HE furnace piping question

Shopping for a new gas, forced-air, downflow furnace.
Weighing whether a high efficiency furnace is worth the upfront investment (taking into account gas company rebate, tax credit). The other options are stay with a simple 80%, get a 2-stage, or get a variable.
But now a new wrinkle.
The guy selling Lennox said he would strongly discourage the high efficiency furnace because this installation is in a closet and the new PVC they would run through the roof would have to pass through 3 or 4 feet of unheated attic space leading to condensation and potential freezing. The guy pushing Carrier never mentioned this (he did say something about a single coaxial pipe taking out exhaust and bringing in combustion air).
So what do the pros say? Is Lennox-guy right? Going through a short stretch of unheated crawl-space / attic a bad thing?
A woman across the street with the same kind of house has an HE of some sort since I can see the PVC on the roof. I suppose I could ask her if she's had any issues with it.
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Big Giant Head wrote:

Hi, Energy cost will rise down the road, I don't think it is getting any cheaper. Here in Canada, they can't sell/install none other than >90% HE furnaces. Venting does not have to go up to roof. You can vent it side ways to out side. But you'll need condensate drainage. If I were you, by all means I'd choose HE furnace and would use rebate money towards beefing up the insulation for the house; attic, walls. windows. weather stripping, caulking, etc.
Here in Calgary, UofC graduate students are in the process of plotting IR image from every single house in the city to see how well the house is insulated. Google map made it possible for them to develop a program for any one to use. Just type in your civic address then it'll show how good your house is air tight. They finished two quadrants of city, NW, SW. I am in SE, can't wait until I can check on my house.
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Unfortunately the furnace/water heater closet is not on an outside wall but about 7' from one. Add a couple feet to get from the back wall to where the furnace outlet would likely be. The intervening room is a laundry/utility room so ugly pipes crossing the space is not impossible but it would be tricky to do this without excessive turns because of how things are layed out. But what makes this a dead end concept is that this is a one story ranch and there are eaves sticking out a couple feet. The exhaust pipe coming out of the wall, even if done at about the same level it comes out of the furnace plus the obligatory sloping, is going to have all that moisture, condensation, and ice there under the eaves unless it juts out beyond.
Best to go through the ceiling and roof and insulate the tubes as required.
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Big Giant Head wrote:

Hi, I think one neighbor did that on his 2 story house from basement go roof. It takes caare of fresh air intake, venting.Pipe straight iup open ended/small one like umbrella handle ont eh side bending down U shaped. Our Carrieer Performance series 96%, 100K BTU reeplaced 8o% Carrier 3 years ago in the summer time(off season) Installer just installed did all the piping work to the side wall. Ran B vent liner up the stack to take care of water heater vent. I did wiring, installed wireless 2 stage thermostat, CO detector. After passing inspection, I paid him 4 Grand. Came with 10 year P&L, HE is life time warranty.
Here in Canada we can no longer sell or install lower than 92% ones. Repairing old one is allowed. Local university is IR scanning every house in the city to see how air tight the house is using Google map. When the project finishes we can go onto the 'net using the app. just input your address and see what you need on your house to make an improvement.
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I have a 2-stage 92% Trane condensing furnace. In my case the pipes goes horizontal about 15 ft and then through the wall to the outside. If you have to go through the roof then I would think that a condensing furnace would be the answer to your problem. It's been about 5 yrs now and the only problem I've has so far has been a flame sensor failure. The colder it is out the warmer the house feels--Obviously because the furnace runs more frequently but it also stays on the 1st stage most of the time. The initial furnace that came with the house was a simple off-on,100K BTU. The current furnace is a 60K BTU and does a better job of keeping the house warm and temperature swings to a minimum. The combustion air now is supplied from the outside vs inside thus reducing the heating bill since you don't have to use air that has already been heated to room temperature. Also, drafts are significantly reduced since you don't have to replace the inside air that goes up the furnace. MLD
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You need to take care of drainage for the condensate from combustion. I have a tee in my supply air pipe right before the furnace. It has a trap and drains into the condensate pump (which handles the heating and a/c condensate). Any condensate in the exhaust line drains through the furnace into the pump.
Any condensation in the attic pipes should likely occur on the inside of the pipes. If you have condensation in your attic, you have a bigger problem.
HankG
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Furnace condensate drains into the condensate pump. From there it is pumped into my washing machine drain line. A/C has a separate line that drains directly from the furnace evaporator pan to the outside through a sloped pipe via gravity. All works quite well. MLD
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MLD wrote:

Hi, Our ac evaporator, furnace, washer, hot water tank share one drain with check valve. They are all in basement utility room.
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