I have a little catalytic heater similar to that that makes a good deal of condensation. It runs on Coleman propane canisters. A Coleman lantern seems to work better for me and yes I have a CO monitor with a cracked open hatch.
So it looks like the consensus to "How unsafe is this?" is: Pretty darn unsafe!
I would go with tallswede's advice for a catalytic propane heater for occasional. We used a small one that takes standard canisters and it definitely helped warm up the cabin in the evening. I wouldn't sleep with one running, and the CO monitor advice sounds smart. I'll get smart on that one next year
Go buy a decent small propane catalytic heater or an origo alcohol heater. keep the hatches cracked, and a fan going, and turn off the heater before going to bed, then shut the hatches but keep the fan running to keep the air circulating.
A fan makes a BIG difference in comfort-it circulates heat throughout the boat.
I know several people who put a clay pot upside down over their stove burner for some heat. Although we have a regular diesel heater, it amazes me how much heat a kerosene lamp contributes. They can be smelly though.
I read some of the report linked by Denise. Thanks. It appears that hypoxia (low oxygen) is a significant danger. It seems like a really bad idea to use this type of heater. If you're going to use one, get a carbon monoxide detector, and you better get one to detect low oxygen levels too. Don't forget that carbon monoxide can pool in an area not serviced by the monitor - like your bunk? I personally would not use an unvented heater.
Reading the test that denise linked, here's the good news- the likelihood of a catalytic heater killing you aboard a ventilated sailboat are very low. From the test:
" test chamber at air exchange rates of 0.0, 0.5, 1.0, and 1.5 ACH. During the test, gas samples were
continually withdrawn from the test chamber and analyzed for carbon monoxide (CO), carbon dioxide
(CO2), oxygen (O2), and unburnt hydrocarbons in the form of propane. Long term testing of two identical
catalytic heaters was also performed to determine if the catalyst degraded over time.
The following is a summary of CPSC staff's findings:
· On average, the catalytic heater operated for approximately 6.5 hours on a 1-pound disposable
bottle of propane. This heater could not be attached to a larger fuel source (i.e., 20-pound tank).
· The peak CO concentration ranged from 68 ppm to 125 ppm and the steady state CO
concentration ranged from 67 ppm to 109 ppm. Assuming a limited exposure time of up to 6.5 2
hours at these CO concentrations, the catalytic heater does not appear to pose a serious CO hazard
to healthy adults when the CO concentration is considered by itself.
· When the catalytic heater was operated in a closed room (ACH ~ 0), the oxygen was depleted
from an ambient concentration of 20.9 percent to 8.8 percent. Because the catalytic heater can
deplete the O2 concentration to such low levels, the heater poses a serious risk of hypoxia. The
degree of hypoxia is further exacerbated by the moderate CO concentration and by an increase in
the carbon dioxide concentration that accompanied the depletion of oxygen.
· As the oxygen decreased in the chamber, the catalytic heater became less effective at converting
the propane and oxygen to carbon dioxide and water vapor. This was reflected by an increase in
the hydrocarbon concentration in the chamber, which ranged from 1,050 ppm to 13,440 ppm (5 to
64 percent of the lower explosion limit of propane in air). The unreacted propane further
increases the degree of hypoxia....."
Be Very aware, cats can be deadly, doesn't take much to accumulate. We lose several people here each year to catalytic heaters, mostly in garages and small rooms keeping people warm in confined space. A sail boat is a very confined space. Not enough cubic feet for combustion air. Like many things some folks get away with it each time. Very sad endings, once your dead!!!. No do-overs.
Please do not put yourself at risk, more important is your family, with a bad decision.
I have one of those Mr. Heater portables with built in fan and O2 sensor. I don't trust it at all. Puts out serious heat, although, up on the hard I can only get the cabin to about 50 degs in dead of winter. Exposed boat is a heat sink, but it's enough to get some work done. However, I always feel like crap afterward. Assume it's the CO.
These cat heaters are fine; when prudently used:
1. NEVER go to sleep with one lit.
2. Crack a hatch, etc. to expel CO & CO2 ... and humidity.
3. NEVER go to sleep with one lit.
4. If you develop the slightest headache, etc, turn it off.
5. NEVER go to sleep with one lit.
6. Use a CO/CO2 detector alarm.
7. NEVER go to sleep with one lit.
.... the exact same rules as when your propane stove/oven is on, or any other 'open flame' device (lanterns, candles, etc.) or any other 'camping heater' is on.
We've been using a similar Cat device for the past 30 years ..... its really good for a quick warm-up of the interior and doesnt 'hog' fuel from your 'main propane' storage/bottle(s).
Use common sense, protect it so it doesnt fall over, and always be keenly aware of what happens with 'misuse'.