HydroHot Engine Preheater Installation
Overview
The Allegro Bus offers hydronic heating, which is an option on the 40' models and standard on the 42' models. This unit is the popular HydroHot system, made by Vehicle Systems. The HydroHot comes in two basic models, the HHE-200 and the HHE-500. Both models are identical in the number of heating zones and on demand hot water heating capability. The only difference is that the HHE-200 also offers an engine preheat system that allows the HydroHot's diesel boiler or electric heating element to warm up the engine's coolant system whereas the HHE-500 does not have this feature. In addition, by plumbing the engine's coolant system and looping it through the HydroHot, the engine can provide heat to the boiler when driving down the road. This allows the heat from the engine to provide free heat to the boiler for coach interior heating needs and minimizes the need to run the diesel burner when traveling in cold weather. This feature is called Motoraide, because the motor aids the boiler by heating it.
Both the HHE-500 and HHE-200 have ports for Motoraide heating. It's up to the coach builder to decide whether or not to plumb the engine to these ports if using the HHE-500, which does not have the engine preheat feature. Tiffin Motorhomes does use the HHE-500 on the 2007 Allegro Bus but they do connect the hose to the engine so the system will utilize Motoraide heating when traveling. If you want to use the engine preheat feature it's simple task to upgrade by adding a few parts. I decided I did want this feature so I contacted Vehicle Systems and they sent me the required parts. The parts ran me around $300 and it wasn't a very complex installation. Therefore I'll detail what I did in the following paragraphs in case anyone else wants to do this.
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Mark Quasius - "Cruzer"
The Installation
This installation wasn't particularly difficult. The wiring harness to the HydroHot switch panel is complete so it's just a matter of connecting a few wires and popping in the switch. You don't have to fish any wires through the coach not do you even have to crimp and wiring terminals. The pump itself is an easy mount and the worst thing was that the engine's antifreeze is in the hoses that you need to remove and relocate. If you don't drain the cooling system you'll need to crimp off the heater hoses or you'll have a big mess. First you'll need to access the curbside of the HydroHot unit. On my 42QRP this is found in the small curbside basement storage compartment just aft of the 3 larger storage hatch covers. The following image shows the preheat pump, which I have just mounted to the right side of the galvanized steel HydroHot housing. He holes are predrilled for machine screws but I drilled them slightly larger and used 1/4" hex head bolts instead. There are two large 3/4" ID heater hoses visible at the left that run from the engine to the HydroHot boiler for Motoraide heating. These two hoses need to be removed and reconnected elsewhere so that's when the coolant will leak out unless you pinch off the lines or drain the system. the black and red wires visible power the pump. This is a simple plug-in operation but don't do this until after the plumbing has been completed because you don't want to run the pump dry.
The image above shows a close-up of the mixing valve. The gray cap on the head of the valve stem shows an upside down number 1. This is your domestic hot water temperature mixing valve. The cap is marked from 1 to 4 in 90 degree increments and the valve's index line is on the far side. This means that the #3 lines up with the index mark. The yellow tag states that the valve is set to 3. If you turn the knob to increase the number there is danger of scalding. So, if your hot water is too hot or too cold, this knob is your adjustment. #3 is the recommended setting.
At the bottom of this image is a heater hose. This heater hose runs to the bottom of the boiler (next to the red oval sticker) and has a formed right angle bend at the end. The HydroHot boiler uses 1/2" hose connection while the heater hose from the engine is larger. Therefore a brass reducer fitting is used and is barely visible in this image where the larger braided hose ends and the 1/2" hose begins at the base of the mixing valve. You will need to remove this hose and replace it with the straight hose that was included in the preheat package. This straight hose then attaches directly to the pump's side port. The port on the boiler itself is now connected to the bottom of the preheat pump with the molded gooseneck hose that came with the pump. Add some hose clamps and the plumbing portion is now done. Refill, or top off if you used the pinch off method, the engine's cooling system and plug in the two wires on the preheat pump. Now it's time to go inside the coach to the control panel.
The electrical part is very easy. Simply unscrew the 4 screws that mount the HydroHot switch panel to the main panel inside the coach. Then remove the blank insert and pop in the Engine Preheat switch. Connect the wires as per the wiring diagram and remount the panel. Then you're all set.
Summary
The engine preheat system is an optional feature. If you are plugged into campground shore power you might just as well use the 120 volt engine preheater that came with the coach as standard equipment. However, if you are dry camping in cold weather, or your coach is stored in cold weather and you want to start the engine, the diesel powered preheater is a nice option to have. Calling Vehicle Systems revealed that the entire package, which included the switch, the two special hoses, clamps, and the pump ran around $300 but the biggest chunk of that was the $250 pump. It does add a bit more capability for those RVers who plan on starting their coach in cold temperatures. For further information the following links will take you to the HydroHot manuals for you to download or view if desired:
This review written 12/1/06
Footnote
Vehicle Systems underwent a name change during the middle of the 2007 model year. They are now called Aqua-Hot Heating Systems and they have also renamed their products. Previously the Aqua Hot was a larger boiler that was designed for Bus conversions and was physically too large to be used in a traditional Class A motorhome. The Hydro Hot was the class A sized unit. When the company changed their name for better brand identity they also dropped the HydroHot name and applied the Aqua Hot nane to all 4 of their products. In addition, the galvanized boiler is now a white painted steel with AquaHot decals. Note that the product hasn't changed, only the nomenclature and model numbers. As of the tail end of the 2007 model year, Tiffin Motorhomes began using the newer Aqua Hot systems and also has them set up with the engine preheat system. Therefore, the aftermarket preheater instalation is only applicable to mid 2007 coaches and earlier.
