If you have an unheated outbuilding or basement and you want to warm it cheaply with minimal installation hassles, the ProCom “Blue-Flame” ML300TBA is an interesting option. A combination radiant/passive-convection heater powered by propane, the ProCom burns cleanly and needs no vent or chimney. A bit more civilized than the brute-force, rocket-engine-style of the previously-reviewed, portable Dyna-Glo workshop heater. Granted the Dyna-Glo has the raw power to bring a very large, cold space up to a comfortable temperature within a minimum amount of time, but it is relatively expensive to run and does make a distracting amount of noise. The Procom, on the other hand, just simmers almost silently in the background.

Those who live in the boondocks may already have a propane tank in the yard. Otherwise, you can buy a cylinder of the same type that you would use with a barbecue grill, and refill it from a propane dealer or by using the cylinder-exchange service at Wal-Mart. To run the heater from a small cylinder you will need a pressure regulator that screws into the top (with a sneaky left-hand thread), plus a piece of hose with 3/8″ female connector on each end, and a male-to-male 3/8″ brass adapter (known in the trade as a “nipple”) to attach the hose to the heater. All of this stuff should be available at your local hardware store, together with Teflon tape, which you wrap around the threads before assembly. Make the joints tight, then squirt a solution of dishwashing liquid on them to check for leaks (which will blow bubbles). If you have any doubts about this, naturally you should consult a plumber. Personally, I had no problems.

Building codes and commonsense advise you to keep propane cylinders outside in case a cylinder leak occurs, which you may not notice since propane is heavier than air and collects at floor level. I made a hole through the wall for the hose and stood an 8-gallon tank outside on the deck. The only problem I found using this size of tank is that the heater’s pilot jet sometimes doesn’t relight easily when the heater is warm and the propane tank is cold (the tank gets colder during use, because of the loss of internal pressure). The instructions do recommend a full-size tank of 150 gallons or larger.

During use, the heater creates virtually no odor detectable by my nose, and because no heat is lost through an outside exhaust, it’s super-efficient. Just to be on the safe side I bought a carbon monoxide detector from Home Depot for around $20, but it has not registered any hazard. Of course any flame will consume oxygen, and for this reason alone you should crack a window a little if you keep the heater burning for prolonged periods. The heater is not recommended for very small spaces, bedrooms, or bathrooms. Be sure to follow all the instructions in the manual.

Initial setup only took me fifteen minutes. It hangs on a single (supplied) bracket, which you fix to the wall with two screws. Model ML300TBA generates up to 28,000 BTU of heat and warms my office area comfortably within about fifteen minutes, starting from 55 Fahrenheit. Since the combustion of propane unites hydrogen in the gas with oxygen in the air, the heater liberates more than one-and-one-half pints of water per hour at its maximum setting, in addition to some carbon dioxide. The water is a bonus for me, as my sinuses prefer some humidity. The heater is silent unless you buy the optional (overpriced) circulation fan. Personally, I prefer to use a ceiling fan to spread heat around more actively.

Is it really safe?

Well, consider the alternatives. A wood-burning stove is generally regarded as benign, yet can emit significant carbon monoxide through incomplete combustion if you use it in “slow burning” mode. Fragrant wood smoke is rich in tars that may be carcinogenic, some escaping into your living room while the rest circulate through your neighborhood via that convenient device, the chimney — which is not only horribly inefficient but becomes a notorious fire hazard as residues accumulate inside it.

You can see where my bias lies. For those who don’t want to install a chimney, or don’t have a municipal gas supply, or don’t want to pay a plumber to extend an existing gas line to an additional location, an unvented heater is a convenient option. Propane is not an irritant, does not cause sensitization, and has no known teratogenic or mutagenic effects. Also, unlike a wood stove, the ProCom heater contains an oxygen depletion sensor that will shut it down if necessary. Of course if you allow it to become very dirty, it may burn less efficiently, creating some carbon monoxide, like a kerosene stove, which is why you may find a carbon dioxide detector reassuring.

Your only problem may be in finding this item. A large local hardware store refused to believe that such a thing could exist. Ventless gas heater? You mean — without a chimney? “Impossible,” they told me. Even a local plumbing/heating specialty place was skeptical. But my local propane supplier told me to mail-order one from Northern Tool, known to many of us as an indispensable source for cheap Chinese hardware, which sells the ML300TBA model heater for $170, excluding external connections.

-- Charles Platt 01/22/09

(Northern Tool also sells a natural-gas version of the ventless gas heater, which I have not tried -- Charles Platt The model reviewed by Charles (ML300TBA) is no longer available at Northern Tool or Amazon. The MD300TBA model, which we link to, seems to be the closest replacement. — editors)