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Old 12-19-2009, 12:39 PM   #1
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Poor performance from 75,000 BTU natgas heil 80% unit?


I have a 2000sq/ft bilevel built in 2001, air filter is new, heat off delay is set 140 sec... it heats the house about +8 degrees per hour...

I run 65 when i'm home (or about 8hrs a day) and 58 the rest of the time, it still consumes 4 ccf's per day! (ambient temps 15-30f), I have 18" of cellulose in the attic, double pane windows no drafts etc

anything to check with the unit itself? I get about 115f out of the registers, blower is set MEDIUM HIGH / ORANGE per the label on the furnace door

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Old 12-19-2009, 12:43 PM   #2
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Poor performance from 75,000 BTU natgas heil 80% unit?


Your temp rise sounds okay at 115 (50F) so the unit is producing the right amount of heat. Ever had an energy audit done on the house to check for leaks etc? Some companies use an infrared camera and it is surprising to see the results. Where do you live?

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Old 12-19-2009, 01:03 PM   #3
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Poor performance from 75,000 BTU natgas heil 80% unit?


Quote:
Originally Posted by yuri View Post
Your temp rise sounds okay at 115 (50F) so the unit is producing the right amount of heat. Ever had an energy audit done on the house to check for leaks etc? Some companies use an infrared camera and it is surprising to see the results. Where do you live?
I live in West Michigan, it's been 15-30f outside during my testing, have not had an energy audit but my house is fairly "tight", I lose 1 degree per hour in the summer or winter if HVAC is turned off regardless of ambient conditions, I wonder if 75,000 BTU is undersized? The house was originally constructed as a bilevel with an unfinished basement... 1000sq/ft upper level and now 1000sq/ft lower level (they just cut into the existing ducting to provide for the basement).... maybe need a larger unit now?
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Old 12-19-2009, 01:17 PM   #4
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Poor performance from 75,000 BTU natgas heil 80% unit?


The only accurate way to know is to do a heat load/loss calculator:Load Calculator
Gaining 8F /hr sounds good to me so I don't think it is undersized. Sometimes we wish we burned less gas but it may be normal.
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Old 12-19-2009, 01:22 PM   #5
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Poor performance from 75,000 BTU natgas heil 80% unit?


Heat loss is X BTU/[(sq.-ft)-HDD], with X ranging from 1.7 for a tight house to 11, with avg. = 6.
For 2000 sq. ft. you'd have avg. 12,000 BTU/HDD.
For 1000 HDD in a month, your avg. loss is 12 MBTU, or 17,000 BTU/hr.
http://www.degreedays.net/

4 ccfs = ~400,000 BTU in 24 hrs. = 17,000 BTU/hr. Do you have 1000 HDD in a month?

At 75,000 BTU/hr your [single stage?] furnace is on 22% of the time.

Last edited by Yoyizit; 12-19-2009 at 01:39 PM.
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Old 12-19-2009, 02:54 PM   #6
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Poor performance from 75,000 BTU natgas heil 80% unit?


Quote:
Originally Posted by ls3c6 View Post
I wonder if 75,000 BTU is undersized?
If it was undersized. It wouldn't be raising the house temp 8 in an hour.
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Old 12-20-2009, 08:25 PM   #7
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Poor performance from 75,000 BTU natgas heil 80% unit?


Confirming a right sized single stage furnace for your house and climate

Find your Outside Design Temperature (ODT) from the link
http://www.hrt.msu.edu/energy/Notebo...by_AJ_Both.pdf

calculate the A factor
A = (65-ODT)/(-100)

then calculate your expected furnace Duty Cycle
in percent
DC = 100(on time)/(off time + on time)
at your outside temperature T

Then,
DC = (T-65)/A
and
T =( DC)(A) + 65



The Outside Design Temperature from the link for Portland, Maine is -6F
Then, A = -0.71

then
T...... DC
65.0F..0%
-6.0F 100%
20F ..63%

and

DC ...T
100% -6F
0% ..65F
30% 43.7F

If you measure your duty cycle over one or two cycles
(should take a few minutes)
and record the outside night time temperature
corresponding to this Duty Cycle,

over several days of varying night time weather,
the average of these readings should be
within a few percent of the predicted values.

Last edited by Yoyizit; 12-20-2009 at 09:11 PM.
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Old 12-20-2009, 10:24 PM   #8
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Poor performance from 75,000 BTU natgas heil 80% unit?


You would need to know the actual output of the furnace.
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Old 12-21-2009, 05:00 AM   #9
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Poor performance from 75,000 BTU natgas heil 80% unit?


Quote:
Originally Posted by ls3c6 View Post
I live in West Michigan, it's been 15-30f outside during my testing, have not had an energy audit but my house is fairly "tight", I lose 1 degree per hour in the summer or winter if HVAC is turned off regardless of ambient conditions, I wonder if 75,000 BTU is undersized? The house was originally constructed as a bilevel with an unfinished basement... 1000sq/ft upper level and now 1000sq/ft lower level (they just cut into the existing ducting to provide for the basement).... maybe need a larger unit now?

Your home in only need 37,500btu.

75,000 is over sized but seems to be the standard form homes 900 to to 1600 sq ft.
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Old 12-21-2009, 02:16 PM   #10
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Poor performance from 75,000 BTU natgas heil 80% unit?


Quote:
Originally Posted by beenthere View Post
You would need to know the actual output of the furnace.
It would seem so, but I don't think it's necessary for this calc.

The single stage furnace is just some "heat machine" that needs to be on
all the time at -6F and
off at 65F, and
on half the time at
T =( 50)(-0.71) + 65 = 29.5F.

This does assume a straight line relationship between furnace output and duty cycle and temperature, so the "on time" should maybe be 10 minutes or more for this to be true.
I asked Bryant about this and they gave me the runaround.

I did a more complicated spreadsheet that should work with multi- or single-stage furnaces, using only input BTUs/hr, based on my January '09 gas bill.
It tells me that my 169K furnace should have been a 69K unit. It's typically on 1/4th of the time, with on times of 6 minutes. ODT to the 99% level = +14F, 3100 sq. ft. including a 900 sq. ft. basement.


Last edited by Yoyizit; 12-21-2009 at 02:23 PM.
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