Gas Furnace

[lowridemazb2200]

What was the efficiency of gas furnaces about 16 years ago. What was the most efficient one and the least efficient? Just curious trying to find out what kind is in my house. I have a metal flue pipe so I know it is not a 90%. It is the original equipment from when the house was built.

[pctech]

Most are 80 to 85% my miller CMF-80PO is 80%
and the 90% i can replace with is double the cost .



What was the efficiency of gas furnaces about 16 years ago. What was the most efficient one and the least efficient? Just curious trying to find out what kind is in my house. I have a metal flue pipe so I know it is not a 90%. It is the original equipment from when the house was built.

[skippedover]

Age of the furnace is not the determining factor. If it has a standing pilot, the annual efficiency runs between 60% and 65%. If it has an electronic ignition and an induced exhaust but is vented through a chimney or with a power vent, it's 80%. If it has electronic ignition and vents with PVC pipe, then it is a 90+. At 16-years, it should not have a standing pilot as all furnaces by then were required to be at least 80% AFUE (Annual Fuel Utilization Efficiency) but it could be a little older, hence the explanation.

[t527ed]

metal vent 16 yrs old 78-80% efficiency.

does it have a draft inducer?:cool:

[lowridemazb2200]

metal vent 16 yrs old 78-80% efficiency.

does it have a draft inducer?:cool:



NO it sure does not!!

[Therm76]

78% natural draft

[t527ed]

78% natural draft



that would be my guess, got a make and model #?

[lowridemazb2200]

A hvac company wants to put in a 90,000 btu 95% goodman furnace in place of the one that I have. The input that is on the old furnace is 100,000 btu. So, if they use this new furnace it well put out more heat into the house correct. What do you think about the Goodman furnace?

[fluepipe3]

Need the make and model of your current unit to tell you the efficiency, sonetimes if you remove the access panel you may be able read that info on a sticker or plate.

As far as if Goodman is good furnace or not, Remember that with very few exceptions most furnaces are constructed from basically the same parts, givw or take a few and sone use propriety boards, or heat exchangers. So it really comes down to properly sized furnace installed correctly and preseason checks done on them yearly to ensure proper operation and catch any potential problems.

[lowridemazb2200]

ok

[beenthere]

Nothing wrong with the Goodman furnace.

But something may be wrong with the contractor.

The new furnace will need to move more air then your old one did.
You will either end up with moisey duct work. Or the furnace will ride the high limit.
Or a combination of both.

You might want to do your own heat loss calc. And find out what size you really need.

[lowridemazb2200]

Yeah I need 80,000 btu's to heat my house. There is not a furnace that is in the size. I only see the 90,000 and 70,000 one's.

[lowridemazb2200]

So, I dont know if I need to go to 90,000 or drop down to 70,000. Any info will be great.

[Senior Tech]

You need to have a reputable company come to your house and do a heat loss/cooling load and they will tell you what size you need and what it would cost to replace.

[beenthere]

You need a load calc done.

Been to a lot of houses with 100,000 BTU nat draft furnaces.

And only need a 60,000 BTU 90% to heat with.

[lowridemazb2200]

You need a load calc done.

Been to a lot of houses with 100,000 BTU nat draft furnaces.

And only need a 60,000 BTU 90% to heat with.



The company gave me a load calc and they said I needed 80,000 btu's.

They want to put in a 95% goodman 90,000btu. I was just wondering if they was choosing the right size furnace. Just want to get some opinions from you guys the experts.

Thanks

[beenthere]

Using a 90,000 BTU input 90% furnace for a house that requires 80,000 BTUs would be proper.

Strange how your heat loss comes out to just about exactly what your old furnace's output was.

Your would be one of the few that was sized right.

Did your old furnace run continuous at your coldest outdoor winter temps.
Or did it cycle.

[beenthere]

How much duct alteration are they doing to handle the higher air volume a new 90,000 BTU 90% furnace needs over an old 100,000 BTU nat draft one.

[lowridemazb2200]

How much duct alteration are they doing to handle the higher air volume a new 90,000 BTU 90% furnace needs over an old 100,000 BTU nat draft one.


They are just going to connect it back up to the original duct work using transitions. So do I go with the 90,000 btu furnace?

[Alden_Sloe]

So, I dont know if I need to go to 90,000 or drop down to 70,000. Any info will be great.
Load calcs are great and all but reality check; how often did your old furnace run more than 75% of the time? What part of the country are you in?

[lowridemazb2200]

Load calcs are great and all but reality check; how often did your old furnace run more than 75% of the time? What part of the country are you in?

North Carolina

Far as how long it has ran, it does not run that long but it does not cycle no more then 4 times an hour.

Can the orifices be changed on the furnace to lower the btu's? I need an 80,000 btu furnace but goodman only has 70,000 and 90,000 btu's. My old furnace has an input rating of 100,000 btu's but it is an 78% natural draft I think. It is about 16 years old. So the furnace was sized correctly from the beginning. I just dont know if a 90,000 btu 95% goodman gas furnace is going to be too much.

[t527ed]

90,000 2 stage would be a good fit.;)

[lowridemazb2200]

90,000 2 stage would be a good fit.;)

you are talking about a 95% furnace right?

[Freezeking2000]

78% natural draft

Less about 5% chimney loss, maybe even lower if it is not sized properly.

[Freezeking2000]

Yeah I need 80,000 btu's to heat my house. There is not a furnace that is in the size. I only see the 90,000 and 70,000 one's.

How do you know you need more than the 78K you currently have?

Does it ever run for periods of more than an hour on the coldest nights?

[t527ed]

don't know squat about the goodman furnace models but if they have a 2 stage 95% furnace 90,000 btu input that would be your best option.

unit would not be short cycling in mild weather and would have the capacity you need for the cold days.

make sure it is one that can use a 2 stage thermostat.

[amd]

Less about 5% chimney loss, maybe even lower if it is not sized properly.

As far as I know, 78% AFUE became the minimum in 1992. Presumably any natural draft unit produced at/after that point must have a vent damper. (Not that they eliminate losses during operation, but manufacturers got away with it - Ex: Lennox G20 has damper, rated at 78)

[Alden_Sloe]

I think what everyone's trying to get at is does the furnace run time indicate you'd have plenty of heat going down in size. I'll get lambasted for even asking :p but what's the square footage and layout of the home?

I can tell you that I'm extremely happy with the 2 stage equipment we installed. I'm guessing NC is similar to Seattle in that the vast majority of the time temperatures are mild and you're only using a fraction of the capacity required for the coldest nights. A two stage furnace will provide much more even heat and can be virtually silent.

Depending on how much you spend per year on gas and what your capital buget is for this I might even consider an 80% 2 stage vs a single stage +90%. Depends on how much you can budget for the job now vs expected savings down there road (and how long you plan to stay in the house).

Here's how it was explained to me here on this forum. Your old furnace blows much warmer air up through the registers than what the current generation of furnaces are designed to do. Hotter air means less volume (CFM) is required to put any given amount of heat into your house. That's why your ducts may be undersized for the new furnace. So, in your situation (max heat only rarely needed) a 2 stage furnace will go a long way in alleviating that issue.

NC sounds like air conditioning country (to a Seattle warm weather wimp :o ). Do you have central AC or do you think you'll be adding it? The CFM requirements for AC might influence the furnace decision (duct sizing aside).

One more thing. What's the current weather tightness of your home. Are there things like better windows, insulation etc. that could increase your energy efficiency? If so then even if the 70,000 BTU furnace is slightly undersized right now you might easily be able to bring it right in line with demand by doing a few of those things and may well have a significant impact on comfort and fuel savings.

[lowridemazb2200]

i just wanted to make sure that I was not going to get an oversized furnace. The square footage is about 2000 square feet. The house was built in 1992. No additional insulation just what was required in building houses in that year. I dont think 9,000 btu's is that much of a difference any ways. But, what do I know I'm just a dumbass homwowner.

[beenthere]

Wow.
A house built in 1992 that needs 40BTU's per square foot.

Your area must get a lot colder then mine.

Are you sure it doesn't run continuous when its 0°F outside.

Around here, thats when the 60,000BTU 90+ furnaces installed in 2000 sq ft homes start to run continuous.

No, you can not put in smaller orfice to down fire it.
Not if you want a warranty.

[Alden_Sloe]

i just wanted to make sure that I was not going to get an oversized furnace.

I think the question to ask is "do you have an oversized furnace?"


The house was built in 1992.
Probably not a lot that can be done economically to increase the efficiency of the insulation and windows. '92 is a new house. Heck, I have cars older than that :cool:



I dont think 9,000 btu's is that much of a difference any ways. But, what do I know I'm just a dumbass homwowner.
That makes you da boss :D It's not so much the BTU's but the CFM. Matching your max airflow to what the old furnace had will likely produce a more satisfactory result. A 2 stage furnace with ECM fan will make a huge difference in overall satisfaction. I wouldn't have believed it if I hadn't experienced it first hand. :)

[lowridemazb2200]

I'm going to call a different hvac company to do a load calc on my house. My furnace does not run that long at all. Also, just to get a second opinion. You guys are making me have second guesses. That company that I had out here the first time seemed like they knew what they was talking about. But, then I again I dont know nothing about this stuff anyways. A 2000 square feet house with a 90,000 btu furnace I think is a little oversized. If I can get an 70,000 btu that will make the price a little cheaper.

[beenthere]

A little.


But, it will get you longer run times. Increase your overall comfort. And perhaps save you a bit more(even at same efficiency), because you won't have that sudden rush of hot air, then no air. making you feel cold shortly after it shuts off.