Last month we were working on a commercial remodeling project. Part of the work involved moving an existing gas fired hot water boiler. When we disconnected the piping and moved the boiler we discovered a big burn hole under the unit. It appeared that the boiler had gotten hot enough to scorch the wood underneath. It had burned through one layer of plywood and started to burn into the layer underneath.
This points out how important it is to keep proper clearances between appliances and devices that generate this type of heat. Zero clearance combustion fireplaces sound like they can be set right tight to wood….but they cannot. You must carefully follow the set back distances from all metal to wood framing.
Boilers and furnaces also have required distances to combustible materials. These units generally do not get so hot that they will scorch wood or combustible material, but they have the potential to do so. If a gas burner malfunctions it is easy for a boiler or furnace to overheat, or direct flames outside of the firebox. If the unit is sitting on or adjacent to combustible mateirals there is potential for a fire.
In the case of the commercial boiler in our remodeling project it apprears that the wood under the boiler never had enough oxygen to support flame. However, it is clear that it smoldered for quite some time. Eventually it could have burned through the last layer of plywood. At that point there would have been a rush of oxygen and the area most likely would have burst into flames….something no one wants to have happen.