Frosty, Toasty or Roasty?

| April 07, 2005

This close-up of a heater core shows the thin metal tubes that carry water through it. These tubes can easily clog with debris or scale deposits if the cooling system is not maintained. Also visible are the closely spaced fins that carry heat to the air circulating in the cab.

Running down the road in the dead of winter can feel like sitting in a vibrating meat locker. But it doesn’t have to if you keep your heater in condition.

The most important single part of your cab warmer is the heater core. The core has headers at either end that look and work a lot like the tank on top of the radiator. They connect all the tubes together so coolant can be distributed evenly. The coolant enters one of the headers and flows through thin metal tubes. The tubes are linked by thin metal fins that conduct heat quickly, and the result is a whole lot of metal surface.

Keep it Clean
Believe it or not, something as simple as keeping your cab clean can help guarantee good heater performance. Some of the air to the unit is picked up off the floor, so any trash or debris lying there could clog the heater core. Rod Laukhuf, chief engineer in body technology at International, has seen candy wrappers inside systems blocking flows. Ray Brown, an instructor with Kenworth Training, says it’s wise to vacuum up the hair frequently if your pet sheds or it can cause trouble in the blower or the fine passages in the heater core. Even a small blockage can mean inadequate heat during the winter.

The heater core works well when everything is clean, but the thin, flat coolant tubes are a prime target for clogging if the coolant gets dirty because there is so little room for flow. And the closely spaced metal fins exposed to air on the outside can easily clog with dust or debris.

Owner-operator Gordon Bow of Oakfield, N.Y., near snowy and cold Rochester, spends a day each year removing the heater core from his vehicles, including his nearly new Kenworth W900L. He then uses a hose to gently force water both through the water passages and through the fins on the outside to remove accumulated dust and dirt. He follows up with compressed air at a moderate pressure to blow anything that remains off those closely spaced fins.

Another great aid to keeping the heater core clean is the filtration often provided on modern trucks, standard on most Class 8s, according to Laukhuf. Check your owner’s manual – many are easily accessible from the engine compartment. Clean metal or plastic filters in mild soap and water. Replace fiberglass filters. David points out that you can purchase and install special anti-allergy filters, if you want. Clean filters keep dirt off the heater core and out of the ducts. Dirty filters will restrict airflow.

Get it ready
Many operators specify shutoff valves so flow through the heater can be stopped in summer. Open the valves and check for leaks, most common at the seal where the stem passes into the valve body.

Laukhuf says you can have a perfect cooling system yet fail to get the heat you need because of a lazy thermostat. Monitor operating temperature on the dash gauge carefully. If it shifts downward, even though the engine still reaches a stable temperature, is not stable or climbs slowly during warmup, the thermostat or its seal has started to fail. With a drop in coolant temperature of 20 degrees F, you could easily lose 20 percent of heater capacity. Replace lazy thermostats or leaking thermostat seals.

Check belt tension to ensure the water pump turns at normal speed. Slipping belts could result in poor circulation through the heater core on cold days.

The coils of this resistor offer two different levels of resistance to the electricity flowing to the blower motor, giving two speeds below full power. The coils may get hot and burn out, creating an open circuit you can see and eliminating one of the blower speeds.

Once you’ve made all these checks, you can see if you’ve got good flow very easily. With the engine idling and at operating temperature, make sure the heater valves are wide open, the temperature control is on hot, and the heater is operating at a higher blower speed. Don’t try to grab the hoses, or you’ll get burned! Very lightly touch the hoses leading into and out of the core with your fingers for just a split second. If you have good flow, both hoses will be quite hot, with one hose just a tiny bit cooler than the other. If one is hot and the other much colder, you have a flow problem.

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