Controlling Excessive Draft (Gas-Fired Boilers)
Sometimes a boiler installation will have excessive draft conditions as a result of oversized chimneys or other factors. Draft conditions in excess of the draft design limits of a gas-fired boiler will reduce its combustion efficiency, resulting in higher fuel costs and possible pilot problems.
A suggested method of controlling the draft on such installations is to install a sheet-metal baffle, or restrictor, in the flue connection at the boiler (Figure 15-82). This baffle may be inserted in a tee, as shown, and secured with stove bolts after the proper draft is obtained as determined by measurements with a draft gauge. If the barometric control is installed in the side of the breeching by means of a draft control collar, the same method of baffling may be used
with the baffle restricting the breeching between the boiler and the barometric control. In this method of baffling, the breeching between the barometric control and breeching should not be restricted.
The baffle installation is correct if the gate of the barometric control is approximately half open while the gas burner is operating and the draft measurement is at or below the operating design of the boiler. The draft limit for a boiler can be obtained from the manufacturer’s specifications.
Always check with the local gas company regarding the use of baffles. Caution should be exercised to ensure against overrestricting the flue. Always be certain that there is a very slight draft after the baffle is secured in position.