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08-10-2010, 09:43 AM   #1
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## What is the CFM for this furance?

I have an Aire Flo direct vent forced air furnance Model Number: AF92MPE125U5B i know some of the codes there...

92 is 92% Effiency U is upflow 125 is the BTU/h 125,000 but what is the CFM of this furnance? The manual says it has a 25X16 filter requirement but then it says if you furnace is over 1,600 CFM then you need two filters (aka two returns?) but how does one determin the true CFM? I know the whole BTU/h divided by blah times blah for condensing formula, but that isn't the true CFM is it? Anyone know the Mfg spec of this furnance?

08-10-2010, 01:11 PM   #2
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Quote:
 Originally Posted by BlueBSH I have an Aire Flo direct vent forced air furnance Model Number: AF92MPE125U5B i know some of the codes there... 92 is 92% Effiency U is upflow 125 is the BTU/h 125,000 but what is the CFM of this furnance? The manual says it has a 25X16 filter requirement but then it says if you furnace is over 1,600 CFM then you need two filters (aka two returns?) but how does one determin the true CFM? I know the whole BTU/h divided by blah times blah for condensing formula, but that isn't the true CFM is it? Anyone know the Mfg spec of this furnance?
Not a PRO, but have plodded the HVAC forums for several years.

I'm assuming that the '5' refers to a 5-ton blower. Being that it's a 125,000 BTU unit, the assumption is reasonable. For air conditoning, tons are used to designate capacity; one ton being equal to 12,000 BTUH. Usually, the blower (for a/c) is set at 400 CFM per ton. It can, however, be adjusted (+/-) a small percentage.

So, if you had a 5 ton a/c, you should be pushing 2000 CFM. For heating, the blower is set to deliver a temperature rise (difference in temp of return air and supply air usually taken in the plenum). You need to check the specs for that unit to see what the temperature rise range is, and adjust the heating speed to deliver it.

Unless you have a McMansion in a cold area, I suspect that the unit is oversized. Depending on your duct system, this could cause problems (shut downs) and premature failure down the road.

As far as 'true CFMs' are concerned, you need to determine the static pressure of the duct system and refer to the chart to see what your 'actual' air flow is. An undersized duct system will boost your static and reduce your airflow.

V

08-10-2010, 02:25 PM   #3
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Quote:
 Originally Posted by veesubotee Not a PRO, but have plodded the HVAC forums for several years. I'm assuming that the '5' refers to a 5-ton blower. Being that it's a 125,000 BTU unit, the assumption is reasonable. For air conditoning, tons are used to designate capacity; one ton being equal to 12,000 BTUH. Usually, the blower (for a/c) is set at 400 CFM per ton. It can, however, be adjusted (+/-) a small percentage. So, if you had a 5 ton a/c, you should be pushing 2000 CFM. For heating, the blower is set to deliver a temperature rise (difference in temp of return air and supply air usually taken in the plenum). You need to check the specs for that unit to see what the temperature rise range is, and adjust the heating speed to deliver it. Unless you have a McMansion in a cold area, I suspect that the unit is oversized. Depending on your duct system, this could cause problems (shut downs) and premature failure down the road. As far as 'true CFMs' are concerned, you need to determine the static pressure of the duct system and refer to the chart to see what your 'actual' air flow is. An undersized duct system will boost your static and reduce your airflow. V

hehe McMansion... that's funny because we have a mansion next to us that is owned by one of the higher ups at McDonalds of all places..... so I litteary have one of those next door

anyways..... the house has 3,700 sq ft of heated / cooled space... but I allways suppected that it is oversized for the house... as in the winter it still got cold in here with it running at 72 degrees but found out that was due to a LOT of leaks in the house along with missing insulation, so I think they where trying to compenstate for that in the past when it was put in by the previous owners 3 years ago... but sealed most of that up now... but I do get near the upper thermal limits when I checked last winter.... this is why i am trying to figure out the CFM... wanted to check the ducts which I know are undersized... the return going to the furnace is only 14x8 that connects to the 24x8 return drop... but then that is even too small.... i dont understand their reasoning... they come off the furance smaller then it should be then right after that the return stack goes into a 24x8 box then that box reduces into a 14x8 duct run down the house never reduces in size past that..... as you can guess the far end of the house gets very little return suction... it cant even hold a kleenx tissue on the return grill without falling off

 08-10-2010, 04:13 PM #4 Hvac Pro     Join Date: Nov 2008 Location: Winnipeg, Canada Posts: 16,124 Rewards Points: 386 Probably a 5 ton blower. Their latest model does 3-5 tons:http://www.aireflo-hvac.com/products..._cg92_0908.pdf __________________ "Cut it twice and it is still too short".
 08-10-2010, 07:54 PM #5 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,591 Rewards Points: 8,168 That a 5 ton drive. 14X8 return duct. thats good for about 1.5 tons of A/C(600 CFM). On that furnace. Even with a temp rise of 70° across the heat exchanger. You still need a min of 1520CFM. So your return is way short. Would be better if you got the temp rise down to 60, and that would need 1775 CFM.
 08-10-2010, 08:18 PM #6 Member   Join Date: Oct 2009 Location: Indiana, PA Posts: 1,371 Rewards Points: 640 alright can someone explain this to me... my furnace installation manual says "number of filteres required 2" and for 125,000 btu/h it says 15.5 x 25" minimum size... but then it says side return sizes that are larger and bottom return sizes... does the 2 required filters mean if you have a central return? because even the 90,000 btu/h ones say 2 filters required ... which seems like a lot yet the instructions to do side returns only shows it set up with one filter
 08-10-2010, 08:22 PM #7 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,591 Rewards Points: 8,168 All furnaces. Need the return to either be through the bottom. Or through 1 side and the bottom, or through both sides. Once you exceed 1750 CFM. The 90, may have a 5 ton drive also. And when its CFM for A/C reaches 1750, it needs the return set up the same way because of the A/C, not its heating BTUs.
 08-10-2010, 08:33 PM #8 Member   Join Date: Oct 2009 Location: Indiana, PA Posts: 1,371 Rewards Points: 640 Ah, I just found the CFM chart for my model.... even if I had a media filter with 0.26 inch wc only with the 125 btu/h when heating it says its pushing 2232 CFM... and that is just with the filter only... even at 0.9 in wc it is pushing 1590 CFM when heating call or cooling is running in high speed (factory default speed for this model)... I guess I need to have a second return added? right now there is only that one... but where exactly does the second return go? do you divide the house up? or does it somehow attach to the single main return plenum?
 08-11-2010, 04:05 AM #9 An old Tradesmen   Join Date: Oct 2008 Location: Somewhere Posts: 34,591 Rewards Points: 8,168 If its running near the high limits trip temp. Your not moving anywhere near 1590 CFM. Your static is probably much higher then .9". Connects to the other side of the furnace. You can add another return to the first floor. And run it to the opposite side of the furnace with its own air filter.
 08-12-2010, 03:56 PM #10 Member   Join Date: Oct 2009 Location: Indiana, PA Posts: 1,371 Rewards Points: 640 anyone have any pictures of a furnance that has two returns going into it? (a 5 ton system type setup) jus trying to get an idea of how they usually set these up... I tried searching google but didn't find any pictures of dual returns at the furnace area and one last question, how do you handle the condensate line when you have two returns on one each side (because there is no more room available vertically to go under)... right now my aire-flo furnace has the condensate line going out right where the return is on the left hand side
08-12-2010, 07:03 PM   #11

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Don't have any pictures.

Use a vinyl hose to run in front of the air filter. Then you just pop it off to change the air filter.

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