I recently got a new furnace for our 1200 sqft house in Denver CO. We are planning on adding a second story in a year and I was going to wait until we did that to replace the furnace. Then the furnace up and died just to mess with my plans. I got a 80,000btu furnace instead of a 60,000btu model in anticipation of the house getting a bit larger. What I didn't think through was just how much air the furnace now wants to move through the house.

I am wondering if it is acceptable in this situation to install a bypass between the supply and return as if the system was a 2 zone.

  • Modern furnaces have variable speed blowers. Typically the speed control setting is left to the installer that should be trained in how to use the control to balance your system. – Tyson Nov 21 '17 at 16:36
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    What's the actual problem you're trying to solve? Noise? Draft? Energy usage? – isherwood Nov 21 '17 at 16:38
  • Even the older furnaces have multispeed fans. Your installer should be able to set the fan to a slower speed. – ArchonOSX Nov 21 '17 at 19:08
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    If the burner is putting out a certain heat rate (BTU/h) and you the flow rate of air through the system is reduced, then the heat exchanger and the air coming off of it will be at a higher temperature. This could cause premature burnout of the heat exchanger. Find out if the burner has an adjustment for gas flow rate. – Jim Stewart Nov 21 '17 at 19:41
  • Just use the unit in normal recommended air flow and see if you can accept that the unit will come on and then go off quickly. Some thermostats have an adjustable temperature difference between on and off to reduce the number of on/off cycles. This allows the temp in the rooms to swing more between limits (not desirable) to reduce the number of cycles per unit time. This is probably mostly designed to reduce wear in the a/c compressor but could be of advantage in the case where the furnace is over-sized for the heating demand. – Jim Stewart Nov 21 '17 at 19:53

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