Silent Heat

Routing of the air tubes is very specific and is planned with the guidance of Truma engineers for maximum efficiency.

By Bob Livingston
January 3, 2017
Filed under Gear Reviews

 

European supplier Truma is raising the bar for efficient comfort heating in smaller motorhomes with its compact Combi furnace and water heater

Staying warm inside a motorhome and the ability to take hot showers are two features that are inherent to any mobile environment designed to closely emulate the convenience of a stationary home. We’re used to setting a wall thermostat for the furnace and flipping a switch to activate a separate water heater. To warm up the interior, a furnace that occupies a fair amount of space is mounted in a practical location and a network of ducting and registers is routed and connected to distribute heated air. The hot-water tank is usually mounted in another location, and it also takes up its share of real estate. Most owners pay little attention to where these appliances are located because there’s usually plenty of space to go around in a typical motorhome.

Called the Combi, which is apropos to its design and function, the Truma combination furnace and water heater is growing in popularity and showing up in a number of motorhomes manufactured in the U.S.

Called the Combi the Truma combination furnace and water heater is growing in popularity and showing up in a number of motorhomes manufactured in the U.S.

As motorhome profiles shrink, shoehorning appliances in becomes more of an art form, and our RV-builder friends from across the pond have become proficient in space management. Truma, one of the largest suppliers in Europe, has offered a combination furnace and hot-water tank for these builders for eight years and has recently made that technology — and compact packaging — available to U.S. manufacturers who specialize in Class B and small Class C motorhomes. There are 80,000 Combi units put into service annually worldwide, and the track record for reliability is exemplary.

Called the Combi, which is apropos to its design and function, the Truma combination furnace and water heater is growing in popularity and showing up in a number of motorhomes manufactured in the U.S. We’ve spent time in RVs with this system in Europe and have experienced its efficiency; recently we repeated our test in a Ford Transit-based Class C to see how our homeland builders are adapting to this technology.

Truma, a German company based in Munich, offers a number of Combi models, including the Comfort Plus (highest output), which is appropriate for motorhomes in the 24-foot range and is pretty much limited to RVs up to 30 feet. It’s rated at 20,400 Btu, which provides good coverage in motorhomes similar to the test unit we lived in to further evaluate the Combi. Outside temperatures were in the low 40s, giving us parameters that were consistent with typical usage of motorhomes in colder weather.

Nuts and Bolts

The Combi is about the same size as a standard 6-gallon water heater found in the majority of RVs, and weighs only 37 pounds. It uses a combustion chamber and fan to produce and distribute heated air and a heat exchanger to make hot water; the two segments can be operated on LP-gas, 120-volt AC power or both. Inside the well-designed case are precision-placed components controlled by sophisticated electronics that also provide onboard diagnostics when hooked up to a computer with the company’s service software. All the components are manufactured to very tight tolerances, must pass rigorous testing and are individually coded so they can be tracked in the event the furnace requires repair. The fit and finish are excellent, and technicians will not have to use Kevlar gloves to prevent cuts to their hands when servicing the components.

 

 

Four outlets in the housing serve the network of ducting, which must be routed to Truma’s specifications. The company plays an active role in designing the integration of the system so that efficiency is maintained. Truma claims that the Combi is 90 percent efficient, which allows its furnace to effectively perform like a 30,000-Btu standard-type unit commonly used in RVs.

The Combi is controlled by a microprocessor that adjusts the flame and fan speed. This eliminates the big swings in temperature experienced by most forced-air furnaces used in RVs. Fan speed is infinitely variable and, combined with a three-stage burner (Comfort Plus model) and temperature monitoring inside the unit, the Combi is able to provide precise heat that maintains a constant temperature inside the rig. And the unit is exceptionally quiet, so it can be mounted virtually anywhere without the fear of annoying residents who are trying to sleep or watch TV.

Installation flexibility is supported by a tube-in-tube exhaust and combustion air intake that can be used up to 6½ feet from the furnace. This configuration eliminates the need to mount the furnace against an outside wall, which gives the designers many options for locating the Combi. The unit we recently tested was mounted under a dinette seat that shared the structure with the galley.

Combi furnace/water heater is lightweight and occupies about the same space as a standard 6-gallon RV hot-water tank. Air-supply tubes (ducting) are affixed to four outlets in the housing.

Combi furnace/water heater is lightweight and occupies about the same space as a standard 6-gallon RV hot-water tank. Air-supply tubes (ducting) are affixed to four outlets in the housing.

Wall-mounted registers are placed in strategic locations throughout the motorhome and can be rotated to adjust the direction of the heated airflow.

Wall-mounted registers are placed in strategic locations throughout the motorhome and can be rotated to adjust the direction of the heated airflow.

Tube-in-tube exhaust and combustion air-intake system allows the Combi to be mounted in locations up to 6½ feet from an outside wall. The wall cowl is unobtrusive and requires little space.

Tube-in-tube exhaust and combustion air-intake system allows the Combi to be mounted in locations up to 6½ feet from an outside wall. The wall cowl is unobtrusive and requires little space.

Cold- and hot-water lines are routed in similar fashion to standard-type installations, and a drain line is provided for winterizing. The hot-water operation is virtually maintenance-free and uses no anode. Hot-water capacity is 2.64 gallons, which is obviously a limiting factor when showering, but the recovery process can be accelerated by operating the unit in the boost mode (more on that later).

The Combi modulates the use of power and its functions based on demand. For example, if the control is set on boost to speed up the water-heating process, the furnace portion will temporarily shut down and concentrate all the energy to making hot water. It takes around 20 minutes to reach 140 degrees Fahrenheit from 59 degrees F when in the hot mode. When the control is placed in the boost mode, the temperature of the water will reach a point higher than 144 degrees F. This decreases the amount of hot water needed to mix with the cold and extends showering time.

Setting the heating and hot-water options is done by a very sophisticated digital control panel called the CP plus. It’s a smart-looking panel with a single control knob that is turned and pushed to make adjustments. Although there are numerous options, the process to input desired settings is user-friendly, relying on icons to program the various functions. It allows the user to set temperature changes based on time of day and hours of operation, which is a versatile tool for those who want to maintain certain temperatures while away from the motorhome.

The CP plus will also be capable in the near future of being programmed via a mobile device tied to Truma’s iNet system with software that can be loaded as an app. The iNet system is currently available in Europe.

Living With the Combi

We were able to confirm that the Combi was indeed as quiet as advertised by Truma after spending a number of nights in the Class C motorhome; the motor and combustion-chamber noise are barely perceptible. We enjoyed the fact that “what you see is what you get” on the readout allowed us to set the temperature inside the motorhome at comfortable levels, without having to fiddle around with various settings to keep interior temperatures from fluctuating. At night we found that the desired temperature was maintained, and we didn’t wake in the middle of the night or first thing in the morning because of drastic shifts in temperature and have to jump out of bed to adjust the thermostat.

Much of the even distribution of heated air can be attributed to Truma’s aforementioned participation in the installation process. When a motorhome manufacturer decides to use the Combi, Truma experts work closely with the manufacturer engineers in establishing the most effective routing for the air tubes (ducting) and registers, and the result is telling.

The CP plus digital control panel uses icons to guide the user through the functions. Beyond the basics of setting temperature, the controls allow programming of time and duration for the furnace. Mobile-device operation will be available in the near future.

The CP plus digital control panel uses icons to guide the user through the functions. Beyond the basics of setting temperature, the controls allow programming of time and duration for the furnace. Mobile-device operation will be available in the near future.

Taking a shower takes some adjustments, especially if you’re coming from an RV with the standard hot-water tank or an on-demand system. It’s best to select the boost setting for water heating, and to wait until the water heating-cycle icon stops flashing before showering, to ensure there’s maximum hot water. We were able to mix the hot and cold water and use the shower-head-wand shut-off valve judiciously to get a full shower, which is still not very long. When we allowed the water to run, hot water ran out well before we were done showering. Back-to-back showers are not possible, but the wait time is only around 20 minutes to get full hot water again. Obviously, users of this system will have to modify their showering habits, but in reality it’s easy to acclimate to the routine.

The Truma Combi is a jewel of a system that makes it possible to enjoy high-end features in more compact motorhomes. There’s little doubt that manufacturers will jump on the bandwagon to bring this kind of luxury to motorhomes that are small in square footage but big on livability.

Truma | www.truma.com


 

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Comments

One Response to “Silent Heat”

  1. Chris Klink on February 16th, 2017 2:20 pm

    looks like a great idea wish I had that on my RV.

    [Reply]

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