Is this how a heat pump is supposed to work?

September 11, 2017 9:09 AM

Recently, our entire hvac system was replaced, but I'm not sure the heating cycle is running correctly.

First, this is in central Indiana, so we haven't had any really cold weather yet. But, there have been a few evenings and mornings where I felt the need to take the chill off the place. The heat pump in question is a new Lennox (not my choice...the home warranty people chose it) The system includes an new air handler with an auxiliary heating unit.

Here's how my system is running...

Let's say the indicated temp on the thermostat is X, and I want it at X+3.
I switch on the Heat cycle, adjust the target temp (X+3) and the heat pump starts doing its thing. It's making good hot air. The temp gets to X+1 pretty quickly. Then, out of the blue, the auxiliary heating unit comes on. The temp eventually gets to X+3 and the auxiliary unit clicks off, but the heat pump keeps running until the indicated temp is X+5. Then, the pump shuts down.

The first thing I think is wrong is that the auxiliary heater comes on. It's not 35 degrees, and the heat pump was making great heat by itself. It would seem to me that the pump should run by itself in this weather and not need the auxiliary heater. I always thought the aux units didn't come into action until the weather got really cold.

The second issue is how it overshot the target temp by two degrees before shutting down. That doesn't seem right. It doesn't overshoot the target temp when in cooling mode.

Should I call the service people and have this thing looked at, or is this typical activity for a modern heat pump?

posted by Thorzdad to Home & Garden (9 answers total)

Overshooting the target temperature is 100% normal. It overshoots by a degree or two, then doesn't come back on until it's a degree or two below. So the temperature is within a degree or two of target at all times, and the system run less frequently.

As to the aux heating running, I assume it kicked in because the temperature didn't come up quickly enough. It might be possible to adjust this so it waits a little longer. I'd ask your HVAC installer about it.
posted by kindall at 9:15 AM on September 11, 2017 [1 favorite]

It should not be overshooting by 5 degrees. I would argue the aux strips on a modern heat pump should not be turning on at all until you get closer to freezing temps. Call for a service check.
posted by LoveHam at 9:34 AM on September 11, 2017

I'm not sure the 5 degrees isn't normal. You probably have a thermostat that's programmed with the desired setback. Just tell the tech you don't want it to go that high. As for the aux heat coming on, well, geothermals don't like big temperature swings. Try going up two degrees, and then another one once it's reached temperature and see if that solves your problem.
posted by kate4914 at 10:25 AM on September 11, 2017

Response by poster: Not meaning to threadsit, but...It isn't a geothermal system. Just a plain vanilla heatpump.
posted by Thorzdad at 10:40 AM on September 11, 2017

Just to remind the folks saying x+5 isn't normal - x+3 is the new setpoint, so it's only overshooting by 2. Which is well within the realm of normal.

The secondary kicking on may be due to a bad setting, but it might also be due to the "aim" point being +5 from the current temperature. And once it's on, it has to stay on for some minimum time.
posted by notsnot at 11:04 AM on September 11, 2017 [2 favorites]

It's probably working as designed and configured, but you should have the techs check the configuration and maybe tune it a bit. I would assume there's some "smart" programming that is meant to make the system seem more responsive when you first turn it on (so it turns on the supplemental heat more quickly, in order to get to your new set point more quickly). I'd also bet that if you only changed the temperature by, say, two degrees instead of three (or even just by one) it would only run the heat pump compressor and never engage the supplemental heater at all. You could probably get it to heat up more slowly (and cheaply) by turning it up a single degree at a time.

There are probably configurable options that your installer could set for you that will change the threshold/time for supplemental heat, as well as the amount it will overshoot. Talk to your installer.
posted by fedward at 11:24 AM on September 11, 2017

Auxiliary heaters are meant to address the issue of heat pumps being slower than other heating/cooling systems. So this comes on so you don't have to wait so long to get to your desired temperature. If you don't mind waiting and you're home, you might change the desired temp a little at a time. Or you can get a thermostat that allows you go turn the Aux on/off independently. I believe some of the smart thermostats have heat pump settings that essentially do the "turn it up one degree at a time" thing automatically (if you want) to keep the auxiliary off in order to save energy.
posted by If only I had a penguin... at 12:26 PM on September 11, 2017

Some thermostats have an explicit Aux heat/emergency heat setting; you might want to double-check that your thermostat isn't set to it. My experience is that Aux heat only comes on by itself if it is extremely cold (like around freezing outside) or if the heat pump is not functioning correctly, but I've never had more than a simple programmable thermostat either.
posted by Aleyn at 4:39 PM on September 11, 2017

If anything, a tech could explain the ideal behavior. Does the aux come on if the desired temp. isn't reached in a certain time? Or if the new setpoint is far away from the original one? Once on, how long will the aux stay on at a minimum?

I'm thinking you're running it in the exact worst performance case - just as you're about to get there without the aux, it kicks on and then has to stay on for some time to prevent quick cycling.

Once you know what the rules are, they can be gamed (a rule generally applicable to many things in life.) Open a window when the aux comes on, so you pass by the setpoint more slowly? Bump it up 2 degrees at a time? etc.
posted by ctmf at 8:09 PM on September 11, 2017