Shut off the A/C or leave it running?

One of the most frequently asked questions is "Should I shut off my A/C when leaving the house, or leave it running?"

You see, it's not that simple...

This is one of those classical cans of worms that a lot of people (HVAC professionals in particular) have an opinion on, but just try to ask them to quantify the problem, and the result has all the chances to be hilarious.

Suppose, you go on vacation and shut off the A/C for a month. You've just saved a lot of money, except now you have to repair the cracked deck in your sixty thousand dollars Steinway grand piano you absolutely forgot about.

Suppose, you get out to walk the dog and shut off the A/C. I will call you paranoid and probably pound foolish, penny wise (all in all, having a dog is an expense you don't really have to incur if you're so concerned about your financial well being).

Suppose, you send the kids to school and go to work at 0800. Kids get back from school at 1400, you get back at 1800. This is where things get interesting.

Pretend the house is a pool. The HVAC is the hose. The walls are the drain. The drain is constantly open. The hose is open as it is needed to keep the pool filled up.

So when you shut off the hose, the pool starts losing water. The longer the hose is shut off, the more water you have to replenish (note that this is pretty much like cooling the house - yes, there will be a point beyond which all the water is gone, and from that moment on you're purely saving money). On the other hand, the longer the hose is shut off, the less you pay.

When you come back and open the hose, the water level goes up (and the house cools down). The bigger the hose is, the faster it will happen.

In other words, the bigger your HVAC is, the more sense it makes to shut it off or adjust the setpoint when leaving the house.

But wait, this is not the end of the story yet...

The drain is nonlinear.
The hose is nonlinear.

The hotter it is, the bigger is the drain, the sooner the water is gone.
The hotter it is, the more strain is put onto the hose (this is even worse for heat pumps in heating mode, because of the balance point).
It may lead to a situation when the inflow can't keep up with the outflow.


In other words, the smaller (or worse) is your HVAC, the less sense it makes to shut it off or adjust the setpoint when leaving the house. If this is your case, bite the bullet and pay the bill. Which reminds me, by the way, that poor people pay more.

But wait, this is not the end of the story either.

The highlighted cases above are for extremes. As the weather gets milder, efficiency considerations fade because the drain narrows (not as much heat is lost or gained). For this case, the longer you are away, the more sense it makes to shut off the HVAC or adjust the setpoint.

Now, let's remember the rule: whatever you cannot measure, you cannot improve. You may spend days and nights guessing, but you will never know whether you improved the situation or made it worse until and unless you have long term trends monitoring set up.

Which brings us all the way back, right here.

PS: Inquisitive readers will notice quite a few threads dangling here. Detailed explanations will go well beyond the point of loss of interest, but those who care to find out have probably figured out where to go to get the answers.