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4.16 Reversing Valve Heat Pump
The reversing valve is the heart of a heat pump. It chang-
es the direction of the refrigerant flow through the coils,
and changes the system from cooling to heating. Duo-
Therm roof top heat pumps have the solenoid energized
in the cool mode. One method of checking the reversing
valve is to feel the refrigerant line at the top of the inside
coil. In the COOL mode, this line will be cool to the touch.
In the heat mode the line will be warm or hot to the touch.
If you do not feel a cold line in the cooling mode, the di-
rection of flow is not correct. Check the solenoid coil for
ohms continuity. An open circuit (no continuity) shows the
solenoid is defective and must be replaced. The ohms
resistance of the reversing valve solenoid coil is approxi-
mately 465 ohms (Coil With Two Pin Connector) or 265
ohms (Coil With Wires Attached) plus or minus (±) 0%.
If ohm reading is within this range, the solenoid coil is
good. If an ohm reading is outside this range, replace the
solenoid coil. If the coil test OK and there is voltage at the
reversing valve, but the unit is heating the inside coil, the
reversing valve is stuck.
SECTION 5
Air Flow
5.1 User Maintenance And Operation
The air conditioner can be installed flawlessly, but if the
user does not maintain it properly, freeze-up can still oc-
cur. Simply not cleaning air filters on a regular schedule
can cause a blockage of return air. This will lower the coil
temperature and freeze-up will result. Filters should be
cleaned approximately every one hundred hours or soon-
er. This will depend upon climate, area, pets, etc. Another
way the user can cause freeze-up is by closing registers
to prevent cold air discharge. This will restrict air flow in
the same manor as a dirty filter.
5.2 Frost On Coil
The formation of a light coat of frost is possible on a prop-
erly operating air conditioner, just prior to the cold (freeze)
control shutting off the compressor. This is normal when
the cold control function is correct; however, this may be
an indication that the unit () is installed improperly, (2)
maintained or operated improperly by the user, or (3) has
a mechanical problem. The first two items listed above are
the most frequent causes of frost formation, and they are
NOT covered under the Dometic Warranty policy.
5.3 Recirculation, Obstructions, Restric-
tions
The 14-1/4" x 14-1/4" (±1/8") opening must be framed to
seal off the roof cavity. Holes used to route electrical wir-
ing must be sealed. The 14-1/4" x 14-1/4" (±1/8") opening
is part of the return air duct and must be finished in ac-
cordance with NFPA standard 50C, Standard for Recre-
ational Vehicles, Section 2-7. Reference Bulletin A20-6B
Frame all sides of
14-1/4" x 14-1/4"
(±/8") opening
AC Power Wire
Seal Around Electrical Wiring
The most commonly found installation problem is the im-
proper sealing of the 14-1/4" x 14-1/4" (±1/8") opening in
the roof cavity. The cooled discharge and warm return air
are mixed in the roof cavity and produce conditions that
are excellent for frost production.
26
Roof Ma-
terial
Base Pan
Roof Gasket
Ceiling
Material
3105007 And
3105935 Return
Air Systems
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