Letters: March/April 2007
A version of this article appears in the March/April 2007 issue of Home Energy Magazine.
Max, I agree with the opinions you expressed in your editorial (“The Cost of Air Tightening May Go Up,” Nov/Dec ’06, p. 2). What do you think about the CFM rates in ASHRAE Ventilation Standard 62.2 when a ventilation system is run intermittently? Do you think that these rates are extremely high?
WSU Energy Program
Author Max Sherman replies:
The short answer is no. I do not think that the intermittent-ventilation rates in 62.2 are unduly high.
The purpose of ventilation is to control exposure to pollutants by diluting their concentration. ASHRAE 62.2 assumes that it is not the instantaneous concentration that is important but the integrated exposure over time. Thus you can trade off periods of overventilation for periods of underventilation.
The intermittent-ventilation section of Standard 62.2 lists one approach for doing that. The committee has also approved a more general approach for doing intermittent ventilation that can be found at http://epb.lbl.gov/Publications/lbnl-56292.pdf or in Technical Report TN 60 through the Air Infiltration and Ventilation Center (AIVC), www.aivc.org. (Note that all U.S. residents are entitled to a free account at the AIVC that allows unlimited download access. Details are on the site.)
Because intermittent ventilation is less effective than constant ventilation at diluting continuously emitted contaminants, more total air is required to control exposure. Fan sizes and air transport energy, therefore, must be larger. Depending on the climate and/or utility prices, however, intermittent ventilation may be more cost-effective for some control strategies, but usually it will not be.
Is Less Efficient Better in the Cold?
I have a problem with my Carrier Natural Gas Furnace Model 58 MVP. This high-efficiency furnace has been installed in my home for three years. Once each year, and occasionally twice each year, my furnace has shut itself off because the exhaust vent and the air intake have frozen up. I live in Saskatchewan, where we have very cold winter temperatures and accompanying wind, making the wind chill a factor. I have two questions that I hope you can answer.
- Could the location of the vent on the outside of the house be a factor in freezing up?
- I have been told that a high-efficiency furnace is not the best choice for climates such as where I live, that is, with occasionally very cold temperatures and wind. I was told a mid-efficiency furnace would work better. Do you have a comment on this?
Moose Jaw, Saskatchewan, Canada
Technical Editor Steve Greenberg replies:
It is quite likely that the location of the vent is a factor under your conditions. The first question to ask is, does the installation conform to all of the manufacturer’s recommendations? If so, does the manufacturer have any further suggestions? It is unlikely that you are the first or last to have this problem
Regarding using a lower-efficiency furnace: It is less likely that you would have exhaust freeze-up problems, because the flue gas is substantially hotter than with the high-efficiency furnace. Of course, a lower-efficiency furnace would use correspondingly more energy than the one you’re using. If climates such as yours with high heating demand are not good applications for high-efficiency furnaces, then where would be? Any furnace that doesn’t work in cold, windy conditions needs a technical fix.
Fan On or Fan Off?
My wife prefers to run the furnace-A/C fan year-round rather than using the “auto” setting; she believes this saves money by keeping air constantly circulating. Please comment.
Steve Greenberg replies:
The fan will use substantially less energy on “auto” than “on.” The amount of energy required to start the fan and get the air moving is small compared to the energy required to keep it moving. Also, if your ducts leak (and all do, to some extent), running the fan also increases the amount of air leakage, causing increased air conditioning and heating load. In the winter, there may well be a reduction in thermal comfort due to the cold-blow effect when the furnace burner isn’t firing. In the summer, there may be some thermal comfort benefit from the continuous air circulation. So, unless your system is specifically designed to run the fan to supply fresh outside air (and few are), you will save energy and money by leaving the fan on “auto.”
We are building a new passive-solar home and have had difficulty finding commercially available low-U, high-SHGC windows. The specs quoted for the windows in “The Little House That Could” (Nov/Dec ’06, p. 24) sound like a dream.
Could you please provide more information on what brand was used and how to go about finding windows with these properties? We have not seen windows with similar specs through Pella, Marvin, Loewen, Jeld-Wen, and so on.
(Jeld-Wen is even listed in Green Building Products as having high-SHGC low-e coatings, but the specs on their windows do not seem to reflect this.) Do major manufacturers not offer this type of glazing? Do we need to request custom windows?
Thank you for your help and the wonderful, practical article!
Shawna and Sherwood Johnson
Author Paul Norton responds:
Here is an image of the National Fenestration Rating Council (NFRC) sticker on the windows of your inquiry, showing the brand, model, and specifications (see photo). Unfortunately, Accent Windows does not appear to supply to your area. I suggest you contact www.accentwindows.com to see if there is any way you can get windows from them. For now, I suggest you call window suppliers in your area and give them the Accent Windows specifications shown in the photo from the “Little House That Could” to see what similar windows they can offer you.
Doing It Right, Doing It Safe
I purchased a used Rheem high-efficiency natural gas furnace. Before installing it in my home I want to make sure that I am doing it right. I hooked it up in the garage with gas near the water heater. When it came on, it appeared to have too hot a flame and the PVC exhaust pipe was getting too hot. The fans were running well and it sounded good, but I wonder if it needs a gas regulator ahead of the gas valve. The valve brand looks like this: @ 36E 37 214 60-23490-01. The listed max pressure is 1/2 psi. Does this valve have a built-in regulator? Thanks for any info you can give.
Steve Greenberg responds:
Yes, this valve has a built-in regulator. To verify that you have the correct firing rate, you can check the burner consumption by clocking your gas meter with only the furnace running. To clock your meter, locate the test dials on your meter. Usually there are two dials (with, for example 1/2 ft3 and 2 ft3 per revolution below the 4 that the utility reads—the least significant of the latter is 1,000 ft3 per revolution). Then, using a stopwatch or a watch with a second hand, clock the smallest test dial, timing several revolutions. Then use the following formula:
ft3 per revolution X number of revolutions / number of seconds
X 3,600,000 = BTU/hour
The resulting number should be within 10 of the furnace rating shown on the nameplate. If it is way high or low, the furnace needs an adjustment.
In the article “Caveat Emptor,” (Jan/Feb ’07, p. 12) the federal limit on the flow rate of showerheads is 2.5 gallons per minute (gpm) maximum, not minimum as stated in the article.
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