Nothing Runs Like a Trane

Where does the other half of the heat go?

Now we are disappointed. Two weeks ago at the peak of our last heat wave of 90+°F days, we installed a small, two stage air source heat pump on the roof of our house, not for cooling the inside air, mind you, but for removing humidity. (A ground or water source heat pump would have been more efficient, but its initial more than double the cost would have made it economically infeasible for the little use ours will get.) The dang thing almost never runs because the indoor air temperature never gets high enough to kick it on. Around 12:30 p.m. that day when the heat pump installers turned it on for the first time, the indoor air temperature had reached a sweltering 73°F (the highest indoor air temperature this year) and the indoor humidity 67% (also the highest humidity level this year). Within an hour the indoor air temperature had dropped to 71°F and the indoor humidity level had dropped to 53%. At that point I set the thermostat to 74°F and the heat pump shut down. I don't know if it had run again since that day.

The heat pump had done an excellent job of lowering the humidity. In the previous year we had used two dehumidifiers running nearly 24 hours a day to control the humidity, at the expense of dumping all their waste heat directly into the living space and pushing the indoor air temperature up to 83°F in late summer. This year the indoor air temperature should never exceed 72°F, and we hope that the heat pump will run often enough to keep the humidity level below 60%.

Here is the situation. Our earth-sheltered house is surrounded by thousands of tons of thermal mass, and that thermal mass slowly sucks up heat that passes through our many window panes and that comes in with air movement through the fresh-air tubes. The nominal temperature of the thermal mass in this early part of June seems to be around 67°F in areas away from the windows and up to 69°F or more in areas closer to the windows, and the nominal air temperature of our living space has recently been around 71°F, being somewhat cooler in the back rooms away from the windows and somewhat warmer near the windows, especially when the sun is shining. That is, the outside air temperature has been swinging between 45°F and 95°F this time of year, but the inside air temperature has only swung between 69°F and 73°F, and that has been with no external sources of mechanical heating or cooling.

Over winter, the indoor humidity remained between 48-52%. In early spring it began to creep up toward 60%. This morning around 9:00 a.m. the indoor air temperature was 71°F, the humidity was 63%, and the thermostat was set at 74°F. The indoor air temperature has remained nearly constant at 71°F for days. The thermostat has a droop setting of 2°F with a target humidity level of 50%. This means that with a 74°F thermostat setting, the indoor air temperature would have to rise to 72°F before the heat pump would kick on to lower the humidity, and possibly the temperature in the process. This time of year, the indoor air temperature seldom rises above 72°F unless the outside air temperature reaches 85°F or so or the sun shines in brightly through the east-facing windows in the early-morning hours, as it was doing this morning.

Even at the 63% humidity level, the house feels comfortable. Because my core body temperature hangs around 96.8°F, I tend to wear sweaters in 72°F ambient air temperatures and wool socks on our 68°F±2°F tile floors. My wife, on the other hand, whose core temperature must be closer to 100.4°F, is constantly complaining of being too hot. (Why do opposites always attract?) Anyway, it would seem a fairly simple matter to kick on the heat pump and get a quick humidity reduction; just pop down the thermostat setting to one or two °F above the current indoor air temperature (the droop), and the heat pump will lower the humidity by 5-10% and the indoor air temperature by maybe one °F before it shuts off. Then restore the thermostat setting.

So at 9:10 a.m. on June 5, with mostly clear skies, outdoor temperature around 65°F, indoor air temperature 71°F, and indoor humidity of 63%, I lowered the thermostat setting to 73°F. The heat pump ran for about twenty minutes and lowered the indoor humidity from 63% to 61%, but it did not change the indoor air temperature from 71°F. At 9:30 a.m. I lowered the thermostat setting to 72°F and noted at 9:40 a.m. that the indoor air temperature had risen to 72°F and the indoor humidity had dropped to 60%. At 9:50 a.m. the indoor air temperature was 72°F and the indoor humidity had dropped to 58%. At 10:30 a.m. the indoor air temperature was 72°F and the indoor humidity had dropped to 56%. At 11:00 a.m. the outdoor air temperature had risen to 72°F, the indoor air temperature was 72°F, the indoor humidity was still 56%, and the heat pump was running. At 11:30 a.m. the outdoor air temperature was 72°F, the indoor air temperature was 72°F, and the indoor humidity had dropped to 55%. At 12:00 noon the outdoor air temperature was 72°F, the indoor air temperature was 72°F, the indoor humidity was 55%, and the heat pump was still running. At 12:30 p.m. the outdoor air temperature was 74°F, the indoor air temperature was 72°F, the indoor humidity was 54%, and the heat pump had shut down. At 1:00 p.m. the outdoor air temperature was 74°F, the indoor air temperature was 72°F, the indoor humidity was 54%, and the heat pump was not running. At 1:30 p.m. the outdoor air temperature was 74°F, the indoor air temperature was 72°F, the indoor humidity was 54%, and the heat pump was not running. (Boring.)

Some thoughts on why the heat pump had taken so much longer to lower the humidity level today as opposed to the day is was installed. Back then the humidity level in the house had risen very rapidly in the three or four days prior to installing the heat pump, and maybe the house contents had had insufficient time to absorb excess amounts of moisture. Since the heat pump was installed, it may not have run more than once or twice or at most a few times, and the humidity level was probably creeping up to the 63% level of today. Now the house contents have had more time to absorb moisture, so the heat pump took longer to lower the humidity because there is more moisture percolating out of everything. The bottom line is that maybe I had better listen to what the Trane man said and not let the humidity level get too much above the target 50% level. I'll keep the thermostat set on 72°F instead of 74°F and see how it does.

So why hasn't the indoor temperature dropped below 72°F? Part of the answer may be that I haven't balanced air flow through the vents yet. The thermostat is attached to the wall in the hallway that opens into the great room. It is about 14 feet from the nearest window and not facing toward any windows. The thermostat faces toward the back wall of a back room and is located about 14 feet from that back wall. A thermometer in the far back room of the house and leaning against the back wall reads 65°F. A thermometer about five feet from the back wall and about ten feet from the thermostat reads 70°F. Of course there is always potential for some error in these inexpensive devices.

And I have some other work to finish on the air handling system. In the colder months, the air handling system is designed to pull heated air from the entryway room located between the house and garage where the masonry stove is, and this room is adjacent to the sunroom where temperatures can reach 100°F on sunny days this time of year. In the warmer months, the air handling system is designed to bypass the entryway room and pull air from the space above the drop ceilings throughout the house. Problem is that I still need to install some grills in the drop ceilings near the windows to return more of the warm air to the cooling coils. And I need to isolate the plenum from the entryway room, because that room tends to run a little warmer in the summer due to its proximity to the sunroom.

I don't know what all logic Trane has put into their thermostatic controls and heat pumps, but the heat pump has cycled off and on several times during the above reported interval, so its total runtime may be considerably less than the elapsed time. It is difficult to tell when the heat pump is running, because it is separated from the living space by nearly six feet of concrete and soil above our heads, and you can't hear anything, except that the air distribution fan runs faster when the heat pump is running. Another thing that I must keep in mind is that air source heat pump cooling is more efficient when the air surrounding it is cooler. So it would likely be better to run the heat pump in the hours before and shortly after sunrise when the air is generally the coolest. The thermostat is programmable, so it looks like I may have to learn how to do that after all.

I have checked the cooling unit and verified that the air filter is relatively clean and that the air is moving at an acceptable rate. The cooling coils are not frosted up and the unit is quite cool to the touch. Everything appears to be working correctly.