Thursday, February 11, 2010

More experiments with air flow

Aimee has been a little perturbed the last few days as I experiment even more with my air circulation problems, causing the temperature to fluctuate wildly in the house.

First I dug out some old rugs and tried covering up more of those forced air vents temporarily. In this stage, I left the oil heater on, mostly because I was worried about the pipes in the basement. This is of course dangerous since the big blower can run and not be able to move air, making the plenum run hot. But the thermostat was set fairly low, at 60 F, and I told myself I would know soon enough if the temperature dropped so much in the house that the heat would kick in. I kicked the rugs aside every night before bed and before going to work. Then we ran the house on the kitchen wood stove and a single electric heater only.

So far so good. At about 15-20 F outside, with the kitchen stove going at about a quarter of its output, and one 1500 W heater on full, we can keep all 1,250 square feet at 65 degrees. But only just. The thermometer was rock-steady at 65 F.

I wasn't happy with this. You need a less marginal solution, and in this case, if the air outside was colder or windier, you'd need more heat. The wood stove stack was running at about 200 F, and you can get it up to 400 F and keep it there easily enough, so we can get more heat. But you need to stoke that stove every hour to keep it at 400 F of stack temp.

I felt that this wasn't good enough yet.

It was as I was kicking the first rug aside the other morning, that I felt the increased strength of the airflow through the vent. This is with all but one other vent covered up. I measured it with a hand held anemometer at 0.5 m/s, or roughly 1.5 feet/second. The vent is a bit less than a square foot, So we're getting around 60 cfm through just one vent!

That did it! That old forced air furnace is making us cold, not warm! I covered all the vents except the one next to the kitchen wood stove, where the air leakage is a source of useful combustion air. I turned the oil heat off completely, and I started up the outside wood furnace again.

Now the house is holding heat nicely. The temperature didn't drop below 60 F last night.

And we can also get the temperature in the house to climb nicely, by running the outside wood furnace in the evenings. I had been hoping to do without this beast because it eats wood, and I probably can when the outside weather warms up another 10F. But I need it for now.

But having it run just in the evening is much better than what we were doing previously before I covered all these vents, which was running it all day and also keeping the oil furnace at 60F in the background.

It's funny how long it's taken me to figure out this airflow thing. I have been concentrating on the building envelope and the insulation, not the interior airflow. But the truth seems to be that the basement acts as a giant air-conditioner, taking warm air from the inside, cooling it down, and then releasing it back to the inside in a continuous flow. There's probably also some fresh air leaking in through the plenum or ducts.

And this is cold, cold air, blasting through our house at at least 60 cfm, and probably twice or thrice that much. Some of it is still coming in, of course, but only right next to the kitchen stove where it gets sucked in for combustion air, and goes right up the chimney.

Much better. And the oil furnace can stay off, except when we want it on, when we're in a hurry to keep the house warm, or when we're not hear and it's cold enough to worry about the pipes.

This weekend I will nail this down fully, with the anemometer and possibly some joss stick smoke tests. I will also send Aimee out to buy some more rugs.

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