80% vs 93% AFUE furnaces

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So a couple of more HVAC contractors came to my house to quote a new furnace and the second guy through a monkey wrench into everything...basically I'm a little confused.
Ok, here's the story. The house is a 1600sq.ft. 2 storey colonial in Michigan. I'm looking to replace my 12yr old Goodman 125000BTW with a 2stage furnace. So far all the quotes have been for 92-93% 80000BTU 2 stage units.
Today however, one of the contractors - recommended on the american standard web site - quoted me a an 80% 80000BTU 2 stage Variable Speed unit. When he asked me what I was looking for I mentionned the 92-93% requirement, but he proceeded to tell me that the 80% variable speed unit is just as efficient....
How can that be? Am I missing something?
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wrote:

He is an idiot, that is the only thing you are missing.
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Buy the 90+ and dont let that idiot any where near your home.
90+ is ONLY way to go
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efficient; but perhaps he thinks it is cost effective for your particular installation. Won't know if you don't ask.
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If you have the money go with the super-efficient model that uses PVC for flue and air intake. First, you'll be as well off as you can if energy prices continue their insane spike, and I think they will go a LOT higher. Second, no more messing with a chimney - one less thing to worry about. Don't know if 92-93 gets it, I think I had a 97 before when I went flue-less.
Now I'm stuck with an 80% Goodman and it's ok but my gas bills are insane.
The 80% guy is an idiot if he said it's just as efficient as 92.
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snipped-for-privacy@yahoo.com wrote:

Yup 92-93 gets the PVC piping as well..

Same here...

That's what I figured... I just wanted to make sure I wasn't missing something really basic. Also the fact that he is more expensive for a less efficient furnace makes me wonder what kind of margin this guy has.
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Do the math!
The difference in your monthly gas bill is from 14% to 16% worse case. And that ignores the fact that the stack provides a little heat. If you can bring in combustion air from the outside, that would "help" the 80% unit. You can decide for yourself whether the fuel savings will make up for the cost.
The main selling point of the 93% units is that they don't need a "hot" stack. They often have an electronics package that can be expensive to replace and can become obsolete well before the mechanicals are gone.
It don't believe that two stage gas heating is worth the cost in terms of comfort gained v. cost. (Multi-stage AC is a VERY good idea.)
If you really want to save money, get a VENTLESS heater. If the slight odor doesn't upset anyone then they provide 100% efficiency and put the heat RIGHT where you need it.
Frankly, I'm surprised that your furnace needs "new" so quickly.

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Two stage gas is more for energy efficiency, in that the hi power burner usually doesn't need to be run.
For best results, you need to upgrade to a 2 stage thermostat. A variable speed blower is the best.
-- If I had something witty to say, this is where I'd say it.
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Yeah, but "heat is heat." It shouldn't make much difference between runing longer at "low" power or not as long at high power.

Again, that stuff is "nice" with AC but for "heating with gas" it still comes down to "heat is heat." The might be a change in the electric bill from a variable speed blower. Your more or less "standard" three speed blower (usually wired for only two speeds) isn't particularly efficient. I understand that some of the variable speed blowers are VERY efficient.

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I think it is more comfort than absolute BTU efficiency. For most of the year, the lower BTU unit is fine.

All I know is that when the heat kicks on high, it is like kicking on the after burners in a jet, it cranks out a lot of heat. And I've not seen it happen yet other than in testing, it takes a multi degree thermometer change to kick it in.
I only have a single speed A/C. I keep the variable speed blower on all the time, it evens out the temp.
-- If I had something witty to say, this is where I'd say it.
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Well, it's your house.
But when you keep the air circulating when the compressor is off some of the condensation water ends up back into the house rather than being taken away. And, unfortunately, those 3 speed fans use a signficant amount of power.

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John Gilmer wrote:

I think it may be a lot more than that. Right now my furnace is a 125000 BTU unit which everyone who has seen indicates that it is oversized. Being oversized it cycles a lot and provides for some good temperature swings. Also, who knows how truly efficient it is in this case - what kind of backpressure is being built up, etc.

What is Multi-stage AC?

That's definitively a thought, but I'ld rather not have the odor...

It's not just the furnace but a combination of items that need fixing. The condenser had been recently replaced but not the A-coil. The filter is a 23+ yr old Sears Electronic Air Cleaner. The humidifier is an older Kenmore unit (with the tumbler wheel). The furnace has been fixed once since I bought the house (the central computer was replaced) and recently has gotten into the habit of popping one of the 5 flame rollout sensors.
I'ld rather get rid of all the junk and get some peace of mind.
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Well, I was simply looking at the difference between two replacement units.
If you have a WORKING furnace, it usually doesn't pay to replace it unless you gain something else like putting in AC.
If you have a large house you might consider setting the thermostat quite a bit lower (say 60 to 65) and use "spot heat" such a ventless gas units or even electric heat. It will cost about $.10/hour to operate a 750 watt heater than can make a real difference in a medium sized family room and if the people are all together your can be a warm as toast.

True. Who knows?
All I say is "cost things out." In general, it doesn't pay to replacement working equipment just for "efficiency." And when you have to replace, use a sharp pencil for your calculations. You will have to decide for yourself when one design or another will result in higher repair costs.
If you remodel you can use plastic pipe for the furnace vent. But you still have gas hot water heat. There are gas water heaters that can also use the plactic pipe but they are expensive. They have a "power vent." If you are doing extensive re-modeling (which also almost never "pays") getting rid of the stack might save a few square feet of floor space.

Oh, these have a two speed compressor. Most of the time the compressor operates at low speed and the fan cycles between low and moderate speed to control humidity. On a HOT day, the compressor runs full speed and the fan speeds (Including the fan on the "outside" unit) are changed for the right balance of comfort and efficiency.

I no longer am a fan of electronic air cleaners. You can get 1" filters that do about as well or better. If I were "starting from scratch" I would have the HVAC guy put in a 3" or 6" filter. It would have to be special ordered but it would last a long time and be quite "efficient." You might even be able to special order a filter that could replace your old sears electronic filter. It would like so long that you could consider "installing" it with duct tape!

Well, the alt.hvac folks are no longer "user friendly" so it's hard to get firm data on reliability anymore.
The ventless units provide a little help with the humidity.
If you put some of your money into "tightening up" your house you will not have as much need to add humidity. Adding moisture can cause enexpected problems in very cold weather when the water vapor condenses and freezes in some out of the way location. When it warms up you might have an ice block melting in you attic or inside an outside wall.

Well, it's your money. Trouble is that you likely will not have the money to truly "start over" and re-engineer and partly replace and modify the duct work. You will still have to make some compromises and/or have very obvious duct work "patches."
Your "peace of mind" is an illusion. If you have smoke and CO detectors and you haven't bypassed any safety equipment you are about as likely to make it through the next year with safety with your old system as with the "latest and greatest."

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A power vent hot water heater won't work if there is a power outage, while a standard one will. Big advantage if you want a hot shower when the lights are off, like after a storm.
In my case, after installing the furnace with PVC venting, they installed a liner and cap in the chimney, so the hot water heater vents through that.

The 4" high efficiency air filters are standard these days, I can get the media at my local hardware store, and the big box stores.

I have an Aprilaire unit, which is tied directly into the furnace fan, and has no moving parts other than a water valve.
-- If I had something witty to say, this is where I'd say it.
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True.
Slight OT: my (our) first house had a thermocouple powered gas valve for the furnace. (This is still standard for gas powered water heaters.)
Were the electric to go out the furnace would still light up as normal. When the insides got hot, the fan controller would still attempt to switch on the fan but without electric the fan would not come up and the insides of the furnace would get hotter and hotter until the over temperature cutoff on the controller opened. When the insides cooled (from convection) the overtemp switch would open again and the cycle would start over.
Needless to say, they don't do it that way anymore.
But it is "nice" to have hot water when the electric is gone.
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He's a basic idiot.
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wrote:

What sort of PVC piping is needed for one of these more efficient models? Just curious..
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Typically 3".
-- If I had something witty to say, this is where I'd say it.
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Rick F. wrote:

horizontally out of doors and a limestone/marble bed provided underneath the outlet. Acidic water is produced and must be effectively dealt with.
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STravis wrote:

Maybe he meant "as cost effective" rather than "as efficient." I have no idea what the numbers are but as you go up in efficiency, the maintenance/repair costs go up, but, energy costs will be less. So if you are lucky the higher efficiency furnace will result in an overall lower cost, especially if energy costs go up. But if you are unlucky, the lower efficiency unit will result in a lower overall cost.
It all depends on chance (repair costs), the weather where you live, and what energy cost will be in the future.
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