Honeywell T874 Benutzerhandbuch

60-2485—8

26

In some regions, during half or more of the heating season, 

the outdoor coil operates below 32°F (0°C) Frost or ice builds 

up on the outdoor coil of a heat pump similarly to the frost 

buildup in a household refrigerator. Eventually, this 

accumulation of ice interferes with efficient heat transfer from 

the outdoor air to the coil and refrigerant. Defrosting is 

occasionally required to remove this ice, and restore the heat 

pump ability to absorb heat from the air.

A heat pump defrosts its outdoor coil by temporarily switching 

to the cooling mode, which causes hot gas from the 

compressor to be directed to the outdoor coil instead of to the 

indoor coil so the heat pump is taking heat from the home to 

warm up the outdoor coil. Defrosting is the greatest detriment 

to heat pump efficiency.

Besides changing over to the cooling mode, defrosting 

requires several more control initiated actions that follow. 

OUTDOOR FAN
When defrosting has begun, it is standard practice to turn off 

the outdoor fan to speed up the melting process. A separate 

defrost relay is required to control that fan. See Fig. 37. Other 

contacts can be needed on the defrost relay to power the 

changeover valve or power part of the auxiliary heat.

AUXILIARY HEAT DURING DEFROST
Most pump manufacturers bring on some auxiliary heat during 

defrost, although this is not a universal practice. Some rely on 

the second stage of the thermostat to call for auxiliary heat as 

needed. Since the heat pump is operating in the cooling mode 

to defrost, it is delivering cold air to the living space. Auxiliary 

heat is used to offset this cooling. It requires another normally 

open contact on the defrost relay. See Fig. 38.

WHEN TO DEFROST
Timely defrosting is an essential component of effective heat 

pump operation. Failure to defrost often enough permits too 

much ice to accumulate on the coil. At the very least, this hurts 

efficiency; at worst, it results in compressor damage. 

Insufficient defrosting is a condition the heat pump 

manufacturer wants very much to avoid.

A 50 percent reduction in outdoor airflow is the maximum that 

would be tolerated. So the designer of a heat pump would 

select a control point that puts the system into defrost when 

airflow through the outdoor coil approaches half its normal 

level. Restricted airflow causes a greater load on the 

compressor; the outdoor coil runs colder, suction pressure is 

lower and the motor runs hotter.

At the other end of the scale, defrosting too often hurts the 

overall energy efficiency of the system. Consider that in terms 

of heating the building, defrosting is a big loss. Not only does 

the system stop heating, but it actually moves heat out of the 

building. If electric strip heat is used, it is a further waste 

because its COP is 1.0 and not the 2.0 or more COP realized 

if the heat is provided by the heat pump.

So, concern for equipment safety suggests fairly frequent 

defrosting while economy of operation argues for fewer 

defrost cycles. Since the equipment manufacturer chooses, the 

balance is usually tipped in favor of more frequent defrosting to 

avoid the possibility of damaging the compressor.

Except in warm climates, all air-to-air heat pump installations 

require auxiliary heat capability. Electric resistance heaters 

can provide this auxiliary heat.

The electric heaters (sometimes called strip heaters) usually 

are supplied in 5 kW units or strips (about 17,000 Btu). The 

indoor unit of the heat pump is designed to accommodate 

various electric heat units so it can be used in variously sized 

buildings in different parts of the country.

The most common heat pump control strategy is for the 

thermostat first stage to switch the compressor and the 

second stage to switch the auxiliary heat. See Fig. 39.

M5839

COMPRESSOR 
MOTOR

CONTACTOR

CONTROLLED BY
DEFROST RELAY

OUTDOOR
FAN MOTOR

M5835

THERMOSTAT

HEAT 2

HEAT 1

DEFROST
RELAY
CONTACT

AUXILIARY
HEAT RELAY

L1
(HOT)

L2

TRANSFORMER