How is the PAHS System Working Today?

As I start writing this blog, it is 12:00 PM, 12/12/12, and sunny, and the outdoor temperature is 49°F. Night-time temperatures have been in the upper teens to low twenties. I sit here next to a south-facing window in the mid-day rays, enjoying temperatures around 85-90°F. I'm looking at a picture of Himalayas, Spiti mountains in the middle of winter on my calendar and not feeling any chill at all.

Two days ago the night-time temperature on the hallway thermostat away from any sunlight was 66°F, and last night it was 68°F. Currently the thermostat reads 74°F, and yesterday at this time it read 73°F. Yesterday and today the humidistat on the thermostat read 37% relative humidity.

I have been running a moderate fire in the masonry stove each morning and evening the last three days. The masonry stove typically raises the air temperature at the inlet to the plenum to between 80-90°F. Currently the air entering the plenum is at 87°F. It was near 90°F earlier and will slowly drop to around 80°F or so until I fire up the masonry stove this evening. The evening firing will not raise the plenum inlet temperature quite as much as the morning firing, because the surrounding air is not heated as much by incoming sunlight.

How is the PAHS system working? Not as good as it should be. And the main reason is that I just hooked it up a few weeks ago. In essence, it was not operational during the previous warm months, so it didn't have a chance to heat up the soil surrounding the house structure. Thus, as we are entering the cooler months, the soil surrounding the house is not as warm as it could have been. Before the PAHS tubes were properly hooked up, the four upper tubes had been closed off so that warm air could not escape through them.

Again, how does the PAHS system work? Or is supposed to work? The soil above and around the house is covered by a 5-inch thick umbrella of expanded polystyrene insulation to keep heat from entering in warm months and escaping in cold months. The concept is that heat inside the house will be absorbed into the cooler soil in the warmer months and heat in the warmer soil will be absorbed into the cooler house in the cooler months.

Well, the soil did not get as warm this summer as it could have, because warm air was not allowed to flow through the upper air tubes. Recall that the four 6-inch diameter warm-air tubes exit through the roof, run horizontally across the roof, then vertically down to floor level, and then nearly horizontally to the outside air. The four 6-inch diameter cold-air tubes exit through the back walls at floor level and also run nearly horizontally to the outside air. When the four warm-air tubes reach floor level, they join up with and run along the top of the cold-air tubes. At this point the two sets of tubes act as one long heat exchanger (over 100 feet long).

Now how is the PAHS system supposed to work? Warmer air rises and cooler air falls. So when the house heats up, warmer air rises and flows out through the upper or warm-air tubes. This creates a vacuum and draws in cooler air through the cold-air tubes. Thus in warmer months warmer air flows out through the upper tubes, and cooler air flows in through the lower tubes. And in colder months, cooler air flows out through the lower tubes, and warmer air flows in through the upper tubes. Does it work that way in our house? Yes in the warmer months but no in the cooler months. That is likely because the PAHS has not been operational long enough to bring the surrounding insulated soil into equilibrium. That is, the surrounding soil is too cold for the PAHS system to work properly this winter. In essence, the inside air temperature is not in balance with the surrounding soil temperature, so air wants to flow out through the upper tubes instead of in through the lower tubes. And this tends to keep the inside air temperature cooler than might be desired. The consequence is that we will have to run the masonry stove a little more than we might have otherwise.

First let's see how the air-tube heat exchanger performs. Early this morning (around 6 AM) the temperature reading on the thermostat was 68°F, and the air temperature at the back room and the kitchen cold-air inlet were each 62°F. Currently (1:15 PM) the back room cold-air inlet temperature is 68°F and the kitchen cold-air inlet temperature is 69°F. What has happened during the day is that the warmer air exiting the house and flowing through the warm-air tubes is heating up the cooler air flowing in through the cold-air tubes. In fact, the heat exchanger was doing its job at night too. In this case, the incoming outside air temperature started around 20°F, and the exiting warm-air temperature likely started in the mid 70°F range. Thus the exiting warm air and surrounding soil warmed the outside air from 20°F to 62°F. Whereas, around this time of day, the incoming outside air temperature is around 50°F and the exiting warm-air temperature is around 85-90°F. Thus the exiting warmer air warmed the entering warmer air to a higher temperature. Perhaps some of the energy of the warmer air went into heating the surrounding soil as well.

So the plan right now is to continue firing the masonry stove morning and evening to hopefully increase the heat reserve in the surrounding thermal mass. While the transient indoor air temperature is what we perceive, it is not indicative of the surrounding thermal mass temperature, which could be several °F lower or higher. When the sun shines in through the windows or warmed air from the masonry stove is blown into the rooms through the ductwork, the air temperature may rise a few °F, but in the course of a day, the thermal mass temperature may have increased only a fraction of a °F. Thus when the sun sets or it is cloudy, or if the stove is not fired up, the inside air temperature will again drop close to the thermal mass temperature.

It is difficult to know exactly what the effective thermal mass temperature is, but I estimate that it is somewhere around 66-67°F. In essence, if the sun was not shining, the air tubes were blocked, the insulated drapes were closed, and the outside air temperature was moderate, the inside air temperature would likely stabilize around 66-67°F. I would like that stabilized temperature to be closer to 70°F, which is why I will continue to fire up the masonry stove, even when the sun is shining brightly and the outside temperatures are moderate, as they are today.

12:00 PM, 12/13/12 Update

Twenty-four hours have passed and the day is again sunny and warm. Outside it is 53°F and inside 75°F. The humidity level is 37%. Air entering the plenum from the masonry stove is holding steady at 90°F. Thermometers at the two cold-air inlets are 66°F in the back room and 68°F in the kitchen.

At 5:30 AM this morning, the two cold-air inlet temperatures were each 64°F, showing a 2°F improvement over yesterday. The thermostat's thermometer was 70°F and the humidity 37%. The air temperature entering the plenum was 80°F. Shortly after 6:00 AM I fired up the masonry stove and noted on passing by the thermostat around 6:30 AM that it's thermometer read 68°F. By 10:00 AM the thermostat's thermometer had gone back up to 73°F, and the two cold-air thermometers read 65°F in the back room and 69°F in the kitchen (a little sun was hitting it). Air entering the plenum at that time was 90°F. It is a bit too warm to sit around the masonry stove at the moment.

By the way, the wood we are burning came from two tall, thorny black locust trees that had been blocking sun from our south-facing windows. (It's pay-back time for them.) I cut them down in April this year, and cut and split everything large enough to burn into ideal sizes for the masonry stove. Then I stacked up the wood and let it season and dry until this fall. A few weeks ago I hauled part of it into our attached garage next to the room with the masonry stove, and I stored the rest in a shed to keep dry.

Black locust is a relatively dense wood and contains lots of heat energy. The masonry stove instructions say that no more than 100 pounds of wood should be burned in one day, and it should be fired no more than twice a day. Overheating the stove can cause the firebricks to crack and the stove to malfunction. Currently I am burning a full armload of wood each morning and night, which is around 50-60 pounds. There is still room in the combustion compartment for more wood if it gets really cold. The wood is very dry, lights easily, and burns rapidly and hot.

Masonry stoves are designed to burn wood rapidly and efficiently and get really hot inside. This minimizes the release of unburned atmosphere-polluting gasses. The frame around the masonry stove's glass door is hollow, and it vents incoming air into the combustion chamber from above and below the door. This incoming air keeps the top-fired wood burning rapidly, and some of it helps to keep the door's viewing glass surface cooler and clear from combustion particles.

However, the masonry stove does have one design flaw that I remedied this fall. After the wood has burned down to a layer of coals 1-2 inches deep, the air supplied by the door-frame openings is less effective in keeping the burn process active. Thus it may take up to three times as long for these coals to completely burn out as it took for the wood to burn down to this point. During this process, the chimney damper must remain completely open until every live coal has burned out, or there will be a risk of introducing deadly Carbon Monoxide into the house. And while the damper is open, precious heat is escaping up the chimney and not heating our house. I want those dwindling coals to burn out as rapidly as possible, so I can close the damper.

The masonry stove has a slotted grate that the wood rests on and later the burning coals. Efficient wood burning uses top-down combustion, where the fire is started on top of the wood stack and progresses downward. Feeding combustion air from the top is ideal for top-down burning, but not for speeding along combustion of the remaining coal bed. However, feeding combustion air from under the grates would hurry along the final combustion process. So I had a mason insert a 2-inch diameter black pipe through the wall of the masonry stove that protrudes into the ash pit just below the grate. In the initial part of the burn phase, I keep a cap on the pipe, and I fully open the air inlet door to the masonry stove's frame air supply. After the wood has burned down into a good bed of coals (about an hour) I partially close this door and remove the cap from the pipe. In about another hour the coals have pretty much burned out. This modification has cut the wood burning time by about a half, which allows me to close the chimney damper much sooner and save more heat energy.