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Draft challenges with oil furnace that is secondary to a heat pump
In a system with a primary heat pump and a secondary from an oil furnace, is it commonplace to need a draft inducer?
I am having issues with this type of a system that is serviced by a conventional 3-story exterior masonry chimney (chimney located on the exterior wall and the furnace is in the basement, dedicated clay-tile lined flue for the furnace). Circa 1989 construction. When the heat pump goes into defrost mode, it kicks on the oil furnace for a few minutes that establishes an acceptable draft at about -0.04" wc in the breech with a gas temp of 500 deg F while the burners are on, but when they shut down, the draft drops off and the flue gasses backdraft and stink up the basement. My thought is that the chimney never gets warm enough to induce a residual draft. This is more noticeable during cold weather obviously when the chimney masonry is cold. The chimney has been tested/inspected ok, as well as the heat exchanger. The flue draft regulator is also operating normally. I have been talking to a few folks, and while this type of furnace configuration isn't super common, they nobody seems to feel that a draft inducer would be required. Just wondering if any of you have seen this issue.