Vented cat heater

[Andy B]

Originally Posted by Mike Magee

2 minutes is negligible, so we can almost forget about the 5 amp draw for that short a time. Figuring .5A per hour, if there's 40A available for use in the battery, that would run the heater theoretically for close to 80 hours straight. But I would think the plat cat would shut off for periods of time when it got warm enough.

2 minutes at 5 amps uses the same amount of power as .5 amps for 20 minutes. If the duty cycle is 10 min on then 10 min off and repeat the startup part of the cycle uses more power than the running part of the cycle. The longer the on/off cycle the less important the start up draw is. The off part of the cycle depends on how quickly the trailer looses heat, not the type of heater. Since the cat heater does not put out a high btu level, maybe it runs for long periods of time or even continuously.

[Darwin Maring]

Years ago we had a gravity heater in a camper and I wish they still sold these heaters because they were ideal for small campers. Vented fresh air and vented exhaust and no noise or electric used.

I rented a Uhaul type of egg back about 22 years ago and think it had one too.

[Mike Magee]

Originally Posted by Francesca Knowles

I'll be interested in their reply, and especially in any independent studies they may cite that support their claims...

.....
Francesca

I received a reply. They indicated that the design has been certified by the National Gas Association, that it does collect the combustion gases and vent them. The plenum at the top sucks the gases up and out. It sucks about double the volume needed to gather the byproducts, which would have the added effect of creating a change of air in the trailer every so often.

For air intake, they say an intake inlet can be rigged if desired; 1.5" is plenty big. Actually an opening of about 8 or 9 mm diameter lets in enough air, apparently, so mere leaky window seals might do the trick. I would have had no trouble with air coming into my Burro... besides the old windows, the side vent behind the A/C was wide open and the air could leak quite readily around the A/C and fridge.

If the vent gets blocked or the fan quits working or combustion isn't right, the unit shuts itself down. Seems like a good safety feature.

[Terry G]

Originally Posted by Mike Magee

I received a reply. They indicated that the design has been certified by the National Gas Association...

I think I'd like a bit more information about this organization since I was unable to Google it up. There are a number of trade groups with names similar to this (such as the National Propane Gas Association) but those organizations seem to be more interested in pushing the use of gas than in certifying heaters.

[Francesca Knowles]

Thanks for getting back, Mike

I'm thinking the same thing as Terry- I wonder what the gas association certification means- I suspect it's more about safe construction than performance claims. I wonder if Consumer Reports has anything?
They told you it the "sucks the gases up and out" ?
I don't get it- are they saying that the heat stays in the trailer, and the water and CO2 go out the vent?
And that no byproducts enter the interior?
That fan would have to suck pretty hard to do that, not to mention be supplied with some kind of magical heat separating device.
I'm still skeptical. I just don't see how it's possible to get rid of all the byproducts without a very substantial amount of heat loss. It's all coming off that exposed combustion surface, and if the fan's moving enough air to vent all the vapor, a lot of heat has to be going out with them.
I'm actually doing much the same thing they're doing with my Eco- Fan setup, and it didn't cost me an arm and a leg to do it. And I'm keeping most of my heat. It's absolutely silent too, which reminds me- if noise is a consideration I'd expect this fan to make just as much of it as any heater fan. It'd be nice if you could hear one work, preferably in a quiet place.
No advantage I can see with the safety features- all cats have low-oxygen shutoffs, and all trailer heaters with fans have low battery shutoffs. Interesting that they claim that a blocked vent will shut off the heater. Is that another switch besides the low oxygen shutoff ?

Francesca

[Francesca Knowles]

FYI:
Here're some pics of my Catalytic heater/Ecofan combo.
The stone silent fan generates its own electricity using the heat off the heater and circulates about 100 cfpm.
Trillium Heater 001.jpg Views: 20 Size: 202.1 KB ID: 35517" style="margin: 2px" />......................Trillium Heater 002.jpg Views: 22 Size: 296.3 KB ID: 35518" style="margin: 2px" />
The two screened cutouts behind the fan are ducted to the outside and supply continuous outside air. I've used this system for four or five years and the only problem I've had has been condensation on the inside walls, though lately I've figured out a better combination of cracked windows, succeeding in expelling more water while keeping more heat.

Ecofan costs begin at $100.00
The pictured RV heater I got brand spankin' still in the box new for $35.00, and I hope it never quits on me-
it'll KILL me to pay full retail for a new one....
The aluminum surround is an old chalk board tray,
and YES, the "hood" is an artistically butchered angel food cake pan!

[Roger C H]

Shudder. I wouldn't be able to sleep at night worrying about being asphyxiated.

Plus there is the problem of the added moisture being added to the trailer. There is a discussion on here someplace about that. Fuels yield about a 1:1 ratio on fuel to H2O, that is, burn a gallon of fuel and you get a gallon of water. With no flue, that water goes into the trailer.

[Andy B]

There seems to be 3 reasons why people choose a cat heater over a more conventional furnace like an Atwood 8012. I compare to the 8012 because it is a state of the art conventional furnace with low electric power consumption, low noise, light weight, small size and good efficiency.

1) Reduced electric power consumption/reduced noise because there is no fan.
2) Increased efficiency (less propane used)
3) Different size and shape that will fit where another furnace might not fit.

Comments:

1) The vented cat heater being discussed here has a fan and it is not clear that it uses less electric power or is less noisy than a conventional furnace. It may actually use more power because of the high start up current requirement. It is also not clear how well it actually vents. If it does vent well, there will be no additional water vapor added to the trailer environment and the efficiency will be lower than the non-vented cats. If a non-vented cat is used there is no electric power consumption, however significant amounts of water, etc is added to a very small room requiring open windows or vents to supply fresh air and remove the combustion byproducts.

2) The increased efficiency that is quoted for cats is mostly because they are not vented. A conventional furnace exhausts the combustion byproducts outside the trailer along with some amount of heat. This heat loss is a major reason why the conventional furnace efficiency is lower than a cat. The higher cat efficiency rating is mostly due to the lack of venting because all the heat ends up inside the trailer. Virtually all cat makers say that venting of the heated space is required, however they do not include the loss of heat from this venting in their efficiency calculation because it is not part of their hardware and they have no control how much venting is done by the user. The high efficiency ratings that they quote are misleading due to this. Exhausting combustion byproducts into a living space where they mix with the heated air and then trying to vent only the byproducts through a window or vent while keeping the heat can never work very well unless something like an air to air heat exchanger is used. An air to air heat exchanger for room temperature air would be bulky and not fit very well in a small trailer. Some states have laws against using non-vented heaters in living spaces because relying on the user to properly vent has shown to be unreliable and has sometimes caused death. The 8012 vents the exhaust outside while drawing fresh air for combustion in and transfers some of the exhaust heat to the intake air by using a co-axial exhaust/intake increasing efficiency and effectively isolating the trailer environmental air from the combustion process.

3) Most cats are surface mount and they take away living space both for the cat itself as well as the required distance to combustible surfaces. Conventional furnaces are usually mounted in a cabinet and take away cabinet space.

[Mike Magee]

I've read before that the Atwoods are quieter, but I have never been able to quantify the sound difference. If anyone has a decibel meter and one of these Atwood heaters, I'd be interested in knowing the decibels at, say, 3 feet away. Or if anyone can find db ratings in the specs, let me know.

Incidentally, in searching for this info I came across a pdf of the installation instructions (with some specs) for the 7900 ii and 8000 ii series Atwoods. The sheet begins with this statement: This furnace design has been certified by the American Gas Association and Canadian Gas Association for installation in recreational vehicles... (emphasis mine)