Are All Air Filter Ratings Equal?

By Dan Morrison

Don't let your lungs pay for bad air filters. Some 99% effective filters are only 10% effective when you change the sampling method. Particle size matters, and so does sampling method.

Our final speaker at Building Science Summer Camp (Day 2), Dr. Dieter Weyel, talked about filters, filtration, and ratings. Turns out, there are some pretty important differences. Particles, like everything else, have to obey the laws of physics. It’s just that our understanding of physics is skewed because we’re big enough that gravity affects us. Gravity affects some particles too -- when they get to be about 10 micrometers. But small particles, say 1 micrometer, can fly. Sort of. They behave like algae in the ocean; they just float around obeying the laws od oceanic currents. Algae are less dense than the salt water they float in, so they float. Small particles are less dense than the air they float in, so they float around moving in response to the air currents.

Big particles fall to the ground and you have to clean them up with a damp cloth or a duster. Small particles float around and you clean them with a filter. As it turns out, lungs are excellent filters. Problem is, clean lungs work better than dirty lungs. That’s where filters come in. There are three ways to measure the effectiveness of a filter:

1. Count the number of particles it catches
2. Count the area of the particles it catches
3. Count the weight of the particles it catches

Dr. Weytal used 10 small particles (1 micrometer) and one large particle (10 micrometers) as an example. If the filter only catches the large particle:

• Method #1 yields a 10% filter.
• Method #2 yields a 92% filter
• Method #3 yields a 99% filter.

For ratings, particle size and sampling method matter
HEPA filters must be judged according to method 1, counting the number of particles it catches. Counting particles isn’t very effective for particles over about 5 micrometers Weyal says, because larger particles tend to overlap.

According to Weyal, ASHRAE measures “sort of by area.” They dump particles into a duct, spray them through a filter, and probe for particles before and after the filter. They shine light through the filter; if there are a lot of particles, not much light gets through. They compare one side of the filter to the other and get a percent efficiency.

As it turns out, particles vary in density too -- a styrofoam peanut and a cork are of similar size, and they both float on water, but they have very different densities. So density matters too.

--Dan Morrison is managing editor of GreenBuildingAdvisor.com

Are Heat Pumps Green?

By Dan Morrison

An article in the Home and Garden section of the New York Times "Time to Worry About Heat Bills" By JAY ROMANO (http://www.nytimes.com/2008/07/24/garden/24fix.html?emc=tnt&tntemail1=y) talks about a winter heating option that will save you money: electric heat pumps. With the price of gas and oil skyrocketing, the article reasons, an electric heat pump will be a money saver this winter and eventually end up "paying for itself."

But is it green?
Not if you're getting your electricity from the grid.

• Coal plants have an efficiency of about 31%; put another way, almost 70% of the energy contained in a lump of coal is lost as heat when it's burned at a coal plant.
• And along with that heat, tons of carbon dioxide are dumped into the atmosphere when the lump of coal is burned.
• And strip mining for lumps of coal leaves a mighty big footprint on the land.

I sure wish I could convince Henry Gifford to let me publish his manuscript on why heat pumps are not such a great idea.

--Dan Morrison is managing editor of GreenBuildingAdvisor.com

What’s wrong with this picture?

a) The rim joist is too heavily notched.
b) Using framing cavities as duct runs frowned upon by codes and professional associations.
c) This passed the framing inspection.
d) All of the above.

The answer is d, all of the above.

While it appears that this rim joist isn’t structural (there are studs below it), The Engineered Wood Association says that when cutting multiple holes in a rim joist, the spacing between the holes should be at least twice the length of the longest side of the longest rectangular hole. And according to Joe Lstiburek, the photographer, this also passed the framing inspection.

This leaves option b, “Using framing cavities as duct runs is a bad idea”. This photo shows both stud and joist cavities being used as return air ducts. In their duct design guide Manual D, the Air Conditioning Contractors of America (ACCA) recommend not using panned joists for return ducts, and the 2006 IRC and IECC codes prohibit joist cavities for supply ducts. It’s too bad that neither does both.

Framing cavities make bad duct runs because the leak and they’re connected to every part of the house via framing gaps and holes for wire and pipe. These leaks suck in air from unintended sources (outdoors, attics, garages, and crawlspaces). Along with this unintended air comes humidity, mold, dust, and whatever else is in these spaces (radon, carbon monoxide, gasoline, paint thinner, pesticides,). Not only does this poison the indoor air supply, but it increases the heating and cooling loads while decreasing the efficiency of the mechanical equipment. A bad idea all the way around.

— Bruce Harley is technical director of Conservation Services Group and author of Insulate and Weatherize (Taunton 2002). Photo by Joseph Lstiburek

Got a dumb building picture? Send it to dan [at] buildinggreen.com

Manual D is not (necessarily) Green

Manual D is about comfort, not energy efficiency. Its requirement in LEED-H makes green certification too expensive to justify the benefit.

By Michael Chandler

I've been building solar and green for thirty years and have built homes that Energy Star certify at 76% more efficient than code and score gold on our North Carolina Green Building Program as well as NAHB’s green building program but I have never built a house that would qualify for even basic LEED-H certification and it doesn't seem likely that I will unless I get a client who specifically requests the LEED-H program over the alternatives. The reason for this is that my homes use oversized ductwork with air flow controlled by butterfly dampers and the LEED-H program requires that airflow be controlled through implementation of ACCA Manual D duct design.

What's the difference between Manual D and Manual J?
There is some confusion in the market about the difference between Manual D, which sizes ducts to best match the equipment and needs of the rooms served, and Manual J, which sizes the equipment to match the actual projected load of the home (and is a basic minimum requirement of Energy Star and most Green Building Standards including the NAHB/ICC National Green Building Standard.) The Manual D duct design standard forces HVAC installers to use 4" insulated flex for smaller rooms and 6" for medium sized rooms and 8" for larger rooms. In my market it’s calculation and implementation adds significant cost to the HVAC system especially on smaller, one-of-a-kind homes that LEED-h is hoping to encourage (the “top 25% of the most environmentally conscious builders” and all that). It is a good system and certainly worth rewarding but doesn’t really fit with the “mandatory minimum for green” in that it is more oriented towards optimizing comfort than saving energy, enhancing durability, or improving indoor air quality in the types of homes that would be reaching for LEED-h certification. It’s a comfort standard, not a green building standard.

The green home I'm building now won't pass LEED-H
We're building an aging-in-place home with a Hybrid Solar-Propane radiant floor heating & domestic hot water system with 15 SEER heat pump for AC and back-up heat that is better than 30% more efficient than code. The house scores gold in NAHB's Model Green Home Building Guidelines and North Carolina's Healthy Built Homes, but it will not qualify for basic LEED-h due to a "lack of comfort" in the AC design that will be used at most two months out of the year. If not for this requirement I think the house would likely be LEED-h Silver but I'm not going to pay to have the house scored when I know that it will fail because of this single requirement.

Seems like a missed opportunity to me.

NAHB is taking advantage of that opportunity
Last summer as we worked on the new NAHB-ICC National Green Building Standard the group discussed following LEED’s footsteps on this issue and decided that we shouldn't disqualify a house for a green rating because the bathrooms and bedrooms might occasionally be slightly less comfortable than the living room. So we awarded points for Manual D implementation but didn’t make it mandatory. The issue is to step lightly on the planet, not to assure that everybody is optimally comfortable at all times regardless of the additional cost.

--Michael Chandler is a home builder and master plumber in Mebane, North Carolina. His website is www.chandlerdesignbuild.com