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Old 11-01-2011, 08:15 PM   #1
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16 seer, 90k btu vs 15 seer 70k btu


I have an option to get a Goodman 95% AFUE gas furnace, dual stage variable fan. I also need a new central AC system to go along with it. For $200 more I can go from a 70,000 btu furnace with 15 SEER AC to a 90,000 btu furnace with 16 SEER AC.

Do you think that $200 is worth it in this situation?

I live in southeast Pennsylvania and want to get rid of my 33 year old oil furnace and 6 seer central air.

Thanks for the help.

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Old 11-01-2011, 08:54 PM   #2
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16 seer, 90k btu vs 15 seer 70k btu


2 different size furnaces? Someone or no one has done the proper load calculation necessary to determine the correct size furnace you need. This must be done to determine your home will heat/cool properly.

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Old 11-01-2011, 09:14 PM   #3
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16 seer, 90k btu vs 15 seer 70k btu


90,000 btu furnace. My first question is, how many floors are you trying to heat or cool. Second is, how many square feet of space to be conditioned. As for the a/c, you may be better to go with a Heat Pump, vs conventional a/c.
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Old 11-01-2011, 09:19 PM   #4
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16 seer, 90k btu vs 15 seer 70k btu


Well I had a few different contractors.

contractor a
Trane XB90, 60k btu and XR15 2 1/2 ton AC
or
Trane XV95, 2 stage 80k btu and XL16i 3 ton AC

contractor b
Lennox SLP98UH090V36C (I think 90k btu) XC14-030 2.5 ton AC

contractor c
Arcoaire 95% 2 stage 9MUX080 (I think 80k btu) with arcoair dx1600 16 seer 2/12 ton AC

contractor d (the most expensive)
Heil 95% 2 stage (I think 80k btu) with 16 Seer AC

and then finally contractor e, which I listed above.

The current furnace is an 80k btu forced air unit (Oil burner) and a 2.5 ton AC.

Contractor E has the best pricing and my up front costs are a big concern. Out of 5 contractors all furnaces were between 60,000 btu and 90,000 btu. Only 1 had a 3 ton AC but he said I could change that out for a 2 1/2 ton AC. I think he put the 3 ton on there because it was a trane and they only came in 1 ton increments for that higher end super quiet unit with dual stage cooling. Don't need that.
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Old 11-01-2011, 09:26 PM   #5
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16 seer, 90k btu vs 15 seer 70k btu


cypherx, you have a bunch of idiots that are contractors. Sorry to say that. What you need, is to find one that will do the proper calculations, vs. just throwing some spec's out there, just because they are trying to make a sale, due to 1) It is Winter/Heating season, and 2) The economy sucks right now, so everyone is desperate and it is showing who the crooks are right now.
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Old 11-01-2011, 09:54 PM   #6
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16 seer, 90k btu vs 15 seer 70k btu


Where you happy with the 80k? If so the 70k will probably work fine, but if you present the furnace with a large setback temperature (for instance, you lower the temp to 58 during the day, you get home from work and want to raise the temp up to 72 very quickly) you may be disappointed. The 60K may take 2-3 hours to get to 72, but it will eventually get there. The 90K will get to 72 very quickly, but be more noisy and less efficient. Depends what you want/need. There is no perfect optimal answer, the different furnace size quotes you are getting are because each contractor favors different things. There is a minimum BTU furnace limit which will be inadequate to heat the house and a maximum BTU limit due to duct work sizing, but a large range exists inbetween these two limits.

Last edited by bungalow_steve; 11-01-2011 at 10:00 PM.
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Old 11-01-2011, 10:06 PM   #7
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16 seer, 90k btu vs 15 seer 70k btu


Quote:
Originally Posted by bungalow_steve View Post
Where you happy with the 80k? If so the 70k will probably work fine, but if you present the furnace with a large setback temperature (for instance, you lower the temp to 58 during the day, you get home from work and want to raise the temp up to 72 very quickly) you may be disappointed. The 60K may take 2-3 hours to get to 72, but it will eventually get there. The 90K will get to 72 very quickly, but be more noisy and less efficient. Depends what you want/need. There is no perfect optimal answer, the different furnace size quotes you are getting are because each contractor favors different things. There is a minimum BTU furnace limit which will be inadequate to heat the house and a maximum BTU limit due to duct work sizing, but a large range exists inbetween these two limits.
If that is how you size units, I would hate to see the repeat service, due to customer dissatisfaction. A furnace if sized properly, it should meet the rise of temp, without short cycling, but should never take two to three hours as you stated. Even if raising from 58 to 72, it can take 45 minutes, could take three hours, but depends on the heat load, or cooling load, if trying to go from 85 to 76 during cooling season.

A system should be properly sized for the structure to meet the demands of both seasons, not just one. Improperly sized, causes repeat service calls, or in the cause of heating season, cracked heat exchangers.
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Old 11-01-2011, 10:30 PM   #8
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16 seer, 90k btu vs 15 seer 70k btu


Well I'm looking between a 70k and 90k. They are dual stage so that number is when it's fully engaged as far as I can tell.

The kind of house I have is very common in the area. Standard bi-level about 1650 sq ft, 4 BR, 2 bath, 1 car garage under the front bedroom. These contractors have done these houses pretty commonly.

Currently the 80k btu oil furnace is pretty loud. I can't imagine a modern day unit any worse. The want to change equipment is to get modern 95% efficiency and get rid of the huge oil tank and dependance on outrageously priced foreign oil (which is approaching $4 /gal). We have an abundance of natural gas, especially in PA. The convenience of not having oil deliveries is going to be great, along with the extra space in my garage from not having a huge eyesore of a tank. In addition in the next year or two I would like to get a gas range, as I like cooking with gas far better than electric.

So in 1978 they determined a Luxaire 80,000 btu oil furnace is sufficient. Now I have a choice between 70,000 btu and 90,000 btu with only a $200 difference. In the summer somehow the 90,000 btu unit gets an additional SEER rating out of the AC system. Not sure how... If it's the coil or something with the airflow or blower, but I guess my main question is that extra SEER worth the $200?
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Old 11-01-2011, 10:42 PM   #9
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16 seer, 90k btu vs 15 seer 70k btu


cypherx, right now Mother Earth is stuck between Fall & Summer. That still does not make it right to not do the proper work to make sure that the hvac system that you are going to be investing money into, is going to be proper for the structure. Either you are going to be throwing money out the window, or saving money.

What is more important to you? Having more money in your pocket every time the unit runs, or complaining about why you never have any money, due to every time you get your utility bill, you keep scratching your head trying to figure out, is it the structure, is it the hvac system, is it the utility. 9 times out of 10, it is improper hvac sizing & maintenance, that costs people more money, next to not making sure that the structure is weather tight.

No one is stopping you in making some individual or company more money, but those of us that have either gone through this, or those in the trade that are on this board, will tell you that the bids you got, if they were just off the top of the head, without Manual J calculations in your hand to go with them, are just bs, and the company just sees a sucker in front of them.
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Old 11-02-2011, 05:30 AM   #10
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16 seer, 90k btu vs 15 seer 70k btu


Cypherx,

The pros here on the board are VERY keen for doing heat loss/gain calcs to properly size a system. I agree that they are valuable to to a point but the problem is that the calcs can give you almost any answer you want depending on the assumptions you put in. So, for me, they are not enough.

I also believe that your long experience with your existing system is a valuable data pont for sizing a new one. i.e., does it do the job? How long does the furnace run to satisfy a typical 1 degree heat call on the coldest night? You probably don't have exact numbers but you will have a pretty good feel for it.

In any case, here are a couple of free heat loss calc tools to play with. Can't hurt to run a few scenarios to get a reality check on your estimates.


This one does the calcs electronically:
http://www.builditsolar.com/Referenc...s/HeatLoss.htm

This one you do manually:
http://www.ci.brainerd.mn.us/buildin...alculation.pdf
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Old 11-02-2011, 08:39 AM   #11
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16 seer, 90k btu vs 15 seer 70k btu


Quote:
Originally Posted by gregzoll View Post
A furnace if sized properly, it should meet the rise of temp, without short cycling, but should never take two to three hours as you stated.
sure it can

Quote:
Even if raising from 58 to 72, it can take 45 minutes, could take three hours,
now you seem to agree

Quote:
Originally Posted by gregzoll View Post
A system should be properly sized for the structure to meet the demands of both seasons, not just one.
A single stage/fixed speed system can only be properly designed for one set point, it's a big compromise for all other setpoints. A typical furnace installation is oversized for most of the season, and yet it doesn't blow up/require multiple service repairs as you state. That is why multi/variable stage, variable speed motors are being introduced to fix the single design point problem, they are still very crude, but it's getting better.

A homeowner has the last word on what is more important, efficiency, noise and/or quick recover time from setback, not the HVAC contractor.

How would you like to go to a FORD dealer and have him tell you exactly what HP engine your getting in your truck based on some daily usage equation that some committee created (and seems to keep changing)?

Last edited by bungalow_steve; 11-02-2011 at 09:09 AM.
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Old 11-02-2011, 12:52 PM   #12
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16 seer, 90k btu vs 15 seer 70k btu


Ok well yeah this is dual stage equipment that will be wired in 2 stage mode to a Honeywell VP 8000 thermostat.

Anyway the 90,000 btu model puts out these numbers:
61,700 btu in stage one
90,000 btu in stage two

The 70,000 btu model puts out these numbers:
46,400 btu in stage one
67,000 btu in stage two
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Old 11-02-2011, 01:02 PM   #13
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16 seer, 90k btu vs 15 seer 70k btu


Do NOT go with an oversized furnace!!!

Manual J and D to determine the right size furnace and duct.
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Old 11-02-2011, 02:00 PM   #14
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16 seer, 90k btu vs 15 seer 70k btu


Quote:
Originally Posted by Technow View Post
Do NOT go with an oversized furnace!!!

Manual J and D to determine the right size furnace and duct.
Ok these homes were built with 80,000 BTU furnaces, so the engineering firm that designed these homes determined that number, and must of determined it correct because there are literally thousands of these same floor plans.

This bi-level style was very popular in the late 70's because of the amount of living space obtained by a smaller foundation thanks to the upper and lower levels right on top of each other.

Problem is, if 80,000 btu is the size... Goodman doesn't make an 80,000 btu (nor do some of the other guys). We have 70,000 btu or 90,000 btu. Would 70,000 be too little or 90,000 be too large? We need 80,000 btu, so what's a guy to do if that is not available.

Other data...
Both have a 1050 RPM Blower, but the 70,000 btu unit is a 3/4 HP motor and 10" x 10" circulator blower size, while the 90,000 btu unit is a 1 HP motor and a 11" x 10" circulator blower size. The speed is variable.

Both units: 95% AFUE
Temperature rise rate: 30 - 60 (F)

GMVC95 0704CX
Vent Diameter: 2"
No. of burners: 3
Disposable Filter (sq in): 384
Min Circuit Amps: 14.1
AC combination nets 15 SEER

GMVC95 0905DX
Vent Diameter: 3"
No. of burners: 4
Disposable Filter (sq in): 480
Min Circuit Amps: 14.4
Looks like approximately 200 CFM more than the other model
$200 more, and the AC combination nets 16 SEER

Also what if 90,000 btu IS too large (by 10k at least). Wouldn't it run in single stage and save money on the natural gas consumption? I had a Lennox dealer tell me that actually with a Dave Lennox Signature unit (98% AFUE). It was much more costly than this one though.

Only other useful data I see is that it appears to me that I get a $500 tax credit for the 16 SEER and 95% AFUE AC/Heating combination, where as the 15 SEER drops the tax credit to $200. So it seems like it pays for itself, just do I want the money up front, or wait until tax time next year (if this change is really WORTH the $200 now).
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Old 11-02-2011, 02:10 PM   #15
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16 seer, 90k btu vs 15 seer 70k btu


Quote:
Originally Posted by cypherx View Post
Ok these homes were built with 80,000 BTU furnaces, so the engineering firm that designed these homes determined that number, and must of determined it correct because there are literally thousands of these same floor plans.

This bi-level style was very popular in the late 70's because of the amount of living space obtained by a smaller foundation thanks to the upper and lower levels right on top of each other.

Problem is, if 80,000 btu is the size... Goodman doesn't make an 80,000 btu (nor do some of the other guys). We have 70,000 btu or 90,000 btu. Would 70,000 be too little or 90,000 be too large? We need 80,000 btu, so what's a guy to do if that is not available.

Other data...
Both have a 1050 RPM Blower, but the 70,000 btu unit is a 3/4 HP motor and 10" x 10" circulator blower size, while the 90,000 btu unit is a 1 HP motor and a 11" x 10" circulator blower size. The speed is variable.

Both units: 95% AFUE
Temperature rise rate: 30 - 60 (F)

GMVC95 0704CX
Vent Diameter: 2"
No. of burners: 3
Disposable Filter (sq in): 384
Min Circuit Amps: 14.1
AC combination nets 15 SEER

GMVC95 0905DX
Vent Diameter: 3"
No. of burners: 4
Disposable Filter (sq in): 480
Min Circuit Amps: 14.4
Looks like approximately 200 CFM more than the other model
$200 more, and the AC combination nets 16 SEER

Also what if 90,000 btu IS too large (by 10k at least). Wouldn't it run in single stage and save money on the natural gas consumption? I had a Lennox dealer tell me that actually with a Dave Lennox Signature unit (98% AFUE). It was much more costly than this one though.

Only other useful data I see is that it appears to me that I get a $500 tax credit for the 16 SEER and 95% AFUE AC/Heating combination, where as the 15 SEER drops the tax credit to $200. So it seems like it pays for itself, just do I want the money up front, or wait until tax time next year (if this change is really WORTH the $200 now).

NEVER assume your house had an engineer design your system. 80k was a popular size back then. I have many customers we have reduced their furnace by 1/2. there is a lot more than "floor plans" that go into a load calculation.

I had an idiot builder tell me he wanted a 5 ton and a 3 ton for his personal house he was building for himself....the "J" calcs said thats what his "stock" plan needed.......well since it was HIS house he added low-e glass, extra insulation, on a shaded lot......No way did he need the "worst case scenario"


Last edited by Technow; 11-02-2011 at 02:16 PM.
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