Heat Pump Auxiliary Heat Control and Lockouts

A customer says their heat pump works fine, but the electric bill has doubled. The heat pump probably is working. The backup heat is running far more than it should, because of a thermostat setting, a stuck sequencer or a compressor that cannot keep up. The opposite fault is just as serious: a house that goes cold because backup heat was locked out too tightly. Control logic decides when each heat source runs. Setting that logic well takes the home's heat loss, the unit's data and the customer's energy rates. Testing strip heaters and gas equipment is work for qualified technicians.

What You Will Be Able to Do

• List what can bring on auxiliary heat.
• Calculate a thermal balance point from real data.
• Size and test electric strip stages.
• Set compressor and aux lockouts that work together.
• Calculate a dual-fuel changeover point from energy prices.

What Decides When Aux Runs

Auxiliary heat, or aux, is backup heat that runs with the compressor. Several things can call for it.

Read the thermostat's installer menu first. Many aux complaints come from a default setting that was never changed.

The Thermal Balance Point

The thermal balance point is the outdoor temperature where the heat pump's capacity equals the home's heat loss. Below it, the home needs backup heat. You can work it out from a heat loss figure and the unit's data.

Worked example: a heat loss calculation gives 40,000 BTU per hour at a 10°F design day, with the home at 70°F. The maker's data shows 36,000 BTU per hour at 47°F and 22,000 BTU per hour at 17°F.

1. Heat loss per degree = 40,000 divided by (70 minus 10), or about 667 BTU per hour per °F.
2. Capacity lost per degree = (36,000 minus 22,000) divided by 30, or about 467 BTU per hour per °F.
3. Test 29°F. Heat loss is 667 x 41, or about 27,300. Capacity is 22,000 plus 467 x 12, or about 27,600.
4. The two lines cross at about 29°F.

The design day also tells you how much backup heat the home needs. At 10°F, capacity is about 22,000 minus 467 x 7, or about 18,700 BTU per hour. The shortfall is 40,000 minus 18,700, or about 21,300 BTU per hour. That is about 6.2 kW of backup heat, since 1 kW equals 3,412 BTU per hour.

This uses a straight line between two data points. Real capacity also falls with frost and defrost, so leave a margin. A cold-climate unit holds capacity much better, which lowers the balance point.

Sizing and Staging Strip Heat

Strip heaters are usually sized in 5 kW steps. A sequencer or relay brings them on one at a time, so the whole load does not hit the panel at once.

Output falls with the square of the voltage. At 208 volts, a 5 kW strip gives (208 / 240) x (208 / 240) x 5, or about 3.76 kW. A building with 208-volt service gets about a quarter less heat than the nameplate suggests. That matters if the backup heat was sized tightly.

To test strips, a qualified technician measures amps on each element while it runs. Every stage should draw close to its rated amps. Check that each stage comes on when called, and drops out when it should. A sequencer that sticks closed runs strips with no call, and an open element gives low output with no fault code.

Warning: Strip heaters run on line voltage, and they get hot enough to start a fire with poor airflow. Never bypass a limit switch or a fusible link. If a limit keeps tripping, find the airflow problem.

Lockouts

Lockouts use outdoor temperature to stop a heat source. They need an outdoor sensor that reads correctly.

Set the two so they overlap or meet. If the aux lockout is set below the compressor lockout, there will be a band of outdoor temperatures where neither source is allowed to run. That means a cold house and possibly frozen pipes.

A good starting point for the aux lockout is a little above the balance point, so aux can help on the coldest cycles but not in mild weather. Use the thermostat's droop setting too, so a quick dip after a door opens does not bring on strips.

Some rules for lockouts:

• Do not set an aux lockout so tight that aux cannot help on a design day.
• Check how emergency heat behaves. On many thermostats, it bypasses the aux lockout.
• Decide how recovery works, and use smart recovery or small setbacks on heat pumps.
• After setting lockouts, confirm the home can still hold temperature on the coldest expected day, or write down the limit.

Dual Fuel

A dual-fuel system pairs a heat pump with a gas furnace. The furnace replaces the strips, and it needs different logic.

In most installs, the indoor coil sits downstream of the furnace heat exchanger. If the compressor runs while the furnace fires, hot air crosses the coil that is acting as the condenser, and head pressure climbs fast. So the compressor and burner must not run together, except as the defrost control allows.

Key points:

• Use a thermostat or control kit that is made for dual fuel.
• Below the changeover temperature, the furnace runs and the compressor stays off.
• During defrost, the furnace usually tempers the air while the coil is in cooling mode. Follow the kit or board instructions, because wiring varies.
• The furnace fan control and the thermostat's fan terminal need to agree, or the blower may start at the wrong speed.

Gas side work and combustion checks belong to a qualified gas technician.

The Economic Balance Point

With dual fuel, the changeover point can be based on cost rather than capacity. You compare the cost of a unit of heat from each source.

Worked example, using sample prices. Use the customer's real rates.

1. Gas is $1.30 per therm, and the furnace is 95% efficient. A million BTU of delivered heat costs 10 therms x $1.30, divided by 0.95, or about $13.70.
2. Electricity is $0.16 per kWh. A million BTU at a COP of 1 needs 293 kWh, which costs about $46.90.
3. Break-even COP = 46.90 divided by 13.70, or about 3.4.

COP, or coefficient of performance, is heat output divided by energy input. Maker data at 35°F might show 30,000 BTU per hour at 2.6 kW. COP = 30,000 divided by (2.6 x 3,412), or about 3.4. In this example, the heat pump only wins above about 35°F. Set the changeover near that, as long as the customer wants the lowest bill. Some customers prefer to run the heat pump longer for other reasons. That is their call, so explain the numbers and record the choice.

Common Complaints

If a system runs aux at mild temperatures, always check the heat pump first. Aux may be covering for a compressor that has lost capacity.

On an A2L system, a refrigerant leak detection system can shut off electric heat as an ignition source. Treat a no-strip call on one of these systems as a possible leak, and never bypass the sensor.

Setting Up a System

1. Confirm the equipment, the thermostat type and any outdoor sensor.
2. Get the heat loss, the maker's capacity data and the customer's energy rates.
3. Work out the thermal balance point. On dual fuel, work out the economic balance point too.
4. Set aux and compressor lockouts, and the droop and time settings.
5. Test each stage of backup heat, in heating and in forced defrost.
6. Record every setting, and leave a copy with the customer.
7. Explain emergency heat, and what the customer will see in normal winter running.

Common Mistakes

Check Your Understanding

A home loses 30,000 BTU per hour at 10°F with a 70°F indoor setting. How much heat loss per degree is that? 30,000 divided by 60, or 500 BTU per hour per °F.

A building has 208-volt service, and the backup heat was sized tightly from the nameplate. Why might the home fall short? Output falls with the square of the voltage, so the strips give about a quarter less heat than the nameplate suggests.

The compressor lockout is set at 10°F. Where can the aux lockout go? At or above 10°F, so the two overlap or meet. Anything lower leaves a band with no heat source.

Your numbers show gas is cheaper below 35°F, but a dual-fuel customer wants the heat pump to run down to 20°F. What do you do? That is their call. Explain the numbers, and record the choice.

A bill has doubled, and the strips draw current with no call for heat. What is the likely cause? A sequencer stuck closed.

Key Takeaways

• Droop, time, recovery, defrost and emergency heat can all bring on backup heat.
• Calculate the balance point from heat loss and the maker's data, and use it to size backup heat.
• Strip output falls with the square of the voltage, so check 208-volt service.
• Aux and compressor lockouts must overlap or meet, so a heat source can always run.
• In dual fuel, the compressor and burner must not run together outside the defrost logic.
• The economic balance point comes from real energy prices and real COP data.