Letters: July/August 2005
With HVAC, Size Does Matter
I have been facing the HVAC-sizing problem for my new house on the central Gulf Coast of Texas for some time now. I had a Manual J run by an independent engineer, and I ran one myself using a commercial software program (I am not an HVAC professional). The numbers were within 1,000 Btu of each other.However, the HVAC professionals I’ve talked with say that there is no way the Manual J sizing will work because of “insufficient air movement.” None of these professionals has run an actual Manual J, and I don’t know what to think other than to insist they run a Manual J (which they are reluctant to do) and explain any differences. My house has 3,567 ft2 of cooling space, and the Manual J calculations call for 40,000–41,000 Btu of cooling (includes both sensible and latent), with 1,300–1,400 CFM of air movement. Should I stick to my guns or listen to the “professionals”?
House Doctor John Proctor responds:
Stick to your guns. “Insufficient air movement” is an old concept that is really applicable to commercial buildings with lots of occupancy. In fact, a well-designed duct system with proper registers will provide good mixing. Putting in a larger unit only reduces the run times with big blasts of air followed by periods of potential stagnation.
Follow up from David Carlton:
I did “stick to my guns” and found a local contractor who was very attuned to my situation and very educational at the same time. He and I spent several hours going over my house,my needs, and the Manual J—and we agreed that 80%–90% of the time a 3-ton unit with a variable-speed air handler would work fine. However, due to limitations of the equipment at this time, this scenario is impractical (the HVAC manufacturers need to do some rethinking in their designs). He showed me four different options that would perform the task at hand and explained the pros and cons of each. Because of my research (and help from people like John Proctor), I quickly recognized the system I needed. Any of the four would work, but I chose the one I thought would work the best and my contractor agreed, although he did
not try to push it on me, as it was the most expensive system. In fact, he said any of the less expensive systems would handle my needs 80%–90% of the time and keep the humidity level down most of the time. I did not care for these percentages or “most of the time” humidity control.
We agreed on a Dave Lennox Signature Series two-stage condenser unit. The unit runs in the first stage (approximately
3 tons of cooling) the majority of the time, but if called for, will run in the second stage (up to 5 tons maximum cooling). This was matched up with a 5-ton variable-speed air handler, which runs at 1,300–1,400 CFM as needed, but will ramp up to 2,000 CFM as the cooling calls for it (large family gatherings during a southeast Texas summer). This was all matched up with a digital thermostat/humidistat. He also installed the ductwork as I requested and designed it following Manual D. The contractor’s name is Paul Blaha, owner of Climate Heating and Air, and he has been in the same location in Angleton, Texas, since 1956.
A word of advice to all those who are in the market for an HVAC system for their home or office. Do the research, and talk to John Proctor and others who are knowledgeable about HVAC and humidity control problems associated with HVAC. Get a Manual J done and find a contractor who will spend time talking about your particular situation and needs. Stay away from the “rule of thumb,”“square foot,” and “I’ve been doing this for 25 years, so I know what you need” crowd.
Thank you, John Proctor, Home Energy, Paul Blaha, and Climate Heating and Air.
Put the March/April editorial on vampire loads (“Vampire Slaying and Other New Directions for Efficiency Standards,” p. 2) together with the article on occupancy sensors (“Code Changes Encourage Innovative Products,” p. 11), and what do you have? The occupancy sensors probably draw at least 1–2 watts 24/7, and when switching on a lamp, you lose some
small amount of throughput power in the switching device. Let’s say 1.5 watts x 24 hours = 36 watt-hours.
Now my bedrooms and bathrooms have a switched overhead CFL lamp that draws 13 watts and might be on for two hours per day or about 26 watthours per day. Certainly many people don’t turn their lights off, and these devices would cut energy use. However, my family and many other families do turn off those lights in unused rooms, and it would not seem to be a good deal to install these vampires.
Do you want to use sensors in commercial buildings? Go for it. Do you want to use them for residential installations? I’m not so sure.
The same analysis should be done for all of the devices proposed for the “Smart Home.”The requirements of the National Electrical Code and other codes include the mandatory use of GFCIs,AFCIs, and smoke detectors, all of which are small 24/7 electrical loads not commonly addressed.
Keep up the good work on an excellent magazine.
New Mexico State University
Las Cruces, New Mexico
I agree with several of Jim Melesky’s points in his article on attic accesses (“Attic Accesses: High Priority or No Big Deal?” Mar/Apr ’05, p.16)—namely, that attic accesses located inside the conditioned space of the house should be insulated and air sealed in a manner durable enough to last 12 years or more.However, I disagree with some of his calculations.
In calculating the average R-value of an R-38 attic with uninsulated pulldown stairs, Melesky uses just the R-value of 1/4-inch plywood for the pull-down stair without considering the film coefficients on either side of the plywood. Doing so would increase the average or effective R-value of the R-38 attic from his calculated value of 17.1 to about 30, which presents a much less ominous situation.
I also question his assertions that $250 spent to upgrade a pull-down ladder will result in an SIR of 6.28, or that an expenditure of $150 to insulate a pop-up hatch has an SIR of 3.71.These figures imply annual savings of about $106 and $37, respectively, assuming a 20-year lifetime and a uniform present value factor based on a 3% discount rate.
I performed my own check on these projected savings using DOE’s National Energy Audit (NEAT), which is used by many states within DOE’s Weatherization Assistance program. For a one-story house located in St. Louis, assuming the same fuel costs reported in the article, I estimated a total annual savings of $14 from upgrading an attic pulldown stair; a savings of $5 from adding R-38 insulation to 10 ft2 of uninsulated attic area (the typical area of a pulldown stair); and a savings of $9 from air sealing that effectively reduces the air leakage rate of the home by 100 CFM measured at 50 Pa (a reasonable estimate of the impact from air sealing an existing pull-down stair that is in poor condition). Furthermore, I estimated total annual savings of just $27 for a house located in International Falls, Minnesota.
Based on my calculations, I must conclude that Melesky’s savings estimates are overly optimistic, even for the worst-case condition implied in the article (40 square inches of leakage area) and if one assumes a severe climate like International Falls. Exaggerated claims can lead to lower-than-expected energy savings, uneconomical use of weatherization funds, and ultimately to mistrust and confusion on the part of the client population we strive to serve.
Do my calculations indicate that attic accesses should not be addressed? No! Assuming 15-year lifetimes for both the added insulation and the air sealing work, my calculations imply that up to about $167 could be spent cost-effectively on labor and materials to upgrade an attic pull-down stair in St. Louis—less if the existing air leakage gaps are not that bad, but more in a more severe climate or if the durability of the measure could extend the lifetimes to 20 years. The challenge for the industry is to develop an attic pulldown stairway product that can be purchased and installed for these costs and that will perform for 15–20 years.
Buildings Technology Center
Oak Ridge National Laboratory
Author Jim Melesky responds:
The crux of Mark's disagreement with what I wrote in the article is about what the justifiable dollar amount should be to insulate and seal an attic access. In his letter, Mark concluded that $167 is a justifiable amount for a pull down ladder, while I indicated that $250 was the right amount. I will first explain how he actually agrees with my numbers using his own assumptions and then show how some of his assumptions dramatically understate the actual energy savings.
Mark calculates that the annual savings of this improvement should be approximately $14/year. Over a 12-year period, this would save the homeowner about $168. If we take the annual savings and extend it to at least 15 years as he suggested, then the savings would justify a solution of $210. His challenge to make a product that will last 20 years justifies a $280 amount for the upgrade. I agree with this standard and I know that there is at least one product available today with a 20 year warranty. Therefore, a $250 investment is easily justified to insulate and seal an attic ladder if the upgrade is based on Mark's assumptions and recommendation.
While I disagree with his calculation of effective R-value, the matter is not yet clear. I agree that there is an increase in R-value on plywood due to the film coefficient. However, in the case of attic accesses, there is also significant air leakage. This allows air flow between the attic and the living area, particularly during the hot and cold weather periods. The air flow affects the film coefficent. I pursued the matter with the thermodynamic departments of some highly prestigious colleges. While there was agreement that air leakage would decrease the R-value due to the film coefficient, there were differing views on the precise effect. I have requested that this be the subject of further research by their students so that an unbiased source could develop accurate data. Even though the matter is not resolved, I used Mark's assumptions for effective R-value in analyzing savings due to an attic hatch retrofit in order to determine the most conservative results.
The assumptions Mark made about the cost of fuel and the amount of air leakage around attic accesses caused him to understate the savings from a retrofit. I used heating costs of $1.20 per gallon of fuel oil in my analysis, while the cost of heating fuel had steadily risen to $2 per gallon by the time Mark’s letter was written. That represents a more than a 66% increase in the cost of fuel. It is clear that my calculations dramatically understate what the energy savings would actually be for this critical component of the SIR calculation. In addition, I didn’t project an increase in the cost of fuel for the period of projected savings. If we look back over the past 20 years, the cost of fuel has increased dramatically. I can find no projections that indicate fuel costs will decrease or remain flat for the next 20 years. Using a cost of fuel of only $1.80 per gallon, which is still far below current market prices, Mark’s model indicates cost savings of $400 for a
pull down ladder retrofit. If we assume that the discount rate is approximately equal to the rate of increase in the cost of fuel, a $250 upgrade produces an SIR of approximately 1.6. An SIR of 1.8 would result at a cost for fuel of $2 per gallon.
I disagree with Mark when he states that a 100 CFM50 air leak in a one-story home due to a pull down stairs in poor condition is a reasonable estimate. He provides no reference or basis for this assertion. I find this to be an unrealistically low reading. A number of our weatherization clients have provided my firm with actual blower door test results from a number of homes with various sizes and designs in a number of different states. They typically record reductions in air leakage that range from 500 to 1,500 CFM50 for a properly upgraded pull down ladder. If we use only a 250 CFM50 reduction (but still use all of Mark's other assumptions) with a cost of heating fuel of $1.80 per gallon, than the annual savings would be $38 and the SIR would be 3. At $2 per gallon, the annual savings will be $42, and the SIR will be 3.3.
I also disagree with Mark that 40 square inches of air leakage is a worst-case condition. As any pull down ladder is used, the plywood will warp over time where the ladder is pulled down. In the worstcase environment, air gaps for the ladder opening will far exceed 40 square inches. I determined the air leakage I used in an example in my article by measuring the actual air gaps that exist in what I believe is a typical pull down ladder. The data I used in the article was entered into TREAT software, which, like NEAT, is also a DOE-approved modeling program. TREAT allows for the use of air leak reduction or measured air gaps.
While I strongly disagree with some of Mark’s conclusions, I do not question his intentions. I think he simply missed some important items that compromised his results. Both my firm and I highly value our good name. We never have nor will we ever put out any bad or exaggerated information knowingly. I thank Mark for his thoughts and offer to work with him or any other reader to delve deeper into the issue, so that we can all continue to provide the highest quality and most cost-effective measures and products for the industry.
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