Sierra Nevada Airstreams -> Owner's Guide -> Understanding

Recreation with vehicles in the Sierra Nevada and American Great Basin areas

HVAC -Heating, Ventilation, and Air Conditioning

The quality of the air in your RV

Once you get a shelter to keep the weather outside, the next step is to make the inside more comfortable. Heating is usually the first consideration as it is easiest to implement. Ventilation is needed to make sure that heating doesn't poison the occupants and to minimize condensation. Cooling is most difficult as it requires the most complex machinery but it sure makes sleeping a lot easier on humid southern summer nights.

Most modern RV's come with furnaces to provide heat, fans and vents to provide ventilation, and air conditioners to provide cooling.

The biggest problem in any heating, ventilation, or air conditioning (HVAC) system is that of moving heat from one place to another. If you can focus your heating to a smaller area then you will need less heat which means less cost. However, any living space needs to have a reasonably even temperature throughout. An RV also needs to make sure that plumbing lines and tanks receive sufficient heat to avoid freezing damage. The furnace uses battery power to distribute heat while the catalytic heater only heats what is in front of it. Furnaces are of a type called forced air heating. Catalytic heaters are of a type called space heaters. There are also other types of heating, such as hydronic that use a liquid to move heat around instead of air.

Basic Thermodynamics

If you take a science class in thermodynamics they will teach you that there are three ways to move heat. Convection means moving air around, conduction means spreading heat through a material, and radiation means moving heat by broadcasting it. Your RV furnace uses conduction to get air hot and then convection to heat the rig by blowing the hot air through a ventilation system. A catalytic heater uses special materials to create a glowing surface that broadcasts heat.

Dealing with heat depends greatly on the properties of materials. Heat is a form of energy. Different materials can store different amounts of heat. Even a particular material can store different amounts of heat depending upon its state. HVAC engineers make use of these material heat properties to design heating and cooling systems.

Temperature does not measure how much heat something has but can be used to determine heat content if you know how much of a particular material is at a given temperature. This is why the BTU, a heat measure, specifies not only a temperature but also a particular quantity of a particular substance (pound of water at given conditions).

When you raise the temperature of a substance you are adding heat to it. To make a substance change from a solid to a liquid and then from a liquid to a gas usually takes a whole lot of heat but won't change the temperature until the change is complete. This is why melting ice is a known temperature of 32F and boiling water is at a known temperature of 212F because these temperatures are characteristic of water.

Pumping heat

An air conditioner is a heat pump. It uses the properties of a coolant under various pressures and in changing from a liquid to a gas to move heat from one place to another. Normally heat will only move from higher temperatures to lower temperatures but a heat pump can make heat move 'uphill' by conditioning a liquid to absorb heat in the colder area and then releasing heat in the warmer area. This is how refrigerators and air conditioners work. They normally require things like compressors and pumps but the ammonia cycle used in most RV refrigerators uses heat driven convection coupled with chemical absorption to move gases and liquids around.

The source of heat

In an RV, the primary source of heat is propane. The other alternatives include solar or electric or perhaps wood, gasoline, or diesel fuel but these usually aren't as convenient, inexpensive, or available as propane for the cold RV.

The key issue is that being warm in an RV means converting the chemical energy stored in a hydrocarbons into heat. This is done by using a chemical process to convert the hydrocarbon – propane, gas, or whatever – into water and other chemical byproducts and heat. Oxygen is needed to do this and that comes from the air which also has other chemicals (mainly nitrogen) that can create unwanted byproducts. Chemical byproducts include poisonous gases that you do not want in your RV. You don't want the water either (see dank!). So you want to keep the chemical process outside if possible and, if inside, as 'clean' as possible to reduce the quantity of the most poisonous gases (like carbon monoxide).

I work for a company [that makes] Fuel Cells. Our fuel cells are supplied with natural gas or propane, and they produce electricity. No I am not trying to sell anything, but the chemical conversion process is identical to that catalytic Coleman propane heater, or one made by any other company. I will not quote any owners manual, but I would like to share the chemical reaction that occurs when propane is oxidized.

The chemical composition of propane is C3H8, 3 carbon atoms and 8 hydrogen atoms. Complete combustion of propane is as follows:
C3H8 + 5O2 --> 3CO2 + 4H2O (the equation is balanced)
As you can see carbon dioxide and water are produced. This the same chemical equation doubled for clarity later.
2C3H8 + 10O2 --> 6CO2 + 8H2O

Complete combustion occurs when there is always an abundant supply of oxygen (or air). Incomplete combustion occurs when the supply of oxygen is less than what is required to complete the reaction. So we adjust the chemical reaction by lowering the number in front of O2, but the chemical equation still needs to be balanced so it would go like this:
2C3H8 + 9O2 --> 4CO2 + 8H2O + 2CO
Here we have less oxygen available to perform the reaction so we get incomplete combustion. And look, we get the formation of CO. And in either case we also get the formation of water (in the form of vapor).

I will not make any recommendations about ensuring that there is an adequate supply of fresh air. Everyone should be smart enough to do that on their own (especially when the owners manual makes the same statement).

If this helps everyone understand why an adequate supply of fresh air is essential, and it can save someones life, think that was worth 3 cents.
- drdouger on rv.net (with permission)

Generating heat

There are several means to combine air and propane chemically to produce heat and exhaust byproducts. Most common is to mix the two and provide a spark. This causes combustion that will continue as long as there is heat, fuel, and air in the proper proportions. Another method is to use a special material rather than heat (the spark or ongoing flame) to stimulate the reaction between fuel and air and just enough heat to keep things going. This is the method used by catalytic (this term is the name for the process of using special materials to stimulate a reaction) heaters and fuel cells. Catalytic processes tend to be 'cleaner' than combustion processes because combustion produces much higher temperatures that can break things apart easier and mix things up.

RV Furnaces have a combustion chamber that is vented to the outside so the fuel burning byproducts are kept outside the RV. A heat exchanger and fan help distribute the heat inside the RV.

The stove and oven should not be used for heating inside an RV except for cooking as they use a combustion process where all of the exhaust byproducts are inside. Crack the kitchen window when using the stove or oven to provide adequate ventilation!

A catalytic space heater also puts its byproducts inside and depends upon inside air to combine with its fuel. Modern heaters usually have sensors to shut themselves off if they detect too high a level of poisonous gases or too low a level of oxygen around them. They also will have sensors to shut them down if temperatures around them get too hot or if they fall over. These safety precautions are a needed backup for careless use or accident.

How much heat?

Heat for RV purposes is usually measured in BTU's (see understanding energy). To get some idea of how much heat is needed for keeping warm, here are some numbers that you can use to compare and contrast to make a scale for RV heat based appliances. Most of the heat ratings are for output. The efficiency of the device can make a big difference in how much it uses for input in order to get the specified output.

For space heating, the key factors are how big a volume needs to be heated, how well is this volume contained, and how much of a temperature difference is desired between inside and outside. A quick look at RV air conditioner and furnace ratings indicates that most rigs are equipped with twice as much heating capability than cooling capability. This makes sense as 20F cooling or 40F warming will keep your rig at a comfortable temperature (70F to 80F) for outdoor temperatures of 30F to 100F which covers most common environmental conditions for the typical RV experience. And you can tack another 10F on each end and still have tolerable inside conditions.

Note that catalytic heaters typically produce less than a third the heat of a furnace. This means that they are designed for heating a smaller area and not for distributing heat throughout the RV. But consider that a catalytic heater has a 100% duty cycle and the furnace duty cycle is less than 50% except for extremely cold weather.

Resources

Owner's Guide: condensation, keeping warm, winterizing,

Senseless Killer - http://www.wnrmag.com/stories/2001/aug01/co.htm – what you really need to know about carbon monoxide and combustion safety

RV Solar Electric - http://www.rvsolarelectric.com/heaters.htm – a nice 'How To' guide – see http://www.rvsolarelectric.com/manage.htm – for power management ideas

Hydronic - http://www.solarhaven.org/HPArticle.htm – how to homebrew an alternate heating system from home power magazine.

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