I have a four year old Bryant Plus 90i gas furnace (replacing the
original gas furnace) and just recently had the same company add A/C.
Part of the A/C install included a furnace tune up because there was
some slight rattling and the thermostat would occasionally shut off
completely. The installers tuned up the furnace but couldn’t diagnose
the exact cause of the thermostat/furnace shut off problem. But during
the troubleshooting we caught the error code that was briefly flashed
on the thermostat (E2) and then determined the error code from the
furnace’s LEDs (33). The E2 is a byproduct of the 33 code at the
furnace as far as I can tell.

33 according to the manual is LIMIT OR FLAME ROOL-OUT SWITCH IS OPEN –
Indicates the limit, rollout switch or auxiliary limit switch is open
or the unit is operating in high heat only mode due to two successive
low heat limit trips. Check for – Improper or misaligned limit and/or
limit shield, Improper low heat gas input adjustment – Stuck high heat
solenoid in gas valve.

They sent a service technician out to do further troubleshooting and he
tried a new limit switch but it didn’t help. He was able to determine
that the limit switch was tripping when the furnace cycled from High
mode to low speed mode. His assessment was that there was inadequate
air flow across the heat exchanger that is causing it to overheat.

Next week the original installer and the install manager are coming out
to see what can be done to provide more air flow. My questions to any
furnace installers out there a

Should the installers have increased the size of the duct work in the
garage that leads to the down stairs and upstairs registers and/or cold
air return?

Is this something that is expected of reputable installers or do they
just marry the new furnace to the existing duct work and assume all is
well?

Is there are way to determine which part of the duct work is not
providing enough CFM? My understanding is that it could be that the
furnace is not getting enough cold air through the return or it cannot
push out enough heat air to the registers. Can I determine which on is
the problem?

What would you do to address this problem?

Thank you for any help you can provide.