Tuesday, December 2, 2008

Beautiful fire pit - Disappointing flame


24" stainless steel fire ring

I got a call from a California contractor today. He built a beautiful fire pit, but the client was disappointed with the project. The match lit fire pit uses a 30" fire ring fueled by natural gas, NG. The lighting ceremony produced a 6" flame. The natural gas is delivered to the pit through a one inch pipe. Why is it so wimpy?

"Is there something wrong with the equipment?" he asked.

Unless there is a crimp in the line to the fire ring, restricting gas flow, this is not an equipment problem. The problem is more likely to be a lower than expected gas pressure at the fire pit. Resolving this problem will not be cheap.

The very first decision in fire pit design is 'what will the flame look like?' Once you have that, then the problem of delivering the necessary gas pressure must be solved. Two very important variables must be considered, where will the supply pipe tap be and how long is the run from there to the pit. Once you tap into an existing gas line you cannot create more pressure than is already there. The second consideration is the length of pipe from the supply to the pit. Gas pressure rolls off over distance (see blog of 10/21/8 for more detail).

I suggested to the contractor that he get a qualified plumber to measure the NG pressure at the pit. Pressure of 150k to the ring should produce about a 14 inch flame. That would give a starting point. Since the pit is already built, going to a smaller ring (the smaller the flame the higher the flame) is probably not a viable option.

If he tapped off from the house, he may have move it and tap off from the gas meter instead if the meter is close enough. If he has tapped from the meter, he will have to resize the pipe to the pit. That is an ugly option if the pipe is embedded in the patio cement.

Always, always start with the fire ring (the burner) before anything else. What should it look like? How will the proper amount of gas be delivered to accomplish that look?

No comments: