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Old 05-31-2009, 04:35 PM   #1
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Is my furnace suitable to my home size? (poor airflow)


I bought this house as a 1400 sq ft ranch style with a Trane XB80 furnace. The airflow was okay, but not great when we bought it. We had the supply system, which was old and falling apart, replaced with flexible ductwork and the same with the return system.
Recently we completed an addition which added 400 sq ft of living space... since that time our heating bills jumped and our airflow seems pretty weak in the old part of the house.
So, I'm wondering
A) Is the XB80 suitable for an 1800 sq ft home...
B) Would an inline duct fan help?

Thanks... I know I'm missing details, so let me know what else you need. A professional installed the ductwork and said the sizes were all standard and appropriate, though I can check on that for you. Oh, and yes, the furnace fan is at max speed right now.

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Old 05-31-2009, 04:50 PM   #2
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Is my furnace suitable to my home size? (poor airflow)


The full model number of the furnae would help.

Not to bust on your installer/"professional", but when someone says the ducts are alll standard size.

Thet don't know how to size duct.

Next time you talk to him. Ask him what static pressure the blower is working against.

I bet he never checked. And may not even know how.

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Old 05-31-2009, 04:55 PM   #3
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Is my furnace suitable to my home size? (poor airflow)


Ok, I'll check the full model number and let you know... I'll also check the duct sizes and let you know. Thanks...
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Old 05-31-2009, 05:30 PM   #4
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Is my furnace suitable to my home size? (poor airflow)


Well, I see no model number on the furnace except (and I messed it up originally) Trane XE80. I took off the panel and see nothing else... The return and supply from the furnace are both 14". The return has a fork which goes to the new part of the house and the rest continues about 45 feet to the original return grills. There are two original return locations which are, oddly enough, 6 feet apart in the same hallway. One is a 14" grill and the other is 12".
As for the supply system, the new part again splits off near the furnace (which is in the garage attic). There are restrictors (blanking out on the actual word) in the new part because it was WAY higher airflow than was necessary. The 14" supply continues into the old part of the home, down a chase in the wall, which I believe is also 14", and then continues in the crawlspace. I'm unsure of the crawlspace trunk size though I'm guessing it's 10 or 12".... off of that comes a bunch of 6" lines to all the registers in the home. That trunk supplies 7 registers....

SO.... theorhetically is that enough size? Also, how helpful ARE those inline duct fans? I'm looking at something like a Fantech FR250 or something... (speaking in complete ignorance here...).

I also must admit that I found a, now priority-sized, problem when looking for this info. I opened my crawlspace and felt a breeze.... a breeze which goes away when the furnace fan is off. SOOOO... after I find the apparent LEAK in my supply lines, are they adequately sized?
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Old 05-31-2009, 05:40 PM   #5
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Is my furnace suitable to my home size? (poor airflow)


The model number is inside the burner compartment.
Without that number, hard to say how badly undersized your duct is.
The XB80 comes in many sizes.
The smallest standard blower size is 3 tons/1200 CFM.

14" flex is adaquate for about 800CFM.
But not on a long duct run. The longer th run, the less air it can carry with out the blower having to work harder.

One of the reasons you get so meuch air to the closest registers, is because the duct is too small to carry air to the old side. So it forces the air out that branch.
The dampers, are closed/choked to force the air to teh old section.

Definately need the air leak in that crawlspce fixed.


I didn't look up the model booster you posted. But, if your going to use it for the old section. You would need a fan that can move the full amount of air the old section needs.
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Old 05-31-2009, 06:09 PM   #6
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Is my furnace suitable to my home size? (poor airflow)


So what kind of CFM would I need, ideally, for 7 registers and 1200 square feet of living space? This is further complicated by the layout of the supply lines...which is nearly impossible to describe in words and makes larger supply lines nearly impossible.
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Old 05-31-2009, 06:15 PM   #7
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Is my furnace suitable to my home size? (poor airflow)


How much, depends on how much heat or cooling the house needs.

Maybe you only need 800CFM, maybe you need 1200 CFM.

That 14" flex, should probably have been atleast a 16".
Alot of guys just look at their duculator, that they have for sheet metal, and use it for flex also. Flex duct doesn't flow the same amount of air as sheet metal duct does at the same size.
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Old 05-31-2009, 06:40 PM   #8
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Is my furnace suitable to my home size? (poor airflow)


okay, so how do I determine how much CFM I need? I'm going to go look at the furnace again but I need to figure out how to fix this if the leak doesn't magically take care of this (which I assume it won't).
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Old 05-31-2009, 06:47 PM   #9
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Is my furnace suitable to my home size? (poor airflow)


The air leak repair itself will help, a little at least.

Since we don't know how much heating or cooling your house needs.
The air flow has to be based on how much air the furnace or A/C needs to move to protect them from damage.

Once you find the model number of the furnace. Its easy to tell how much air the furnace must move to be protected from over heating the heat exchager, and cracking it.

Also, if you have A/C, you should post the midel number of the outside unit.
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Old 05-31-2009, 07:06 PM   #10
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Is my furnace suitable to my home size? (poor airflow)


Ugh.. NO clue where the model number is and the internet won't tell me where to find it. All I can find is this number which is highlighted in a list of numbers: DD080C936D and that turns up NOTHING on the internet. I see nothing in the burner compartment.... the AC is TTR030C100A3.
Even the Trane website doesn't tell me where to find the model number of the XE80.

EDIT: OK, so I guess the model is that DDxxxx with a "T" on the front... yay internet. http://www.unitedenergy.ca/brochures/trane_xe80.pdf

Last edited by knottshawk; 05-31-2009 at 07:11 PM.
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Old 05-31-2009, 07:34 PM   #11
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Is my furnace suitable to my home size? (poor airflow)


Both your A/C and heat should move 1000 CFM.

Wiht the descroption of how your trunk runs to teh crawlspace. It should be a 16" line to start with at the furnace. And then reduce down shortly after the branch T/Y that goes to the addition.

Your return should be larger also.

You might be able to have him run each line(the one for teh addition, and the one for the old section( a seperate runs right from the plenum. This would help a lot. And allow you to keep the 14" that runs to the existing section 14".
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Old 05-31-2009, 07:39 PM   #12
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Is my furnace suitable to my home size? (poor airflow)


Ok.... I'll talk to him about that. Thanks so much for the help.
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Old 05-31-2009, 08:18 PM   #13
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Is my furnace suitable to my home size? (poor airflow)


What about the possibility of feeding the 3 furthest registers in the old house with a NEW 14" run? Essentially leaving it all the same except having two 14" runs from the plenum.... the new one could stay in the attic, making it a shorter run and easier to install.... would that work well? The existing run in the crawlspace could then have 3 of it's 6" runs capped.
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Old 05-31-2009, 08:23 PM   #14
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Is my furnace suitable to my home size? (poor airflow)


If you go to large. The air will move too slow, and can cause uneven temps also. Because it can pick toomuch heat in summer, or lose too much in winter.

A 10" to feed those 3 may be ok.
Weather or not the 14" to the crawl will have enough velocity(speed) is questionable.
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Old 06-01-2009, 08:42 AM   #15
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Is my furnace suitable to my home size? (poor airflow)


Do you mean the 14" to the crawl may be negatively affected by adding a 10" run, or do you mean the 14" run is questionable regardless?
I'm just trying to figure out the best solution here...

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