Why auxillary limit switches on downflows?

[georgelass]

I was googling about and some site said ALL downflows
have auxillary limits switches? Why?

(I would think that downflows would have warmer return
air than upflows, and so a high return air temp would be
more likely to occur and thus less likely to be a problem.)

What do auxillary limit switches cover that high limit ones
don't?

Thank you.

[kangaroogod]

If the blower fails, heat rises and you need aux limits at the top of the furnace as the lower limits are not too effective.

[BaldLoonie]

Sometimes with a blower motor failure, for example, the main limit on a downflow doesn't open fast enough so they use the aux limits in the blower compartment as extra protection.

[georgelass]

Crap. That sounds so logical. Now I feel like an idiot.
Thank you.

[2141]

Crap. That sounds so logical. Now I feel like an idiot.
Thank you.

Don't feel bad, I've always wondered why there was a limit on the blower housing.

[slctech]

Many downflows use the same board as an upflow. The default blower OFF delay maybe set for too short of a duration. I will usually set the Blower OFF Delay to the longest duration available. Reason being, if the blower on a downflow does not run long enough to purge the residual heat from the heat exchangers, this heat will rise and cause nuisance trips of those auxiliary switches. Not to much of an issue with the auto-rest type, but a sure PIA if you have the manual reset types.
Just an FYI

[54regcab]

IMHO all heat blower off delays should be set at maximum unless you get complaints for end of cycle cold drafts.

[mike3]

One problem that occurs is a power outage. If the furnace is in operation at the time of the outage the limit will open.

[jumpit]

Yep and some manufactuers put manual resets on them which is terrible for the homeowner during power failure.