I have one of those tall propane outdoor heaters…..( fumes)?

You need to consult the CERTIFIED operating instructions of any heater you attempt to use for this area to ensure it is suitable for that application. If it is not certified for indoor use and it causes a fire or hurts anyone, the manufacturer will take no responsibility and insurance companies will deny any claims because the heater was used “In a manner other than what it was intended”.

If this is a patio heater, I think I can safely say that Yes, it would be dangerous to use it. The legal ramifications aside, the burner on those heaters is fairly high up and you don’t want that anywhere near the buildings ceiling structure.

As a general comment, you need to make sure you have enough fresh air for an appliance to operate safely and efficiently. A propane burning heater requires combustion air to burn properly, but it also requires ventilation air to allow the products of combustion to escape. Propane burns clean, but if you REBURN those products of combustion you make carbon monoxide in a big way!

The general rule of thumb we use in the industry is 1 square inch for every 1000btu’s. So if it is a 20,000 btu heater you need to have at least a 2″ x 10″ (or 20 square inch) opening for safe and proper operation. That doesn’t mean propping a door open, or opening a window in the area. These can easily be shut, cutting off the fresh air supply.

I recommend you look at a direct vent wall furnace like an Empire DV35 or something for this application. Direct vent means that they draw air from outside, burn it with the fuel and then put the products of combustion back outside. They are not cheap, but it’s the safest way to go.

Hope this helps!