Here’s the formula:

Where:
ACH_n is the removal and replacement of a volume of air equal to the volume of the building in one hour.
CFM₅₀ is the rate (measured in cubic feet per minute) at which air is entering the building when under a constant pressure of -50 pascals, or in other words, the leakiness of the building when a fan is blowing air out of it.
N is the correction factor for building height, wind, etc.
V is the volume of the conditioned space in the building.

Calculating the volume sounds like it should be easy. Take the area of the footprint, times the ceiling height, times the number of stories, correct for sloped ceilings, knee walls, cathedral ceilings, etc., and you have the volume. However, if you measure the interior volume of each room and add them up, you can get a significantly different number. And what do you do with a semi-conditioned basement or other semi-conditioned rooms in the building? Is that included in the volume measurement or not? These decisions can drastically change the ACH_n number.

Take for example, a building I recently measured. Using the footprint method I came up with a volume of 24,630 ft³. Using the room-by-room method I came up with 20,986 ft³. Adding in the semiconditioned basement I got 30,580ft³ and 26,936ft³ respectively. That gives a range of ACH_n values between 0.4 and 0.6.

Debating the shortcomings of this method is well and good for energy geeks but quickly becomes a substantive issue when it’s added to building codes. Here in Massachusetts, the code will follow the IECC’s (International Energy Code Council) recommendation of ACH_n of 0.35 or less. But how will volume be calculated?

I guess we’ll have to see what happens.

Here at Energy Metrics headquarters, I monitor electricity usage with The Energy Detective and custom software I put together (you can see the result here). Recently, I was inspecting a plot of the electricity the furnace uses when I noticed that the burner was turning on and off multiple times each time the thermostat called for heat. Not having seen this before, I wondered why the furnace was designed to do that. As it turned out, it wasn’t. But I’m getting ahead of my story.

Here’s the plot I was looking at (click on the image to enlarge it):

I’ve labeled the major electrical events. To understand what’s going on, you need a little background on how furnaces work. When your thermostat calls for heat, the burner in the furnace turns on and starts heating the air in the heat exchanger (the part of the furnace where hot combustion gases transfer their heat to household air). When the air heats up enough, a blower turns on to push the air through the supply ducts into your living space. As the hot air flows into the house, cooler air near the floor is sucked back into the return duct by the blower and sent through the heat exchanger again. In this way, the air flows in a circle, from the furnace to the living space and then back to the furnace to be heated again.

Once the living space reaches the desired temperature, the thermostat signals to the furnace to turn off the burner. The furnace does this, but it doesn’t stop the blower immediately; there’s still hot air in the heat exchanger — why waste it? So the blower keeps running until the hot air is gone.

Nothing I’ve said so far explains why a burner would turn off before the setpoint on the thermostat is reached. So I looked a little more into how furnaces work and found that burner cycling is certainly not a normal condition. Rather, it is a symptom of a furnace starved for air.

If the air being moved through the heat exchanger doesn’t move fast enough, it gets heated up too much. When this happens, the furnace wisely shuts the burner down to cool itself off. This safety mechanism protects the furnace from getting dangerously hot if the blower stops working, and, as well, if the blower can’t pull enough air through the return duct.

Looking for a reason that my furnace might be starved for air, I recalled that when we remodeled our kitchen five years ago, the new floor cabinets ended up covering the return duct. “No problem,” said the cabinet maker, “I’ll just make a slot in the kick plate of the cabinet so the air can flow under the cabinet and into the return duct.” And when I checked the flow of air into the slot with my Wizard Stick, there seemed to be plenty of air moving into it. But was it enough?

Apparently not. When I measured the temperature in the return and supply ducts, I found that the supply duct air was getting much hotter than recommended, confirming my diagnosis. To my family, the furnace was working fine — after all, we hadn’t noticed any problem in five winters since our new kitchen was installed. But the cycling of the burner, probably thousands of times over those five years, wasted who knows how much heating oil. It was like we were driving in stop-and-go traffic instead of cruising at a steady 40 miles per hour.

What did I do to fix the problem? Well, temporarily, I’ve removed the cover on the blower compartment. This lets the blower draw air from the basement and it fixes the cycling problem. But it also carries some risk: sucking basement air into the furnace blower could depressurize the basement enough to backdraft the furnace flue — a dangerous condition in which combustion gases are pulled into the basement instead of going up the flue. I certainly don’t want that to happen and I’ve installed a loud carbon monoxide detector near the furnace to alert me if it does. But this is a problem that should be fixed right away by adding another return duct.

What does it take to diagnose appliance problems with your energy monitor? Obviously, an interest in how things work. But if that’s not you, have a fixit-type friend or a service technician look over your energy plots. Maybe they’ll spot a problem.

The second key to successful diagnostics is good time resolution. Your energy monitor needs to sample your electricity usage at least every second. Anything less is like trying to peer through a fogged up window. You might know how much electricity your furnace draws in total but you won’t see the burner and blower turning on and off.

Monitoring the energy in your home is like watching the gauges in your car. Most of the time, things hum along as they should. But every once in a while something goes wrong. It’s then that paying close attention can save you energy and dollars.

[An earlier version of this post appeared on the Energy Circle blog.]

Our main theme is residential and small business energy and how to manage it. But don’t hold us to that. Occasionally, we’ll stray into other issues like alternative energy technologies, energy policy, or even cultural barriers to reducing energy usage.

We welcome discussion of our posts from all points of view, but please stay away from raw political controversy. Just like in our home, we’d like to keep the temperature down.