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.

Okay... I received updated AHRI reference numbers (and copies of the certificates) from the Bryant dealer for both the 60K and 80K furnace options. One thing I am concerned about is the clearance for the stack of equipment in the basement utility room. There is only a clearance of 63 inches to the bottom of the main supply duct. The Lennox dealer mentioned this, and the Bryant dealer took note of it as well, but here are the options the Bryant dealer has presented me with: 60K Unit AHRI # 4891891: Condenser: 187BNA036 Coil: CNPVP3717ATA Furnace: 355CAV04260 80K Unit AHRI #4891910 Condenser: 187BNA038 (same) Coil: CNPVP4821ATA Furnace: 355CAV060080 The coil for the 60K unit is 5 inches taller than the coil for the 80K setup. The total height of the stack (including a 2 inch base) for the 60K unit would be over 69 inches - that doesn't seem like it would fit. The 80K setup that they have specified still has a 64 inch height, which would mean the coil sticks part way into the supply duct - is this a significant problem? I have found the Bryant 2010-2011 product handbook so that is helping me to put together what I think is the ideal system. If I understand it, ideally I need for my use a 3 ton condenser, a coil that can handle at least 3 tons of cooling, a furnace that can provide at least 3 tons of cooling airflow (it looks like Bryant has two options here - 3.5 tons of airflow (042 series) or 5 tons (060 series). I need the combination to be less than 61 inches tall (to allow for the 2 inch base underneath and I want it to be eligible for the cooling tax credit (which I think means minimum 16 SEER/13 EER rating). I THINK I should be able to put that together, but I need to cross reference the overall design. Just a quick question - the coil and cooling airflow capacity can be oversized without adversely affecting things, correct? When we talk about correct sizing we are referring to the condenser and the furnace output right? Thanks again for any input. Ted

If the breaker is 15 amps, the heater is most likely rated no more than 12 amps. That said, it's not a great idea to use a thermostat rated right at the heater's running current. An overrated thermostat will be more reliable. FWIW, speaking of reliability, I tried electronic (digital display) baseboard heat thermostats 20 years ago. I replaced them all with good old fashioned mechanical thermostats within a few years, after some failed. No failures since. I hope you find that the newer electronic thermostats are more reliable.

"People who do not have a multimeter or have one and don't understand how to use it shouldn't monkey around with electricity." The only way you learn to trace circuits and use test gear is by doing it. It's nice to have expert guidance, but you can also get help from books, which is how I learned to use a VOM as an adolescent. (No DMMs in those days. Also, no internet. We had libraries, though. :) But here we are more than 2 weeks after the original post, and if the OP has done any circuit tracing, I haven't seen reference to it. My guess (and I hope he'll correct me if I'm wrong) is that he doesn't want to get into the project quite that deeply, and is hoping he can just buy a solution. His proposed purchase is a thermostat which has a similar configuration to the old one. That seems like it ought to work, but a digital thermostat still works on a completely different principle from a mechanical one. So the NEW new thermostat may work when connected like the old one was. Or it may not work at all. Or it may work only partly. Or it may go pop or BANG. Or it may just quietly release a little puff of smoke and stink. There's even a slight chance he could get hurt or damage something else, the worst case scenario. I just think that if he doesn't want to do the hard technical work of tracing things out (and there's nothing wrong with having different priorities), it's probably best to turn this over to a pro, and get it over and done with and WORKING.