Undersized Furnace? Greek Town, Toronto.

[zimme]

Hello: I had a keeprite model C9MPD075F12A2 with 75,000 BTU, single stage furnace until it ran out of its service life (2024). Installer replaced it with a Lennox ML296UH070XV36B, 70,000 BTU 2 stages furnace. Other than the furnace, no changes (size of house, basement & duct works) took place.

After the installation, I experienced in the winter of 2024/2025 I had to use portable heaters on my 2nd floor to supplement the heating. And in this 2025 summer I have to turn on fans to circulate the air conditioning.

Is the 2 stages Lennox undersized? Installer recommended the 2 stages Lennox.

Should / Could I exchange for a higher BTU 2 stages furnace or go back to a 1 stage furnace?

I do not mind to pay for the labor but I have no use of the furnace if replaced. Could the installer take it back for credit?

Thank you. Best regards.

[zimme]

Note: installer had already adjusted the furnace to max CFM.

[Admin]

That model of furnace comes set to work with a two stage thermostat. Are you using a two stage thermostat and is it wired for two stage?

If you're only using a single stage thermostat then you're probably stuck on low fire and need to adjust the dip switch so it's set for a single stage thermostat.

I would also double check that he adjusted the cooling airflow properly as they have separate dip switches.  He may have bypassed the Y terminal at the furnace and connected the thermostat directly to the AC and that's why the cooling airflow will not engage.  Turning the thermostat FAN ON is only engaging the lower continuous speed, not the higher cooling speed.  Y from the thermostat and AC should both connect to the furnace Y terminal.

[zimme]

I am using an ecobee3 lite single stage thermostat. While testing after installation I can hear the furnace kicked into the higher speed by sound and informed as such by installer.

Per Lennox manual: "The automatic heat staging option allows a single stage thermostat to be used with two stage furnace models. To activate this option, move the jumper pin (see Figure 60) to desired setting (5 minutes or 10 minutes). The furnace will start on 1st stage heat and stay at 1st stage heat for the duration of the selected time before switching to 2nd stage heat."

It does not explain the meaning of "None"

Installer set my automatic heat stage to 5 minutes. Also the "Adjust" jumper is set to "+" with "Heat" jumper set to "A").
 
However, manual has a table apparently saying setting "A" (1175 cfm – 2nd stage heat speed) is not allowed for whatever reasons. Does that mean it defaults back to "B", 1085 cfm?
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All in all, given the poor heating in last winter, I like to upgrade to a Lennox ML296UH090XV48C. My house is about 1800 sq ft (second & main floors) but with counting in the basement (ie: counting all rooms/spaces that have register air outlets) is about 2300 sq ft.

Should I worry about oversizing with this ML296UH090XV48C unit? Thanks. Best regards.

[Sergroum]

The dip switch you are describing changes the delay between 1st and 2nd stage when using 1 stage tstat.


But there is a seperate dip switch that tells the furnace whether the thermostat is 1 stage, or 2.