Sunday, June 22, 2014
One Cold but Comfortable Winter
Where has the time gone? Sixteen months have passed since I last posted. Since then we have come to know our house much better, and so far our opinions have been quite positive. I've gotten a better understanding of the PAHS system and thermal mass and will try to give an update. I'll also talk briefly about the moisture problem in our house, which is likely not what most would think.
Last winter was probably colder than we've had in decades, and it damaged many of our fruit trees and flowers. Numerous nights we had temperatures between -10°F and -20°F, and some days the temperature barely climbed to 0°F. But we stayed toasty inside with lots of sunny solar-heating days and burning 2 1/2 cords of hedge wood. We ran out of wood in the middle of March, but we still stayed comfortable through the unseasonably cold spring. It is ironic that our indoor spring temperatures averaged maybe 6°F colder than our indoor winter temperatures.
Since late May our outdoor temperatures have been pushing close to 90°F, and our indoor temperatures haven't been taking notice. If anything, our indoor temperatures have stayed on the cool side, and this is the source of our peculiar moisture problem. Our house structure is very water resistant, and we have never seen moisture leaking in anywhere. In fact, the indoor humidity level has run in the 25-40% range all late fall, winter, and early spring. Only when the outdoor temperatures and humidity levels start to increase in late spring do we see indoor humidity levels begin to rise. Moisture comes in with the fresh air brought in through the air-circulation tubes. By early June our indoor humidity passed 60%, and we knew it was time to get out the dehumidifier and fire up the heat pump. Problem is that the heat pump doesn't run enough to lower the humidity, because the inside air doesn't warm up enough. So the dehumidifier does most of the work and adds a little heat.
I got to wondering why our house was so warm and comfortable in the winter months and so much cooler in the summer months. Then I started thinking about the PAHS concept of annual averaging of heat energy storage and usage. In our area, the average annual temperature of the earth is 52°F, which would be uncomfortable to live in on a daily basis. We prefer to live in an annual average temperature of about 72°F. Thus there is about a 20°F difference between the average annual temperatures of the thermal mass outside and inside. In essence, we have used an umbrella of insulation to isolate hundreds of tons of thermal mass surrounding our house from the environmental thermal mass. So we want to keep the average annual temperature of our isolated thermal mass at 72°F, while the average annual temperature of the thermal mass surrounding it is 52°F.
But what has happened these last six months? The outdoor temperatures averaged maybe an extra 10°F colder, so that pushed the apparent average annual temperature maybe 5°F colder to about 47°F. Now, instead of making up for a 20°F difference we were making up for a 25°F difference. However, with the cold winter temperatures came more sunny days with ample solar heating, and I often fired up the masonry stove twice a day. I sometimes pushed the indoor temperature up to 78°F, which was quite comfortable and welcome. The extra heat energy flowed into our protected thermal mass. But if I had realized then that we needed to make up for a 25°F difference instead of a 20°F difference, I would have pushed the indoor temperatures even higher.
When spring came along, a number of things became apparent. We had run out of wood, which wasn't a big deal. (I cut and split 5 1/2 cords of black locust and hedge this spring, but we didn't burn any of it.) The sun was higher in the sky, so its heating effect inside the house was less significant. The low temperatures inside the house in late April and early May were 62-64°F, whereas in winter they were 68-70°F. As the outdoor temperatures pushed up later in May and into June, the low temperatures inched up. Currently the low temperatures are 70-71°F and high temperatures 72-74°F. This makes controlling the humidity difficult for the heat pump, because it doesn't run long enough to do the job. However, the dehumidifier and heat pump together keep the humidity in the comfortable and safe 50-55% range.
Getting back to the thermal mass issue, we have hundreds of tons of isolated thermal mass whose average annual temperature we would like to maintain at 72°F, and we have much more thermal mass surrounding that whose average annual temperature is somewhere between 47-52°F. The thermal isolation between these two masses is far from perfect, so a significant amount of energy in the 72°F thermal mass part is escaping into the cooler 47-52°F thermal mass part. This means that the average temperature of our 72°F thermal mass is continually decreasing, and the bigger the difference between the two thermal mass temperatures, the more energy that must be added to our inside thermal mass. The best time to add that extra heat energy to our thermal mass is in the colder months when the humidity is lower and solar heating is plentiful, and when the extra heat feels good.
So even though we had used up our wood supply in March, I could have gotten more. But I chose not to add more heat to the house, because I didn't realize that our thermal mass had a substantial energy deficit from the extremely cold winter. I should have kept adding heat from the masonry stove into early to mid May to keep the low temperatures closer to the more comfortable winter range of 68-70°F. Then the heat pump would have done its job and consumed more energy.
Are there any alternatives? I'm looking into installing an Energy Recovery Ventilator (not to be confused with a Heat Recovery Ventilator) inline with the air circulation tubes. The two sets of air circulation tubes run through a 4 foot by 4 foot pit in the back corner of the garage, and I could set the ERV right over the pit. The following image shows that the ERV uses a rotating drum to transfer heat and moisture from one air stream to the other. Thus in the warmer months it acts like a dehumidifier and in the colder months it acts like a humidifier. Since we tend to run 15-25% low in humidity in the colder months and 15-25% high in the warmer months, this type of device may provide a better balance year-round. The device does use some energy to rotate the drum and operate a fan to overcome air-flow resistance through the drum and ductwork.