Servicing Diesel Aqua-Hot Systems

Hydronic heating is an option or a standard feature in an increasing number of diesel-powered motorhomes. It’s popular among many coach owners because it provides clean, quiet, evenly distributed heat and unlimited, on-demand hot water. Hydronic heating systems transfer heat by circulating fluid through a closed system of pipes.

Most of these units are made by Aqua-Hot Heating Systems Inc., an FMCA commercial member (C2723). The company offers propane-powered and diesel-powered models. Like any complex system, diesel-powered hydronic systems require regular maintenance and service. We’ll describe that service in this article. But first, we’ll briefly explain how these systems operate.

Traditional heating systems rely on one or two propane furnaces to warm a coach’s interior, with a separate propane/electric water heater supplying the domestic hot water. A hydronic heating system replaces these items with one common unit that handles both functions.

The heart of the system is a boiler that contains propylene glycol boiler antifreeze. The antifreeze is heated in the boiler and circulated through various heat exchangers throughout the coach. A fresh-water coil passes through the boiler to provide domestic hot water as needed. Most units, such as the Aqua-Hot 450D, are designed to fit into the same space as a traditional water heater, so motorhome manufacturers can include hydronic heating as an option without requiring major changes to the coach design.

The boiler itself can be heated by a number of sources. When minimal heating is required, the system’s 1,650-watt electric heating element uses 120-volt campground power to provide approximately 5,600 Btu of heat. But when dealing with especially cold weather, or when continuous hot water is desired, you’ll want to fire up the diesel burner. Depending on the model, the diesel burner produces between 50,000 and 65,000 Btu — plenty of heat to handle the demands of the Aqua-Hot system.

A “motor aide” feature also offers some free heat while the motorhome is being driven. A loop from the engine’s hot coolant circulates through the boiler, using the engine’s water pump to transfer up to 12,000 Btu of engine heat to the boiler antifreeze.

That same engine coolant loop can be used in reverse when the motorhome is parked. An engine preheat switch activates a pump that circulates the engine coolant through the boiler when the engine is not running. That allows the heat from the Aqua-Hot boiler to warm up and preheat the engine for those cold-morning starts.

Hydronic heating offers a number of benefits, none more popular than continuous, uninterrupted hot water. As long as the diesel burner is switched on, water in the shower can flow all day and never run cold. Because it’s an on-demand system, there is no tank to preheat. Water is heated as needed.

Another plus is multiple-zone heating. The Aqua-Hot system is capable of five separate heating zones throughout an RV, although rarely are they all used. Heating zones for the front and rear of the motorhome are each controlled by their own thermostat. Another zone, linked to a thermostat in the water tank area, provides basement heat to prevent freeze-ups. Some manufacturers, such as Entegra Coach, also add a zone for heated tile floors.

For those who like to camp in cooler weather, hydronic heating offers efficiency and economic benefits. A 50,000-Btu diesel boiler consumes 0.35 gallon per hour. No. 2 diesel fuel has a rating of 139,200 Btu per gallon. Propane has a rating of 91,500 Btu per gallon, so it’s only 66 percent as efficient as diesel fuel. That means more propane must be burned to achieve the same amount of heat output as a diesel burner.

Let’s assume that it’s really cold and the Aqua-Hot system needs 12 hours of boiler run time throughout the day. The system will consume four gallons of diesel fuel to deliver 556,800 Btu for that day. To meet that same 556,800 daily Btu requirement, a pair of propane furnaces will burn six gallons of propane. The price of fuel often fluctuates, but four gallons of diesel fuel is cheaper than six gallons of propane. Also, using this same scenario, consider that a coach with a 150-gallon diesel fuel tank has a maximum run time, theoretically, of 37.5 days, while a 32-gallon propane tank filled to 80 percent will be empty after 4.3 days. So, the length of stay is seriously limited with propane.

Like any mechanism, a hydronic heating system eventually will require service. Almost all such systems in RVs are virtually identical, so we’ll cover how to keep the typical Aqua-Hot system running at peak efficiency. Service manuals for each model can be found on the Aqua-Hot website; however, the Webasto burners used in these models are basically the same, so the service techniques are practically identical for all units. Once you understand the basics, refer to the service manual for a particular model.

Aqua-Hot recommends an annual service schedule, regardless of how much the system is used throughout the year. Fortunately, servicing the burner isn’t all that difficult once you know the basic steps. If the burner has no major issue and you are fairly handy with tools, you may not need to enlist a trained professional. And that will save you money.

The typical service consists of replacing the fuel filter, replacing the burner nozzle, and closely inspecting the burner. Combustion chambers tend to get sooty over time, so this is the time to clean them as well. If the Aqua-Hot is running well, changing the fuel filter and replacing the burner nozzle may be the only necessary steps.

The fuel filter’s lifespan will depend greatly on whether dirty or clean fuel was used, but an annual replacement is a good starting point. The burner nozzle is a finely machined orifice that sprays fuel at a 60-degree angle in a cone-shaped pattern, at a rate of 0.35 gallon per hour. As fuel passes through the nozzle, this hole erodes over time. The more fuel that passes through it, the greater the erosion. When the hole gets bigger, the fuel flow rate increases. The result is a burner that runs rich, develops an unburned fuel odor, and probably smokes. Aqua-Hot recommends changing the nozzle annually. If the system runs extensively, this service may be needed more often than once a year.

When it’s time to service the burner, a few basic tools will be needed. And, before turning any wrenches, have a new fuel filter and a replacement nozzle on hand.

The 0.35-gallon-per-hour burner nozzle is small compared to nozzles used in home heating applications, so that item won’t be found at local heating supply houses or home improvement stores. It might be available from a local RV dealer who performs service work on hydronic heating systems. Or, it can be ordered directly from Aqua-Hot Heating Systems or from an Aqua-Hot servicing dealer.

A spin-on fuel filter from General Filters is the most popular choice. It is available from Aqua-Hot or from an RV dealer, but I was able to find a comparable Wix filter (part number 33109) that was available locally. Depending on the filter model, either a No. 2 Phillips screwdriver or a 7/16-inch wrench will be needed to remove the cover from the unit. In addition, you’ll need a 3/8-inch drive ratchet with a long extension and a 10-mm deep-well socket to loosen the eyebolt nuts that hold the burner assembly to the unit; a 5/8-inch wrench to remove the nozzle; a 3/4-inch wrench to hold the nozzle holder; and an oil filter wrench to remove the spin-on fuel filter. These tools are the bare minimum.

A voltmeter and fuel pressure gauge may be needed if more work or testing is done. (Webasto recommends a fuel-pressure test annually.) A brush will be needed to clean out the burner if it’s sooty, and a vacuum or air hose to blow out the loose soot or vacuum it up. A copy of the service manual is an indispensable reference when checking tolerances and voltages.

The first task is to replace the fuel filter. As noted, most systems use the General spin-on fuel filter, which can be removed easily using a standard oil filter wrench. The filter housing threads have a square gasket or O-ring seal. A new gasket or O-ring probably came with the new filter, so replace it. If there is no new O-ring, leave the old one on; usually, there is nothing wrong with it. When installing a new O-ring, be sure to remove the old one; otherwise, the rings will stack up and the filter will not tighten properly. Before spinning on the new filter, rub a little fuel oil on the gasket to prevent it from pinching and binding when the filter is installed. Fill the new filter with clean diesel fuel to keep air out of the line. Use locking pliers to pinch off the rubber fuel line to eliminate excessive leakage while changing the filter.

Once the filter has been changed, air must be purged from the fuel system. Before we delve into that, though, it may be helpful to understand how the diesel burner cycles through its steps.

When the diesel burner is switched on, the burner motor runs for approximately 15 to 20 seconds before ignition begins. This is a purge cycle, designed to blow cool air through the combustion chamber so that hot spots are cooled, unburnt fuel is purged, and the fuel pump has enough pressure for the system. After the initial purge time has passed, the burner’s electronic controller module allows the fuel solenoid valve to open, and fuel flows through the nozzle. At the same time, the transformer energizes, sending high voltage to the electrodes, creating a spark, and igniting the fuel. The electrodes stop sparking once the photocell determines that the burner has been lit. When it’s time to stop heating the boiler, the fuel solenoid closes and the burner flame stops. The blower motor continues to purge air through the combustion chamber for another minute or two in order to cool down the unit.

It’s important to note that the electronic controller handles timing and switching of the various components within the burn cycle. The fuel pump, however, is constantly turning and pumping fuel from the fuel tank supply line and back to the tank via the return line whenever the blower is running. The spraying and atomizing of fuel through the nozzle doesn’t happen until the fuel solenoid valve is open.

To purge air from the fuel system, the fuel pump must be run for a while. To do this, switch on the diesel burner. Allow it to run for 5 to 10 seconds to initiate the pre-burn purge cycle, and then switch it off. The burner will enter the post-burn-purge-and-cool-down cycle and will continue to run for a minute or two. During this time, the fuel pump will pump fuel and recirculate it to the fuel tank via the return line. If you listen carefully, you will hear a change in the sound of the motor as the rpm drop once fuel finally gets to the pump. This cycle may have to be repeated a second time if all the air has not been purged from the fuel lines. All air must be purged from the lines in order to test the burner’s operation or to check the fuel pump pressure.

Once the filter has been changed and the air purged from the system, you can tackle the burner itself.

To take out the burner, start by removing the fasteners from the Aqua-Hot cover, lifting off the cover, and setting it aside. Fuel lines then can be seen entering the unit at the top right. One is a supply line that comes from the fuel filter, which is connected to the fuel tank via a dedicated fuel pickup tube within the tank. The second line returns unused fuel to the fuel tank.

The large black item is the Webasto burner. This assembly contains the burner motor, ignition transformer, fuel pump, blower fan, fuel shutoff solenoid, nozzle, electrodes, and photocell sensor. A controller box, attached to the left side of the burner, controls the burner motor, fuel solenoid, ignition transformer, and photocell operations. The controller performs all of the timing and switching operations needed during the purge and burn cycles. The two larger round items at the bottom left are the circulating pumps for the boiler antifreeze; the square electrical box above them covers the connections to the 120-volt electric heating element.

The burner assembly must be removed from the main unit in order to be serviced. Begin by sliding the control box off of the burner housing and unplugging it. Using a 10-mm deep-well socket with some long extensions, loosen the two nuts that hold the burner to the boiler housing. The nuts do not need to be removed; just back them off to the end of the threads and swing the eyebolts out of the way so that the burner assembly slides out of the boiler. If the fuel lines are rubber, they may not have to be removed during this process. Pinch off the fuel lines on the outside of the Aqua-Hot unit to prevent fuel from leaking while performing this service. Steel lines will have to be removed; so, keep a wad of paper towels handy to catch any fuel that dribbles out of the exposed lines. When reinstalling the burner, be careful not to overtighten the nuts holding the burner in place; 20 to 40 inch-pounds is recommended.

The nozzle, which is the most critical item on any oil-fired burner, is the number one service point on the Aqua-Hot system. The nozzle is finely machined to spray a regulated amount of fuel in a predetermined spray pattern. Even the smallest spec of dirt that enters the nozzle can plug the vanes or orifice and cause it to spray erratically. So, it is critical to deliver clean fuel to the burner nozzle. It’s just as important to use clean practices when servicing the burner and handling the nozzle. Do not touch the tip of the nozzle, drop it, or allow dirt to get on the inlet screening.

To replace the nozzle, place a 3/4-inch wrench on the nozzle holder to prevent it from turning. Then, place a 5/8-inch wrench on the nozzle and unscrew it in a counterclockwise direction. Carefully insert the new nozzle and tighten it using the two wrenches. Once it is tight, back the nozzle off one-quarter of a turn. Then, retighten it. This reseating of the nozzle is necessary to ensure that the threads properly seal and that no fuel will leak past the nozzle threads.

The electrodes should be properly gapped and located. If the gap is too large, the spark may not jump across it, and the burner will fail to ignite. If the gap is too small, the spark may not be bright enough to ignite the fuel. Also, the gap should be placed just outside the fuel spray cone, so the electrodes must be properly located in relationship to the nozzle. Fortunately, an electrode gapping gauge is supplied with the main unit. Usually it is attached to the side of the burner and can be accessed by removing the screw that holds it to the motor housing. A missing gauge can be replaced by ordering online via the Aqua-Hot website.

To adjust the electrodes, slip the tool over the nozzle. The tips of the electrodes should rest flush in the notches that are cut into the electrode gapping gauge. If the electrodes do not line up, loosen the electrode retaining clamp bolt and move the electrodes to the correct position. Overtightening the bolt may cause the clamping bar to bend or the electrode’s ceramic insulator to crack. If that happens, the electrodes must be replaced. Normally, the electrodes will not have to be adjusted unless the unit has accumulated enough running hours that the tips have eroded away as a result of the arcing. (Aqua-Hot, however, recommends adjusting the electrodes every time a service is performed, since they will move while changing the nozzle.) Certainly when the electrode tips appear to have widened the gap beyond 1/8 inch, it is time to adjust the electrodes. If the gap is extreme, they might have to be replaced.

A combustion chamber check is the final step in servicing the burner. The combustion chamber insert consists of an aluminum swirler and a steel tube. The insert slides into the boiler cavity and is easily removed by hand once the burner motor has been removed. After the combustion chamber insert has been removed, use a wire brush and shop vacuum to eliminate any soot buildup from the boiler cavity and restore the burner to peak efficiency. On newer models, the combustion chamber is bolted in and does not need to be removed; if it is removed, a new chamber gasket is required.

A burner that is running well will need only a filter and nozzle change and a good cleanup. But if a burner isn’t performing as it should, even after the above steps, further testing and adjustment may be required. This can become very complex, and unless you want to remove the unit for bench testing, it’s best to have it serviced by a professional. To determine whether that’s necessary, three items easily can be checked: fuel pressure, voltage, and air intake.

Fuel pressure can be checked when servicing the burner. A fuel pressure gauge, available through the Aqua-Hot website, is needed. It’s a bit pricey, so I chose to make my own gauge. I drilled out an old burner nozzle, tapped for 1/8-inch pipe threads, and inserted a 1/8-inch pipe and air pressure gauge with a range of 0-200 psi. It works just as well as the manufactured gauge and saved me quite a bit of money. Even if you do purchase one, it costs far less than paying a service tech and is a good investment if you perform your own service work.

To test the fuel pressure, remove the nozzle and replace it with the gauge. Place the burner assembly on its side so that you can see the gauge without handling the unit. When the fuel solenoid opens, the ignition will create a high-voltage spark across the electrodes, so you don’t want to be handling the unit when that happens. Then, switch on the diesel burner and watch the gauge. Newer models have steel fuel lines and require an external fuel source to perform this fuel-pressure check.

When the burner first runs, it will begin a purge cycle as the blower fan pushes air through the combustion chamber. After about 15 to 30 seconds, the fuel solenoid will open, allowing fuel to flow to the nozzle. Once this happens, the needle on the gauge suddenly will move to the 145-psi position. If the gauge shows 145 psi, the pressure is correct, and the burner can be switched off. It will then enter another cool-down cycle with the blower fan continuing to run and finally coming to a stop.

If the pressure is not up to specs, it can be adjusted by turning a small screw on the side of the fuel pump. Only very slight adjustments are needed to bring it to 145 psi, so take it easy. This must be done while the burner is running, so remove the high-voltage wires from the electrodes or ignition coil in order to prevent voltage from jumping around and stinging you. If the pressure does not come to specification, or gradually drops when the burner is running, other issues must be dealt with. It might be that air is being sucked into the fuel lines, the fuel pump strainer screen is bad, the motorhome has less than one-fourth tank of fuel, or the fuel pump is defective. Regardless, it’s time for a higher level of service. Refer to the troubleshooting section of the service manual or have a professional handle the problem.

The speed of Aqua-Hot’s diesel burner motor hinges on having enough voltage to obtain its rated rpm. If the voltage is too low, the burner motor will run slowly, and airflow through the chamber will be insufficient to provide clean burning. The Aqua-Hot requires voltage between 11.5 and 14.0 volts DC. Check for proper voltage using the leads from a voltmeter at the hot terminal on the controller board.

Any clean-burning flame requires the proper ratio of air to fuel; the Aqua-Hot diesel burner is no different. The air intake tube attaches to the base of the burner, which includes a baffle that can be rotated to increase or decrease the size of the air intake port. The baffle is held in position by a small Phillips-head locking screw. When fully open, the port is one-half of a circle, or 180 degrees. Aqua-Hot systems are generally preset at the factory to a 90-degree opening, which is the middle of the adjustment. Sometimes this can vary, and you may find a red stripe that marks the initial opening position of the baffle adjusting screw. Opening this farther allows more air; reducing the aperture means less air. To some extent, you can smell the difference when the burner is running, but a proper adjustment of this opening requires some sophisticated exhaust gas sampling equipment, which is covered in the service manual but is beyond the scope of this article. In most cases, this setting is preset, and if you leave it alone, it should be fine.

Properly maintaining an Aqua-Hot heating system will provide many hours of clean-burning operation and maximum heating efficiency with minimal fuel consumption.

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