Lukewarm air until thermostat is bypassed

Lyra

Hi I have a Tappan natural gas forced hot air furnace (late 2009 intall) in a duplex (up/down) I believe it's a two stage- I think the model # is 7090470.
In August I had the old Honeywell thermostat in the downstairs apt connected and disconnected the programmable one in upstairs apt. hoping that would solve the problems I had last winter-with downstairs tenant being cold (like 63) while upstairs tenant was comfortable. Then when thermostat was turned up for him it became 78 in downstairs apt.
This unbalanced system was the result of a repair in 2017 during which all the dampers were adjusted/closed. They have since been opened and readjusted several times but I have not been able to balance it (but note it was balance from 2010 thru 2017 so it is possible). I don't live there so it's complicated getting dampers adjusted.
Saturday when the heat came on for the season it felt lukewarm to upstairs tenant. Sunday a tech came but left before I could arrive. The tenant tells me he bypassed the thermostat (bc tenant with thermostat was not home) and at that time the furnace was putting out hot air (ducts were hot to the touch). He concluded that dampers to the downstairs needed to be decreased to send more hot air upstairs. Monday afternoon the upstairs tenant notified me that the heat never warmed their apartments Sunday night .
I went last night found that the furnace is rapid cycling-so air is not hot.

Why would the furnace only work when the tech is there bypassing the thermostat but not when the thermostat is regulating? The flame comes on briefly then shuts off, then after about 5 mins it restarts only to shut down again.
The repair company owner (who has been there many times since 2010) said by phone the only solution is zoned heat. Others have suggested 2 furnaces. Neither seems like the only solution because because it was balanced for 7 years.

I've had nothing but issues with this furnace (and the programmable thermostat) and I'm thinking about replacing it for something more reliable. In the meantime what would cause the rapid cycling when the tech isn't there? Where do I start?
I replaced the filter-it did not seem dirty. The water drain tubes seem full of water but there isn't any water near condense pump nor did I hear the condense pump go on in the 20 or so minutes I was there. The green and red lights on the control panel are lit, the amber one looks like it's not-indicating "normal" and "no flame". I don't know if the amber light was on when the flame was lit.
I've considered the flame sensor, pressure switches and venting issues, but wouldn't all those cause it to also quit while the tech was there? What changed? Any ideas would be greatly appreciated. Anyone looking to buy a duplex?

Jamie Hall

I think we're all replying to your other thread. Which is talking about the same fundamental problem. You need a tech. who actually knows what he or she is doing with the controls.