It’s wintertime in the Northern hemisphere and that means we’re heating our homes. The new year started off a bit on the warm side, with the temperature at nearly 70° F (21° C) here in the Atlanta area. In terms of heating degree days (HDD), we’ve gone from about 12 HDD/day at the beginning of the month to 24 HDD/day in the past week. With only four days to go in the month, we’re at 515 HDD and probably won’t hit our average of 684 HDD for January so it may be another down year for heating degree days here.
Still, with temperatures dropping into the low 20s Fahrenheit for the next couple of nights, we’ll have lots of demand for heating in the ATL. And nearly all of that heating will come from one of these three sources.
This one’s common in many places that do significant heating. It’s the furnace that extracts the heat from burning natural gas or propane, the boiler burning natural gas or fuel oil, and woodstoves burning wood. Fuels like natural gas (which is mostly methane), propane, fuel oil, and wood contain chemical energy. The process of combustion releases that energy in the form of heat. Then the heat is distributed through the living space with air ducts and hydronic tubes.
Burning a fuel and using the heat in your home will be less than 100% efficient. Furnaces, boilers, and woodstoves send exhaust gases up the flue and some of heat of combustion goes right out of the house along with the exhaust.
If you’re a clever person, though, you may be thinking to yourself, “Hold on a minute, Allison. What about unvented space heaters or ventless gas fireplaces inside the living space?” Every BTU of heat stays in the home as there is no flue to carry away the exhaust gases. Right? Not so fast. Actually, a 96% efficient condensing furnace is more efficient than a ventless gas fireplace. The key to understanding that lies in the water vapor created during the chemical reaction of combustion. See my article on the topic for more detail.
But let’s also remember that unvented space heaters of any kind, including ventless gas fireplaces, are a bad idea. Pretty much anyone who understands indoor air quality will tell you they can be a problem even when operating properly because of the water vapor and nitrogen dioxide created.
So in the type of combustion system that has a flue for the exhaust gases, you’ll always lose some heat to the outdoors and thus your efficiency will be less than 100%. The best furnaces and boilers these days are up in the high 90s, though, so you can capture most of that heat. But that doesn’t count what you might lose in distributing that heat if your air ducts or hydronic tubes are not completely in conditioned space.
Another way to get heat is to move electricity through a type of conductor that has a high electrical resistance. That converts the electricity to heat…and it does so at 100% efficiency. Don’t get too excited about that, though, because the third method below gets more than 100%.
A lot of homes have electric resistance heat (also called strip heat) built into the central forced air HVAC system. That’s the case in the photo above. If it’s used as supplemental heat in a heat pump, it can be OK. If it’s your primary source of heat, your heating bills will be higher than you could be paying because you could get much more heat from that same electricity in a heat pump.
Sometimes that electric resistance heat in a system malfunctions, too. The worst is when it comes on while your air conditioner is running in summer. Not only will your bills be way too high but you may have trouble cooling the house.
Outdoor air, ground, or water
The best way to heat your home is by capturing heat from the outdoor air, ground, or water. (Yes, I do have an opinion.) You may be thinking, how the heck can you get heat from cold outdoor air? Great question! It’s basic physics.
The second law of thermodynamics says that heat flows from warmer objects to cooler objects. If you want to wring heat out of air that’s 20° F, say, you need to have something colder than 20° F in contact with the air. And that’s what heat pumps do.
Regarding efficiency, electric resistance gives you one BTU of heat for each BTU of electricity. (Yeah, electrical energy is measured in kilowatt-hours but both are units of energy and the conversion is straightforward.) With a heat pump, you generally get more than one BTU out for each BTU in. The ratio of BTUs out to BTUs in is typically around 2 or 3 but can be higher with better heat pumps or lower in really cold weather.
Heat pumps do have the drawback of their heating capacity going down as the temperature goes down, and that’s what you need supplemental heat for. As mentioned above, electric resistance is often used as the supplemental heat source in heat pumps but it’s not your only option. A hydronic coil connected to the water heater can be a really good way to provide that extra heat you need on the cold nights.
Also, the technology now is much better than it was in decades past. Inverter-driven mini-split heat pumps can provide their full heating capacity down to the single digits Fahrenheit. We’ve got Mitsubishi mini-split heat pumps in our office in Decatur, Georgia and they’ve been great, even on those days where the temperature has dipped into the 20s. (Disclosure: Mitsubishi is an advertiser here.)
What about solar heating?
Yeah, a few homes are heated with solar energy. Back in the ’70s, that was a thing. Here’s a really cool solar home built in the early ’70s by architect Richard Levine, and which he still lives in. I got to visit him there a few years ago and I can tell you, it’s quite the spectacle.
But capturing solar radiation for heat turned out to be mostly a deadend. As Martin Holladay has shown, superinsulation won the battle between more windows or more insulation (pdf). Now we know it’s better to use that solar radiation to make electricity.
In the end, if you’re wondering where the heat in your home comes from, it’s almost certainly one of the three main sources: combustion, electric resistance, or the outdoor air, ground, or water.
This article was updated on 1/29/19 to correct the original statement about unvented space heaters being 100% efficient. They are not. See the combustion section above.
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