Heat Pump Comparison Flawed?

Another difference is that the electric baseboard heat in RSDP homes is zoned, while the heat pumps are ducted to the whole house.

The comparison, then, is between high-tech heating systems (the heat pumps) in moderate shells and low-tech (electric-resistance) heating systems in high-tech shells.

Jim Maunder
Missoula Electric
Missoula, MT

Author Jeff Stein responds: Mr. Maunder's point is well taken. Drawing conclusions about heating system efficiency by comparing actual heating or whole-house energy consumption can be like comparing apples and oranges, if the houses have unaccounted-for differences in insulation levels, occupancy behavior, etc. But it is also the best way we know of getting an accurate picture of field performance and actual savings. Thus we strive to find similar controls (retrofits are usually the best) and to normalize for as many differences as possible. We have recalculated the Montana heat pump and control homes using total UA, rather than floor area, and found that the heat pump homes do indeed save about 13% of total energy consumption. This is still considerably less than the 35% savings predicted by the COP measurements made at the heat pump homes.

It should be noted that Mr. Maunder is one of the few people in the country bothering to carefully monitor the performance of homes that he helped sponsor. His comments about the heat pump homes are especially important because, about ten years ago, he was involved in building the homes that became the controls in our comparison.

Give Heat Pumps Their Day in the Sun

As the authors contend, it is clear that if a geothermal (or fossil fuel) system is poorly installed, it will not perform up to design specifications. Our new construction energy conservation program, the Energy Crafted Home program, requires all HVAC contractors installing geothermal systems to perform an approved Air Conditioning Contractors Association Manual J equipment sizing calculation and Manual D duct system layout and sizing calculation. We have found that, when properly designed and installed, geothermal heat pump systems reduce customers' fuel costs, build positive load for the utility, and help preserve the environment by reducing toxic and greenhouse gas emissions.

John Livermore
Energy Crafted Home Program
Northeast Utilities
West Springfield, MA

Editor's reply: Ground source heat pumps are relatively energy efficient, but their real-world performance is still being studied. The EPA report is based on simulations when heat pumps perform to specifications and ducts don't leak. Actual performance is considerably below expectations in many cases, so the environmental benefits are less certain. A utility program, such as Energy Crafted Home, with good contractor training--and inspection--is the best way to achieve maximum environmental benefits from ground source heat pumps.

A ground source heat pump "harnesses ... solar energy" by using electricity most often derived from nonrenewable resources. Unless that electricity is being made from photovoltaics, implying that they use "solar energy" is misleading and reduces the credibility of ground source heat pump promoters.

Zebra Moisture Mystery

What explains the striping over the studs in summers? Could the thermal mass of the wall be storing heat overnight and preventing the surface of the exterior siding from reaching the dew point temperature? The home is about three years old and is well insulated and sealed for our climate. The wall construction is (from interior out) painted gypsum board, stud framed wall construction, R-13 fiberglass insulation with kraft facing, exterior 0.5-inch polyisocyanurate with radiant barrier on both sides and the seams taped, and 0.5-inch exterior Louisiana Pacific (LP) lap siding. All electrical and plumbing penetrations were foamed, and the soleplate in the wall was caulked. A 6-mil polyethylene vapor barrier is also installed in the crawlspace. Relative humidity levels inside, in the attic, and in the crawlspace were within a few points of 74%.

There are problems with moisture in this type of LP siding, but the homeowner wishes to find out about the condensation before replacing the siding or covering it with vinyl.

Greg Mullis
Tri-County Electric Membership Corporation
Gray, GA

Editor's reply: Maybe the siding is condensing water out of the air surrounding the house. The siding that can "see" the night sky is cooled (by radiation to the cold sky) to a temperature below that of the surrounding air, which then drops below the dew point and condenses water onto the siding. What matters isn't the outside air temperature, but the temperature of the siding surface, which is being cooled by radiation to the night sky to a temperature lower than the outside air.

Since the porch-sheltered siding can't see the sky, it doesn't have this problem; this also explains why it happens year-round in the early morning, when relative humidity is typically at a maximum. If the insulation were not as good as it is, heat leaking out from inside would warm the siding enough to prevent this effect; in fact, that is what is happening at the studs. A test for the theory: shield more of the siding from the night sky (perhaps with a tarp attached to the roof edge and held out like an awning). If the condensation goes away under the cover, then this is probably the cause.

I'm not sure what to suggest as a solution, but if my theory is correct, no harm is being done. It's just dew forming on the outside of the wall. If the siding is going to be replaced, enough extra rigid foam could be installed over the existing insulation (under the new siding) to mask the thermal bridging of the studs, making for a more uniform outside surface temperature. Vinyl siding alone might provide enough extra insulation to get the dew to form over the whole surface. Another option is to plant trees that will shield the walls from the night sky.

Reflections on 
White Roofs

Peter Gollon
Huntington, NY

Editor's reply: While different colors might absorb more or less of the visible light hitting your roof, they may not make much difference in how much solar energy the roof absorbs. As we reported in "White Roofs for Cool Homes" (Nov/Dec '96, p. 28), even the best white asphalt shingles still only reflect 35% of solar energy. They do help--black shingles rise 90°F over ambient air, while white ones rise only 60°F (under the test conditions we reported). But to really make a difference, get a coating engineered to keep the house cool. These coatings can reflect as much as 80% of solar energy and stay only 15°F hotter than ambient air on hot days.

If this sort of coating is too expensive, you might consider a radiant barrier, which can reflect energy that would otherwise heat up the second-story ceiling and ducts running through the attic.

Insulating Floor Trusses

John Webb
Weaverville, NC

Editor's reply: We understand what you mean about drafts--the trusses allow air flow through them. Even with conventional joists, air can flow along them, with the same result, though not as severe. The best way to insulate floors, with the possible exception of spray-on insulations, is to completely fill the joist (or truss) cavities with insulation, leaving no space between the insulation and the subfloor. (There are fire safety reasons for leaving no gap as well; see "Does Floor Insulation Performance Meet Expectations?" Mar/Apr '94, p. 8.)

This usually means using more insulation than simple R-value cost-effectiveness analysis recommends, but you're much more likely to get effective insulation. A less effective alternative is to use spring ("lightning") rods, sometimes called tiger teeth, but they have been known to fall out over time.

Ceilings and Roofs, Reprise

Paul Fisette's two articles ("Out Out, Dammed Ice" and "Roofing and Siding Get an Energy Fix," Nov/Dec '96, p. 25, p. 33) are excellent, as usual. However, I'm certain that given where Paul lives, he has seen that the wide aluminum strips on the eaves, with or without embedded heat wires, don't help. If, after the winters of 1993-1996, anyone still believes that most attic ventilation devices, including some ridge vents, work, or that the code-required ratios provide adequate attic ventilation, they are indeed blind.

The sidebar on roof venting doesn't deal with the fact that many manufacturers of ridge vents claim that internal baffles or no baffles on ridge vents work as well as external baffles. (I fully concur with Paul's position that external baffles are best.)

Gene Leger
New Ipswich, NH

Pilot Lights Use 
More Energy

I live with my family in a 3,300 square foot house. Our annual space heating consumption in a very energy conserving house is the equivalent of 270 therms. Were we to install a gas fireplace, the annual pilot light gas use would increase our space heating by 27%. Ouch!

Could I make the following modest proposal? Let's ban all pilot lights. Period.

Rob Dumont
Saskatoon, Saskatchewan

Editor's reply: Thank you for your vigilance. We regret the error.
 
 

Errata
In "Budgets, Deregulation, and Big Scissors"  (Jan/Feb '97, p.7), we misidentified Daniel William Fessler.  He was with the California Public Utilities Commission, not the California Energy Commission.
 
 

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