A few settings decide a lot
A heat pump can be efficient or expensive depending on how its thermostat is set. Most of the difference comes from a handful of settings in the installer menu and a couple of habits.
Bluffline Memphis Heat Pumps sets them with you at start-up on every installation and writes the key ones down.
The settings at a glance
| Setting | What to choose | Why |
|---|---|---|
| Equipment type | Heat pump, with auxiliary heat (or dual fuel) | Lets the thermostat run the reversing valve and stage backup heat |
| Reversing valve | O or B, to match the equipment | Wrong choice swaps heating and cooling |
| Outdoor sensor | Installed and enabled | Lets lockouts work |
| Aux heat lockout | About 30 to 35 degrees | Keeps strips off on mild days |
| Compressor lockout | Below the balance point, or off | Avoids running the heat pump when it can’t keep up, where configured |
| Recovery | Adaptive or slow recovery | Warms the house with the heat pump, not strips |
| Fan | AUTO | Better summer humidity |
| Dehumidify | On, if offered | Slower airflow when humidity is high |
The equipment type
The thermostat has to know it is controlling a heat pump. A thermostat configured for a furnace and AC can run the system, but it won’t stage auxiliary heat correctly and may call strips when it shouldn’t.
The reversing valve setting
Heat pumps energize the reversing valve in either cooling (the O terminal, most brands) or heating (the B terminal, some brands). If the setting is wrong, the system heats when you ask for cooling. It is set once, at installation.
The outdoor sensor
Without an outdoor temperature, the thermostat can’t tell a 50-degree day from a 20-degree one. It brings strips on whenever the house falls behind, which on a heat pump is much of the winter.
Sensor options
- A wired sensor on a shaded north wall, the most reliable.
- The outdoor unit’s own sensor, on communicating systems.
- An internet weather feed, on some smart thermostats; it works but reads the airport, not your yard.
The strip lockout
With an outdoor sensor in place, the thermostat can keep strips off above a set temperature. In most Memphis houses, about 30 to 35 degrees works: above it the heat pump carries the house, below it the strips help.
How to choose
The right lockout sits a little above the house’s balance point, the temperature where the heat pump alone stops keeping up. The load calculation and the heat pump’s performance data give that number. The sizing guide explains how, and the electric furnace to heat pump page shows how strips are sized to match.
The aux and emergency heat guide explains why it matters on the bill.
Setbacks and recovery
Setting the temperature back at night or while you’re away saves energy with a furnace. With a heat pump it can backfire. A big setback means a big morning recovery, and many thermostats bring strips on to speed it up.
What works better
- Small setbacks, about 2 to 4 degrees.
- Adaptive or smart recovery, which starts warming early and slowly with the heat pump alone.
- No setback on very cold nights, when recovery would need strips anyway.
In summer, setbacks are less of a problem; a few degrees warmer while you’re away is fine.
The fan
Summer is when the fan setting matters most in Memphis.
AUTO
The fan runs only when the system runs. After each cycle, water on the indoor coil drips into the pan and drains away.
ON
The fan runs all the time. Water on the coil evaporates back into the house between cycles, raising indoor humidity. Some owners use ON for air filtration or mixing; in a Memphis summer it usually costs comfort.
Circulate
Some thermostats run the fan for a set share of each hour. It’s a middle ground, but in humid weather AUTO is still better.
Humidity settings
Many newer thermostats show indoor humidity and offer a dehumidify mode that slows the blower when humidity climbs. In Memphis summers that helps, especially with two-stage and variable speed systems. The humidity guide explains why.
Dual fuel settings
On a dual fuel system the thermostat also chooses between the heat pump and the gas furnace. The key setting is the switchover temperature: above it, the heat pump heats; below it, the furnace does.
Where to set it
On MLGW’s typical rates gas is cheaper at every temperature, so the switchover is a comfort choice. The switchover guide works through the options, and the dual fuel page covers the system.
Emergency heat
The emergency heat setting turns the heat pump off and runs backup heat alone. Use it only if the outdoor unit is broken or frozen. Leaving it on by accident is one of the more common reasons for a shocking winter bill.
Heat, cool or auto?
Most thermostats offer HEAT, COOL and AUTO modes. In AUTO the thermostat switches between heating and cooling on its own to hold a range.
When AUTO helps
Memphis springs and falls can need heat in the morning and cooling by afternoon. AUTO handles that without anyone touching the thermostat. Set the heating and cooling setpoints at least a few degrees apart, so the system doesn’t flip back and forth.
When to avoid it
In the middle of summer or winter, fixed HEAT or COOL is simpler and prevents an unexpected switch on an odd day.
Where the thermostat sits
A thermostat reads the air around it, so its location matters.
Places that mislead it
- A sunny wall, which reads warm and under-heats the house.
- Near a supply register, which reads the air coming out, not the room.
- An exterior wall, especially an uninsulated one, which reads cool.
- Near a kitchen, lamp or TV, which add heat.
- A hallway with no return, which can sit in still air.
When a new thermostat goes in, the old spot often stays because the wire is there. If the old spot misleads it, moving it a few feet, or adding a remote sensor, can make the whole house more comfortable.
Common mistakes
Most expensive heat pump bills trace back to one of these:
| Mistake | What it costs |
|---|---|
| Thermostat set up as furnace and AC | Strips run when they shouldn’t |
| No outdoor sensor | No lockout; strips on mild days |
| Emergency heat left on | Strip heat all day |
| Fan left ON in summer | Higher humidity |
| Big nightly setbacks in winter | Morning strip heat |
| Heating and cooling setpoints too close in AUTO | Constant switching |
Communicating thermostats
Many variable speed systems use the maker’s communicating thermostat, which talks to the outdoor unit and air handler over a data connection. Those thermostats set much of the above automatically, but the lockouts, recovery and dehumidify settings still deserve a look.
A start-up checklist
At start-up, every item below should be set and confirmed:
- Equipment type and reversing valve set correctly.
- Outdoor sensor reading checked against a thermometer.
- Aux heat lockout set with you.
- Recovery and setback settings chosen.
- Fan on AUTO; dehumidify on if offered.
- On dual fuel, switchover set and written down.
The commissioning checklist covers the rest of start-up, and the Memphis buying guide puts the decision in order.