SIGforum.com Main Page The Lounge HVAC Quotes Received -- Which Load Estimation Method Is To Be Trusted?
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SWMBO suggested (?) that we needed to focus our home upgrades on the foundations of the home, rather than the cosmetics. She charged me, Engineer Husband, who used to design AC compressors for Copeland, with determining a path to replace the 20 year old systems. One system has been leaking R22 (gasp) very slowly, and had its compressor replaced already, some five years ago. Engineer Husband inquired of three contractors, who visited. Vendor B was chosen based on their reputation for good system design, Vendor U was chosen since that lead technician is known to us from our neighborhood friends who used him, and Vendor W was chosen based on a direct mail ad promising reasonable pricing during a winter season. Vendor B came first, and spent two hours on site. All windows were measured. All window locations were measured. A full home floorplan was measured and sketched. The compass direction was marked, to show the hotter side of the home. The return vents were opened and inspected. Sizes of ducts were measured. The crawl space was entered, on a rainy day, to look at those supply ducts. Quality assessment of existing ducts was made, including routing, leakage, etc. Full inspections of indoor and outdoor units was made. Vendors U and W came next. Both U and W spent about one hour on site. They both looked at the existing visible supply and return ducts, and counted them. Both inspected units indoors and out, shaking their heads with amazement that the system actually worked. With factors in mind for each size duct, they calculated a volumetric flow rate for all ducts, and compared that flow rate to a known unit size in tons. They determined that there were insufficient returns and supply ducts for that existing size units. They then concluded that more ducts would be required to match the air flow capacity of the ducts to the existing size of the unit. With all three quotes in hand, it was a surprise, albeit a pleasant one, that Vendor B determined that both upstairs and downstairs AC units were 1/2 ton OVER size, and that the correct sizes were 2 tons up and 1.5 tons down. The other two vendors quoted the same size units as the builder installed 20 years ago. Ductwork revisions to improve flow and thermal management were foreseen. Vendor B also supplied five quotes, giving me two cost options for downstairs and three cost options for upstairs. If Vendor B load calculation is correct, I can afford a more efficient system for the same cost of equipment and installation, and also do my part to save the planet by using less electricity (1/2 ton, 6000 Btu/h while operating would consume less energy). If the smaller units were operating for the true home thermal load, it would seem they would also be operating for the correct duty cycle (on / off time), maintaining a more uniform temperature. If Vendor B load calculation is not correct, I think I would be in a world of hurt. But, the Vendor would have to come back out and fix it under their initial warranty period. Vendor U would use Rheem, Vendor W would use Daikin, and Vendor B would use Lennox. Given the attention to detail in Vendor B proposal, as well as the equipment, is there any reason not to choose that contractor? ------- Trying to simplify my life... | ||
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I worked for a Lennox Dealer back in the early 70s and they were top of the line units. Looks like vendor B has taken more into consideration with window and door heat/cooling loss in his decision that the other two. HVAC has come a long way since I was an installer. NRA Life member NRA Certified Instructor "Our duty is to serve the mission, and if we're not doing that, then we have no right to call what we do service" Marcus Luttrell | |||
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I currently own or manage 5 properties from a 1 bedroom condo to a 10,000 sq foot house with 12 central a/cs. You couldn't PAY me to put another Rheem in one of my properties. They're pure garbage these days. Last one I installed May 2010 in a 4 plex I own. It has been through 3 air handler blower motors, and one of the condensing fan motors. Other friends that manage properties have had the same or worse experiences with them. Lennox makes good equipment, I'm not familiar with Daikan. My preference is Trane or American Standard and pretty much all I buy. That being said, let's get to your house. In summer time, what do you keep your thermostat's at and how long per hour would you say the a/c's run per hour? If your current a/c's are running 60% of the time or more during the middle of the day in summertime, I'd say keep them the same size.....If they're not, then it might make sense to go down a size. Here in Florida the a/c's put in 20+ years ago almost always were undersized and won't hold 78F in the summer in most houses. You definitely don't want to go to small. But it sounds like Vendor B, did all of the calculations and so forth. | |||
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No | |||
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Nosce te ipsum |
Generally speaking, 100cfm per duct is the most you get. Sometimes as little as 50cfm. 400 cfm per ton is our usual multiplier. The return drop is sized 25% bigger than the supply, just the last 6-8 feet. I do some old-school inspection stuff before whipping out the duct-sizing chart and generating an actual load calc. Ask about hot and cold spots. Take a hard look at the ductwork design. You want your system sized so that it runs all or near full capacity. Oversizing a system leads to the coil getting too cold and a raft of other issues. On a recent install two winters ago and this summer, we removed 100k and 120k furnaces and installed a 60k and a 40k (down/upstairs). Fought the owner every step of the way. The two-story house heats perfectly now, and we have two winters with zero callbacks. In July, from the same house, we removed 4 & 3.5 ton condensers and installed 3t and 2.5t units. Perfect cooling and dehumidification. There is a lot to be said for a modern, careful load calculation. The guy I work with produces a 20+ page document which will stand up in court if need be. Vendor B will supply his load calculation after a contract is signed and deposit received, I'd bet. Sizing is nothing anyone of that caliber fools with. They trust the math. | |||
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| Dances With Tornados |
Slight thread drift, but how is your insulation? Better insulation will absolutely go hand in hand with HVAC performance, comfort and financial benefit. | |||
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| Get my pies outta the oven!  |
I had a Lennox system put in last July and am VERY happy with it. It replaced a 30 year old Trane. FYI: the issue that has come up with discussions here about Lennox units gettting leaks due to their copper coils, that issue is gone now that they’ve gone to aluminum alloy coils. | |||
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The issue spread through several manufacturers who decided to put aluminum fins on their copper coils (versus copper coils/copper fins). The galvanic corrosion between the 2 dissimilar metals would eat the metals and spring a leak. (Same as using aluminum gutters with copper roof flashing). Aluminum fins/aluminum coils doesn't experience this, nor does copper coils/copper fins. | |||
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Need to ask vender b what temp ranges he used on the calculations. If he used say 92 for summer and 30 for winter and you have a real hot summer or colder winter equipment is going to run harder to keep up. May have him rerun with higher and lower temp see if it still calls for smaller size equipment | |||
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+1 Anyone paying attention to details is good to go in my book. Those who give comments without supportive facts and understanding is of questionable motivation. Disclosure: My knowledge of load calculations in minimal. The fact that one is being utilized is a cut above the typical hack. The early days of the long form and the new software of today still has a significant " fudge factor" in my opinion. This doesn't negate having one done. In fact, I highly recommend that load calculations be done, especially if the original structure has updates or the current system has operational issue history. I believe Fairfax county requires a load calculation with permit. A true professional will perform a load calculation on all replacements and or new installs. | |||
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| I'd rather have luck than skill any day  |
I agree with this. Had a reputable dealer install Lennox system in office. Called him after the fact, he pointed out that the "system" is only capable of 20 degree deferential. Shit, this is Georgia; we easily get into upper 90s for long durations; that's not good enough for me. That was 5 or 6 years ago. The Lennox has provided good service. I added another system; happens to be Bryant. I like it better. It's variable speed heat pump. I can hang meat in here now!  | |||
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| As Extraordinary as Everyone Else  |
I will agree with our resident HVAC expert on this. From your description it sounds like he calculated the proper size by completing a “Manual J” calculation. You should ask him if this is what he did. In the old days my HVAC company would just figure 600SF per ton. Now with our energy efficient construction methods AND doing a complete Manual J calculation we are approaching 1200SF per ton but it varies per house. If the company is willing to stand behind their work I don’t see any issues. ------------------ Eddie Our Founding Fathers were men who understood that the right thing is not necessarily the written thing. -kkina | |||
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| Ermagherd, 10 Mirrimerter!  |
I prefer slightly undersized to grossly oversized, humidity has a large effect on comfort An oversized unit won’t run long enough to wring out a decent amount of moisture. Depending on budget, go with a variable system like the Carrier Infinty, or Trane/Lennox equivalents. Slightly oversized isn’t an issue, and they make for some of the most comfortable conditions I’ve seen. The downside is upfront cost and expensive parts. I quit school in elementary because of recess.......too many games --Riff Raff-- | |||
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PLEASE, provide me the brand and model number of ONE piece of residential HVAC equipment which used copper fins on their evaporator coils. The real issue was formicary corrosion with the thinner copper tubing used for better efficiency ratings. | |||
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Who said anything about Evaporator coils? I was speaking about condensing coils, which have a much higher failure rate than evaporator coils, at least in a Southern state. I've seen many people over time replace a bad condensing unit and not change the air handler/evaporator and it went the lifespan of the 2nd condensing unit. The 1983 Trane 2 ton Heat pump I replaced in 2010 had copper coils/copper fins on the condensor. My mothers house had a Rheem condenser from around 2001 that by 2007, the fins disintegrated completely. It had copper coils/aluminum fins add moisture (rain water) and you get galvanic corrosion. Anywhere coastal (near the ocean) the condensing unit fins will almost always fail first (regardless of construction material). Copper coil/copper fins is the most efficient at disapating heat from the condensing, but cost is prohibitive, so they went copper tube/aluminum fin in the 80's, then GE went aluminum tube/aluminum fin first for even more cost savings. The manufacturers did figure out how to isolate the copper tube/aluminum fin from galvanic corrosion better and utilize special coatings on them to keep oxygen from them as time went on, early 2000s from what I've seen. | |||
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I'll be putting together a small spreadsheet with a lot of detail. Then, I'll ask Vendor B to confirm a few details. I agree the load calculation has to be at 105 F summer for Virginia. I also want to know all about the duct renovations needed, particularly the routing of the somewhat collapsed return duct in the attic. I also need to get the upstairs thermostat located where it is not subject to the upflow from the entry foyer. I hope that the Lennox thermostats work well with WiFi and mobile phones. I do plan to put more cellulose insulation in the attic, but all three vendors remarked that it looked fine. ------- Trying to simplify my life... | |||
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Jimmy, you are simply wrong. No copper to copper exist in either evaporators or condensing coils. And PASig was taking about evap coils, as they have went to aluminum evap coils. Nice try though. | |||
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Vendor B did manual J at 92 F to calculate 1/2 ton smaller systems. When temp was raised to 100 F and recalculated the system size went up 1/2 ton. With most days being well below 100 F here, I can't see giving up 1/2 ton lower capacity and all the advantages of being well matched to actual loads. ------- Trying to simplify my life... | |||
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| Member |
Residential HVAC Load Simulations. This sounds like the Thursday Brown Bag seminars that I pass on at work. Four to five kW demand? Please! Talk to me when you get to megawatts! I vote contractor B P229 | |||
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https://higherlogicdownload.s3...ign-Conditions-1.pdf | |||
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SIGforum.com Main Page The Lounge HVAC Quotes Received -- Which Load Estimation Method Is To Be Trusted?
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