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Thread: Temperature rise lower when on HI fire

  1. #1
    DIY Junior Member MarkBarrett's Avatar
    Join Date
    Feb 2012

    Default Temperature rise lower when on HI fire

    Hi everybody,

    I'll start by saying I am not knowledgeable in HVAC. Never worked in the field. I have decent analyzing skills, as I am a newly retired system integrator in industrial electronics. My question is why is my temperature rise approximately 15 degrees F less on HI fire than on LOW fire?
    Here is a run down of my furnace specs:
    1, Furnace is a Goodman GMV90703BXA
    2, BTU output 64,200 High fire, 44,600 on Low fire
    3, Temp rise F 30-60
    4, Skuttle model 216make up air unit
    We bought this newly constructed house in 2006 as a second home. Since the summer of this year it is our main home.
    Observations of furnace performance:
    1, Many hot air plenum leaks. No tape on top, an 8 inch hole that wasn't covered on top of plenum etc. These were fixed.
    2, Temp. rise not even 30 degrees. Temp rise taken at just before filter on return side and out of the line of sight on the supply side.
    3, Thermostat wired for 2 stage operation, but wasn't programed for 2 stage op. Rectified that. Ran on Low fire for 5 years!
    4, With a 35 degree outside temp, recovery time is approximately one hour to raise house temp 4 degrees (64-68 on low fire).
    5, When calling for high fire, inducer blower and circulation blower run at high speed.
    Things I changed and checked:
    1, Slowed blower speed for a temp rise of 50-60 degrees on low fire.
    2, Using a multimeter, got 24vac at N.O. contact of high fire pressure switch to 2 stage valve when on high fire.
    3, Make up air unit adjusted for low speed blower operation.
    Sorry for the long post. Any ideas as to why temp rise is less on high fire. Your help is appreciated.


  2. #2
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    You should not be changing blower speeds without also check air flow ( CFM). One explanation for the rise is that with a higher blower speed, moving more air, the in/out ērise may be lower, but the house would heat up faster as more "heat energy" is being produced.

    Here is a brief section from your install manual:
    Temperature rise must be within the range specified on the unit
    rating plate. An incorrect temperature rise may result in condensing
    in or overheating of the heat exchanger. An airflow and temperature
    rise table is provided in the Specification Sheet applicable
    to your model*.

    This furnace is equipped with a multi-speed circulator blower. This
    blower provides ease in adjusting blower speeds. The Specification
    Sheet applicable to your model* provides an airflow table,
    showing the relationship between airflow (CFM) and external static
    pressure (E.S.P.), for the proper selection of heating and cooling
    speeds. The heating blower speed is shipped set at “B”, and the
    cooling blower speed is set at “D”. These blower speeds should
    be adjusted by the installer to match the installation requirements
    so as to provide the correct heating temperature rise and correct
    cooling CFM.
    Use the CFM LED (green), adjacent to the integrated control
    module fuse to obtain an approximate airflow quantity. The green
    CFM LED blinks once for each 100 CFM of airflow.
    1. Determine the tonnage of the cooling system installed with
    the furnace. If the cooling capacity is in BTU/hr divide it by
    12,000 to convert capacity to TONs.
    Example: Cooling Capacity of 30,000 BTU/hr.
    30,000/12,000 = 2.5 Tons
    2. Determine the proper air flow for the cooling system. Most
    cooling systems are designed to work with air flows between
    350 and 450 CFM per ton. Most manufacturers recommend
    an air flow of about 400 CFM per ton.
    Example: 2.5 tons X 400 CFM per ton = 1000 CFM
    The cooling system manufacturer’s instructions must be checked
    for required air flow. Any electronic air cleaners or other devices
    may require specific air flows, consult installation instructions of
    those devices for requirements.
    3. Knowing the furnace model, locate the high stage cooling
    air flow charts in the Specification Sheet applicable to your
    model*. Look up the cooling air flow determined in step 2
    and find the required cooling speed and adjustment setting.
    Example: A *MV90704BXA furnace installed with a 2.5
    ton air conditioning system. The air flow
    needed is 1000 CFM. Looking at the cooling
    speed chart for *MV90704BXA, find the air flow
    closest to 1000 CFM. A cooling airflow of 990
    CFM can be attained by setting the cooling
    speed to “C” and the adjustment to “-” (minus).
    NOTE: Continuous Fan Speed will be 56% of
    high stage cooling.
    4. Locate the blower speed selection DIP switches on the
    integrated control module. Select the desired “cooling”
    speed tap by positioning switches 1 and 2 appropriately.
    Select the desired “adjust” tap by positioning switches 3
    and 4 appropriately. Refer to the following figure for switch
    Last edited by jimbo; 02-25-2012 at 05:16 PM.

  3. #3
    DIY Senior Member Runs with bison's Avatar
    Join Date
    Aug 2009


    Seems odd to be that far off and in that direction (it is the reverse of what my 2 stage Ruud is configured for where the high fire spec delta T is greater than low fire because the flow doesn't increase proportionately with the fire rate.) From the Goodman install manual: "An airflow and temperature rise table is provided in the Specification Sheet applicable to your model." Reading from the spec sheet http://www.alpinehomeair.com/related...eet%206.09.pdf it looks like delta T is supposed to be the same for high and low fire. Furthermore a check of the high and low fire output and cfm table shows the ratios to be identical, which should yield essentially the same delta T.

    The blinking LED will give an indicated flow, not an actual flow. That is useful for telling you if the logic board is at least making the correct calls and if it is hooked up and jumpered correctly (since you can compare to the tables.) Your initial post indicates that the blower speed is rising as it is supposed to. I assume that when it goes to hi stage you can hear the speed increase, feel the higher flow, and see a blink pattern that matches the expected flow rate for your jumper settings. (Correct on all three?) If these aren't matching one another then thermostat/connection or type, or the blower connections would be suspect. Could also be a problem with the board.

    Another possibility is that it is kicking up the 2nd stage air flow, but is not kicking in the hi stage gas valve--or if it is the actual gas flow is not changing (stuck/obstructed?) This would give a low temp rise. Actually, the size of the delta T is such that this is a reasonable hypothesis. Guess it could also be some sort of gas supply pressure problem such as undersized or overly long gas lines to the unit that starve the unit on high fire.

    I've not measured temp rises on these and the devil is in the details when performing this kind of measurement. (I've done a lot of this for chemical plants/pilot plant R&D, process design, and operations so I appreciate how easy it is to get misleading readings, even from instruments that seem to check out and methodology that seems good.) Your difference in delta T's suggest a problem, but I wouldn't obsess over a measured temp rise *if* the other aspects seem to be working properly. Most likely you will measure less temp rise than is actually occurring in both lo and hi. If it is still within the design range, then a lower delta T is likely to be a little more efficient with respect to gas use--but with a higher electric use for the blower.

    What tonnage is the AC and what switch are you using for that (A,B,C,D)?

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