Honeywell T874 Benutzerhandbuch

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60-2485—8

Some manufacturers of heat pumps have previously controlled 

their units so the compressor was locked out below a specified 

outdoor temperature. This was done because of the stress 

placed on the compressor by the very cold temperatures, and 

the fact that efficiency drops off at low temperatures.

Other manufacturers say that although efficiency is low at cold 

temperatures, the problem of starting the cold compressor is 

the most critical point. It is more important to keep the 

compressor running than to shut it off and try to restart it when 

the temperature warms up to +10°F (-12°C). Also the 

crankcase heater, which can run only when the compressor is 

off, compensates for the lower compressor efficiency. So, it is 

equitable to let the heat pump run even with a coefficient of 

performance (COP) slightly less than one, rather than to turn it 

off and have to run the crankcase heater.

NOTE: To determine the COP of a heat pump, use the fol-

lowing formula:

COP =

Btu Out/Btu we pay for or Btuh Capacity
Unit Wattage x 3.413 Btu/Watt

Another factor favoring this control strategy is that newer heat 

pump designs maintain a level of efficiency even at outdoor 

temperatures well below zero. Some brands do not reach 

1.0 COP until -25°F (-32°C).

Virtually all manufacturers now let the heat pump run 

continuously rather than lock it out at cold temperatures.

Staging residential sized heat pumps is a requirement that 

developed from the need to make heat pumps as efficient as 

possible. One of the methods used to improve overall 

seasonal efficiency is to reduce the amount of time the 

machine is operating in a transitional mode. Transitional mode 

is starting and stopping in addition to recovering from defrost. 

Under light loads, when the heat pump is cycling on and off, 

this can be a significant amount of time and can result in a 

significant efficiency reduction. Capacity control is one way to 

reduce the cycling rate and improve the efficiency.

Additional capacity control methods used on other 

refrigeration systems (unloading, multiple compressors) are 

not as likely to be seen on residential heat pumps.

With a single-stage heat pump, the compressor is normally 

controlled by the first stage and the auxiliary heat by the 

second stage. The T874G, N and R Thermostats are used for 

this purpose.

With a two-stage compressor, the two thermostat switches 

control the individual compressor stages. Auxiliary heat is 

controlled along with the compressor stage-two, by the 

thermostat second stage. It comes on with the compressor 

high speed if an outdoor thermostat—wired in series with it—

is made. See Fig. 35.

The outdoor thermostat is set at the balance point with both 

heat pump stages running. So if the heat pump can control the 

heating load down to +20°F (-7°C), the auxiliary heat comes 

on with the second stage of the compressor.

In effect, this control strategy changes the heat input of the 

second stage, depending on the amount of heat needed. 

Additional stages of auxiliary heat can be added with or 

without the control of more outdoor thermostats, See Auxiliary 

Heat section.

Another way to control a two stage heat pump is with a three-

stage thermostat. The T874W is designed for this application 

as well as others. See Fig. 36.

The advantage of this system is that no auxiliary heat is used until 
there is an actual demand from the controlled space.

One way to control capacity is to use a two-speed 

compressor. The compressor runs at low speed for stage-one 

heating and cooling. The compressor runs at the low speed 

instead of cycling the heat pump on and off under some light 

load conditions. 

The high speed is stage-two cooling. The compressor runs on 

high speed when loads increase so the low speed can no 

longer handle the cooling load. This reduces cycling rate and 

improves capacity.

M5838

THERMOSTAT

HEAT 2

HEAT 1

OUTDOOR
THERMOSTAT

COMP. 
STAGE 2

AUXILIARY
HEAT 

COMP. 
STAGE 1

M5837

COMP. 
STAGE 2

THERMOSTAT

HEAT 2

HEAT 1

AUXILIARY
HEAT 

HEAT 3

COMP. 
STAGE 1