving forward, proving that it's real and possible today, so others will follow. We can't wait for the government to make it happen, so this is how we can do our part.

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