This article was originally published in the March/April 1997 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
| Back to Contents Page | Home Energy Index | About Home Energy |
Home Energy Magazine Online March/April 1997
Heat Pump Comparison Flawed? In the article on ground source heat pumps (Do You Dig Ground Source Heat Pumps? Nov/Dec '96, p. 33), Jeff Stein's comparison of efficiency studies on p. 37 states that 28 homes in Montana with heat pumps were found to use about the same heating energy as 40 similarly built homes with electric-resistance heat. That's incorrect. The homes did not have similar construction. The 40 Residential Standards Demonstration Project (RSDP) homes with electric-resistance heat, built in 1984, were superinsulated. Their average UA (U-value multiplied by the combined area of the floor, ceiling, and walls) was about 199, thanks to walls averaging R-38 and R-60+ ceilings. The heat pump homes, built in 1993-94, meet super Good Cents standards--R-26 walls, R-49 ceilings, R-38 crawlspace, R-21 walls below grade, R-10 under slab--but are not superinsulated. They have an average UA of about 339.
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.
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 There are several issues and omissions in Do You Dig Ground Source Heat Pumps? that we at the Energy Crafted Home program do not agree with. Of particular note, the authors do not discuss one of the most compelling advantages of geothermal heat pump systems: reduced air pollution. The EPA, in their report Space Conditioning: The Next Frontier, found that emerging geothermal heat pump systems typically produce the lowest CO2 and NOx emissions of any conventional space conditioning system. Geothermal systems harness a natural and renewable energy resource--solar energy--and as such are an environmentally responsible step toward reducing our country's reliance on nonrenewable fuels and achieving President Clinton's Global Climate Challenge.
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.
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 I have been called upon to help a customer deal with a moisture problem. The outside of this house sweats in the morning, year-round. It only sweats between the studs, and not underneath the porch overhangs, so it looks like zebra stripes. We have considered thermal bridging, but cannot understand why it would sweat in this manner throughout all seasons. Although our climate is very humid, our summer temperatures in the early mornings are rarely lower than the indoor design temperature.
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.
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 I am replacing the gray shingle roof on a two-story split-level single-family house with full attic. I'm in Long Island, New York. I am trying to decide between a light- and a dark-colored roof, and one consideration will be the effect on the summer heat loading. If the light roofing absorbs about 70% of the incident visible solar spectrum (as measured with my camera's light meter), and the darker shingles absorb 90%, how much difference in attic and second story temperature can this make during the height of summer? Could it make the difference between having to run an air conditioner or not?
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 My home was built using floor trusses, so the framing members against the floor are laid flat, unlike conventional floor joists. What is the recommended technique of insulating? When the insulation is suspended flush with the lower edge of the trusses, it is impossible to seal off drafts.
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 Regarding Only Soggy Ceilings Sag (Letters, Nov/Dec '96, p. 4). The strapping or furring out of ceilings with 1 x 3s is peculiar to New England. For years I have spaced this furring 24 inches on center and blown in 15 to 16 inches of cellulose. No ceiling has ever sagged.
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.)
More Energy I read with great interest the article on gas fireplace pilot lights in the Jan/Feb '97 issue (Fireplace Pilots Take Gas Use Sky High, p. 6). Those Canadians do great work! However, the article understates therms by a factor of ten. The annual energy use of a typical pilot light is 73 therms, not 7.3 therms as stated; the 14 homes with significant fireplace use but a less than continuous pilot used 67, not 6.7 therms per year, and the homes with continuous pilot lights and insignificant fireplace use consume 70 therms, not 7.
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.
Editor's reply: Thank you for your vigilance. We regret the error.
Home Energy can be reached at: firstname.lastname@example.org
- FIRST PAGE
- PREVIOUS PAGE