This is a question I get asked all the time - which is better, gas or electric? There's not a single answer to this question, it depends what your criteria is for "better". Most people want to minimize operating cost, but some people want to use less energy or have lower carbon emissions. Many people would like to do all of the above. In this post, I'll answer the question in generalities, and then follow up with a couple posts that have actual numbers. Here are the major differences between the fuels:
Equipment efficiency: The efficiency of gas furnaces and water heaters can't ever go above 100%. In reality, 95%-97% is the highest efficiency out there. This is because they burn the fuel to generate heat, and you can't get more heat energy than is in the fuel. The same is true for electric equipment (like most water heaters and baseboard radiators) that use the electric to directly generate heat. However, there is one big difference: electricity is generated by burning fossil fuels at a power plant, and then delivered to you at about 35% efficiency. So, a 100% efficient baseboard radiator is really only 35% efficient in terms of "source" energy use. This is why carbon emissions are often higher for electric equipment. Heat pumps are an exception to this rule. Instead of using electricity to generate heat directly, they use electricity to move heat from one place to another. For every 1 unit of energy that a heat pump uses, you can move approximately 3 units of heat energy into your house. This is equivalent to being 300% efficient. Combine that with 35% distribution efficiency and you're still at 105%, which puts them back into competitiveness with gas in terms of source efficiency and emissions. One downside with heat pumps is that when it's very cold outside, they don't work very well and have to use "backup heat" (which takes us back to generating heat directly from an electric coil) - if a home is in a cold climate, unless you have a special type of heat pump, this will reduce the overall efficiency throughout the year.
Fuel cost (usage, service charges, and hookup): Electricity is one of the most "price stable" forms of energy. Its cost varies a lot depending where you live, but it does not tend to fluctuate like some other fuels do. Another advantage is that every home has electricity coming into it. So, you would be paying the monthly surcharge for this utility anyway (most utility bills have a monthly "base fee" that everyone pays regardless of usage - usually $10 or so). For a new home, you would be paying to hook into the electrical grid anyway (or installing solar and a lot of batteries). Natural gas price is more volatile than electricity, but not as volatile as propane. However, it it not available as a utility in every location, and if it is available owners of new homes may have to spend several thousand dollars to hook up to the gas utility. There is usually a monthly service charge to have gas service to the home (typically about $1o per month). So, to be more cost-effective, efficiency gains have to be large enough to save at least $120 per year plus recoup installation costs (this almost never happens in affordable housing, which is smaller and has lower demand for heating to begin with). If natural gas isn't available, propane is the next option. Most people either purchase a tank for an up-front charge, or they rent a tank for a monthly fee (similar to the service charge for other utilities). Propane prices have been very volatile over the past three years (varying from $1.30 to $3.30 per gallon) in our area. At the higher range of these rates (over about $250 per gallon), we have not been seeing any circumstances where propane is the lower-cost option.
Equipment cost: In our area, most homes have air conditioners. A heat pump is simply an air conditioner that can work in reverse in the winter-time. If a home is going to have air conditioning anyway, "upgrading" to a heat pump is very inexpensive. There is cost to add a gas furnace. Also, with furnaces and gas water heaters, there can be some added cost to install gas piping.
Comfort: Some people find that gas heat makes them feel "warmer". Technically, heat pumps and gas furnaces can both "meet" your thermostat setting and produce the same indoor temperature. The difference is that heat pumps supply air at lower temperatures than gas furnaces do. If you feel 85 degree air blowing out of a register, it will feel cool, but if you feel 120 degree air blowing out, it will feel warm. Ideally, the HVAC system will be designed and installed so that you don't feel air blowing out of the registers, and this will be a minor issue. A side effect of this is that the gas furnace will have a shorter "recovery" time. If you go on vacation and set the heat down to 50 degrees, when you come home you can fire up a gas furnace and be warm very quickly. With a heat pump, it will take longer. Some programmable thermostats now let you program in your return date so that the house can be ready for you.
Indoor air quality: All homes with any gas appliance should have carbon monoxide detectors. In addition, "sealed combustion" equipment is safer and less likely to backdraft combustion by-products into the house. This type of equipment is also more efficient, so there are a number of reasons to install it. Using electrical appliances means that combustion takes place at the power plant rather than in your home. This affects outdoor air quality, which is where our indoor air comes from and also a major issue on its own. In the "green building" world, if you choose electricity as your primary fuel source, then you should also actively support the "greening" of the electrical grid to include more renewable energy.
Carbon emissions: Natural gas, propane, and electricity (in our area generated mostly from coal) have different carbon emissions. Per 1 million Btu, electricity generates about 360 lb CO2, natural gas about 120lb, and propane about 140 lb. That means that to get equal carbon emissions, electricity must be used 3 times as efficiently as natural gas, and about 2.5 times more efficiently than propane.
Saturday, July 18, 2009
Monday, June 22, 2009
The problem with energy modeling net-zero homes
What's the biggest problem? Accuracy. A big part of home energy use depends on the occupant. And those darned occupants don't behave as regularly as we'd like them to. We have quite a bit of information what "average" occupants do and how they behave on an average day. The problem is this: how many people do you know who are home every day at the same times, do the same amount of cooking every day, and use the same appliances? Compare that to the number of people you know who don't cook anything at home for a week straight and then invite 6 friends over and cook up a storm on the weekend.
At best, energy models usually let us schedule internal gains and occupant activity on an hourly basis on "weekday" and "weekend" schedules. So, you have 4 people in the house, they go to school or work, they cook, they watch TV, etc. You have a typical weather year, which we may or may not be actually having. Most of the estimates that I've seen are that a typical home's energy use can vary by about 20-30% based on occupant behavior. When I model homes that are in the "typical" or somewhat less energy than typical range, and when I get follow-up data for comparison, I'm usually pretty close. Often closer than 20-30%. But really, I consider within 20% to be pretty good considering the degree to which the input data is an estimate.
"Net-zero" homes pose a couple of distinct problems. First, the occupants are almost never ordinary. Let's face it - most of us don't live in net-zero homes because we either just plan can't afford it yet, or haven't made it a big enough priority in our lives to figure out how to afford it. Unless we're quite wealthy or have a very unusual site, it always makes sense to spend money and effort on extreme conservation before adding more energy generation to get to net-zero. So, the internal heat loads for net-zero houses aren't going to be well predicted based on "average" occupants. The second major problem is that the lack of precision with which we know how occupants behave becomes much more important when you get into very low net energy situations. To put some numbers on it, let's say that a given homeowner's behavior uses 10% more energy than the average person. If that person lives in a house that is just built to code (HERS rating of 100), then it really functions at 110% (HERS rating of 110) and we're not too far off. But if you take that same person and put them in a house that's close to net zero (say it has a HERS of 5), then it really functions more like a HERS of 15 and we're off by a factor of 3. What gets even more interesting is that unless this person is Mr. Boring, he probably has weeks where he's a 20 and other weeks when he's out of town and is a 0.
This can start to matter a lot when designers try to adopt a strategy that counts on these internal gains as part of a function of the whole system. For example, if you're counting on the internal gains to replace part of all of the heating system in the house, you need them to be there. One of my earliest exposures to green residential building was just such a project. What happens when the family that moves in happen to be smaller than average, cooks less than average, some of them go on business trips, and you get a long stretch of cold weather? You sit around and brainstorm ways to make more internal gains: oven cleaning, anyone?
At best, energy models usually let us schedule internal gains and occupant activity on an hourly basis on "weekday" and "weekend" schedules. So, you have 4 people in the house, they go to school or work, they cook, they watch TV, etc. You have a typical weather year, which we may or may not be actually having. Most of the estimates that I've seen are that a typical home's energy use can vary by about 20-30% based on occupant behavior. When I model homes that are in the "typical" or somewhat less energy than typical range, and when I get follow-up data for comparison, I'm usually pretty close. Often closer than 20-30%. But really, I consider within 20% to be pretty good considering the degree to which the input data is an estimate.
"Net-zero" homes pose a couple of distinct problems. First, the occupants are almost never ordinary. Let's face it - most of us don't live in net-zero homes because we either just plan can't afford it yet, or haven't made it a big enough priority in our lives to figure out how to afford it. Unless we're quite wealthy or have a very unusual site, it always makes sense to spend money and effort on extreme conservation before adding more energy generation to get to net-zero. So, the internal heat loads for net-zero houses aren't going to be well predicted based on "average" occupants. The second major problem is that the lack of precision with which we know how occupants behave becomes much more important when you get into very low net energy situations. To put some numbers on it, let's say that a given homeowner's behavior uses 10% more energy than the average person. If that person lives in a house that is just built to code (HERS rating of 100), then it really functions at 110% (HERS rating of 110) and we're not too far off. But if you take that same person and put them in a house that's close to net zero (say it has a HERS of 5), then it really functions more like a HERS of 15 and we're off by a factor of 3. What gets even more interesting is that unless this person is Mr. Boring, he probably has weeks where he's a 20 and other weeks when he's out of town and is a 0.
This can start to matter a lot when designers try to adopt a strategy that counts on these internal gains as part of a function of the whole system. For example, if you're counting on the internal gains to replace part of all of the heating system in the house, you need them to be there. One of my earliest exposures to green residential building was just such a project. What happens when the family that moves in happen to be smaller than average, cooks less than average, some of them go on business trips, and you get a long stretch of cold weather? You sit around and brainstorm ways to make more internal gains: oven cleaning, anyone?
Saturday, June 6, 2009
2x4 or 2x6?
My intention is to use this blog to discuss questions I am asked all the time. So, the 2x4 vs. 2x6 debate seems like a good opening post.
There are 2 major issues: how much wood is used and the R-value of the wall assembly.
In terms of wood used, if both have the same stud spacing, the 2x6 obviously uses more wood. But if you can go up to 24 o.c. for the 2x6, it's about the same volume of wood used. (About 1/3 less due to spacing, but 1/3 more due to thickness). Some drywall installers complain, but I've seen it work just fine.
In terms of R-value, 2x4 will get you between R-13 and R-15 for most cavity insulations, and 2x6 can give you R-19 to 23. From an insulation standpoint, you're obviously better off with the higher R-value. But it turns out that an R-13 wall with R-5 exterior sheathing is almost exactly equivalent (in terms of overall assembly R-value) to R-20 in a 2x6 wall. So, you could get almost the same performance. In the past, R-5 exterior sheathing has been tricky for a number of reasons: structurally you have use OSB at the corners or use metal bracing, and siding guys aren't crazy about having to locate the studs to nail into. But there is a new product out from DOW that combines structural sheathing with an R-5 all in one product - eliminating all these problems. Of course nobody's saying you can't also use R-5 sheathing on a 2x6 wall...
There are 2 major issues: how much wood is used and the R-value of the wall assembly.
In terms of wood used, if both have the same stud spacing, the 2x6 obviously uses more wood. But if you can go up to 24 o.c. for the 2x6, it's about the same volume of wood used. (About 1/3 less due to spacing, but 1/3 more due to thickness). Some drywall installers complain, but I've seen it work just fine.
In terms of R-value, 2x4 will get you between R-13 and R-15 for most cavity insulations, and 2x6 can give you R-19 to 23. From an insulation standpoint, you're obviously better off with the higher R-value. But it turns out that an R-13 wall with R-5 exterior sheathing is almost exactly equivalent (in terms of overall assembly R-value) to R-20 in a 2x6 wall. So, you could get almost the same performance. In the past, R-5 exterior sheathing has been tricky for a number of reasons: structurally you have use OSB at the corners or use metal bracing, and siding guys aren't crazy about having to locate the studs to nail into. But there is a new product out from DOW that combines structural sheathing with an R-5 all in one product - eliminating all these problems. Of course nobody's saying you can't also use R-5 sheathing on a 2x6 wall...
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