Everyone who has driven a vehicle without Air Conditioning on a sweltering Summer day during a thunderstorm knows that even with the vehicle's heat controls in the "off" position, there is still some heat conveyed into the cab by virtue of the fact that hot radiator water is always circulated through the heater core regardless of the position of the cabin heat / air controls. This can be felt as "radiant" heat on the floor of driver and passenger when the windows are rolled (zipped) up during a hot Summer day downpour.
This simple modification might be called "The Summer Valve". Installed in the water pump heater output hose, this valve when closed, prevents hot water from the radiator from being circulated through the cabin heat exchanger. When cold weather returns, the valve can be opened to restore functionality to the heater. The valve can be purchased at Home Depot for about $10.00. You will also need two small hose clamps to fit around the heater hose.
The valve in the stock photos below is shown in the "open" position. To shut off the heat completely, move the valve 90º to the closed position.
NOTE: With the heater core cold, you will lose windshield defrost as well as cabin heat. You may also notice that your coolant temperature gauge reads about 20º lower than normal. This is because more coolant is now flowing through the radiator (instead of the cabin heat exchanger) where the cooling is much more efficient.
To prevent fogging (condensation) on the inside of the windshield, treat the inside of the windshield with Rain-X AF21212 Anti-Fog and to maximize visibility in pouring rain, treat the outside of the windshield with Rain-X Glass Treatment (original).
In order to provide complete cutoff, you should install valves in BOTH
lines - be advised that these hoses are likely to be different diameters
requiring two different size PEX valves. Installing two valves also
gives you the capability to completely isolate the heater core should it
happen to start leaking. If that happens, you will need quite a bit of
time to replace the heater core,
since you must disassemble the dashboard to get to it.
Determine what size PEX valves you need. My '97 Jeep Wrangler required a 1" PEX valve; these are available (from Amazon) in ½", ¾" and 1" diameter. You should also purchase a 4-foot length of heater hose in the appropriate size (available at Pep Boys, Auto Zone, etc.) and replace the hose (or both of them if needed) instead of just tapping into the existing hose with the valve. Install one end of the hose onto the valve, cut the hose to reach the firewall nipple, from where you want the valve to be, then close the valve.
WAIT UNTIL ENGINE COOLS COMPLETELY
Locate the OUTPUT hose on the engine's water pump. This hose is usually connected to the top front of the water pump.
Remember to wash away any coolant spillage when you're done, with a garden hose.
In order to minimize spillage of anti-freeze (ethylene glycol) which is toxic to humans and animals, pinch-off the heater hose at the output of the water pump. Use a heavy piece of plastic (I cut a suitable piece of plastic from the lid of a Chinese Won-Ton soup container) to keep the Vise Grips from damaging the hose. We will be replacing the output hose, and you should consider also replacing the return hose if these hoses have not been replaced in 5 years. If your hoses are fairly new, you can just tap into the hose with the valve at any convenient point.
Removal of the heater hose from the firewall heater core nipple will not be easy unless your vehicle is fairly new. Use an Xacto® Knife knife to cut down the length of the hose about 2 inches, then GENTLY pry the hose off the nipple with a flat-blade screwdriver. DO NOT attempt to twist or pull the hose off; you may bend or crack the nipple. Clean any residual rubber from the nipple with solvent or fine sandpaper in order to avoid leaks when the new hose is installed. Coolant will leak out, so be ready to put the hose on with attached valve in the off position.
With more water flowing through the Jeep's radiator where it gets cooled by incoming air, the engine runs 30 to 40 degrees cooler. This photo was taken when the ambient air temperature was 96 degrees, and the humidity was beyond stifling. I was driving in moderate traffic and the heat was sweltering. I noticed that the engine temperature remained significantly below where it normally is on a cool day.
For purpose of clarity in the photographs, I used colored zip ties for illustration (left to right):
Red tie secures custom wiring for battery connections for
fog and spot lamps.
Yellow ties secure the PEX valve to the engine compartment transverse rod.
Orange tie secures wiring to fog lamps (not visible in the photo)
Light Blue ties secure the
Spectre Air Filter.