My wife and I just purchased a home built in 1900. It has two separate HVAC systems, with a package unit downstairs, and a traditional compressor/furnace combo in the upper. Most of the windows are in need of replacement as they allow quite the draft. Also, the original hardwoods are not insulated underneath the house. Everything aside, my problem lies with the package unit downstairs. With the unit turned on there is very little heat coming out of the 2 or 3 vents closest to the unit, and no heat or anything coming out of the vent further away. Upon inspection of the unit, I found what appeared to be a fresh air intake hastily screwed to the unit. There was quite a suction coming from the opening where the panel should have been. I replaced the panel that was meant to go there, which I hoped would allow the air to circulate through the house, as it should. Still, after this, no luck. Ive been under the house and checked the ductwork to find the same thing as in the house, no air flow. My thoughts on the matter are, if the return side has such a large amount of suction, the supply side MUST be putting air out as well, so why no air through the duct?