Author Topic: how does low staging affect AFUE and longevity?  (Read 115 times)

Offline mdhare

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how does low staging affect AFUE and longevity?
« on: December 30, 2018, 09:25:19 AM »
Thanks to those who chimed in on my first thread re: carrier ECM blowers.

We ended up having a 60kbtu 59MN7A installed and so far we're quite pleased with the furnace.

A few questions

1) If BTU needs are being met, is it 'safe' to lock the furnace on low gas staging?  We run the continuous fan on low and the thought is if I'm going to run the fan why not have continuous heat?  Low fan is electrically 3x more efficient than high for us (pressure per thermostat is 0.11 vs 0.4).  Is it important to the furnace (and system as a whole) to occasionally run on high?  By safe I mean am I compromising the integrity of the system (heat exchangers, burners) by running on low staging for hours on end (coming back from setback).

2) How does staging affect AFUE?  Low is electrically efficient but what about gas?  Some discussion I've read online suggest low flame means less condensate removal per therm implying a lower AFUE.  But at 40% gas the fan CFM is also at about 40%; to me this implies a similar heat rise and transfer potential. 

3) I could probably answer this with an amp meter but does anyone have information on the inducer power draw at various loads?  Obviously this affects overall efficiency.  Obviously longer run times means more inducer usage.