By Matthew Power Combine 24-inch, on-center framing with R-19 fiberglass batts. Then blanket the whole structure with taped, caulked 1-inch EPS foam. Next, carefully install and seal your low-E windows and doors. Result: a house with a predictable microclimate.
Which is exactly what engineer Joe Lstiburek of the Westford, Mass. based Building Science Corp. had in mind when he drew up plans for the Atlanta Homelink project's HVAC system.
Based on the home's tight shell, Lstiburek arrived at an estimate for the home's HVAC needs. That figure came in at about half what Carrier Corp. and David Swenson, the HVAC installer, had in mind for equipment. Everybody compromised. Carrier plugged in its ultra-efficient, two-speed condensing unit, capable of adjusting to any demand between 2 1/2 and 4 tons. And Lstiburek got his unusual, minimalist duct system.
"What you have in this house is really a duct system that's independent of the mechanicals," notes Lstiburek. "We wanted to simplify and downsize the system, so we made sure all of the ductwork was kept inside conditioned spaces. That makes the system more efficient, which means it can be smaller."
One of the key components of the duct system is the use of passive airflow in lieu of multiple return air ducts. As Lstiburek explains it, because the home's envelope is so tight, the flow of air can be carefully controlled.
The resulting infrastructure adheres to principles Lstiburek has long held dear--low-tech, well-designed ducting.
"The biggest problem homes in the South have is leaky duct systems," he says. "You can deal with leaky duct runs by tightening them up, or by simply getting rid of them, which is what we did. Of course this will only work in an ultra-efficient building."
Homelink contains only one return duct. Everywhere else in the house, air moves passively.
"We have grilles in stud cavities, and short little ducts in the floor system and ceilings that connect rooms," Lstiburek explains. "It works because we have such an efficient delivery of the supply side air. We get a uniform mixture."
In the zone
The secret to that delivery lies in the eight electronic dampers installed in the supply ducts. "That's where I believe the real money savings is," says installer David Swenson, with A-OK Heating and Cooling in McDonough, Ga. "Dollar for dollar, zoning really makes sense."
Stroh Brand, a project manager with Carrier Corp., adds that the extra cost of zoning (about $1,500 installed) should be a non-issue with homeowners. "I've never done a scenario where upgrading to a zoning system didn't save you money from the moment you walk in the house," he says. "If it saves you $18 a month on electric, and adds $7 to your mortgage, where's the problem?"
This zoning system also has "smart" capabilities. It will ultimately be tied to a whole-house interface that carefully controls which rooms most need attention--for instance, favoring the sunny side of the house on a hot day.
Clearing the air
All parties involved in the home's mechanicals agreed on the need for mechanical ventilation. An energy recovery ventilator (ERV) has lots of perks: It efficiently replaces fresh air in the house and keeps pollutant levels down--but homeowners have to be told how they work.
As a footnote, Brand adds that many home buyers who don't understand the devices often misuse their ERVs. For example, they shut them off at times allowing moisture to build up in the home, or they leave them on and open the windows. "Either choice creates major inefficiencies," he explains. "If you turn the system off, it might take an hour and a half to suck out all that humidity when you restart."
All this state-of-the-art equipment costs more, of course, but the Homelink HVAC team argues that between savings on ductwork, payback from energy savings, and comfort improvements, the price is right for consumers.
"I honestly believe a good heating contractor should encourage people to do things like this," says Swenson. "The first thing people ask me these days is 'Can I get a more efficient air conditioner?' I try not to sell on economics. I sell on comfort levels."