Low Temperature Lockouts; Heat Pumps With Dual Compressors; Two Speed Heat Pump - Honeywell T874 Product Data

Multistage thermostats and subbases
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Low Temperature Lockouts

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:
Btu Out/Btu we pay for or Btuh Capacity
COP =
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.

Heat Pumps with Dual Compressors

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.
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
THERMOSTAT
HEAT 2
Fig. 35. Two-stage heat pump with two-stage thermostat.
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.
THERMOSTAT
HEAT 3
HEAT 2
Fig. 36. Two-stage heat pump with three-stage thermostat.

Two speed Heat Pump

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.
25
HEAT 1
COMP.
STAGE 1
COMP.
STAGE 2
OUTDOOR
THERMOSTAT
AUXILIARY
HEAT
HEAT 1
COMP.
STAGE 1
COMP.
STAGE 2
AUXILIARY
HEAT
M5838
M5837
60-2485—8

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