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Hydronic Heating
Operation and Maintenance Tips


HydroHot Boiler               AquaHot Boiler

There are two hydronic heating systems used in the Allegro Bus. Partway through the 2007 model year Vehicle Systems replaced the HydroHot system with a new Aqua-Hot system. This was merely a name change and cosmetics so for practical purposes we'll use the term HydroHot universally here. For more information on the name change refer to the Hydronic Heating Systems Overview page in this library.

Prior to 2007 Vehicle Systems made the HydroHot models HHE-500-07M and HHE-200-07E. The difference between these two is that the HHE-200 had an engine preheat pump installed while the HHE-500 did not. Tiffin installed the HHE-500 model in the Allegro Bus. If an owner wanted to add the engine preheat feature to an HHE-500 it was possible by purchasing the circulation pump, a few hoses, and the extra rocker switch from Vehicle Systems. Adding these items effectively turned the HHE-500 into an HHE-200. For details on my installation of this upgrade visit my tutorial on www.rvcruzer.com for detailed information and photos.

In the middle of the 2007 model year, Vehicle Systems changed the branding over to the Aqua-Hot model name. The Aqua-Hot 450 comes in two variants. The Aqua-Hot 450-DE includes the engine preheat function while the Aqua-Hot 450-DM does not. Tiffin is using the 450-DE now so the Allegro Bus will have the engine preheat feature going forward.


How It Works:

The HydroHot system is made of a central unit as well as slaved heat exchangers located throughout the coach. The central unit consists of a boiler filled with antifreeze, a burner system, an electric heating element, a domestic hot water heat exchanger, and a series of circulating pumps and solenoid valves to circulate the hot antifreeze to the heat exchangers. A remote control panel consists of two illuminated rocker switches (three if the Engine Preheat function exists). These switch control the burner and electric heating elements.

Whenever the rocker switches are off, the boiler will not be heated. If the Electric switch is in the On position the electric element will be on and will heat the antifreeze in the boiler. Whenever the Diesel burner switch is in the On position the diesel burner will be active to heat the boiler using diesel fuel. Once the boiler's antifreeze reaches the set temperature limit switches will shut off the electric element and diesel burner to prevent the boiler from overheating. As the antifreeze cools off, either from sitting too long or from circulating through the various heating zones as demanded, the boiler will heat up again. Both electric and diesel switches can both be on at the same time. Doing so will minimize the amount of time the diesel boiler runs and save on diesel fuel consumption.

Okay, so far we see that there are a couple of rocker switches and limit switches that make the boiler hot. But, how do we get the hot antifreeze to circulate and heat up our coach?

First off, the boiler itself is not affected by your RV's thermostat. There is no connection to it whatsoever. In order to get heating distributed to your coach's interior you need to get the hot antifreeze to the heat exchangers located throughout the coach. Each heat exchanger is a small radiator with a 12 volt fan that blows the warm air from it through your heating ducts. There are a number of these in your coach and you probably have two separate heating zones - one in the front and one in the rear. In addition there is a third heating zone that feeds a dedicated heat exchanger in the basement to keep your water lines and tanks from freezing. In order to get the hot antifreeze to these areas a series of circulating pumps and solenoid valves are employed, as well as a pair of poly antifreeze lines to and from each heat exchanger. Your RV's thermostat communicates to the logic board on the HydroHot so that the various circulating pumps and solenoid valves can be opened in response to requests for heat from that particular zone. At the same time, the fan motor in the heat exchanger will turn on as the hot antifreeze flows through it.

There are a number of interfaces that prevent unwanted circulation. If the antifreeze is not yet up to the proper temperature no solenoid valves will open, no circulating pumps will turn on, and no heat exchanger fans will be blowing. Once the antifreeze is hot enough for operation, everything kicks in. This prevents blowing cold air around the coach or from running batteries down when the boiler isn't even turned on. Each zone is unique so if your thermostat calls for heat in one zone but not the other, only that particular zone will have circulation and fan operation.

The basement heat is a third zone. This zone is not controlled by your upstairs thermostat. Instead, you'll see a small brass valve unit with a knurled knob in your basement compartment. This thermostat has a remote probe attached to it that goes to the water tank area. This thermostat is independent of your upstairs thermostat so even if you have the upstairs thermostat in the Off position it will still signal the HydroHot if it needs heat and the solenoid valves and fans will kick in for the basement zone. You do have to have the HydroHot boiler enabled with a rocker switch or two though in order to produce any heat.

The HydroHots are also plumbed with a MotorAide feature. This uses the coolant in your RV's diesel engine to help heat the boiler. Whenever your engine is running the engine's coolant circulates from the engine to the HydroHot boiler and back to the engine again. This is a closed loop system. The engine's antifreeze/coolant does not mix with the antifreeze in the HydroHot boiler. Instead it runs through a tube that transfers the engine's heat to the HydroHot, in effect giving you free heat while you drive down the road. There are no special circulating pumps necessary for this. The natural flow from the engine's water pump will do the job as long as the hoses are connected to the engine in the right place.

If you have the Engine Preheat feature on your HydroHot system a small circulation pump is added to the MotorAide hoses and a third rocker switch is added to the control panel. The engine preheat function works similar to the MotorAide heating function except in reverse. When the engine preheat switch is in the ON position the circulating pump will pump the engine's coolant from the HydroHot to the engine and then back again. As it does it will transfer the HydroHot's heat to the engine coolant and warm up your engine for ease of starting. This works in similar manner to an electric engine block heater except you do not use electricity and the higher BTU rating of the HydroHot provides more heat to the engine and warms it up much faster. You do have to have the HydroHot's diesel boiler switch turned on in order for this to work or else you'll just be circulating cold coolant.


Operation Tips:

Sources of Heat:

The HydroHot boiler is heated by three different sources. These can be used independently of each other or combined. The main source of heat is the diesel burner. This burner uses diesel fuel in order to generate 50,000 BTUs of heat. The burner will consume .35 GPH of fuel if it runs non-stop. However, the design of the boiler means that it won't be running non-stop. Average consumption is expected to be around 4 gallons of fuel per day. This heat source is selected by turning on the rocker switch on the HydroHot's control panel.

A second source of heat is the electric heat element. This element consumes 1,650 watts of power, which relates to 13.75 amps of shore power. It only puts out 5,630 BTUs of heat though so your diesel burner will still be the primary source of energy when heating the boiler. If the temperatures don't get that cold you'll find that the electric heating element will provide enough energy to keep your coach warm. However, once the temperatures drop down to around 40 degrees this won't be enough energy and you will need the diesel burner. Note that you can run both the diesel burner and electric element at the same time. This will help the diesel burner along and it won't have to kick in as often. The electric element is also controlled by a second rocker switch on the control panel.

The third source of heat is the engine itself. The MotorAide feature of the HydroHot circulates the engine's hot coolant through the HydroHot boiler to provide "free heat" while driving. This feature contribute approximately 10,000 BTU so it's not enough to keep the coach warm while traveling in cold weather so you will still need the diesel burner in those conditions but it will add a significant amount of free energy to minimize your diesel burner's fuel consumption in cold temperatures.

Control Panels and Thermostats:

The main HydroHot control panel consists of two rocker switches, or three if the optional Engine Preheat feature is present. The Diesel switch turns on the diesel burner. The burner will cycle on and off as necessary to keep the boiler antifreeze hot. The second switch is the Electric heat switch. Don't get this confused with the heat pumps on the roof. All this switch does is turn on an electric heating element inside the HydroHot boiler.

If you have the Engine Preheat feature, a third switch is present on the panel. When this switch is turned on a circulating pump will be activated to circulate the engine's coolant through the HydroHot boiler to preheat the engine. Note that in order for this function to work, you also need to have the diesel burner switch turned on or else you'll just be circulating cold coolant.

All of the above switches merely control the HydroHot boiler. They do nothing to circulate that heat through the rest of the coach. That function is controlled by your thermostat.

Your RV's thermostat has a number of operating modes. Typical modes are Air Conditioning, Fan Only, Gas Heat (or Furnace), and Electric Heat (or Heat Pumps). Your thermostat will talk to the rooftop air conditioning units when in the Air Conditioning or Fan Only modes. When you select the Electric Heat mode it will control the heat pumps, which are part of the rooftop air conditioners. Do not confuse this mode with the HydroHot's electric heat mode. The only communication with the HydroHot is when you have selected Gas Heat (or Furnace). The thermostat doesn't care whether or not the HydroHot is heated by the diesel burner, electric heating element, or nuclear power. It just thinks of it all as gas heat. This is because your thermostat does not control the HydroHot boiler. All it does is control the circulating pumps that pump the warm coolant to the heat exchangers radiators) located throughout the coach. Your thermostat normally has two zones however 42-43' coaches will have 3 zones - one for each air conditioner. But you will find that the furnace or HydroHot heat only operates on two zones. Zone 2 (the middle zone) is skipped on the 42' coaches and is only used for A/C or Heat Pump operation.

Whenever a given zone calls for heat a signal is sent to the HydroHot. A circulating pump kicks in and a solenoid valve opens. At the same time a fan kicks in and blows the warm air from the heat exchanger into your coach. There are normally a number of heat exchangers located within a zone to facilitate even distribution of the warm air. If the boiler temperature is too low the circulating system will wait until the boiler heats up again rather than blow cold air into the coach.

A third zone is the basement heat. This zone is not controlled by the upstairs thermostat. Instead, a brass thermostat with a knurled knob is located in the basement to control that zone independently. This zone keeps the water tanks and lines from freezing and the thermostat has a range of somewhere between 40 and 55 degrees. As long as you have the boiler turned on, this thermostat will keep the basement warm regardless of where your upstairs thermostat is set. However, if you allow the upstairs to freeze while the basement is kept warm you'll still have damage and cracked water lines but they'll be upstairs rather than downstairs. So it is important to leave your upstairs heat on as well if your coach is not winterized and you are in freezing temperatures.

Hot Water Heating:

Normally a hot water heater is present on an RV without hydronic heating. These heaters are a tank that stores water, usually around 10 gallons of it, and then uses an electric heating element or a propane burner to keep it warm, much as a residential hot water heater does. The HydroHot systems not only eliminate the propane furnaces from the RV but also remove the hot water heater. The HydroHot boiler handles all of the domestic hot water heating tasks. Rather than use a storage tank design, the HydroHot is capable of providing continuous hot water. It's on demand heating system is powerful enough that it heats the water as it is required rather than storing it in advance. A big benefit of this is that you won't run out of hot water when taking a shower.

In order to accomplish this, the HydroHot boiler contains a series of tubes that circulate the domestic water through the hot boiler antifreeze, heating it as it passes through. Whenever domestic hot water flow occurs the HydroHot will temporarily shut off the circulating pumps to the heat exchangers so that all energy can be devoted to heating the water as it passes through. As soon as you shut off the hot water faucet the heat exchangers will kick in again. It's also important to run the diesel burner when doing this because the electric element won't be able to keep up under heavy water flows.


Service and Maintenance:

Winterizing - When you winterize your motorhome's fresh water system it's recommended that you do not rely on the blow out method to winterize your system. Vehicle Systems recommends pumping the pink RV antifreeze through your water system. Apparently the intricate twists and turns of the domestic water heating coil inside the boiler don't allow for adequate removal of all of water in it. Remaining water can settle out in the low points of this coil and expensive freeze damage can result. Use the pink stuff.

Domestic Hot Water Temperature Control - On the passenger's side of the HydroHot unit you'll see a bunch of hoses. There is also a large mixing valve that controls the temperature of the domestic hot water. If you turn this too far in either direction you'll get either cold water or scalding hot water.

Maintaining The Boiler's Antifreeze Level - Under normal conditions your HydroHot system will not lose any coolant. Just like the cooling system in your RV or towed vehicle there is a sealed cooling system with an expansion/overflow bottle to retain and expanded antifreeze and return it to the system when cold. It's a sealed system so the level should only vary from temperature variations. When the boiler is hot the antifreeze level will rise in the overflow bottle. When the system is cold and has been resting for a while the level should be low. It should remain between these two extremes.

If you find that your antifreeze level is continually dropping you have one of two things going on. First of all, on a new system there is generally air in the system. AS the system operates this air will bleed out and antifreeze will replace it, thereby lowering the level in the overflow bottle. You may need to add some antifreeze for a bit after you first get your new coach or after your system has been disassembled or serviced. After a while though this level will stabilize.

If the level still drops, you have a leak someplace. A common place is right underneath the boiler. The plastic drain valves have been known to cause problems in the past. You could also have leaks in the plumbing lines or at the heat exchangers. It will take some exploring to find where the leak is. It could just be a hose clamp that's loosened up.

Antifreeze Type - The HydroHot system does not use regular automotive antifreeze. It requires a boiler type of antifreeze. This stuff is available at Camping World, RV dealers, from the Tiffin parts department, directly from Vehicle Systems, or from your RV dealer. Vehicle Systems recommends the Camco Boiler Antifreeze - 100. Rather than an automotive coolant, this antifreeze is designed for heat transfer and freeze protection down to 100 degrees below zero. The key is that it's a residential boiler antifreeze so you're not going to find it at an auto parts store but would more likely find it at a heating supply house.

Diesel Burner Requirements - The diesel burner will require annual maintenance. The burner requires a fresh fuel filter and burner nozzle every year. If you ignore this your HydroHot will tend to smell because it'll be running rich with partially unburnt fuel out the exhaust. This maintenance is not expensive but it is something that should be done and it's fairly easy for an owner who is halfway mechanically inclined top do on his own.

Exhaust Fumes - The exhaust pipe generally sticks out the driver's side of the coach, near the middle. If your burner is running rich there can be a noticeable smell to it. Generally this is not noticeable but if you are camping in tight quarters to a neighbor, there could be an issue. The biggest challenge is that when the exhaust bounces around near the vehicle it can filter back into the coach. In this case you need to seal up any air gaps in your coach. These RVs are built with lots of holes to pass the various hoses and wiring harness through. These holes are sealed up with expandable foam. However, not every coach gets an airtight seal jib. It depends upon who did it and what day of the week it was. If you start smelling your exhaust fumes inside the coach it's time to inspect the various basement compartment areas for places that it's getting in. Also check your sewer hose access panel in the basement water service bay. The hole is larger than the hose so if your hose is run out that hole there will be a gap there. If you are in tight quarters you may want to leave the hose off and just connect it when you need to dump or else stick some foam pipe insulation in the gap to seal it up.

Another situation is the direction that the exhaust is pointing. The exhaust pipe has a curved extension on it. This extension should be pointing backwards band slightly down. If your extension is aimed at the ground it'll bounce back at the coach rather than getting thrown away from it. If you are in really tight quarters or the wind is blowing tit back at you may want to consider the Camco Genturi, which is a handy little exhaust stack kit that assembles to raise the exhaust to over the roof line when parked. While originally designed for generator exhausts, it has proven well in hydronic heat because any noise and smell are exhausted up high where it can easily dissipate if you are in tight quarters.


Submitted by Mark Quasius - 2/06/06

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