Do I need a new Furnace?

[HoosierBuddy]

When your new furnace is 40 years old how efficient do you think it's going to be?
Sometimes it's just not practical to install a 90 plus furnace,

By that time it will have paid for itself a few times over. My gas usage went down by 1/3 when installed, and after that was when NG went to $14/DTH on the NYMEX (currently it is less than $2/DTH for comparison).

Also...regarding efficiency of an engine and a furnace you are talking apples and cantaloupes. If your engine makes more HP pulling in cold air, that does not make it more efficient. Your increased power comes from the ECU (or the tuner if using a carburetor) being able to add more fuel to the engine because of the increased air density. AND...before you get too upset, I'm not saying you are wrong about the furnace. I'm just saying it's not analogous.

I don't mean to hide behind a definition but so we are on the same page, efficiency to an engineer is energy in over energy out. If you are able to add more fuel and air and get more power out of the same sized engine...that is awesome....but it doesn't in itself indicate if "efficiency" went up, down, or stayed the same.

To know for sure someone (like the manufacturer) would have to test it both ways. I've seen several furnace manuals that specify they can be installed either with or without exterior combustion air and none of that literature indicates the manufacturer claims an efficiency gain because of it just comparing the heat input (heat content of the gas times run time) vs the heat output (delta T on the downstream side of the heat exchanger X volume of heated air). Maybe they haven't tested it...or if they have they are keeping that information to themselves.

Phil

 

[Showkey]

News flash chief,I've been putting really nasty small block and big block chevys together for a lot of years.
Cold air is cold air its denser and burns a whole hell of a lot better than hot air.
It doesnt matter if it's in a furnace in your basement or in the combustion chamber of an LS6 or 7 454.
Mean while go out and service/install hvac equipment for 30 years ,then come back and talk to me about furnaces and air conditioning.

News flash chief ........the furnace does not have fuel mixture adjustment for temp or air density.........your modern vehicle adjusts on the fly for a dozen variables. The old chevy had to tuned to take advantage of the temperature and density.

Does a camp fire burn hotter on cold damp day ?

 

[Jim greengo]

By that time it will have paid for itself a few times over. My gas usage went down by 1/3 when installed, and after that was when NG went to $14/DTH on the NYMEX (currently it is less than $2/DTH for comparison).

Also...regarding efficiency of an engine and a furnace you are talking apples and cantaloupes. If your engine makes more HP pulling in cold air, that does not make it more efficient. Your increased power comes from the ECU (or the tuner if using a carburetor) being able to add more fuel to the engine because of the increased air density. AND...before you get too upset, I'm not saying you are wrong about the furnace. I'm just saying it's not analogous.

I don't mean to hide behind a definition but so we are on the same page, efficiency to an engineer is energy in over energy out. If you are able to add more fuel and air and get more power out of the same sized engine...that is awesome....but it doesn't in itself indicate if "efficiency" went up, down, or stayed the same.

To know for sure someone (like the manufacturer) would have to test it both ways. I've seen several furnace manuals that specify they can be installed either with or without exterior combustion air and none of that literature indicates the manufacturer claims an efficiency gain because of it just comparing the heat input (heat content of the gas times run time) vs the heat output (delta T on the downstream side of the heat exchanger X volume of heated air). Maybe they haven't tested it...or if they have they are keeping that information to themselves.

Phil

You're comparing your energy savings between a 40 year old furnace and a new 90plus furnace.
Your energy savings between that 40 year old furnace and a new 80% furnace would of been pretty impressive also.
And you could take the extra money and dump it into insulation in your attic,which saves energy year around.
Not just a few bucks a month in the winter.
I used the comparison of a furnace and a car engine both benefiting from cold air versus hot air because it's a pretty simple fact that to anyone with a 3rd grade education who's ever worked on these things knows its true.
What happens when you heat most materials?
They expand,when you cool most materials including air it contracts.
Since theres only so much room inside of a burner compartment for air.
And you're also not using heated air that you already spent money on to heat to support combustion to make more hot air .
So anyway do what you think is best for you.

 

[Jim greengo]

News flash chief ........the furnace does not have fuel mixture adjustment for temp or air density.........your modern vehicle adjusts on the fly for a dozen variables. The old chevy had to tuned to take advantage of the temperature and density.

Does a camp fire burn hotter on cold damp day ?

I can do a whole lot of adjustment on a gas valve and regulators to compensate for things on a furnace.

Put the camp fire in a sealed room and see what it does,who's talking apples and oranges now?

 

[HoosierBuddy]

I used the comparison of a furnace and a car engine both benefiting from cold air versus hot air because it's a pretty simple fact that to anyone with a 3rd grade education who's ever worked on these things knows its true.

I went to the third grade. My teacher taught us a lot things, but not that.

Then I went on to one of the top 5 undergraduate programs in Mechanical Engineering in the country and my thermodynamics professor taught me a lot of things too...and also not that.

Again Jim...I'm NOT saying you're wrong. I'm saying I'd need to see the data. It's a lot more complicated than you are making it seem. You're comparing straight air-fuel combustion in the furnace with an otto cycle or some approximation thereof...apples and rock candy. Not the same.

Regarding insulation...Yup. Did all of that I could. Added blown in insulation in the attic. Sealed up everything I could. Replaced all the single pane windows that the previous owner hadn't already replaced (just a few small windows in the cellar). I completely agree with you that those items are the low hanging fruit that can save a lot of money for anyone that buys an older home.

Phil

 

[Jim greengo]

 

[LS6 Tommy]

Does a car engine make more hp sucking hot air from a sealed up engine compartment,or from a functioning coal induction set up bringing in colder air from outside?

You best stick with HVAC........cuz comparing a car engine to furnaces, your way out on limb. One is internal combustion, compression with air density, temp adjusted mixture control and the other is external combustion.

News flash chief,I've been putting really nasty small block and big block chevys together for a lot of years.
Cold air is cold air its denser and burns a whole hell of a lot better than hot air.
It doesnt matter if it's in a furnace in your basement or in the combustion chamber of an LS6 or 7 454.
Mean while go out and service/install hvac equipment for 30 years ,then come back and talk to me about furnaces and air conditioning.

I've been building engines and involved with multiple forms of racing up to the Pro level since I was 13 and have been in the HVAC trade over 35 years, including as an instructor. Jim greengo and I may not always agree on things, but this time we definitely we do.

Carbureted and mechanical FI high performance IC engines have no automatic compensation for density, or temp. I have long forgotten the numbers, but there is a definite increase in power per degree drop in intake temp.

Tommy

 

[mrobins297aaa]

Has the furnace been properly cleaned? I'm talking about removing and cleaning the burners, preferably by washing them with water, if they are rusty and in bad shape they may need replacement. This should be followed by a proper combustion test, you simply cannot do a proper tuneup on a fuel fired appliance without using a combustion analyzer. Heat exchanger needs to be visually checked for cracks, holes, etc.
A failed heat exchanger on a 16 year old furnace is not uncommon, however I would bet that a proper cleaning fixes your problem, the fact that it started after drywall work is a good clue.

Sent from my SM-T510 using Tapatalk

What exactly is there to adjust on a 90+ furnace other than the gas pressure.
I have not installed any for 12 years since I retired but all we ever did was set the gas pressure and visually looked at the burners and called it good.
I never even heard of a combustion analyzer up until a few years ago.
So what's it going to tell you and how do you adjust things if there wrong.
I'm not being a smart *** I'm just asking a question.