Heres a Link: http://www.greenbuildingadvisor.com/blogs/dept/musings/saving-energy-manual-j-and-manual-d Even when they do the calculations, the results are usually wrong In his posted comment, Bailes raised an interesting point: the problem of “garbage in, garbage out” (GIGO) Manual J calculations. Several energy experts have commented on the GIGO problem, including John Proctor, Hank Rutkowski, and Allison Bailes. Hank Rutkowski, the mechanical engineer who wrote the original Manual J document, was recently interviewed for a profile posted on the Green Building Talk Web site. The piece notes, “Rutkowski estimates that only 10% of heating and cooling equipment sizing decisions are based on some type of Manual J calculation and that less than 1% of the jobs are based on an aggressive implementation of the recommended design procedures.” Rutkowski explains the GIGO problem this way: “Manual J is a good tool if you use it aggressively. Follow the rules and it will give you a reasonable margin of safety. But the average contractor says, ‘Better safe than sorry,’ so he fudges here and there. He adds 5 to 10 degrees to the summer and winter design temperatures, calls the building ‘average’ instead of ‘tight’ and doesn't take credit for shading by interior blinds and drapes. Then when he finally comes up with a load number for sizing, he throws in an extra half-ton of AC just for the heck of it.” For a high-performance home, sizing by Manual J may still result in oversizing Several years ago, I had a conversation with building scientist Joe Lstiburek about air conditioners installed in Building America homes in Las Vegas. “HVAC designers are committed to the institutional oversizing of air conditioning equipment,” Lstiburek told me. “The average system is sized at 150% to 200% of the requirements of Manual J. They oversize because they don’t know what they will get for a building envelope, and to compensate for duct leakage and inappropriate refrigerant charge. If you size according to Manual J, there is already a fudge factor built in. But most designers then add another fudge factor.” According to Lstiburek, their experience proved the high-performance Building America homes in Las Vegas could have had HVAC systems sized at about 60% of Manual J. To be conservative, his team recommended installing systems sized at 80% of Manual J. Does oversizing matter? There are strong arguments against routine oversizing of HVAC equipment. The best argument is simple: oversized equipment usually costs more than right-sized equipment. Oversized equipment suffers from short cycling. For example, an oversized furnace brings a home up to temperature quickly, and then shuts off. A few minutes later, it comes on again, only to shut off quickly. Many homeowners find the see-saw sound of a short-cycling furnace to be annoying. When air conditioners short cycle, the units don't run long enough to achieve much dehumidification ��" at least in theory. (During the first few minutes of operation, an air conditioner cools a house. But not enough moisture has collected on the cold coil or on the pan below for any water to have actually gone down the drain. When the air conditioner turns off, all of the moisture in the pan and on the coil just re-evaporates.) The conventional wisdom may be wrong, however Increasing evidence shows that energy experts have exaggerated the negative effects of equipment oversizing, however. Studies have confirmed that oversized furnaces don't use any more energy than right-sized furnaces. Moreover, newer modulating or two-speed furnaces operate efficiently under part-load conditions, solving any possible problems from furnace oversizing. Although there are ample reasons to believe that oversized air conditioners are less effective than right-sized equipment at dehumidification, at least one field study was unable to measure any performance improvements or energy savings after replacing an existing oversized air conditioner with a new right-sized unit. The bottom line The main reason to choose right-sized equipment is to avoid paying too much money for equipment you don't need. A Manual J calculation will ensure you don't spend more than necessary for your furnace, boiler, or air conditioner. Moreover, a Manual J calculation will provide room-by-room heat loss and heat-gain information that is essential to good duct design. Without good duct design, you're running the risk of comfort complaints.

Thanks, tigerdunes... We currently have a Lennox furnace model #G24M. As for size, the inside plate says "rated output: 82,000 BTUH AGA/CGA; 73,000 CGA only"; is this the size? Several of the contractors we've gotten bids from have also suggested a HP, so we are getting quotes on those, as well as solo ACs. I'm not ready to chuck gas altogether, especially given that we occasionally have really cold stretches and I don't want to count solely on a HP...but the combo with gas sounds like a good option. I guess ultimately my question is whether I should take this opportunity to upgrade the furnace to get a variable speed blower in order to make all systems more efficient? And, if so, would it be worthwhile to go with a high efficiency 90%+, or stay with an 80%? Another quandry I'm facing is that all 5 bids we've gotten have suggested a zoned system. The system as it exists covers basement and first level, but the addition of a sunroom 13 years ago has thrown the air flow off to be less that comfortable. The sun room faces south, has 3 walls of windows and 4 skylights, and a vaulted ceiling. At the time, the ac was upgraded to a 3 ton unit, but the manual Js we've had with the current bids show that the addition alone requires a 2 ton unit. We've gotten opinions all over the board whether the ducting is a large part of the problem, though most agree that they would have done a better job of ducting during the addition if they had handled the job. The opinions differ on whether things are worth fixing. I've read threads about the pros and cons of zoning, and it looks like the cost would add $2.5-3000 to whatever else we do with the system. I'm not really picky about temps (we run 77 in summer, 68 in winter simply to not waste resources). It has also been suggested that some manual dampers might be a good first step to see how it balances the cooling load (an imperfect, though vastly less expensive option). Do you have any words of wisdom for me? We haven't gotten all the bids back yet, but assuming they're comparable for similar work, I have a good level of confidence in the Bryant, Trane, and American Standard dealers. Thanks so much for your opinion! -Cider

garyg, It is an either or situation XL14i with variable speed air handler, aluminum evaporator coil and Trane touch screen thermistat. Include 10 years parts and labor. No need to replace the lines because they are large enough. Heat Pump $ $9105 includes 2 -5K heat strips. or hybrid below 2 XL14i condensor with 2 Stage variable speed furnace $8208. XR13 with variable speed air handler, aluminum evaporator coil and Trane touch screen thermistat. Include 10 years parts and labor. No need to replace the lines because they are large enough. Heat Pump $ 7150 includes 2 5K heat strips. or XR13 condesory with 2 Stage variable speed furnace $6985. Gas rates are somewhat confusing for two reasong 1. They under reported July and caught in August. 2. we have distrubtion rate, gas supply rate,weather normalization adj rate, base rate adj and EE cost rate. All of which is different.

I certainly would want a load calculation performed. Insist on it. Get it in writing. And you set the design temps in and out for both heating and cooling. I would think an 80 K high eff furnace would be fine especially with a zoned system. And if your budget allows, I would go with complete Carrier Infinity/Bryant Evolution with their high end modulating furnace and matching zoning controls. IMO