Learn the basics of taking care of your motorhome’s Aqua-Hot hydronic heating system.
By Mark Quasius, F333630
October 2012
Hydronic heating systems are a highly desired feature in diesel-powered Type A motorhomes. If your coach has hydronic heating, odds are that it is equipped with an Aqua-Hot heating system. A number of models within the Aqua-Hot brand and earlier units were known by the Hydro-Hot model name. However, all of these are nearly the same. When a heating system is referred to as an Aqua-Hot in this article, this includes models badged as Hydro-Hot as well.
Hydronic Heating Overview
A traditional non-hydronic heating system relies on one or two propane furnaces to supply the motorhome’s interior heating needs, plus a separate propane/electric water heater to provide domestic hot water. An Aqua-Hot system eliminates the need for multiple appliances, replacing them with one common unit that meets both heating and hot water needs. It does this by utilizing a boiler that contains propylene glycol antifreeze. This antifreeze is heated in the boiler and circulated through a number of heat exchangers, which are located throughout the motorhome to provide clean and quiet heat. A fresh water coil loops through the boiler to provide domestic hot water as needed. The boiler is designed to fit into the same space as a traditional water heater, so coach manufacturers easily can incorporate hydronic heating as an option without making major structural changes to the motorhome design.
The boiler itself can be heated by a number of sources. A diesel burner has an output between 50,000 and 65,000 Btu, depending on the model, and provides plenty of heat to handle the demands of the Aqua-Hot system. For those times when minimal heating is required, an AC electric heating element is also provided. The 1,650-watt electric element supplies only about 5,600 Btu of heat, though, so for really cold-weather use or extensive hot water needs, you’ll want to fire up the diesel burner.
A motoraide feature also offers some free heat while you are driving. A loop from the engine’s hot coolant circulates through the boiler, transferring some of that engine heat to the boiler antifreeze. The motoraide feature offers approximately 12,000 Btu of energy when driving. That same engine coolant plumbing can be used in reverse when you are parked. An engine preheat switch will activate a pump that circulates the engine coolant through the boiler when the engine is not running. That allows heat from the Aqua-Hot boiler to preheat the engine for those cold morning starts.
Hydronic heating offers a number of benefits, but the most popular is the continuous hot water supply. With the diesel burner on, you can shower as long as water is available without having it run cold. Because it’s an on-demand system, there is no tank to preheat. Water is heated as needed.
Multiple-zone heating is another plus. Separate thermostats control the front and rear zones within the motorhome. In addition, a separate zone is provided for basement heat to prevent any freeze-ups below. A dedicated thermostat in the water tank area monitors the temperature there. If your motorhome is equipped with heated tile floors, an additional zone can be configured to monitor that system.
For those who like to camp in cooler weather, hydronic heating offers another benefit. A diesel boiler consumes 0.35 gallons per hour to produce 50,000 Btu of heat. Number 2 diesel fuel has a Btu rating of 139,200 Btu per gallon. Propane has a Btu rating of 91,500 Btus per gallon, so it’s only 65.7 percent as efficient as diesel fuel, meaning you’ll need to burn more propane to get the same amount of heat output as with a diesel burner.
Let’s assume that it’s cold outside and your Aqua-Hot will need 12 hours of boiler run time throughout the day. In this scenario, it will consume four gallons of diesel fuel to deliver 556,800 Btu for the day. In order to run a pair of propane furnaces to meet that same 556,800 Btu requirement, you would need to burn six gallons of propane. The price of fuel fluctuates, but you can see that four gallons of diesel fuel is going to cost you less than six gallons of propane. Also, consider that a motorhome with a 150-gallon diesel fuel tank will give you a theoretical maximum run time of 37.5 days, although no one would ever run it that low. A motorhome with a 32-gallon propane tank filled to 80 percent will be empty after 4.3 days, so the length of your stay is seriously limited with propane.
Maintaining The System
Like any mechanical system, an Aqua-Hot heater requires some service. The majority of the company’s RV systems are virtually identical, so we’ll cover what’s involved with maintaining the typical Aqua-Hot system to keep it running at peak efficiency. Service manuals for your particular model can be found on the Aqua-Hot Web site (www.aquahot.com). The Webasto burners used in these models are basically the same, so the service techniques will be nearly identical for all units.
Aqua-Hot recommends an annual service schedule, although this can vary depending on how much your Aqua-Hot system is used throughout the year. Servicing the burner isn’t difficult once you know the basic steps. It is time-consuming, so having this work performed by a service professional will cost you some money, mostly for labor, not for the service parts used.
The typical service involves replacing the fuel filter, replacing the burner nozzle, and inspecting the burner. Combustion chambers tend to become sooty over time, so this is the best time to clean them. If your Aqua-Hot unit is running well, all you’ll probably need to do is change the fuel filter and replace the burner nozzle. While it is recommended that the burner be serviced annually, many service techs agree that you should leave it alone if the unit is running fine. The annual recommendation is based upon average use. After so many gallons of diesel fuel have run through the fuel filter, it will need to be replaced, but this will depend on the cleanliness of the incoming fuel supply. The burner nozzle is a finely machined orifice designed to spray fuel at a 60-degree cone and at a flow rate of 0.35 gallons per hour. As fuel passes through the nozzle, this hole will erode over time. The more fuel that passes through it, the greater the orifice will erode. As the opening becomes larger, the fuel flow rate will increase and result in a burner that runs rich, develops an unburned fuel odor, and probably will begin to smoke. If you don’t run your heating system often, there may be no need to change the nozzle on an annual basis. Conversely, if you use the system extensively, this service may be needed more than once a year.
Getting Started
Once you determine that the diesel burner needs to be serviced, following are the parts you will need. A fuel filter and a replacement burner nozzle should be on hand prior to turning any wrenches. The 0.35 burner nozzle is small compared to nozzles used in home heating applications, so you probably won’t be able to find this item at your local heating supply house or home improvement store. You may find it at a local RV dealer who performs service work on hydronic heating systems, or you can order it directly from Aqua-Hot or a number of online Aqua-Hot dealers. The most popular fuel filter is a Garber spin-on type. It is available from Aqua-Hot and RV dealers.
You also will need some tools: a #2 Phillips-head screwdriver to remove the cover from the unit; a 3/8-inch drive ratchet with a 10-inch-long extension (or a 6-inch and a 4-inch extension joined together); a 10-mm deep-well socket to remove 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 secure the nozzle holder; and an oil filter wrench to remove the spin-on fuel filter. Various other tools may be needed, but those are the bare minimum. If you want to do more work or testing, you may need a voltmeter, a fuel pressure gauge, etc. A brush also will be needed to clean out the burner if it’s dirty, along with a vacuum or air hose to suck up or blow out the loose soot. Naturally, a copy of the service manual is indispensable for reference when checking tolerances and voltages.
Changing The Fuel Filter
The first thing to do is to replace the fuel filter. As noted, most systems use the Garber spin-on fuel filter, which can be removed easily with a standard oil filter wrench. There is a square gasket or O-ring seal on the filter housing threads. A new gasket should be included with the new filter, so replace it. If a new O-ring is not included, leave the old one on. Chances are there’s nothing wrong with it. Be careful not to add a new O-ring without first removing the old one, as they will stack up and your filter will not tighten properly. Prior to spinning on the new filter, rub a little fuel oil on the gasket to prevent it from pinching and binding when you install the filter. You can use a pair of soft-jaw or special pinch-off vise-grip pliers to pinch off the rubber fuel line to eliminate any drainage when the filter is being changed, or you can add a petcock valve.
Purging Air From The Fuel System
When you switch on the diesel burner, the burner motor will run for approximately 15 to 30 seconds before ignition begins. This is a purge cycle designed to blow cool air through the combustion chamber so that any hot spots can be cooled and any unburnt fuel can be purged. After the initial purge time has passed, the burner’s electronic controller module will allow the fuel solenoid valve to open, and fuel will flow through the nozzle. At the same time, the transformer will energize, sending a high-voltage current to the electrodes, which create a spark to ignite the fuel. The electrodes will 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 will continue to purge air through the combustion chamber for another minute or two in order to cool down the unit.
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It’s important to note that the electronic controller is responsible for timing and switching on and off the various components within the burn cycle, but the fuel pump is constantly pumping fuel whenever the blower is running. The only difference is that the fuel is not atomized through the nozzle until the fuel solenoid valve is open.
Now that the filter has been changed, you will need to purge the air from the fuel system. In order to do this, you just need to run the pump for a while. Switch on the diesel burner, allow it to run for five seconds to initiate the preburn purge cycle, and then switch it off. The burner will enter the postburn 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 send it back to the fuel tank via the return line. If you listen carefully, you will hear the motor sound change as the revolutions per minute (rpm) drop once the fuel finally reaches the pump. You may have to repeat this cycle a second time if the air is not purged from the lines. All of the air must be purged from the fuel lines if you want to test the burner’s operation or check the fuel pump pressure. Once the filter has been changed and the air purged from the system, you can move on and tackle the burner itself.
Removing The Burner
The burner access cover is held in place by four Phillips-head screws. Remove the screws, lift the cover off the unit, and set it aside. You will see the fuel lines 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 is a return line that sends unused fuel back to the fuel tank. The Webasto burner is a two-pipe system that constantly pumps fuel from the inlet to the return line and back to the tank. This is necessary to ensure that any air left in the system after the tank has run dry or the fuel filter has been changed is removed.
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 silver controller box is attached to the left side of the burner. It controls the burner motor, fuel solenoid, ignition transformer, and photocell operations. It 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 4-inch-square electrical box with the yellow warning label covers the 120-volt-AC electric heating element. Also, note the hose clamp around the rubber hose at the bottom of the burner. This rubber air intake hose extends through a hole in the floor. It is designed to feed outside air to the burner for combustion purposes. It has a reputation for collapsing during operation, which restricts the intake air to the burner and makes it run rich. It can be removed with no ill effects, but doing so means the electrical wiring and components in your housing are no longer protected from rodent intrusion. Many owners remove the hose and then attach a piece of 1/4-inch wire mesh screen over the hole to prevent this. In my case, I just put some mesh inside the intake hose to prevent it from collapsing.
The burner assembly needs to be removed from the unit in order to service it. First, slide the silver control box off the burner housing and unplug it. Next, remove the hose clamp that secures the air intake hose to the bottom of the burner. Using a 10-mm deep-well socket with the extensions, loosen up the two nuts that hold the burner to the boiler housing. You do not need to remove the nuts. Just back them off to the end of the threads and swing the eyebolts out of the way so that you can slide the burner assembly out of the boiler. If your unit has rubber fuel lines, you may not need to remove them when doing this. If your unit has steel fuel lines, you will need to disconnect them, in which case a wad of paper towels is handy to catch any fuel that drips out of the open lines.
Electrode Adjustments
The electrodes need to be properly gapped and located. If the gap is too large, the spark may not jump across the gap, and your burner will fail to ignite. If the gap is too small, the spark may not be bright enough to ignite the fuel. The gap needs to 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 unit. Generally, this tool is attached to the side of the burner and can be used by removing the screw that holds it to the motor housing. If it was removed during the last service and not replaced, you will have to find it or purchase another one. They are available online via the Aqua-Hot Web site.
To adjust the electrodes, slip the gapping tool over the nozzle. The tips of the electrodes should rest in the notches that are cut into the tool. If the electrodes do not line up, loosen the electrode retaining clamp bolt and move them to the correct position. When tightening the bolt, do not overdo it or you may bend the clamping bar or even crack the ceramic insulator on the electrode. If that happens, you will need to replace the electrode. Normally, you will not need to adjust the electrodes unless your unit has accumulated enough running hours that arcing has caused the tips to erode. When these tips appear to have widened the gap beyond 1/8-inch, it is time to adjust the electrodes. If the gap is extreme, you may need to replace them.
Nozzle Replacement
This is the most critical service point on the Aqua-Hot system or any oil-fired burner. The nozzle itself is finely machined to spray a regulated amount of fuel in a predetermined spray pattern. If even the smallest speck of dirt finds its way into the nozzle, it can plug the vanes or orifice and cause it to spray erratically.
It’s critical that clean fuel be delivered to the burner nozzle. It’s just as critical 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 counterclockwise. Carefully insert the new nozzle and tighten it using the two wrenches. Once it is tight, back the nozzle off one-quarter turn. Then, retighten it. This reseating of the nozzle ensures that the threads will properly seal and that no fuel will leak past the nozzle threads.
Cleaning The Combustion Chamber
Cleaning the combustion chamber is the final step in servicing the burner. The combustion chamber consists of an aluminum swirler and a steel tube. This unit slides into the boiler cavity and can be removed by hand easily once the burner motor has been removed. Once the combustion chamber has been removed, use a wire brush and a shop vacuum to eliminate any soot buildup and restore the burner to peak efficiency.
Taking The Extra Step
If the burner is running well, the only service that should be required is to change the filter and nozzle and to clean out the combustion chamber. However, if you’ve been having problems with the burner and it still doesn’t operate properly after servicing it, further testing and adjustment may be necessary. This can become very complex, and unless you want to do some serious work and remove the unit for bench testing, you would be advised to have it serviced by a professional. However, there are three items you can check before deciding whether a technician needs to evaluate the problem. These three areas are fuel pressure, voltage, and air intake.
Fuel pressure is easy to check when you are servicing the burner, but you will need to have a fuel pressure gauge to do this. Aqua-Hot offers a fuel pressure gauge on its Web site, but I chose to save money by making my own gauge. I did this by taking an old nozzle and drilling it out; tapping it for 1/8-inch pipe threads; then inserting a 1/8-inch pipe and air pressure gauge with a 0-to-200-psi range. It works just as well as the “official” gauge. Even if you do need to purchase one, it will be far less expensive than paying a service tech to do this task for you. It’s a good investment if you choose to perform your own service work.
To test the fuel pressure, remove the nozzle and replace it with the gauge. You will need to place the burner assembly on its side in such a position 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. You then switch on the diesel burner and watch the gauge. When the burner first runs, it will begin a purge cycle as the blower fan pushes air through the combustion chamber. After approximately 15 to 30 seconds, the fuel solenoid will open, allowing fuel to flow to the nozzle. When this happens, the needle on the gauge should swing to the 145-psi position. If the gauge shows 145 psi, the pressure is correct and you can switch off the burner. It will then enter another cool-down cycle as the blower fan continues to run before coming to a stop.
If the fuel pressure is not up to specs, a small screw on the side of the fuel pump can be turned to adjust it. Make only small adjustments to the screw to bring it to 145 psi. This needs to be done while the burner is running, so you will need to remove the high-voltage wires from the electrodes or transformer in order to prevent voltage from jumping around and giving you a shock. If the pressure does not come into tolerance or drops while the burner is running, you likely have other issues that need to be dealt with. The unit may be sucking air into the fuel lines, may have a faulty fuel pump strainer screen, or the fuel pump may be bad. If you cannot get the pressure right, you will need to refer to the service manual or have the problem dealt with by a professional.
It’s critical that your Aqua-Hot system has adequate voltage present. The speed of the burner motor depends on having enough voltage to obtain its rated rpm. If the voltage is too low, the burner motor will run slow, and it won’t get enough airflow through the chamber to provide clean burning. Also, the fuel pump may not develop full pressure because of lower rpm. To check for proper voltage, take the leads from a voltmeter and test for adequate voltage at the hot terminal on the controller board. The Aqua-Hot requires between 11.5 and 14.0 volts DC.
Any clean-burning flame requires the proper ratio of air to fuel, and 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 will be one-half of a circle — in other words, 180 degrees. The systems generally are 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 position of the baffle-adjusting screw for the initial opening position. If you open this up farther, the unit gets more air; reduce the aperture, and it will get less air. You may be able to smell the difference when the burner is running, but in order to properly adjust this opening, you will need some sophisticated exhaust gas-sampling equipment, which is covered in the service manual but is beyond the scope of this tutorial. In most cases, the preset should be fine.
Properly maintaining your Aqua-Hot heating system helps to ensure that it will provide many hours of clean-burning operation, maximize the heating efficiency inside your motorhome, and minimize the amount of diesel fuel consumed.
Aqua-Hot, 15549 E. Highway 52, Fort Lupton, CO 80621; (800) 685-4298, (303) 659-8221; www.aquahot.com.