I have discovered that my furnace limit switch was tripping because of high temperatures within minutes when one or two of the three zones are calling. I called my hvac contractor and the tech came and confirmed that the limit switch was tripping after checking that the filter is clean and that the furnace drain is not clogged. Then he recommended a bypass to fix the issue. No other checks were done.
I am not convinced that the bypass will fix the issue. I think there is a serious issue especially after I have also found that even with all zones calling for heating, the furnace eventually exceeds the maximum temperature rise and trips the limit switch as well, although it takes over 25 minutes to get there. I called the hvac contractor again to update with my findings, but unfortunately I haven't gotten anywhere, and seem to still think bypass will resolve the issue.
I have an appointment in a week with another hvac contractor that has many positive references. I am hoping they will address the root cause.
Meanwhile, I wanted to get some input if possible and make sure I can make sure I pick the right hvac contractor who will spot the root cause.
Furnace is 100,000 BTUh input and 95,000 BTUh output. Nameplace temp rise range is 45-75F and maximum outlet air temp is 170F. ECM blower is set to highest speed with 1570cfm@0.5inWc.
My suspicion is that the ducts are undersized. The ducts are located in the attic, and there are long runs on supply side because most of the attic is finished.
I measured the Total External Static Pressure for the furnace, and I got 1.07 inWc. Nameplate max is 0.5 inWc.
- Return plenum (before filter) static pressure is -0.43 inWc.
- Filter is adding 0.14 inWc.
- Evaporator Coil is adding 0.20 inWc.
- Supply plenum static pressure is +0.28 inWc.
It seems that the return has high static pressure (hence undersized). To confirm this, I opened the filter cover and removed the filter and kept it open. Immediately, the supply temp dropped, and furnace TESP dropped to 0.82 inWc (without a filter). For some reason the pressure after Heat Exchanger increased from 0.48 to 0.67 inWc.
I wonder if that also means that supply ducts are also undersized if the return restriction is resolved?
I have disabled multi-zones on this system. I think I need to get the furnace to work properly with all zones first. I didn't feel comfortable relying solely on limit switch to prevent Heat Exchanger from reaching maximum temp limit.
Am I on the right track to make sure that hvac contractor should first get the furnace temperature rise within the design range before even suggesting a bypass to solve zoning issue?
And once the furnace is working within spec, then we can look at how to get multi-zone working which could be dump zone, bleeding, or bypass if other options do not work.
I am not convinced that the bypass will fix the issue. I think there is a serious issue especially after I have also found that even with all zones calling for heating, the furnace eventually exceeds the maximum temperature rise and trips the limit switch as well, although it takes over 25 minutes to get there. I called the hvac contractor again to update with my findings, but unfortunately I haven't gotten anywhere, and seem to still think bypass will resolve the issue.
I have an appointment in a week with another hvac contractor that has many positive references. I am hoping they will address the root cause.
Meanwhile, I wanted to get some input if possible and make sure I can make sure I pick the right hvac contractor who will spot the root cause.
Furnace is 100,000 BTUh input and 95,000 BTUh output. Nameplace temp rise range is 45-75F and maximum outlet air temp is 170F. ECM blower is set to highest speed with 1570cfm@0.5inWc.
My suspicion is that the ducts are undersized. The ducts are located in the attic, and there are long runs on supply side because most of the attic is finished.
I measured the Total External Static Pressure for the furnace, and I got 1.07 inWc. Nameplate max is 0.5 inWc.
- Return plenum (before filter) static pressure is -0.43 inWc.
- Filter is adding 0.14 inWc.
- Evaporator Coil is adding 0.20 inWc.
- Supply plenum static pressure is +0.28 inWc.
It seems that the return has high static pressure (hence undersized). To confirm this, I opened the filter cover and removed the filter and kept it open. Immediately, the supply temp dropped, and furnace TESP dropped to 0.82 inWc (without a filter). For some reason the pressure after Heat Exchanger increased from 0.48 to 0.67 inWc.
I wonder if that also means that supply ducts are also undersized if the return restriction is resolved?
I have disabled multi-zones on this system. I think I need to get the furnace to work properly with all zones first. I didn't feel comfortable relying solely on limit switch to prevent Heat Exchanger from reaching maximum temp limit.
Am I on the right track to make sure that hvac contractor should first get the furnace temperature rise within the design range before even suggesting a bypass to solve zoning issue?
And once the furnace is working within spec, then we can look at how to get multi-zone working which could be dump zone, bleeding, or bypass if other options do not work.
source https://hvac-talk.com/vbb/showthread.php?2222717-Furnace-limit-switch-tripping-on-high-temperature&goto=newpost
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