TOPIC: How long should furnace cool before applying heat?

Greetings-

At a recent inspection, the outside temperature was over 90 degrees. The Air Conditioner had been running for some time. (It felt wonderful inside). I shut off the air conditioning to allow the furnace to warm back up, before testing the furnace.  It seems that applying the furnace heat to a cold unit could cause thermal shock and perhaps damage the unit.  How long should the air conditioner be shut off before safely starting the furnace?

If the furnace is installed in a conditioned space, then after shutting off the AC, the most "warm" that the furnace (particularly, the heat exchanger) can possibly get is whatever the inside air temp happens to be, and the quickest, "gentlest" way to get the heat exchanger to that temp after just having the AC on is to simply run the fan for a couple minutes without any heat or AC on.  This would be the ideal way of doing it, but keep in mind that mid-winter de-winterizing, firing up after repairing with the house cold, and unconditioned space (attic) installations all have the furnace getting started relatively cold - sometimes below 0 degrees F - which are greater "shocks" to the furnace than what you're talking about.  Vacation settings and deep night setbacks in the heating season are more like the situation you've described.

Still, the greater the difference between heat exchanger temperature extremes, the greater the stress on the heat exchanger metal, and, cycle after cycle, that stress can affect the unit.  Is it the best thing for the furnace to go instantaneously from AC to heat "one time?"  Probably not, but in the realm of possibilities, could "something" happen? Unfortunately, yes, though that possibility is remote, and it would depend on other factors, like age of unit, extended period return air temp, whether or not the furnace has been starved for return air (way too common, whether due to a clogged filter or undersized return ducts or both), etc.

Personally, I prefer to not ignore what's in the realm of possibilities....but that's me.

Thanks, Scott-

I knew there was a safe way of getting the unit warmed up.....just never thought about running the fan.  Now I know!!

Dang, thought about it again, Steve, and realized that in most furnaces, the evaporator coil is after the heat exchanger, which means that the exchanger is likely already pretty near the inside air temperature.  Going instantaneously from AC to heat shouldn't be worse for the furnace than going away for vacation in winter and leaving the t-stat set at 55 degrees F.

Now, before I stick my foot in my mouth again, anybody else want to comment about the effect on the evaporator coil, refrigerant, oil, or any other part of the air conditioning system when one goes instantaneously from AC to heat?  It's tempting for me to try to explain what happens when the burners fire while the A-coil still has some liquid refrigerant being fed into it and the overall effect on the AC....

Oh, I'll still run the fan for a minute or so going from AC to heat, and after I'm done with the heat, I'll still let the fan switch shut off the blower before switching the AC back on.

A Heat Pump goes from as hot as it can get to as cold as it can get every 45 min in the winter in .5 sec...

...then back again in <5 min.

Don't listen to those Home Inspection School  "old wives tales"...

A Heat Pump goes from as hot as it can get to as cold as it can get every 45 min in the winter in .5 sec...

...then back again in <5 min.

....Great point, but is that ability designed into (or just inherent in) all heat pumps, even "one-way" heat pumps?  Now, I've never heard of manufacturers of furnace/AC units warning against going from AC to heat instantaneously, but has anyone else heard of that?

I personally think that the heat "generated" by the heat exchanger can't be transferred fast enough to the A-coil to cause a harmful rise in pressure, but I can't help but think that manufacturers would advise against what we're talking about, even if it's actually "mostly safe."  In my mind's ear, I can just hear them saying something like "we don't recommend it" or "if you do this, it'll void the warranty."

Not saying anyone who makes the sudden switch is wrong.  Heck, I stand on the top step of all my ladders, and I use my hammers without safety glasses, but those are situations where I'm in control.  For me, careful turns into CYA when other people are in the equation...but that's me.