26 year old Trane furnace died.. need replacement help / suggestions!

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  #1  
Old 02-20-12, 05:21 PM
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26 year old Trane furnace died.. need replacement help / suggestions!

My 26 yr old Trane XL90 80K BTU 92% single stage recently developed a crack in the heat exchanger causing flame roll out. I pulled the old furnace out and once I set a new one in will have a licenced plumber do the final gas hookup and LP conversion (I will not do the final install myself for all the obvious reasons). While my furnace out I cleaned the AC coil (quite a mess with build up etc). I currently have the AC coil unit is suspended in placed with straps.

I also have a whole house humidifier on the supply that feeds the return. The sensor for the humidifier is higher up on the return (old dial style)

One HVAC contractor came out (before I pulled the old furnace out) and recommended a Rheem RGRC-10EZAJS 95% 105K BTU single stage. After reading the last couple days I do not believe this is the way I want to go. I believe 105K is too much for my house and the 80K unit I had previously worked great for the 10 years I have been here.

What I am now looking at is the Rheem RGTM09EZAJS 95% two stage / variable spd (~$1600 plus the LP kit)

1) I only have a 5 wire T-stat cable pulled to my furnace. Since my basement is finished, pulling a new cable to my t-stat would me a nightmare if even possible without some woodwork (I have pine ceilings in the basement). What are my options to take advantage of a 2 stage thermostat? Wireless? 5 to 6 wire adapter box? Could a good wireless system be setup to control my humidifier too? Recommendations on a good t-stat?

2) The furnace I pulled out was quite tall (18"x48.75"x31" HxWxD), the newer furnaces are all much shorter. The Rhreem is only 34" tall. What is the best way to fit this in? Build a 14" base, shorten the return and have the plumber reduce the length of the gas piping or put the unit on the floor, attach to existing return, make brackets to secure the AC coil in place up to the joists and have a 14" supply duct extender made?

Any insight / suggestions are greatly appreciated


Original furnace:



Original unit info:




Unit removed:




Central air condenser info:




Thanks!
 
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  #2  
Old 02-20-12, 05:31 PM
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I would get the honeywell Vision Pro IAQ it will also control your humidifier.
 
  #3  
Old 02-20-12, 05:48 PM
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Get a manual J done so you know what the right size is for your home! The 100k is obviously too big since your 80k 92% did fine (effectively ~72K). Once you know the manual J calcs, you might want to see if the AC is oversized; too much and you get short cycling and reduced dehumidification.
You also need the blower unit able to push the right amount of air for your A-coil so matching the A-coil to the furnace blower is important. Get your HVAC guy to do the tinwork needed to mate the old to the new; he/she will do a better job IMO.
 
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Old 02-20-12, 05:58 PM
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Do or have done a Manual J before buying new equipment. You will probably be surprised at how small of a furnace you need in terms of heating capacity.

I'm sure there are wireless stats which will work for you but having only installed one wireless in my career, I'm not familiar enough with them to suggest one.

I'd suggest replacing the a/c at the same time.

I never like to see a furnace sitting directly on the floor. Look at all of the rust from yours.
 
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Old 02-25-12, 12:01 PM
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So I decided to calculate myself based on info from this site I bought HVAC-CALC. The Heating loss came out to 60K so with the saftey factor of 25% sized me at 75K BTU (much closer to my original 80K furnace and much less than the 105K they wanted to install!).

I do have a question on CFM.. RHEEM offers (2) 75K 2-stage variable speed motor versions.. a 600-1200 CFM and a 1000-1600 CFM blower version.. How do I know which one would work better for my home?

My origianal furnace was setup for 1610 high and med-lo at 1100.

thanks much for all the help!

-Bill
 
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Old 02-25-12, 01:24 PM
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the blower must be rated to handle your A/C, what size is it? You need 400cfm per ton of cooling. 1 ton = 12,000 btu If you dont know what size it is then post the model number of the outdoor unit.
 
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Old 02-25-12, 04:46 PM
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Bill,
Don't include any additional capacity for a "safety factor". Manual J is VERY conservative all by itself.
One thing to bear in mind is most gas furnaces are rated on input rather than output. For example you could buy a furnace rated at 100,000 in 80% & get 80,000 out or a 95% & get 95,000 out. Your old furnace likely had an output of 60-64,000 having an input of 80,000.
Your A/C would appear to be a 3 ton so you would need a furnace capable of moving 1200 CFM.
 
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Old 02-25-12, 06:20 PM
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The old furnace had an output of around 69,000 and an input of 77,000 on LP. Grady is correct about not over sizing the unit.
Per your indoor coil you have a 3 ton A/c as Grady said, but I like to make sure by verifying the outdoor unit. Sometimes larger coils were installed on air conditioners.
 
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Old 02-25-12, 06:25 PM
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My old furnace had a label on it showing 77K BTU after LP change and was 92% so I was figuring about 70K.. New one would be 75K (not sure after LP change) and 95% so I am figuring about 68K which should line up with what I need.

As far as CFM, my AC is 3Ton so sounds like 1200 CFM will work, but is this number normally a min required? The two options I have is 600-1200 CFM and a 1000-1600 CFM variable speed motor. Is it better to have the lower range or the higher range? and what are the variables to consider when selecting (other than needing at least 1200CFM for the AC)

thanks much.. having some insight into this feels good.
 
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Old 02-26-12, 01:25 PM
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It seems strange to only have a 600 CFM range on the blower, especially a variable speed.
Are the ranges quoted (600-1200 & 1000-1600) the maximum air flows at various static pressures? If so, I would certainly go with the 1000-1600 CFM blower. On the other hand, if they are the total range of the blower, I'd go with the 600-1200 CFM unit. You surely don't need 1000 CFM in the heat mode with a 75K furnace.

Something I just thought of while looking at the pictures: Make sure there is a damper in the humidifier connecting duct & you close it off during the A/C season. If you don't, you are short circuiting cool air back into the return. That could be the cause of all the rust on the floor.
 
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Old 02-26-12, 06:04 PM
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Hi Grady,

Not quite sure I understand the specifications.. the models I am looking at are the 07-M and the 07-Y

http://www.rheem.com/documents/rgrm-specification-sheet

I am leaning towards the 07-M since I am not sure I need more than 1200CFM with the 3 Ton AC. Is there any advantage to running higher CFM if my duct work can handle it?

Thanks much about the damper, I will be sure to let the installer I want one put in!

-Bill
 
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Old 02-27-12, 06:15 PM
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In looking at the specs, I tend to agree the 'M' would fit your needs better. The 'Y' just moves too much air in heating. You wouldn't get very warm air, especially on low fire which is where it will run the vast majority of the time.

What do you think, hvactechfw?
 
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Old 02-27-12, 06:30 PM
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I agree. You don't need more than 1200 cfm. So stick with that as your top end of things. Make sure you get a filter rack installed that is at least 20 X 25 X 1. I would recommend at least a 4" width filter however.
 
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Old 02-27-12, 06:32 PM
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Good point about the filter. The 4"-5" filters do a really good job without being super restrictive.
 
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Old 02-27-12, 07:06 PM
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Installer recommended "airbear" filter which looks pretty good to me. http://www.trioniaq.com/getfile2.aspx?fileid=8987

Thanks for the help guys.. I ordered the 07M. Great forum here!

I will let you know how it all works out.
 
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Old 02-27-12, 07:15 PM
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Never have seen the "right angle" style but I've used the traditional AirBear & been happy with them.
 
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Old 03-05-12, 08:47 PM
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Your current a/c is 3 tons - 1200 cfm needed.

According to the label, it's a 1986.

Don't use a plumber - call a real hvac contractor. There's more to setting up a furnace properly than connecting the gas line; the manifold pressure has to be set correctly. (gas meter should be clocked to verify proper input)

It's advisable to pull combustion air from outside.

The installers will adapt the old plenum to the new furnace with a transition.

5 wires is enough for a two-stage stat if it's battery powered.
 
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