I expect to replace these bricks from time to time, so they are not cemented in place.
Additional notes: Stuff some scraps of ceramic blanket in the collar around the burner tube. This will prevent heat from escaping and avoid the chimney effect where hot air is drawn into the burner.
Open up the door to let some fresh air in and start it up.
Using a hand-held propane torch I lit the burners and cranked up the gas. The initial burn is quite rich. The large yellow flame is indicative of unburnt fuel.
Other Things of Interest
If your burners are running rich you can shoot some additional air into the intake to improve the fuel-air mixture. Remember we need about 1:100 fuel to air and that's a lot of air. If a burner is running rich, the additional air will make it more efficient and save you on LPG. The correct ratio will also reduce the carbon monoxide (CO) output of the forge. CO is bad.
LPG vapour is relatively safe and needs the right amount of air to burn. Unlike gasoline where combustion is possible with a wide range of mixtures, LPG needs the ratio to be just right
It's highly recommended that you get a carbon monoxide detector for your shop if you are going to forge indoors. CO is colourless and odorless and poisonous. Outdoor forges are ventilated and CO buildup is normally not an issue.
Storage of Cylinders
Legally in my locale (may vary by juristiction) propane is not to be stored indoors.
As per the Propane Storage and Handling Code, no propane cylinders may be stored or used inside any structure unless specified in the code, and shall not be stored or installed under any fire escape, stairway, or ramp used as a means of egress form a building.
This means your tank needs to be outdoors.
Orientation of Cylinders
The normal 20 lb BBQ cylinder must be stored and used in the upright position. The vapour is meant to be in the top of the tank. If the tank is on its side, there is a risk of liquid coming out of the tank's pressure relief valve or the main valve. One drop of liquid will vapourize to 270 times its volume. When mixed with enough air, this can become an explosion hazard.
In cold winter areas this may become a problem as the low temperatures may prevent the vapour from boiling off inside the tank. The rate of boiling is a function of the temperature and surface area where the liquid is exposed to the ambient temperature.
If you are concerned about accidentally hitting your gas hose with a super hot piece of steel, you can make a leather wrap for the hose. Optionally, you can buy a stainless steel hose braid to cover the hose or use foil for that matter.