furnace size

______________________________________________________________________ I’ve got a 15 year old, 60/65% eff, builder model Rheem gas furnace that I’m going to need to replace one day soon. It is a 14″ wide, updraft, RGDA model, with 75,000 btu input, 3 ton air exchanger. It heats my 2,600 sq ft house just fine. The A/C evaporator plenum is also a Rheem but is 17.5″ wide, so it overhangs the furnace on both sides a bit, but it is well sealed. The evaporator coil was just replaced and the A/C has always worked fine. I’m considering going with a Rheem Criterion II, two stage 80% eff, furnace, but I noticed that the 14″ width unit only comes in a 50,000 btu input model. I’ll be losing 5000 btu on the change out (.60 * 75k = 45kbtu vs .8 * 50kbtu = 40kbtu). Is this significant? If I go the next size up I’ll have to change some ductwork to accomodate the 17.5″ width, and I’ll end up with 75,000 btu input resulting in about 60,000 btu output. With that kind of output I’ll be getting more heat on my first stage than I get with my current low efficiency furnace, which I don’t need. Should I just stick with the 14″ width and go with a little smaller capacity? Would I realize much advantage with my cooling if I went with a wider furnace to match the plenum better? Replies: williamgeorge “Re(1):furnace size” , posted Thu 25 Sep 00:16 ______________________________________________________________________ Experts recommend 90% furnaces for most homes. A 90% or 92% AFUE furnace has enough efficiency to merit state or utility energy conservation rebates in many states, even though initial costs can be $1,000 more than an 80% furnace. ———————- williamgeorge consumer generated media Tinmantu “Re(1):furnace size” , posted Sat 13 Aug 15:09 ______________________________________________________________________ if you already have the 17.5 coil case, then go with the 17.5 furnace…it should be a simple changeout and greater blower capability….we are selling many of the 2 stage Rheems right now as they are a small price over single stage…I too recommend a 2 stage stat, but the board is designed to kick in the second stage if the stat isn’t satisfied after 12 min. or so. Then it goes to high fire. Depending on where you live, how long you are going to live there, what natural gas prices are (and what they might become based on these rising fuel prices) you might consider a 92% furnace if you have a way of running new vents. That gets back the space and maintains your btu. MQuillen “Re(2):furnace size” , posted Sat 13 Aug 16:47 ______________________________________________________________________ Tinmantu, I see what you’re saying about the 90% efficiency furnace. Since I’m not experiencing any heating issues now, it would make sense to go that route and I can really save on my heating bills and enjoy the benefits of two stage. It seems to me that the higher efficient furnace would pay for itself pretty quickly in central Ohio. What kind of extra venting is needed? Tinmantu “Re(3):furnace size” , posted Sat 13 Aug 17:05 ______________________________________________________________________ It would be pvc venting seperate from your water heater…it can go out the sidewall or roof…lots of options if you have an unfinished basement MQuillen “Re(4):furnace size” , posted Sat 13 Aug 17:24 ______________________________________________________________________ Thanks Tinmantu, it looks like that is the way to go since I’ll be living here at least 5 more years. I think the smallest, 45k btu unit will work. Full stage operation will give me what I currently have + – 3000 btu. Should be no problem and big saving and more comfort. What multistage thermostat is best. I like the Honeywell VisionPro but I’m concerned there is no sensitivity adjustment and they cycle the equipment within 1 degree variation. MasterTech “Re(1):furnace size” , posted Sat 13 Aug 14:12 ______________________________________________________________________ Go with the 17.5″ unit. Reason its a 2 stage unit. First stage will probably be 40k*80%=32k and second will be 75k*80%=60k. I believe with a 2 stage unit its best to be a little bigger for the really cold nites that happen ever so often. Most time u will run first stage(make sure you use 2 stage T-stat). Another advantage is that normally the wider units make less air noise. MQuillen “Re(2):furnace size” , posted Sat 13 Aug 16:34 ______________________________________________________________________ The 14″ furnace is a two stage as well, but I really like the idea of the 17.5″ being quieter. My concern is that at single stage I’m not saving anything over my current system. My current output is about 45k btu, and the 1st stage of the 2 stage, 17″ Rheem is 70% of 75k btu, which is 52k btu. I’ve never had any heating issues with my current system so I think a first stage output of 52k btu might be overkill. CoolTiger “Re(3):furnace size” , posted Sun 14 Aug 11:50 ______________________________________________________________________ What make you think you got a 60% efficiency furnace in first place? Does it really say it or someone told you so? Easy way to tell is to look at the label in the unit… divide the BTU output by the input… should come out to 80%… I got an old 1972 boiler and from the label, it is 80% due to calculation and yes, it loss efficiency after all these years… but heck… after home renovation and airtight insulation along with new windows, the each floor of duplex uses less than $120 a month of gas… 1200 sq. ft. and before renovation, the gas bill was average $250 a month and at least $200 a month. Northern NJ. Back to you… efficiency means alot to your pocket… at 80% efficiency, for every dollar you spent, 20 cents goes out the chimney. At 93% efficiency, only 7 cents goes out the chimney. Two stages will save you even more money as you don’t need full throttle acceleration all the time when half throttle will do. In the end, at least choose high efficiency over single or dual stage which your pocketbook dictates. I would truly recemmends have someone do the heat loss calculation for you… because if you buy undersized furnace, you will not be able to heat up your house… and you are right… oversized is no good either… but both will cost you more money than the price of furnace alone in fuel bills. MQuillen “Re(4):furnace size” , posted Sun 14 Aug 12:34 ______________________________________________________________________ You’re right, Cool. I need to confirm actual output. The label doesn’t give output and I can’t find a spec sheet. It is a 1989 Rheem RGDA series, 14″ width model, with 75k btu input, 3 ton air handler. Maybe I’m wrong on the efficiency of these. Oh, it would be embarassing and expensive to underspec my furnace, but I’ve always wondered if my wife could be more frigid. MasterTech “Re(5):furnace size” , posted Sun 14 Aug 14:06 ______________________________________________________________________ Need more model #. RGDA is not enough to give an accurate. Varies from 62% to 80% MasterTech “Re(4):furnace size” , posted Sun 14 Aug 12:15 ______________________________________________________________________ Good question cooltiger. If u got the model # i might be able to tell you what the rating is. MQuillen “Re(5):furnace size” , posted Mon 15 Aug 09:40 ______________________________________________________________________ The specific furnace is RGDA-07EC-ER. I’m just trying to get an idea as to what model I’ll go with in the future so I’m not up against a short decision deadline in the future, like I was with my A/C this year. We have a couple parrots in the house, and a small crack in a heat exchanger could be a bad thing for them. I do keep CO detectors on both floors of my home. Any of you that have experience with these 16 year old Rheems, how much more life can I get out of it here in Columbus, Oh.? I know the 90+ efficiency furnaces can be upsized a little without too much problem, and a little larger isn’t that much more expensive. I think the Rheem RGRK series looks interesting in the two stage, 90+. The Rheem modulating furnace looks interesting, but I’ve never been one to adopt the newest design, and the associated problems that go with them. Thanks again for your help. MasterTech “Re(6):furnace size” , posted Mon 15 Aug 19:54 ______________________________________________________________________ Rated at 69%, 75kbtuh 1988-1992 unit 51kbtuh output. MQuillen “Re(7):furnace size” , posted Tue 16 Aug 08:48: ______________________________________________________________________ Thank you Master Tech. So if I went with a 45k btu 90+ furnace, I’d take a 10k btu loss on capacity (maybe too small, maybe not?). The next furnace size up is 60k btu, which would leave me with 55k btu output at 90%, or 42k btu on 1st stage. My first stage would be about 18% less capacity than my current furnace’s total capacity. Does that seem reasonable? I can really see the beauty in one of those modulating furnaces with the ecm motor, and the 40% low stage heat capability. Ever notice that everything you really want is just a few (hundred) dollars more. What kind of reliability do the modulating furnaces have? [this message was edited by MQuillen on Tue 16 Aug 08:54] CoolTiger “Re(8):furnace size” , posted Wed 17 Aug 11:36 ______________________________________________________________________ I would definitely go with the 60K 90% unit to replace your old one… I thought Ohio is a cooolllldddd state… I would still get a heat loss calc first before I buy one. You can do it yourself if you want… $50 for two month use… just search for HVAC calculator on Yahoo. Be very specific and exact on what you input… or hire a pro to do it for you. MQuillen “Re(9):furnace size” , posted Wed 17 Aug 11:42 ______________________________________________________________________ Thanks to everyone here for your help.