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Old 03-13-16, 03:49 PM   #11
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yes combustion air from outside the conditioned space is definitely the way to go. Each Cubic foot of gas requires 10 cubic feet of air, so 100kbtu would exchange all the air in your house several times over. Thats why outside combustion air is part of the energy code. Mine are located in the unconditioned open air attic crawl space so there's no loss there. What I suggest is preheating the combustion air which presumably you are bring in from outside, which is colder because you are running the heater. You already have a flue and need combustion air why not preheat it? Testing a double wall flue last week, 3' from the furnace still very hot, couldn't keep my hand on it. about 4' from it it was warm to touch, maybe 100-105f. If I wanted to get max heat out of it I'd use single wall vent. Unfortunately winter's over here, my hvac has been off for almost a month, so might not happen for me until fall. Maybe someone in a colder climate might want to try it before then.
Would it? 100kBTU is 1ccf or 100 cubic feet, so based on your numbers, that would be 1000 cubic feet of combustion air. That would be an air exchange for 125 square feet. 24 hours of continous runtime for a 100kBTUhr unit would be 24,000 cubic feet which would be a single air change for 3000 square feet. Figuring that this system is probably oversized and I figure it wouldn't even run 1/3 the time of a day, that would an air exchange for a 1000 square foot building. That doesn't seem like many times over.

Granted, it is still an unwanted air exchange and I'd prefer it be sourced from the outside too, but it isn't as much air as you are implying.

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Old 03-15-16, 06:40 AM   #12
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Figuring that this system is probably oversized and I figure it wouldn't even run 1/3 the time of a day, that would an air exchange for a 1000 square foot building. That doesn't seem like many times over.
100k furnace would run 1/3 of the time on a design day, mush less on a normal day. Look at Therms burned in a month to get a better look at actual run time.
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Old 03-15-16, 09:33 AM   #13
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Thanks, if I can get to 85% and getting to theoretical maximum I'd be happy. Materials and time invested would be minimal compared to time and investment to replace with 95%. Someone using propane or oil might see a better return. I guess I'll know if I over achieve and it starts condensing. Unfortunately yesterdays high was 92f, winters been over for a month already.
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Old 03-15-16, 07:06 PM   #14
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To hit 85% you have to get the air/fuel ratio exactly right. At 83% you start having condensation problems. You’re talking about modifications to get THREE PERCENT. In TEXAS you will NEVER see the difference in your gas bill. Is 92f already, you use the heat 3 times a year?

We went from an 88k 80% to a 44k 90% in Oklahoma, didn't see any real world difference in gas bills. Cutting the furnace size in HALF did make a HUGE difference in comfort, no more wide temp swings between cycles.

Your efforts are much better spent on improving AC efficiency, there’s a LOT more on the line. REAL MONEY.... Looking at your signature you have really tightened up the house, when it comes time to replace the AC install HALF the typical 500sqft per ton. 1,000sqft per ton may sound nuts for TX, but does your house not have about 1/2 the typical heat gain for your area?
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Old 03-16-16, 09:18 AM   #15
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Its not one 100k furnace its 2x 50k units, the match up for their 2 ton a/c counterparts. I get about 15 seer out of the a/c units and don't plan on replacing them too soon. 600 sqft per ton is about the max I care to go, we still have 100F weeks. I prefer to have the hvac off when the house is unoccupied and require more btu to catch up when we get home. Additionally my wife is requiring the house cooler as we age. We used to get by with 78f indoor, last year that went down to 76f, this year shes complaining about 74f. The good news is with my improvements our power usage still went down, thanks micro solar. I know this won't effect my gas bill by much, its more of a curiosity. Preheating the air makes sense, mixing burning 100f air vs 60f air should yield a more heat.

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