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From another thread ...
Background:
The contractor set the MABS2EZ zone controller to go to Stage 2 after ten minutes, and the T8602D stats at 6 CPH. So the furnace would very frequently go to stage 2, then the stat would reach the cycle timer limit and shut it down. So the only time high fire would run long was when there were overlapping calls from both zones.
Maybe that was by design but all that switching to high fire for 30 seconds then going to purge just didn't sound right. Since our zones have staggered start times there is only a bit of overlap: Downstairs weekdays we go from 60F to 68F between 7 AM and 8:30 AM then 5 PM to 10 PM. Upstairs we go from 60F to 68F from 6 AM to 8 AM then 7 PM to 10 PM. Unless it's in the tens or below zero the furnace only runs during those hours and generally it takes an hour to heat up either zone.
I have a related question - I changed my programmable stats from 6 cph to 3 cph. If I understand this, 6 CPH means a maximum run time of ten minutes, then purge and restart and 3 CPH means twenty minutes. What is the theory behind this cycling? How should it be applied in a zoned system?HVAC Doc said:As any manufacture will tell you, when using a properly sized 2 stage furnace operates at a lower capacity most days except in extremes when high fire is needed. Just because it is in low stage does not mean it "increases or doubles" its run time. You are only using the capacity needed to heat your home at that given time.
Background:
The contractor set the MABS2EZ zone controller to go to Stage 2 after ten minutes, and the T8602D stats at 6 CPH. So the furnace would very frequently go to stage 2, then the stat would reach the cycle timer limit and shut it down. So the only time high fire would run long was when there were overlapping calls from both zones.
Maybe that was by design but all that switching to high fire for 30 seconds then going to purge just didn't sound right. Since our zones have staggered start times there is only a bit of overlap: Downstairs weekdays we go from 60F to 68F between 7 AM and 8:30 AM then 5 PM to 10 PM. Upstairs we go from 60F to 68F from 6 AM to 8 AM then 7 PM to 10 PM. Unless it's in the tens or below zero the furnace only runs during those hours and generally it takes an hour to heat up either zone.