High Efficiency Furnace

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An out-of-town friend has a two-story condo with an old gas, forced-air furnace in a pantry off the kitchen. She wants to replace it with a high efficiency furnace, taking her from 80% efficiency to 95% efficiency,for the fuel savings.
A contractor came to estimate the job and said that she couldn't put a high efficiency unit in that space but didn't give a good reason, at least not one that she understood. One complicating factor is that she thiks that there is just one chimney outlet on the roof which is shared by the two adjoining condos but I haven't actually seen the unit as it's hundreds of miles away.
Does anyone have any ideas about all this? It seems that you should be able to just remove the old unit and put in a new one unless the high efficiency units are somehow larger, hotter or whatever.
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Check with other contractors, and get estimates for their installation of a high efficiency furnace. The chimney should not matter, as high efficiency furnaces don't need one. The location may be in a difficult location making it a problem to install the plastic exhaust pipes as they are limited by length and location, or maybe she will need to get someone to clear a path and then cover the pipes once they are installed. The original contractor probably didn't want the job or he is incapable of explaining the problem in lay-man's terms, which means you don't want him either.
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Pavel314 wrote:

Tell your friend that given the low price of natural gas, it will take 50 years for the cost of the new furnace to equal the savings in natural gas used.
And new furnaces are a repair nightmare compared to older ones. More sensors, delicate electronic motor.

High efficiency furnaces don't exhaust the combustion gas through the chimney. You have to run a new exhaust pipe (made of plastic) through a wall to the outside. The layout and current location of the furnance must mean that it would be a bitch to run this new exhaust pipe.
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On Wednesday, October 3, 2012 5:53:23 PM UTC-4, Home Guy wrote:

I know you don't have to have a chimney but why couldn't you use an existing chimney anyway? It doesn't seem like it would hurt anything, but I really don't know much about them; I have an old fashioned hot water radiator/boiler system.
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The 90 percenters, the exhaust isn't cold enough to naturally rise. The cold exhaust (also a lot of humidity) would not dependably exit the building.
Christopher A. Young Learn more about Jesus www.lds.org .
wrote in message
I know you don't have to have a chimney but why couldn't you use an existing chimney anyway? It doesn't seem like it would hurt anything, but I really don't know much about them; I have an old fashioned hot water radiator/boiler system.
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If by use an existing chimney you mean just route the PVC exhaust pipe to the chimney and tie it it, no you can't do that. A chimney relies on hot gas to create a rising air current. A condensing furnace has air that is barely warm. A related problem is if you have a gas water heater on the same masonry chimney as a gas furnace and then replace the furnace so that the chimney is left with the orphan water heater. That leaves a chimney too big for just the water heater alone. In winter in cold climates the gas will condense and the acid formed will destroy the chimney over time. That can be solved by putting in a chimney liner.
If by using the existing chimney you mean routing the PVC pipes from the new furnace up, inside it, essentially using that as just a conduit, that might be possible, if nothing else, eg gas water heater uses the chimney, but I've never seen it done. Every one I've seen uses PVC that is run as short a distance as possible to the outside, which is usually easier done directly.
For the condo in question, the essential question is what the path would be to run two PVC 3" pipes to the outside via either a wall or through the roof. They also have concentric termination kits where the two pipes can be combined into one where they go through the wall if that makes it easier.
There are also restrictions on how close the exhaust can be to a window that opens, a door, another appliance exhaust, an overhang above, etc. For details, you can pull up the install manual on any one of the commonly available furnaces and they will have diagrams and rules.
Also the poster that claimed a high efficiency furnace will only lower your gas bill by 10% is wrong. Let's compare two furnaces, one at 80%, the other at 95%.
I put 100 units of energy into the 80% one and get 80 units out. To get that same 80 units with the 95% furnace I need to put in 84 units. That's a savings of 16%. And that assumes that the old furnace is indeed operating at 80%. If it's 25 years old and not in great shape, it may be less.
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On 10/3/2012 6:07 PM, snipped-for-privacy@optonline.net wrote:

I had hoped could do that on the replacement here--I've thehe problem outlined above of the water heater exhausts thru it as well and I figured surely there was a liner that could allow it to avoid the other penetration but was told "no"...not allowed to share even if separated flows...

Like if it's natural draft and that old it's likely only 50% by design, not 80%, anyway...
Good post; outlines the pertinent points well...
Guess: Problem OP's friend has is quite possibly the condo rules on modifications of units that prevent doing what otherwise could be done. Recommend to OP to investigate whether that's the real hangup instead of actually a real physical installation issue. If so, may need to approach several others and broach the subject in force to get necessary changes in covenants...
--
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That depends on a whole host of factors not placed into evidence. Like the location. Georgia and Michigan are very different. And you have to factor in rebates, utility credits, etc that can offset the cost. But that can and should be done to get an idea on the payback. Also, if the existing furnace is 30 years old, there is also the question of how luck you feel. Many would prefer to replace it when there are rebates, incentives, tax credits, good deals available as opposed to having to do it in Jan in a crisis.

If you want. you can buy many high efficiency condensing furnaces that have a simple PSC motor, just like the existing furnace.

You would think that if that were the reason, the contractor could have explained it. The fact that the contractor couldn't give an explanation suggests the need for a new contractor.
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On Wed, 3 Oct 2012 16:18:07 -0700 (PDT), " snipped-for-privacy@optonline.net"

Other factors come into play. Last year, at work, we replaced a 1970's boiler with a new one. With the old boiler, the heat would come on and make my office nice and comfy. Then the heat would go off and I'd feel the cold air down my back until it cycled on again. With the new boiler, it circulates lower temperature water for a longer time and I never feel the draft. An outside temperature sensor helps to determine the proper water temperature.
Did I mention the $3000 rebate?
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My forced hot air furnace and air conditioning is variable speed. It doesn't go on and off. It adjusts to whatever level is needed to mainatin the set point temperature.
Don. www.donwiss.com (e-mail link at home page bottom).
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Ed Pawlowski wrote:

There is (or was) nothing stopping you from adjusting the gas-valve (or thermostat) on the old boiler so that it too would have circulated lower-temperature water, giving you the same result.
I do exactly the same thing with the 36-year-old furnace in my house. I adjust the gas dial-valve in the furnace (the electrically-operated valve that has the pilot-light setting and the "ON" setting) and instead of turning it full-on, I adjust it much lower (just like you would on a barbeque). This has the effect of putting out less heat and results in a longer run-time for the fan - and better "comfort" in the house. Probably gives better efficiency as well (less waste heat going up the flue).
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Maybe the boiler is in a locked room, and Ed doesn't have permission to work on it.
Christopher A. Young Learn more about Jesus www.lds.org .
Ed Pawlowski wrote:

There is (or was) nothing stopping you from adjusting the gas-valve (or thermostat) on the old boiler so that it too would have circulated lower-temperature water, giving you the same result.
I do exactly the same thing with the 36-year-old furnace in my house. I adjust the gas dial-valve in the furnace (the electrically-operated valve that has the pilot-light setting and the "ON" setting) and instead of turning it full-on, I adjust it much lower (just like you would on a barbeque). This has the effect of putting out less heat and results in a longer run-time for the fan - and better "comfort" in the house. Probably gives better efficiency as well (less waste heat going up the flue).
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wrote:

Not exactly. If it was adjusted to a lower flame, it would be at that constant. This boiler adjusts according to need as determined by the outside temperature. What works at 40 degrees may not be adequate at -10 degrees.
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Ed Pawlowski wrote:

It's my experience that many residential forced-air gas furnaces built / installed 25+ years ago are ridiculously over-sized, especially when you take into account upgrades to the home like better windows and draft sealing and more insulation.
Over the past few years, I've found that the throttling-down of my gas valve on my 36-year-old furnace has served me well over a wide range of outdoor temeratures (including the few nights we might see -5 f) - and it hasn't resulted in the phenomena that you suggest.
And even if it did, it's no problem to go down to the furnace and tweak the dial to give a stronger flame and leave it at that setting for the next month or so until it gets warmer outside.
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wrote:

Beyond being over-sized, actual efficiency wasn't much in the calculations. Too much heat for the exchanger, that heat being lost up the stack. I throttled the gas on my old hot water system and it cut my gas bill considerably. Adjusted the air to keep good flame. It cycled more, but that's what I wanted.
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On Thu, 04 Oct 2012 12:11:30 -0500, Vic Smith

which IS what you want. Ideal furnace size will maintain temp on the coldest day you can expect to experience by running over 80% of the time. (100% would not be able to recover temperature)
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On Thu, 04 Oct 2012 17:08:35 -0400, snipped-for-privacy@snyder.on.ca wrote:

Yes, that's what I meant. Ran more, for longer cycles.
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If a 120K BTU is rated at 93% efficient, then it's 93% efficient. If a 70K BTU furnace is 93% efficient, then it's 93% efficient. Any differences in heat exchanger are reflected in those numbers.
Now, having a furnace that is too large may lead to the house having temp swings and some inefficiency related to the furnace having short cycles. But it has nothing to do with too much heat for the heat exchanger.
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" snipped-for-privacy@optonline.net" wrote:

Efficiency is related to BTU output.
In other words, if the design goal of a furnace is to be able to put out 120k BTU and do it at 93% efficiency, then it's heat exchanger is designed to accomplish that (by adjusting it's surface area, thickness, length, air-flow speed, etc).
If that same furnace can alter it's BTU output (multi-stage or variable burners) then it's efficiency will not necessarily be 93% at other BTU settings. Most likely it will be MORE efficient at lower BTU outputs.
Same way that a car is spec'd to have a certain MPG under certain well-controlled conditions - primarily vehicle speed.
In the case of a heat exchanger, all else being equal, you can increase the efficiency of the heat-exchange process simply by dialing down the BTU output. The extent that you can do that depends on whether the amount of heat coming out of the furnace is high enough to match the heat-loss (of the house) to it's surroundings.
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Yes, I agree with that. If it's capable of firing at a lower rate, it should be slightly more efficient at the lower firing rate. So, if it's 93% efficient at 100%, firing at 70%, maybe it's 94% efficient. But it's the basic overall design that makes the big difference. You can't turn an 80% furnace into a 93% furnace by just lowering the firing rate. With a furnace that is rated at 93% when firing at 100%, it's already extracted almost all the heat available. The steam is condensed and the air coming out is barely warm. Lowering the firing rate isn't going to extract much more heat because there just isn't much left to be extracted.

Except that the difference is there can be a huge variation in vehicle speed, grading, load, etc. which gives a big difference in fuel consumption.

And with a condensing high efficiency furnace almost all that available heat has been extracted, even when it's firing at 100% output.
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