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Diagnostic Chain

THINGS TO CHECK IF THE UNIT WILL NOT DEFROST:

MOST LIKELY CAUSES:

Defrost controls vary in type and design.
Some older ones are as much mechanical as they are electrical and will have
 a remote tube with a gas filled bulb on the end that will sense for the ice 
 build up on the outside coils and an internal (or external) relay to do the 
 electrical switching. The bulb will be strapped strategically on the coils.

The defrost mechanism has a built-in timer that will periodically initiate
a defrost.  This will only occur if the remote bulb is sensing ice on the 
coils.  If not, it will just do nothing and revert to the standard heating 
mode.  If it does sense there is ice on the bulb, then it will turn off the 
outside unit fan motor and reverse the flow of the refrigerant via the 4 
way valve.  

The compressor continues to operate with the hot gas now routed 
through the outside coil which will defrost the ice.  Usually this process 
is set to run until the ice is defrosted and is sensed by the bulb as it is 
warmed or the defrost will continue for a maximum of 10 minutes or so.

The above process is normally enough to defrost the ice build-up, but 
occasionally during periods where the temperature is near or at freezing 
with rain, the ice build-up will be excessive and defrost will terminate 
before removing all the ice.  I highly recommend you should not be running 
the heat pump during these conditions anyway.  

At temperature below about 35 or 37°F (4°C), you should have 
switched the thermostat to the EMER. HT position which disables the heat 
pump and will supply quite adequate heating from the auxiliary source 
which is usually electric heat strips or gas or oil heat from an inside 
unit. Besides, the heat pump has become inefficient at these temperatures 
and the indoor air feels drafty.

Yes, the cost of operation will be slightly greater, but what do you want 
when it has gotten cold outside: warmth or draftiness? What's more, likely 
as not, the heat pump will have not been supplying enough heat at this 
juncture to maintain the temperature you have the thermostat set to operate 
and periodically the indoor heat will have been running to keep the house 
warm.

The problem here is that most thermostats will control the auxiliary 
heat to shut off three degrees below the setpoint (where you are running the 
heat pump). When you switch to EMER. HT, most thermostats will switch control 
of the auxiliary heat to the primary control and will maintain the temperature 
you set it to hold. Now you have comfort again.  Isn't this the objective?

In any event, if you continually get icing of the coils, then just replace 
the defrost mechanism. There is little or no repair possible on the devices.

TIP: The outside unit will probably continue to ice more and precaution 
should be taken before you turn back to the heat pump mode as the weather 
warms again.  You will have to manually defrost the outside unit.  Be 
especially careful not to energize the unit in heat pump mode with ice 
in and on the fan blades.

Newer units have printed circuit boards (with relays onboard) and remote 
thermistor sensors. Thermistors sense temperature very accurately and will 
send resistive circuits back to the microprocessors for interpreting. Some 
manufacturers will use sensors with tiny contact points that will open or 
close when a thermal disc senses the ice build-up. Many of the PCBs have 
the sensor wires and sensor as part of the board.

You will most likely have to replace this printed circuit board and sensors 
if you continually have icing.

4-way valves sometimes will stick in one position 
or the other and will not respond for energizing demand or the release of 
demand.  Check to see if the heating mode requires the 4-way valve coil to 
energize or to be de-energized.   You may have to give the model and make 
to a dealer and ask which is the usage.  If it isn't reversing when it needs 
to, then it will not defrost and the unit will ice badly.

 			4way valve

Replacing the valve will require the services of a professional with all the 
fancy equipment for reclaiming the refrigerant and a very skilled use of an 
oxygen/acetylene torch.  Don't try it yourself! Don't even think about it.  
It's a bear.
LESS LIKELY CAUSES:
Low refrigerant can sometimes cause the coils 
to ice and this is one concern. Measure the line temperature of the hot gas 
line as it comes out of the unit and it should be approximately 100°F 
(38°C) above the ambient (outdoor) temperature when correctly charged 
and running in the heat pump mode.

Dirty outside coils, which in the winter time 
have become the cooling coils, will not allow enough conductivity of heat 
transfer and the coils will ice.  Just wash the coils until nothing else 
will come through them but clear water.  Be careful not to bend the fins.  
Of course, this will have to be done at temperatures above freezing.
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