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  1. #1

    Appropriate output temps for 355CAV on heat

    My furnace has recently developed never before seen problems, consistent with overly high output temperatures. I'm not yet sure whether my HVAC company considers them resolved. After I last spoke with them, I thought of a few questions that will help me to better understand the situation and what they've told me. Perhaps you could help me.

    My 3-stage 355CAV-060's tag lists air temp rise as:

    Low: 35-65 F
    Med: 50-80 F
    High: 35-65 F

    If you wanted to arrive at appropriate output temp ranges, would you simply add return temp to those? IOW, if we assume a properly measured return temp of 70F, can we do this:

    Low: 35+70 to 65+70 = 105F to 135F
    Med: 50+70 to 80+70 = 120F to 150F
    High: 35+70 to 65+70 = 105F to 135F

    to project what the corresponding, properly measured supply temp ranges would be?

    The tag also lists "Design Max Outlet air temp":

    Low: 170F
    Med: 170F
    High: 170F

    I haven't found a definition for that. Might this be the absolute maximize output temperature the equipment is designed to withstand, and therefore a temperature which basically should never be seen on the supply side during a heat rise measurement?

    I haven't found information about the limit switch and the temperature at which it should open. Does anyone know where I would find that info, or might someone have it handy?

    Thank you in advance, I'll followup up.

  2. #2
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,046
    Quote Originally Posted by WhatsThatSound View Post
    My furnace has recently developed never before seen problems, consistent with overly high output temperatures. I'm not yet sure whether my HVAC company considers them resolved. After I last spoke with them, I thought of a few questions that will help me to better understand the situation and what they've told me. Perhaps you could help me.

    My 3-stage 355CAV-060's tag lists air temp rise as:

    Low: 35-65 F
    Med: 50-80 F
    High: 35-65 F

    If you wanted to arrive at appropriate output temp ranges, would you simply add return temp to those? IOW, if we assume a properly measured return temp of 70F, can we do this:

    Low: 35+70 to 65+70 = 105F to 135F
    Med: 50+70 to 80+70 = 120F to 150F
    High: 35+70 to 65+70 = 105F to 135F

    to project what the corresponding, properly measured supply temp ranges would be?

    The tag also lists "Design Max Outlet air temp":

    Low: 170F
    Med: 170F
    High: 170F

    I haven't found a definition for that. Might this be the absolute maximize output temperature the equipment is designed to withstand, and therefore a temperature which basically should never be seen on the supply side during a heat rise measurement?

    I haven't found information about the limit switch and the temperature at which it should open. Does anyone know where I would find that info, or might someone have it handy?

    Thank you in advance, I'll followup up.
    Your numbers and assumptions are correct.

  3. #3
    Thanks for the reply. Based on that math the maximum supply temp would be 152F for a 72F return (estimate for mine, forget what tech said he measured) and 160F for a 80F return (maximum per spec, can't imagine it applying to me). So maximum per spec would be 10F below that 170F "Design Max Outlet air temp".

    For some yet not known/explained reason, my furnace melted two 165F fusible links in a fire damper just above the evaporator coil (upflow arrangement). Based on math and specs, it sounds like the supply temperature definitely should not have gotten so high as to do that.

    I'm not sure my interpretation of "design max outlet air temp" was correct. In that, there are two numbers of interest... 1) maximum outlet air temp if everything is operating within spec, 2) maximum outlet air temp the equipment/materials can withstand without actual damage. I think you'd want #1 and the limit switch chosen so as to prevent the system from ever reaching #2. I was interpreting "design max outlet air temp" to mean #2 but now I'm leaning towards #1. IOW, I'm now thinking that if the limit switch is operating correctly it will prevent the outlet air temp from exceeding 170F. I hope someone can tell me if this is correct or straighten out my interpretations.

    FWIW, I located my limit switch or at least one of them. It is a L200F-40, HH12ZB200 (pic & location diagram attached). Descriptions I found suggest it opens at 200F. I'm a bit confused to see 200F given the 170F max outlet. Might that be due to the location of the limit switch being naturally hotter than the outlet?
    Attached Images Attached Images  

  4. #4
    Join Date
    Jul 2007
    Location
    Phoenix, AZ
    Posts
    205
    Is your air filter dirty?

  5. #5
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
    Posts
    4,656
    Did you tech check for cracks in the secondary coil?
    That things a rustbucket down there.

    Maybe your getting to hot.

    BTW, what kind of engineering do you do?
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  6. #6
    The HVAC company provided filter (fits into my 20x20 filter grille, a step up from the cheap fiberglass kind but not nearly as restrictive as the pleated ones) was checked and is fine. Note: I've used the more restrictive pleated ones during past winters. There have been no ductwork changes since I bought the unit (fall 2011). I don't know exactly what the techs have/haven't checked. Particularly last time I tried to give the tech some space. I know the last gentleman made heat rise measurements and before an adjustment they were too close to 165F for him to continue. After an adjustment the supply temps still seem too high on Low & High. Actuals were Low: 140.8F, Med: 147.9F, High: 149.3F. I'm not certain what he changed, but I suspect it may have been switching to efficiency and/or turning on Low Heat Rise via Evolution.

    Shortly before the first fusible link melt, the condensate line got plugged by ice some distance from the furnace. No freezing in/around the furnace. A pump workaround was installed. I was told that event shouldn't have played a role in creating high output temps days later, but obviously I wonder since fusible links happened to melt after the frozen condensate line event.

    I am expecting to hear back from the company though, so perhaps we'll get to the bottom of things. This discussion is helping me to get oriented and prepared for that call. I greatly appreciate the replies and welcome more if you think it may help!

    I'm a computer hardware engineer turned software engineer. No experience in the HVAC field or similar applications.

  7. #7
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
    Posts
    4,656
    How old is the furnace?
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  8. #8
    Manufactured: Sep 2011, Installed: Nov 2011.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,764
    The high limit is effected by radiant heat. So it has to have a higher rating then the max outlet temp.
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    How many times must one fix something before it is fixed?

  10. #10
    Thank you for that explanation beenthere. I have some additional questions if someone has the time to reply...

    If I assume these don't apply:

    1) Ductwork or filter changes caused reduced airflow: No such changes, filter declared OK
    2) Faulty or damaged components: None identified and reported to me thus far

    I'm left wondering about these:

    3) Unusually cold outdoor temps pushed me over an edge I was too close to
    4) Settings were somehow changed

    Regarding #3 and outdoor temp, one question that comes to mind is what effect this can have on the natural gas supply and whether this can affect furnace output temps. Are there adjustments that should be made during (very) cold temperatures? Might, for example, a furnace installed and checked during barely cold temperatures be "miscalibrated" for more extreme cold?

    Regarding #4, I'm not aware of any settings changes having been made before the fire damper fusible links started going. However, I suspect there may be a difference between the way the Evolution control is configured to run the system and the way the furnace board is configured to run the system.

    Evolution Control...
    Comfort/Efficiency = Efficiency (increasing airflows)
    Low Heat Rise = On (increasing airflows)

    Furnace DIPs...
    Comfort/Efficiency Adjustment = ON (NOT increasing airflows)
    Low/Medium Heat Rise Adjust = Off (NOT increasing airflows)

    I'm not familiar enough with this equipment to be certain of things. However, my thinking is that in such an arrangement: if the Evolution control Staging setting were changed from "System" to "Furnace", my furnace would operate with decreased airflows and perhaps those sufficient to melt fusible links. If that were the case, I think we'd want to "play it safe" and make sure *both* the furnace board DIP settings and Evolution control settings "matched" and reflected my airflow needs. Comments?

    Perhaps more relevant to what happened, I believe the recently (just before fusible links started melting) added condensate pump was wired into the 4-wire link between furnace and Evolution control. Based on descriptions, I'm thinking there is a relay in the condensate pump that will open if it can't evacuate itself, and that relay is now between the furnace and Evolution control on the D-wire (24 VAC). I don't know what opening the D-wire would cause to happen though. I thought the idea was that if the condensate pump couldn't evacuate itself it would cause the furnace to shutdown. However, to explore possibilities, I wonder if the modification of the D-wire connection or some kind of related glitch could possibly cause a change in operation from "Evolution is in control and commanding higher airflows" to "Furnace is in control and commanding lower airflows"? Comments?

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,764
    Opening the D wire would shut the furnace down.

    Infinity control should over ride furnace settings.

    You were probably always near the line of tripping the high limit.
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  12. #12
    Quote Originally Posted by beenthere View Post
    You were probably always near the line of tripping the high limit.
    Ugh. Are there options for further bringing down heat rise and supply temps that don't involve ductwork changes? Note: I ask for informational purposes only. I would of course discuss such changes with my HVAC company.

    As previously mentioned, the Evolution control is currently set to Efficiency and Low Heat Rise is On. These are the best settings to reduce those temps, right?

    Staging is set to System. I don't set back, and my heat loads are far below that which would require high stage. However, my counts are:

    Low Heat: 28910 cycles (3660 hours)
    Med Heat: 8113 cycles (496 hours)
    High Heat: 7549 cycles (418 hours)

    So for some reason I am using High and even Medium. I don't know if changing staging to Low, or Low+Medium is worth considering but it comes to mind.

    High Stage Timer is set to 10 min. I'm inclined to think that bumping that up might help, but note the "a demand of 5F or more will override the staging timer".

    Are there any gas pressure, mix, burner type adjustments that can be made to bring heat rise down?

    I think I have the SYSTXBBUID01-C Evolution control. I know my control displays software version CESR131470-19. Would a newer control and/or newer firmware bring some more options to the table?

    Anything else?

  13. #13
    Join Date
    Dec 2005
    Location
    Cincinnati, Oh
    Posts
    4,656
    Oversized furnace for the home, undersized duct for the furnace.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

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