Preheating furnace combustion air
 

I'm considering building a preheater for my furnace's combustion air. Winter is almost over so unless weather gets weird it might not be a factor until the fall. I currently have 2x 50kbtu natural gas furnaces with 80% afue existing in the same attic space. This attic space has a super insulated floor and 3-4 layers of radiant barrier and a ridge vent. Something I always notice is the roof above the adjacent attic space will frost while this one doesn't. Seems like there is a lot of heat energy left in the 20% going out the flue. Because of our short and warm winters 80% furnace are the norm and I have 5-10yrs left on my equipment's life span.

I have found a few papers on using combustion preheaters to increase industrial equipment efficiency and possibly reduce some emissions. On a small scale it might not do much, but as we know here gaining 1-2% overtime adds up. My current plan is to wrap my 4" flue double wall type b vent with 6" single wall snap, nearest to furnace connect a 4-4-6 wye and feed 4" aluminum flex into my furnace. My Ruud unit has a stub for a combustion air duct. As the furnace burns it will draw fresh air from the 6" pipe wrapped around the flue. If I really want to go all the way I could replace the double wall 4" with single wall, this pipe gets way hotter. Double wall flue clearance is 1" from combustibles vs 4-5" for single wall.
http://energy.gov/sites/prod/files/2..._preheated.pdf
http://energy.gov/sites/prod/files/2..._preheated.pdf

single wall wye
https://www.theductshop.com/uploads/.../reg/TY1-M.JPG

GTO,

There will be some efficiency to gain, but I see a few hurdles.

One, even if your furnaces have combustion blowers, they are still draft furnaces requiring certain flue attributes as described HERE. Be mindful of the requirements of other devices, like water heaters, that share a flue. You wouldn't want to create negative draft conditions that might invite carbon monoxide in the house!


If you lower the flue discharge temp, you are likely to get more corrosion of the flue since more condensation will occur and the flue won't be as hot at shutoff to reevaporate it.


You probably aren't going to take an unacceptable amount of heat away from the flue with the short section you describe, but you will certainly change the air/fuel ratio of the furnaces, since they will be drawing through a restriction instead of free air, and the inlet pressure your furnaces see will be lower still as the combustion air is trying to rise out of the inlet since it is now warmer and buoyant. The same volume of air now has fewer molecules of oxygen since it is warmer. If you do this, be sure the furnace runs at stoichiometric under all operating conditions.

Lastly, you will gain WAAAAYYYY more efficiency by relocating equipment and as much ductwork as possible into conditioned spaces (even building a conditioned space around them), or replacing with more efficient systems if you can. There is a frighteningly large amount of air leakage in equipment casings and air ducts, plus so much surface area to lose/gain heat through even if it is well sealed.

I removed both my furnaces and installed hydronic retrofit heat plate warm floors powered by a 95% efficient water heater, plus a 23SEER Nordyne inverter air conditioner which can vary its output from 1.5 to 3.5 tons, allowing one unit to cool the whole house. Rearranging the ducts to do this got all my equipment out of the attic and reduced the amount of ductwork in unconditioned space by half.

Even though I was now heating/cooling 3 floors AND a garage instead of just 2 floors in a 5br/3.5ba house, the cooling demand for kwh went down to 15kwh/day from around 60kwh/day, and my gas use during heating season went from 300-350 therms/mo to 175-225/mo.

Frost on the inside of a roof is due to warm moist air leaking from the conditioned space. Fix those air leaks (there is always more than one) and you will save much more than by preheating furnace intake air.

I replaced an 85% efficient water heater hydro air system with a 95% efficient boiler with indirect water heater. My gas consumption decreased 24% for the same number of degree days.

The 6" around the 4" isn't going to be enough flow area either, and 4" supply duct may not be enough. Manufacturer and code requirements differ but you will probably want to go with the most conservative NFPA 54 requirement of 1sqin of air opening per 2000btu of burner unless you carefully research otherwise, 20sqin per furnace in your case. The area between a 6" pipe and a 4" is about 6.5sqin, and the area of a 4" duct is 12.5sqin.

If you move or replace equipment, make sure to provide external combustion air or have sealed burners that draw only outside air for combustion. The draft caused by drawing in cold air to replace combustion air wastes a lot of heat, and could risk CO poisoning if your house was tightly sealed.

Frost was on the outside, on top of the shingles above the attic without furnaces. Attic space with furnaces, no frost on the shingles. My ducts are reasonably tight, lots of mastic, rebuilt 2 plenums when I moved one for a remodel last year. Again its warm here, my January gas usage was high at 86 therms compared to December's 52 therms. Saving 2% on $30 worth of gas won't do much for me, its more of an experiment for a cheap improvement. Maybe a feather in my cap. Both heaters are power vented with their own dedicated runs. I don't think there will be much of a restriction for the combustion air, 2" pipe is all 50kbtu furnaces need. The ruud installation manuals always show this as an installation option in tight or restricted spaces. The factory concentric vent kits for the 90%+ furnaces use a similar idea, flue gas in a center pipe while outside combustion air flows around it in an outer jacket. And the combustion air pvc pipes are also 2" on the 90% 50k and 70 kbtu units. Because my heaters are in the open attic cold air draft from combustion make up aren't really a problem.
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You should be good with your heat exchanger idea since you have separate runs for each, as long as the units are designed to use a more restrictive metal coaxial air/flue combination. You mentioned 80% efficiency, so I wasn't sure if they were the type of non-condensing power burners which might need the flue to act like a draft flue as described in the link I posted.

90%+ efficiency furnaces condense, therefore exhaust temps are lower and most can vent through plastic pipe. Those burners are designed for that restriction.

True, you only need outside combustion air provisions if you move the units into enclosed or conditioned space.

My mechanical engineer buddy got one of those condensing higher heating value 100,000BTU/hr natural gas furnaces that can be vented through PVC, one that can be installed in a trailer home. He opted to install his new furnace trailer style in the basement of his house, where the furnace draws air from out side the house.
That way the furnace is not drawing air from inside the house, which intern does not draw cold air into the house.
I think that would be the bigger efficiency improvement as compared to prewarming the air.

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.

85% That's the MAXIMUM efficiency you can get out of a non-condensing gas appliance. The other 15% of the combustion process is latent heat, and you must condense the combustion gasses to capture it. The 5% difference between 80% furnace and the maximum 85% theoretical non-condensing efficiency allows a safety margin so flue gases don’t condense.

Combustion Efficiency and Excess Air

Mobile Master Tech is SPOT ON with his advice.

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.

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.

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.

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?

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.