Hi. Thats the same heater I have. We had a problem with it shutting down as well, but discovered it was because the greenhouse was too airtight. Its a safety function. So after allowing a small amount of fresh air in it was fine. We just have to keep it vented. Just one possibility.

These heaters are very sensitive to the oxygen content of air in their area. You do need to be sure there is some outside air coming in; recommendation is for 1 sq. in of opening for each 1000 BTU output, so you would need about a 2x3" opening for this heater. You also need to have a bit of air movement within the greenhouse, or you'll find that the air right at the heater is depleted, even though there is sufficient air intake. A small fan helps to disperse the heat as well as move in the oxygen.
Renais

Most likely your model has an oxygen sensing pilot. That is one that flames out when the O2 level gets too low. Most ventless heaters in the last 5-10 years do.

There are a few things which can cause this: not enough ventilation (as mentioned), too high an altitude (>4500' can be problematic), and dirt in the pilot assembly.

I have a similar model, and I had a lot of problems with the pilot going out until I cleaned the pilot tube (Qtip and some alcohol). Even though it was brand new, it had some dirt/grease in there. After cleaning it it has worked much better.

Since yours is going off fairly quickly, you might want to watch the pilot flame for a while. Does the flame base travel away from the end of the tube and then go out? If so, that is the symptom of O2 depletion going on. It may be lack of vent air, or possibly dirt in the pilot tube. Both cause the same symptoms.

You can read up on oxygen sensing pilot lights online. There is lots of info out there. They are intentionally very hard to modify/adjust. But quite simple in operation. Usually just a cleaning will fix them. But it is possible that you got a defective unit (but unlikely).

Thanks gang, will try it again with the small vent open and see what happens!

I have two tank mounted Mr. Heaters. The old model works perfectly and cost $30. Then the epa mandated now controls and the same heater cost $80...and it wouldn't stay lit. (although My leaky house had no co2 problems) I called the factory and they would replace the sensor....it was 10 degrees that night! I took a radiator clamp and locked down the starter plunger. It now works just fine.

The orifice size is critical, and I have heard that it is not uncommon for a small spider to crawl in and leave minute amounts of debris that can interfere with proper operation. This is particularly true when the heater is unused for months and stored in a place where bugs can get in. Try cleaning the hole with a fine wire or replacing it.