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  1. #1

    Carrier Furnace short cycling in a dual zone system

    Hi everyone,

    I'm new to this site and I have a question that nobody seems to be able to answer. I have a dual zone heat/ac system in my 3100 sqft 2 story home.
    Recently my heater has been short cycling, but only when the heat is turned on downstairs. (Upstairs has been working great).

    Two story home, approx. 1600 sq. ft. upstairs, and 1500 sq. ft downstairs.

    2 zone control.

    Carrier Model 58 RAV
    115K BTU
    2000CFM

    Honeywell EMM-3 Minizone Controller
    White-Rogers 1F78 thermostat

    2 16" supply ducts into 2 J-boxes
    1 18" return duct
    11 in-ceiling registers upstairs various sizes
    9 in-ceiling registers downstairs various sizes

    PROBLEM: heater short cycles about every 7 min. (downstairs only)

    When heating downstairs only, error 33 high limit switch trips open. I measured the heat at the sensor, and it trips at 210F as designed, so I know my furnance is overheating.

    I can run the upstairs indefinitely, and the heater doesn't cycle.

    I believe I ruled out any electronic issues with the unit, and I believe that I may have airflow issues with the downstairs ducting.

    I have adjusted the upstairs damper to stay open about 30 degrees when I operate the downstairs control only. This has lengthen my cycle time to over 30 minutes ( I only tested to this time, as it may run longer before cycling)

    What else should I check? Would it be safe to operate given that I have adjusted the upstairs damper to get better airflow?

    Thanks!

  2. #2
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,738
    Your ductwork is too small. Properly installed and you may get by with no by-pass or at the very least a much smaller by-pass.

  3. #3

    Overheating

    Thanks guys for your reply.

    I verified that the downstairs damper is full open when that zone is selected.

    There is no bypass damper installed in the system. Is that something I should look into? Is that an easy modification?

    I have measured delta T when the high limit trips, and it is within spec. on the high side, so that would indicate an airflow problem, I think.

    Tony, if the manifold pressure is set too high, wouldn't the heater also over heat when using only the upstairs zone?

    Thanks!

  4. #4
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    A 2 stage furnace and AC should be used with any zoning application. A bypass damper might help some, but you might still hit hte high limit.

    I wonder if you could set a minimum position for both the zone dampers, rather than have them close completely.

    Ideally, you need a larger return, larger supplies and/or a smaller furnace. That furnace seems a little large for that size home. Zoned systems can normally use a slightly smaller furance as well.


    Since it's a 80% furnace, unless you're in the south, it might be worht looking at replacement soon anyway. But opening the dampers a little may help. Some have srecw you can install to hold the dampers cracked open a little... or thre's a set screw to adjust the actuator.

  5. #5
    Quote Originally Posted by motoguy128 View Post
    A 2 stage furnace and AC should be used with any zoning application. A bypass damper might help some, but you might still hit hte high limit.

    I wonder if you could set a minimum position for both the zone dampers, rather than have them close completely.

    Ideally, you need a larger return, larger supplies and/or a smaller furnace. That furnace seems a little large for that size home. Zoned systems can normally use a slightly smaller furance as well.


    Since it's a 80% furnace, unless you're in the south, it might be worht looking at replacement soon anyway. But opening the dampers a little may help. Some have srecw you can install to hold the dampers cracked open a little... or thre's a set screw to adjust the actuator.
    Thanks motoguy128.

    I actually adjusted the other zone to remain open a bit, and it has helped. What I can't figure out is why only the downstairs overheats while the upstairs works fine?

    The only difference is that the upstairs has 2 more registers, but I have operated the upstairs with 3 registers closed, and had no problems.

    Is it possible that one or more feeder supply ducts have collapsed?

    Thanks!

  6. #6
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    The downstairs likely needs more heat and has much longer run times. I know in my home, which is 3200sqft, but 1700 is downstairs + basement, 1500 upstairs, the downstairs requires roughly 2-1/2 times the amount of heat. Depending on convection currents and stack effect in your house (heat rising in winter) you might need at least 2X the amount of heat downstairs so it will run 2X as long. The longer a furnace runs that's short on airflow, the more likely it will trip on high limit. A furnace heat exchnager doesn't reach peak operating temp (equilibrium) for at least 10 minutes I'd guess.


    The difference might also be related to the main branch and how it's distributed.


    Again, I'd keep all the supplies completely open unless you're really roasting in aparticular room, and increase the amount of air you let bypass when a damper is closed.

    Honestly, even with both zones open, your ductwork is still marginal with a 60F temp rise and 700fpm. That assumes the flex uducts are pulled tight and so forth. I'd considering opening the zone damper at least 30% or so. Having a furnace shutting down on the high limit switch is bad. Over time you will cause damage to teh heat exchanger. So if you're not ready to replace it now, you will be soon. IF anything it will make yu more comfortable with longer run times, less duct noise and if you get a vairable speed unit, you existing AC will run about 3-8% more effeciently since the blwoer will use less power.

  7. #7
    Quote Originally Posted by motoguy128 View Post
    The downstairs likely needs more heat and has much longer run times. I know in my home, which is 3200sqft, but 1700 is downstairs + basement, 1500 upstairs, the downstairs requires roughly 2-1/2 times the amount of heat. Depending on convection currents and stack effect in your house (heat rising in winter) you might need at least 2X the amount of heat downstairs so it will run 2X as long. The longer a furnace runs that's short on airflow, the more likely it will trip on high limit. A furnace heat exchnager doesn't reach peak operating temp (equilibrium) for at least 10 minutes I'd guess.


    The difference might also be related to the main branch and how it's distributed.


    Again, I'd keep all the supplies completely open unless you're really roasting in aparticular room, and increase the amount of air you let bypass when a damper is closed.

    Honestly, even with both zones open, your ductwork is still marginal with a 60F temp rise and 700fpm. That assumes the flex uducts are pulled tight and so forth. I'd considering opening the zone damper at least 30% or so. Having a furnace shutting down on the high limit switch is bad. Over time you will cause damage to teh heat exchanger. So if you're not ready to replace it now, you will be soon. IF anything it will make yu more comfortable with longer run times, less duct noise and if you get a vairable speed unit, you existing AC will run about 3-8% more effeciently since the blwoer will use less power.
    motoguy128 thanks again!

    I have the upstairs zone damper adjusted to stay open about 30 degrees open position (approx. 67% closed) when the downstairs zone is only used. That seems to really help, as my runtime is > 30 min. I have only tested for 30 min., as it may run longer before cycling. Without the damper adjustment, the furnance will short cycle every 5 min.

    I think this may be a short term fix until further assesment. Do you think a bypass damper may help? Would installing a DATS help the wear and tear on my furnance? My home is only 7 years old, and it would be really expensive to replace the HVAC unit. I guess I could try to see if the home builder might help, but that is doubtful.

  8. #8
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by mffarrell View Post
    motoguy128 thanks again!

    I have the upstairs zone damper adjusted to stay open about 30 degrees open position (approx. 67% closed) when the downstairs zone is only used. That seems to really help, as my runtime is > 30 min. I have only tested for 30 min., as it may run longer before cycling. Without the damper adjustment, the furnance will short cycle every 5 min.

    I think this may be a short term fix until further assesment. Do you think a bypass damper may help? Would installing a DATS help the wear and tear on my furnance? My home is only 7 years old, and it would be really expensive to replace the HVAC unit. I guess I could try to see if the home builder might help, but that is doubtful.
    I'm not sure a DATS will do you much good without 2 stage equipment. It mgith at least shut the system down before it trips the limit switch. You want a dats for sure if you were to add a bypass damper. But I don't think a bypass damper is a good idea if you're already near maximum tmep rise on the furnace.


    Don't hold you breath on the builder...not 7 years later. He has no liability now... and could care less. Especially if it's a tract home. You mgith have a small window if it's a local builder and a custom home. But I'd expect a custom home would have had a 2 stage furnace, even 7 years ago.

    How cold is it when it runs for 30 minutes? I'ts been a pretty mild winter, so I'm a little suprised it runs for even that long given hte size of your house and age.

  9. #9
    Quote Originally Posted by motoguy128 View Post
    I'm not sure a DATS will do you much good without 2 stage equipment. It mgith at least shut the system down before it trips the limit switch. You want a dats for sure if you were to add a bypass damper. But I don't think a bypass damper is a good idea if you're already near maximum tmep rise on the furnace.


    Don't hold you breath on the builder...not 7 years later. He has no liability now... and could care less. Especially if it's a tract home. You mgith have a small window if it's a local builder and a custom home. But I'd expect a custom home would have had a 2 stage furnace, even 7 years ago.

    How cold is it when it runs for 30 minutes? I'ts been a pretty mild winter, so I'm a little suprised it runs for even that long given hte size of your house and age.
    It's about 35F outside, and 59F inside. I can run the furnance from 59F to
    63F in < than 30 minutes.

    Thanks!

  10. #10
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,619
    Wood floor grills?
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  11. #11
    Quote Originally Posted by Freezeking2000 View Post
    Wood floor grills?
    All grills are ceiling mounted.

    Thanks!

  12. #12
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    5,520
    Quote Originally Posted by mffarrell View Post
    It's about 35F outside, and 59F inside. I can run the furnance from 59F to
    63F in < than 30 minutes.

    Thanks!

    I ran some numbers on my own home to work into an equipment size based on gas useage. From Dec 10th to Jan 10th I used 214 therms in my home. That means I used on average 28,763BTU's per hour input (gee, that's not a lot... consider how little heat is needed for a house with concrete or stone walls???) The average outdoor temp was 37F. My design temp is -2F outdoors and with setbacks I keep it on average about 65F indoors. Considering that, on average I should have been utilizing 41% capacity on average. That tells me that at design counditons, I roughly need 70,154 BTU's to maintain indoor temps on average of 65F. OF course, You need to maintain 70F, and this also factors in solar gain, and average infiltration. You need to still design for design conditions. So we add 10.5% for the higher indoor design temperature. Then use a 10% safety factor on top of that... and we have 85,272 BTU's.. in equipment size required or 78,450 in heat loss for a 92% furnace that I have. Coincidientally, that is very ,very close to my heat loss calculations that told me I need 60kBTU downstairs and 45k BTU (smallest available) furnace upstairs.

    Now, would you beleive me if I told you that I have a 3200sqft 86y/o 2 story home with orignal windows covering 27% of the total wall surface area, no wall insulation, moderate air leakage (est.) and only R20 spray foam on the roof deck in the attic with a full uninsulated basement with attached garage in half of it and higher than average ceilings???

    Many homes need a lot less heat than we think. And the larger the equipment you install, the more problems you run into since run times get shorter and ductwork gets larger.

  13. #13
    Quote Originally Posted by motoguy128 View Post
    I ran some numbers on my own home to work into an equipment size based on gas useage. From Dec 10th to Jan 10th I used 214 therms in my home. That means I used on average 28,763BTU's per hour input (gee, that's not a lot... consider how little heat is needed for a house with concrete or stone walls???) The average outdoor temp was 37F. My design temp is -2F outdoors and with setbacks I keep it on average about 65F indoors. Considering that, on average I should have been utilizing 41% capacity on average. That tells me that at design counditons, I roughly need 70,154 BTU's to maintain indoor temps on average of 65F. OF course, You need to maintain 70F, and this also factors in solar gain, and average infiltration. You need to still design for design conditions. So we add 10.5% for the higher indoor design temperature. Then use a 10% safety factor on top of that... and we have 85,272 BTU's.. in equipment size required or 78,450 in heat loss for a 92% furnace that I have. Coincidientally, that is very ,very close to my heat loss calculations that told me I need 60kBTU downstairs and 45k BTU (smallest available) furnace upstairs.

    Now, would you beleive me if I told you that I have a 3200sqft 86y/o 2 story home with orignal windows covering 27% of the total wall surface area, no wall insulation, moderate air leakage (est.) and only R20 spray foam on the roof deck in the attic with a full uninsulated basement with attached garage in half of it and higher than average ceilings???

    Many homes need a lot less heat than we think. And the larger the equipment you install, the more problems you run into since run times get shorter and ductwork gets larger.
    Thanks motoguy128 for sharing. Me living in Cali, which probably is alot warmer where you live, and the fact that we have about the same size house, it looks like my HVAC is way too big. 115K BTUs @ 2000 CFM.

    After I adjusted my damper, so the upstairs stays open 30 degrees when only the downstairs zone is in use, this morning the outside air temp. was
    32F.

    I started the heat cycle at 58F to 65F this morning. It took about 70 min. to reach the set point of 65F. The furnance did not overheat, and my delta T was 57F.

    Do you think I am good to go with the way I have adjusted the damper? Maybe I am not running as efficient as possible?

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