Home Energy Monitoring: Surprises, Malfunctions, and Fixes

By David Fay*

We all know the benefits of real-time energy monitoring: determining your electrical baseload, figuring the cost of daily household activities like doing the laundry, discovering that someone left the attic lights on, and so on. But there's another benefit that is often overlooked. Energy monitoring can uncover malfunctioning household appliances. Fixing them can save money. Here's a story from my own household.

I regularly monitor my home electricity usage with T.E.D. The Energy Detective paired with some custom software I put together (energy.shadypixel.com). Recently, I was inspecting a plot of the electricity used by my furnace. It's a forced hot air furnace ordinary in every way except that it burns oil instead of natural gas and its best years are behind it (It's 30+ years old). I noticed that the burner was turning on and off multiple times each time the thermostat called for heat. Not knowing much about furnaces at the time, I wondered why they were designed to do that. As it turned out, they weren't. But I'm getting ahead of my story.

Here's the plot I was looking at:

I've labeled the major electrical events. To understand what's going on, you need a little background in how household 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 from the burner 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 where it is 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.

I didn't know any of this at the time. Looking for some explanation for the burner going on and off, I ran across a discussion of how furnaces malfunction and discovered that burner cycling is definitely 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 by the burner. When this happens, the limit control on the furnace wisely shuts down the burner to let it cool off. This safety mechanism protects against overheating when the blower stops working altogether, and, as well, when 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 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."  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. Interestingly, the furnace was working fine as far as my family was concerned - in the five winters since the remodel, we hadn't noticed a problem. If not for the blip on our electricity monitor, I would never have known.

Beyond the satisfaction of tracking down this malfunction, what was the benefit? For one, starting the burner several times for each call for heat ages the burner at several times the normal rate, because much of the stress on an electric motor and oil pump comes in starting it up. Sooner or later, that burner would have failed prematurely. Beyond that, cycling wastes electricity:  The blower, which uses most of the electricity in the furnace, has to remain on longer than it otherwise would, waiting several times until the burner comes on again. All of this is hard to quantify, but appliances will be most efficient and reliable when they operate as designed.  A household electricity monitor provides an excellent window into the functioning of your appliances.   

What did I do to fix the problem? Well, for the time being, 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 long term, I need to add another return duct. By the way, the burner problem described here is not unique. My energy monitor has also helped me detect a clogged filter in my washing machine and an overworked blower in my other furnace.

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, everything's operating normally. But every once in while something goes wrong and it's then that paying close attention can save energy and dollars.

*David Fay retired recently after a long career in telecommunications research at Bell Labs, GTE Labs, and Verizon Labs. He satisfies his new passion in energy monitoring by serving on his town's Energy Advisory Committee, by taking part in weatherization "barn raisings" and by instrumenting both town buildings and his own home. His household electrical monitor can be found at energy.shadypixel.com.