Two Stage Furnace Problem

[MoonDogGuy]

House has the common problem of upstairs being much hotter than downstairs in the summer and much colder than downstairs in the winter. Thermostat is located downstairs. To compensate, I’ve always left the main damper to the upstairs full open and kept the downstairs dampers one-half to three-quarters of the way closed. Works pretty well in balancing things out between the two floors. About 15 years ago, I had a two stage Trane furnace and Trane air conditioner installed. Problem is that when it is around 30 or 40 degrees out during which the unit runs on the low speed, the air just won’t get to the upstairs and hence too cold upstairs no matter how I set the dampers. Works fine in really cold weather when the unit runs on high speed. Didn’t have this problem with a one stage furnace with only a high speed. I am thinking about replacing the system (have started to have other problems) and maybe going back to a one stage furnace this time. When I looked at the specs on a 90 percent efficient American Standard single stage furnace, it indicated the furnace had a multi-speed fan. Only the burner was single stage. Is this common? Now I’m worried that if I go back to single stage furnace, I will still get a fan that will run on slow speed during certain temperature ranges and I will still have the problem with air not reaching the upstairs. Yes, I know this is more of a duct sizing problem, but I don’t plan to open up my walls and replace the ducts. Is it possible to get a two-stage furnace and set it so it only uses the high setting for fan speed, but still use both the high and low settings for the burner? Any downsides to that? Any other ideas?

- Thanks

[beenthere]

A single stage furnace doesn't chage speeds in heating.
It runs on what ever speed the installer set it to.

Your old furnace probably had a multispeed blower also.

Your current furnace. Can have its blower speed increased in both first and second stage heat.

Since your having trouble in low stage.

I'd guess your return is undersized a lot.

[HVAC9900]

If you have the room,and the $,and if the duct can be split appropriately , suggest you also consider seperate furnace /ac for each floor.
Would likely save operating costs,and lengthen service life.

[motoguy128]

You have a ductwork problem. Different equipment can help, but it doesn't address the source of your issues.

You'd be best as someone else mentioned to split it into 2 systems, size properly for each floor... or fix the ductwork and/or go to a zoned system that can better distribute the air.


It's also possible the furnace is oversized which is compounding the problem.

Also, balancing airflow at the registers, doesn't work very well. You rally need a combination of proerly sized ductwork and manaul dampers located where the supply branch comes off hte main trunk.

It might also be possible that jsut increasing hte return upstairs and the size of the first 10-15' of supply trunk will solve soem of the balancing issues.


It's very common, and cheaper for most low budget installers (common in developments), to use hte same ductsize for the entire main trunk. In most cases, it's a little undersized at the plenum at the start of the system, then oversized at the end of the system. You want ot maintain a constant velocity throughout the system for balanced airflow. The main trunk needs to step down in size after each or several supplies branch off.

[sktn77a]

Try running the fan all the time to mix the air better. This should help. Or have the furnace/thermostat wired to bring on the second stage first. Cheaper than getting a new system!