Amy's energy efficiency blog

Ventilation options for green homes

2010-03-10T05:35:00.000-08:00

This is a frequently asked question - what are the options for fresh air ventilation for Energy Star, Healthy Built, and LEED, and what are the pros and cons of each?

There are basically three options for fresh air ventilation - I'll offer a short description of each below and try to summarize the pros and cons that I hear most frequently discussed. While I would love to see everyone install an ERV, it isn't always feasible.

1) ERV (Energy Recovery Ventilator): This is a device that has its own fan that brings in fresh outside air and exhausts some air from the home. It can be tied into the home's HVAC ductwork, but it doesn't have to be - it can be separately ducted as well.

PROS:
Pressure-balanced system. This is mainly a concern for super-tight homes.
You know where the air is coming from (because you choose the location of the outside air intake).
Energy recovery recaptures some energy from the outgoing air.
The ERV fan has a smaller fan than the air handler (usually about 100W, but some new models are as low as 40 W).
Can work with non-ducted systems (like radiant floor).
Versions available with high-efficiency filters.

CONS:
Most expensive option - probably $1500-$2000 installed.
Some less expensive versions don't have timers or speed controls and may give you more air than you want.

2) Supply air only - This system involves a small outside air intake duct that connects to the return side of the air handling unit for the home. To qualify as a ventilation system, there MUST be an automatic damper in the duct and a controller. The controller is set to operate for a specified number of minutes each hour, and will cycle the air handler fan if it does not run for this period of time in any given hour. If the fan runs longer, the damper will close to avoid over-ventilation. The "air cycler" and a similar controller by Honeywell are the most commonly installed local versions of this system.

PROS:
You know where the air is coming from (because you choose the location of the outside air intake).
Intake air passes through the filter in the air handling unit, which could be high-efficiency. It's also a good idea to have a coarse filter right at the intake duct.
Since it uses the air handling fan, this also mixes air in the home to dilute pollutants. This can reduce occupant exposure to some types of pollutants (those emitted in occupied rooms).
Less expensive to install (~$300-400 typically).
Puts house under positive pressure. Some people feel that this is better than negative pressure, but the pressure is fairly small, and it's not clear to me that this is the biggest issue at play in our location.

CONS:
No energy is recovered from the outside air.
The air handling fan uses a lot of power (~300-400W), so it doesn't need to cycle for much extra time before a lot of energy is used. There is little additional cycling in many climates but because Asheville has extended periods of mild weather, there could be a lot of fan cycling here. This system uses the most overall energy of the three options.
Cannot be easily be used with non-ducted HVAC systems.

3) Exhaust only - This system involves using an exhaust fan that either runs continuously or on a timer to exhaust air from the home. There are bath fans available that do "double duty" as this type of fan (they run on a low, continuous setting all the time and then kick up to the normal flow rate when it's used as a bath fan). The fans have to be low sone. The Panasonic Whisper Green is often used locally; Broan/Nutone also has models that allow numerous fans to be controlled together to perform this function. There is also a switch called the "Smart exhaust" that will convert any low-sone bath fan to this function.

PROS:
Can be used with non-ducted systems.
Very low installation cost. Bath fans with this capability run approximately $150, and the smart exhaust switch is available locally for about $50.
Overall operating cost in our climate is relatively low. Although there is no heat recovery, the fan wattage is typically very low (10-20 Watts), so the overall energy consumption is almost as good as most ERVs.
Usually involves a bath fan upgrade, which increases the likelihood that the bath fan will function well as a bath fan and remove moisture from the home. Can be very helpful in densely occupied homes to keep indoor humidity levels down.

CONS:
Incoming air is not filtered.
Puts house under negative pressure. Some people don't want to do this, but the pressure is fairly small, and this is probably most important if you know the home has a problem crawlspace, etc. There are limits to how much air you can move at a time with this system in hot/humid climates, but it's usually possible to comply. Asheville is not considered a hot/humid climate. (it's mixed/humid)
No energy is recovered from the outside air.
You can't control where the air is coming from, so it is important to seal the house well from garages, crawlspaces, etc. A house with a lot of surface area that is attached to something like a problem crawlspace might not be a good candidate for this system.
The system does not mix the indoor air.

Energy Star media opportunity

2009-10-22T05:33:00.000-07:00

For builders or buyers of Energy Star homes: the EPA is planning some media "events" for its millionth home celebration and want stories about Energy Star homes and the people who live in them. This is a great opportunity for some national attention.

http://www.energystar.gov/index.cfm?fuseaction=mil_homes.showSplash

RENOVATION tax credits and incentives

2009-10-19T12:22:00.000-07:00

For existing homes, there are also credits and incentives. Here are links to sources of further information - read the rules carefully and discuss with your accountant if necessary. Be aware that in some cases, the home must be your primary residence.

Federal renewable energy tax credits (solar thermal, solar PV, wind, micro-hydro, and geothermal) and credits for installing new insulation, HVAC, windows, and more:
Federal: www.energystar.gov/taxcredits

North Carolina State renewable energy tax credits:
http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=NC20F&re=1&ee=1

Progress Energy Home Energy Improvement Program: rebates for insulation, new windows, upgraded HVAC, and duct tightness testing:
http://progress-energy.com/custservice/carres/efficiency/programs/heip/index.asp

Summary of NEW HOME energy efficiency incentives

2009-10-19T11:57:00.000-07:00

There are a lot of energy efficiency incentives for new homes out there - these are the relevant ones that I know of right now:

Federal tax credits:
(1) $2000 federal tax credit to the builder of a home that used 1/2 the heating and cooling energy of a home just built to code. Note that the home has to exceed Energy Star by a substantial margin, and that the credit is based only on heating and cooling and does not include water heating or appliances. The same process used to certify homes for Energy Star is used, but our computer program prints out a certificate for homes that qualify, and we present it to the builder upon completion of the project.
Note: congress is currently considering extending this and may add further incentives when they do so.
(2) Renewable energy: 30% federal tax credit for renewable energy systems including solar thermal, PV, wind, micro-hydro, and geothermal.
http://www.energystar.gov/index.cfm?c=tax_credits.tx_index (this site also has a good FAQ)

State tax credits - North Carolina:
(1) Renewable energy: 35% state tax credit for solar thermal, PV, wind, micro-hydro, and geothermal. There are caps on each, depending on technology. There is also a tax credit for passive solar homes that allows you to take a credit for a portion of windows, thermal mass, and control systems. However, the rules of the credit are very specific and homes MUST be designed with its requirements in mind to qualify. Also be aware that the total tax credit taken in a year can't exceed more than 50% of your total state tax liability - but you can roll the credits forward for up to 5 years.
http://www.dsireusa.org/incentives/incentive.cfm?Incentive_Code=NC20F&re=1&ee=1 (NC renewable credit overview)
http://www.dsireusa.org/ (links to incentives for all 50 states)

Local municipalities:
(1) City of Asheville: $100 permit fee rebate for Energy Star new homes. Additional $100 permit fee rebate for certified Healthy Built Homes.
(2) City of Black Mountain: $500 permit fee rebate for new Healthy Built homes certified at the Bronze level or better.

Utilities:
(1) PSNC gas company: $0.05 per therm rate discount for Energy Star homes, and $100 rebates for high efficiency gas furnaces, boilers, and water heaters.
http://www.psncenergy.com/en/residential-services/customer-service-center/rate-plans/default.htm#discount
http://www.psncenergy.com/en/save-energy-and-money/appliance-rebates/default.htm
(2) Progress Energy: 5% discount on electricity and a $400 rebate for new Energy Star homes with 14 SEER or higher AC or heat pump. The rebate is increased by an additional $300 per unit (max 2) for 15 SEER or higher, and an additional $600 per unit (max 2) for geothermal.
Note: 5% discount is to the homeowner, rebates go to the builder.
http://progress-energy.com/custservice/carres/efficiency/programs/ha/details.asp#faq
(3) Duke Power: 5% discount on electricity for Energy Star new homes.

Disclaimers:
When trying to determine whether you will quality for a rebate, pay attention to these things:
1) check with your accountant to make sure you will be eligible (you have to pay taxes to take tax credits!)
2) check the eligibility rules - we have heard that it can be tricky for owner-builders to take the $2000 federal builder tax credit. If something says it's for builders, an owner builder may not be eligible. We don't make the rules, we just provide the paperwork!
3) make sure the home you are buidling qualifies. Energy Star homes must be inspected throughout construction - once drywall is on, homes can't be certified.
4) We've done our best here to outline the pitfalls, but check each of the websites yourself to make sure you have current and accurate information.

Great progress energy workshop

2009-09-16T10:12:00.000-07:00

For builders - there's a great progress energy workshop on Energy Star coming up. $40 to attend, and you get a $400 coupon you can use for a rating. Pass it on!

http://www.advancedenergy.org/buildings/courses/registration/es_pe/

North Carolina renewable energy tax credits

2009-08-24T15:33:00.000-07:00

The state legislature appears to have just passed what will be a game changer.
http://www.ncleg.net/Sessions/2009/Bills/House/PDF/H512v5.pdf
First, they have renewed the 35% renewable energy tax credit for active solar, PV, passive solar, and other renewables. Coupled with the 30% federal tax credit (which now thanks to the stimulus has no cap for individuals), this is now a 65% off discount on these technologies if you pay enough taxes to use the credits.

The game changer is that they have also included geothermal for the first time. There is a cap , but it's pretty high ($8400 on the credit) so many small/medium systems won't reach the cap. At 65% off, the price of a geothermal system can look a lot like the price for a high-efficiency air-source heat pump. And a geothermal system with a water to water heat exchanger to replace a boiler system powered by oil or propane can look like a fast payback. I can't really just throw out numbers because different sites will vary a lot, but if you pay taxes, geothermal should be very seriously considered for both new homes and upgrades of existing homes.

There are also Progress energy rebates if you are in their territory:
Home Advantage (for new homes)
http://www.progress-energy.com/custservice/carres/efficiency/programs/ha/index.asp
and Home Energy Improvement Program (for existing home upgrades):
http://www.progress-energy.com/custservice/carres/efficiency/programs/heip/details.asp

SEE Expo presentation

2009-08-21T13:56:00.000-07:00

This is for everyone who came out to see my presentation at the SEE Expo today - titled "Energy efficiency on a budget". We listed all of the things everyone at the office could think of that are easy and cheap and save energy, and we categorized them by how easy they are to do (couch potato, small effort/cost, medium effort/cost) and then estimated the carbon emissions reduction. Here they are in order with estimated tons of carbon and effort. They all add up to 10.1 (although everyone can't do all of them, and some would be double dipping), but the average car creates 6.3 tons of carbon, so you can easily take the equivalent of a car off the road. The "couch potato" ones add up to almost 1/2 a car!

Rank Item CO2 reduced (tons) Cost/effort
1 Insulate attic ceilings 1.5 medium
2 Heat pump water heater 1.4 medium
3 Don't run HVAC fan continuous ON 1.05 couch potato
4 Insulate floors 1 medium
5 CFL bulbs 0.7 small
6 Don't turn AC on and off. 0.6 couch potato
7 Clothesline 0.54 medium
8 Get rid of un-needed things (xtra fridge) 0.46 couch potato
9 Air sealing 0.4 small
10 Low flow fixtures 0.35 small
11 Kill phantom loads w/ power strips 0.28 small
12 Monitor power 0.28 small
13 Blinds/shades/trees on S, E, W sides 0.25 small
14 Close windows before running heat/AC 0.2 couch potato
15 Energy star appliances 0.19 couch potato
16 Turn down water heater 0.17 couch potato
17 Programmable thermostat 0.16 small
18 Pipe insulation 0.15 medium
19 Seal/cover attic fans, window AC in winter0.1 small
20 Keep garage door closed 0.1 couch potato
21 Turn down thermostat 0.053 couch potato
22 Fix attic kneewalls 0.05 medium
23 Lock windows to get better air seal 0.05 couch potato
24 Insulate hot water tank 0.04 small
25 Insulate + w-strip attic hatch 0.03 small
26 Fix water drips and leaks 0.001 small
27 Dual flush toilet conversion 0.001 small

Energy cost comparison for different fuels and systems

2009-08-21T13:47:00.000-07:00

My previous post shows the results we obtained when we ran annual energy simulations for the same house in Asheville, NC. Obviously the actual energy use depends on the house design, but this shows how systems perform relative to one another in the same house.

You can see that solar and heat pump water heaters are by far the most inexpensive to run. This is followed by on-demand natural gas, which is about $65 per year ahead of a regular storage tank electric water heater. Alone, that's not enough to justify the service charges to have gas in the house. At $2 per gallon, the propane tankless is barely better than the electric tank, and at $3.30, it's much worse. Usually I only recommend propane tankless for second homes where usually no one is using it, but on occasional weekends 14 people in a row need to use the shower.

In terms of energy cost, geothermal wins for space conditioning. Using a heat pump with furnace backup is less expensive than using a heat pump alone - here it was 22% better, or $100 savings. Again, it's not a compelling economic case to pay gas service charges and install another piece of equipment. With propane at $2 per gallon, the savings are smaller, and with propane at $3.30 per gallon, you'd be better off with the heat pump alone. The gas furnace is easier to justify if you want a fast recovery time, or if the home occupant is really sensitive to moving air and likely to complain about a heat pump. But most of us would do just fine. The systems that use just a furnace or boiler alone always cost more to operate. If these are augmented with solar, this could turn around, but most homes would need a lot of solar. One or 2 panels isn't likely to do it.

2009-08-21T13:45:00.000-07:00

Here's the energy cost comparison that I ran for a prototype house in Asheville, NC.

Gas or electric?

2009-07-18T07:39:00.000-07:00

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.

The problem with energy modeling net-zero homes

2009-06-22T19:36:00.000-07:00

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?

2x4 or 2x6?

2009-06-06T20:13:00.000-07:00

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...