Simple HVAC Question

I have a programmable thermostat (a basic, not-Smart). When the room temperature shown on the thermostat matches the set point temperature on the thermostat, the A/C turns off. However, the fan keeps running. Yes, the fan is set on “Auto” vs. “On”. Shouldn’t the fan stop running, too?

I did an experiment, and when I bump the set point up 2 degrees higher than room temperature, the fan will stop running. But, shouldn’t the fan stop when Troom = Tset?

Am I wrong about that, or should I have someone look at this?

When you say the fan keeps running, is it running continuously? Long enough so that it’s running until the compressor cycles on again? Or does it run some interval of time after the compressor cycles off.

The latter is normal. That is, the fan runs 60 or 90 seconds after the compressor cycles off in order to extract the remaining coolness of the refrigerant in the evaporator coils after the compressor cycles off…for efficiency.

It’s winter on this end of the earth. On my AC, the fan won’t turn on until the heating coils / burner come up to temperature – to avoid blowing cold air into the room. And the fan goes off when the burner goes off – which is really sad, because the ducts are full of hot air, and I wish it would blow that through, rather than wasting it.

But if your fan only goes on when the heat/cool is off, and always goes off when the heat/cool is on, you’ve got a problem.

You sure about that? I’ve never seen an HVAC system that shut the blower off at exactly the same time as the burner. Just like with AC, the blowers I’ve seen always run for another thirty seconds or so after the burner shuts off.

From a physics standpoint, that heat is not being wasted. It will dissipate from the ducts into the surrounding structure, heating things up just as much as if the air had been blown into the vents. It’s just a slow process that you don’t really notice. But no waste, unless your ducts run along exterior walls and you are heating the great outdoors.

HVAC ducts often run through uninsulated space. At least, in both the houses I’ve owned, they do. Whatever isn’t blown out of the ducts is heating/cooling my attic and crawlspace.

I’m not an HVAC guy, I just know how my own house is built. No crawlspace, and no ductwork in the attic or other uninsulated space. Not sure why there would be ductwork in an attic. Our top-floor vents are in the floor.

They’ll often install the vents on the ceiling when the primary goal is cooling. It’s common in Arizona.

Only the return air ducts should be uninsulated. The ducts supplying the hot/cold air should have insulation wrapped around them.

Ours are in the ceiling. Makes sense to me to have most stuff running in the attic or crawlspace, since it makes it easier to get to than if it were between floors. But I’m also not an HVAC guy. :man_shrugging:

My vents are in my attic. My attic is not sealed. The heat is not dissipated into my home: the vents are above the ceiling insulation. I don’t have crawlspace: I have a floating slab (a concrete slab poured onto the dirt).

Also, yes, the fan goes off at the same time the flames go out.

When we came to Australia, my father regarded the lack of central heating as barbaric. But he never wished to return to a climate where good heating and insulation was a requirement. Heating, cooling and insulation has improved in Australia since then, but it’s still less important than many places in the world, and the technology (and the capital investment) reflects that.

So true. Like living in the dark ages here (I’m also in Melbourne). I couldn’t believe how cold houses got here inside in the winter, coming from Canada, where most houses are insulated and heated pretty well.