Vent Free Heater Cautions
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Your RV comes with a vented furnace that exhausts all combustion products outside, and draws oxygen from outside as well.  However, on a cold night when no hookups are available, it can run down the batteries overnight.  As well, the furnace will go through a lot of propane in freezing temps for prolonged periods.
As a result many have turned to auxiliary unvented heaters to save a bunch on propane in winter, and to be able to have heat at all while boondocking.
I choose not to use any unvented heaters that burn fuel in my rig.  But many do.  This article contains information on what you need to consider to operate them safely in the close confines of an RV.
There are a lot of myths and "mythconceptions" about their safety and use.  Yes thay can be used safely, but require a lot more than just turning them on to be safe.
This article touches on the major points you need to know to choose the type of auxiliary heater if any. 

First on safety.
There is no such thing as a clean burning appliance of any type that burns everything and emits no toxic fumes in the combustion gases when using carbon based fuel, be that wood, petroleum, charcoal, or propane.
Your RV furnace vents all combustion by products to the outside.  Therefore no toxic gases or particulates are vented to the inside.
This is just like the gas water heaters, dryers, and gas or oil fired furnaces in stationary homes.  They are required to vent the combustion products to the outside.
Having done considerable research on this subject, and having in hand the actual laboratory tests for several of the vent free heater types, and having gone over the accident reports from each of the main three types, vented, catalytic, and open flame, there are some cautions in order with all three to be used safely, and all three can, barring accident, armed with a bit of knowledge.
First of all combustion by definition is oxidation of the substance combusted, so oxygen will be used up at different rates depending on the amount of combustion occurring.
Most types of heaters can be used safely, assuming the user observes all the safety precautions of each type.
Going from least amounts of toxic by products introduced into the rig by a properly functioning appliance, to most: Vented - none, Catalytic some, Open Flame heaters more.
Vented heaters.
The furnace that exhausts all of the exhaust gases outside.  Many folks assume that it can be used indefinitely and you will be safe.  Not true.
Your furnace uses a combustion chamber made of sheet metal, and it has the gases inside, while air from your RV is circulated around that very hot sheet metal, heating the air that goes back into the RV.  The gases inside the chamber are contained during heating of the chamber then vented outside.  So far totally safe.
However, many buy older used RVs to save money, and some have RVs they bought new ten years ago that they still use today of every type.
If any part of the combustion chamber begins to rust and develops pinholes in the chamber, it will let the combustion gases mix with the air inside with results ranging from mild burning of the eyes, to low level CO poisoning, to death, depending on not only the amount of gases let in, but the physical health and makeup of the people inside.
Your RV furnace should be removed and the combustion chamber inspected every few years at least, to assure it is still in good working order.  Replace any heater with any pinholes/damage to the combustion chamber at all.
Vented heaters are the safest, rarely fail but can with no owner knowledge of the inspections required.
No ventilation is required with vented heaters as they draw their oxygen for combustion from outside, never depleting the O2 inside the RV.  You can shut the windows and no gases or large amounts of moisture will be produced inside by the furnace. It is all vented outside.
Older furnaces should be inspected to assure the chamber is still in good shape.
A good CO detector should still be used as a developing problem with the combustion chamber may be caught before a health/life threatening condition occurs.

Unvented heaters.
Vent-Free Warnings:
-Don't use if anyone in the house is pregnant, diabetic, anemic or suffers from heart or respiratory problems
-Cannot be used as a sole or main source of heat
-Cannot normally be installed in a bedroom or bathroom
-Cannot be installed in a "confined space" where fumes may not be properly dispersed

Catalytic and Open Flame.
Catalytic heaters use a catalyst mat that looks similar to a fiber A/C filter on the front, except it is made of metals impregnated with platinum which actually is where combustion takes place.
Catalytic heaters use a mat that causes the propane to glow at about 800 degrees, instead of the 1000 degrees plus of an open flame heater.
Both catalytic heaters and open flame heaters burn inside the rig, and release all combustion products inside the rig.  The catalytics less than the open flame heaters.
I have seen much confusion with non-catalytic heaters being considered catalytic because of the confusing terminology used by heater manufacturers.  To make it simple, whether a heater is called a radiant heater, ceramic gas fueled heater, brick heater, they are open flame heaters.  Whether they hide the flame behind a block, screen, cone, or any other device to change the perception, they are still burning an open flame somewhere inside and are not catalytic.
Both types of unvented heater produce CO, and many other combustion by products in the RV.  Along with a lot of moisture.
Many confuse the ratings of unvented heaters for RV use with the ratings for house use, where there are many more thousands of square feet to disperse and dilute the combustion by products released inside.
They can build up quicker in an RV.
Having said that, unvented heaters can be used safely, if all venting and other operational instructions are followed to the letter.
I would go so far as to state that any unvented heater must be equipped with an ODS (Oxygen Depletion sensor.)  How it operates is in the first link below with pictures of it shutting off the appliance.
In addition a CO alarm and Propane alarm should be installed and tested regularly in case the ODS (Oxygen Depletion Sensor) or the heater itself malfunctions.
In addition to water, CO2, CO and other combustion byproducts produced by all heaters that burn any fuel, as opposed to an electric heater with no combustion, unvented heaters can produce a lot of CO if they burn inefficiently due to a burner malfunction, rust, or something out of alignment from bumpy roads.
Thus my recommendation to have a CO alarm and Propane alarm just in case.
Never have a propane source/bottle inside the rig.  It is best if any unvented heater is connected to your main propane source, tank or cylinders, to prevent propane leakage problems in addition to the others associated with burning fuels inside unvented.
Now to the big fallacies.
Furnaces vent much of the heat outside and use a lot of the battery power to run the fans. Thus are not 100% efficient.
This is true as far as it goes.  But when the unvented folks say their units are almost 100% efficient, because all the heat goes inside, they usually neglect to mention three things.
1.  That to be safe, you have to open vents or windows to replenish the oxygen and vent out the moisture and other pollutants the unvented systems generate. 
2.  That ODS sensors sense CO or O2, they don’t.  They are simply thermocouple distance/flame sensors that cut off the fuel if the flame is not right on the thermocouple due to bad combustion from increasing CO2.  They never explain that CO production starts to go up as soon as oxygen begins to deplete.  (See first link below)
3. That both bring an increased fire hazard INTO THE RIG.  Even a catalytic can ignite a drape if it is lying against it for time.
OK, both Catalytic heaters, and open flame heaters produce CO, even when operating at peak efficiency.  Catalytics at 6 PPM (Parts per Million) and open flame heaters at a range of from 9 - 25 PPM.  The EPA maximum exposure rate is 9PPM.  Many times, in cities, the CO rate is higher from the "fresh air!" 
Add more inside?
So if you choose to save some bucks using an open flame or a catalytic heater, and they do save a lot of propane in winter, here are my safety suggestions, followed by source documentation links for my statements.
1. Read and follow all the manufacturer's instructions, especially the ones about minimum venting.  If the venting makes it too cold, never reduce the venting or you will be at risk.
2. Have a good CO detector with LCD readout of CO in PPM in case you get the initial symptoms of CO poisoning.
3. Also have a propane detector to alert you to any leaks while using any propane appliance.
4. Understand and apply the preventative maintenance procedures outlined in the owner's manual for your unvented device.
5. In all cases, never use an unvented heater, especially in a small space like an RV, if you have the capacity/financial resources to use a vented alternative, as the EPA and the CPSC both state.
6. Catalytic heaters should be covered from dust when not in use.  If the catalytic mat becomes “poisoned,” which means contaminated, it will produce much higher amounts of CO, moisture, etc.  Keep them covered up when not in use.
7. Open flame heaters also can go way up on CO and other substance production if the device is altered by road or user error such as having a cracked or rusty burner, or being out of adjustment due to soot.  Inspect them and the flame regularly.
OK now for the links for those wishing to go further.
The ODS, and a pic of how it works, scroll to the bottom:
The EPA page –detectors not a substitute. Scroll down to the section on CO detectors.
Good discussion from Canada on ventless gas heaters and safety
This question and answer from a heater company and why they refuse to handle ventless heaters and fireplaces, and why.
Where to place a CO detector? This link from First alert, who does not certify their CO detectors for RV use, I know, I bought one that failed. But a good explanation of the weight and rising characteristics of CO. Go here:
If you use an unvented heater read the manuals and thoroughly understand the warnings and operating instructions.
Understanding CO, and understanding which unvented heater type you have, and the unique operating requirements of each unvented heater, is critical to your safety.
Here's an excerpt from the CPSC or Consumer Product Safety commission statistics on CO deaths for one year- 1998. I can find no current estimates, but with the explosion of RV sales and the increasing numbers of fulltimers selecting this lifestyle for both retirement, and mobility for younger contractors, I think it is imperative that we get the word out about the dangers of unvented heaters when used by uninformed folks, especially in the smaller square footage of an RV, when compared to the build up of CO in an average sized house.
An average of 18 percent of
deaths took place in temporary shelters, such as tents, recreational vehicles, campers, and trailers. In 1998, 26 percent of CO deaths, the highest percentage for the five-year period, took place in such temporary shelters. LP gas camping heaters were the products most frequently associated with these deaths, followed by charcoal grills.
The complete report is here:
Carbon Monoxide Poisoning Deaths Associated with Camping -- Georgia, March 1999
Much important info. Unfortunately, the victims can't post that they "heard" it was safe, and made a mistake.
And, unfortunately, online, in some cases (current company excluded of course), the "long reads" and links tend to be ignored in favor of three line answers.
Winter brings out the auxiliary heating systems for both the part timers that use their RVs for ski trips and winter camping, and even the fulltimers in the South. (Yes, it freezes in a lot of our areas too) Getting the real skinny out to our fellow RVrs, even if it takes a few extra minutes of reading and/or research, if it can save one life, to me, is worth the slight effort.
While some may take my article as being “anti” unvented heater, my sole purpose is to be sure that those using them or considering one are aware up front about the care required to BE ABLE to operate them safely.  And they can.  For the above reasons, and because I rarely boondocked, I chose an electric ceramic supplemental heater.  Safe too?  Not necessarily.  I had one catch fire inside the control box of one of the safest types, the oil filled radiator type!
Just being aware can go a long way to keeping safe, regardless of your choice in auxiliary heating, if any.
With any combustion device in the home, just having a CO detector is not enough as it may fail too.  Having propane and CO detectors, as well as knowing the limits of safe operation of any type of combustion heater, and doing the inspections and maintenance necessary, are all required to assure your safety.
Safe travels!

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