I have single pane windows and one was broken this weekend. I covered it in plastic and found that I had a significant negative pressure in the house, drawing in outside air. After inspecting the duct work, I found a significant leak where the package unit heat pump (outside) connects to the duct work. Taping this up corrected the negative pressure in the house. This can cause significant performance problems, especially if the only source of air is drawn in from the attic space through the exhaust fans openings and other leaks in lighting or attic access doors.
IMy understanding is the HVAC system should be like a pump in a pool where the pump is taking water out and returning it to the pool.
My return takes air out of the house. It heats and, then then returns the very same air back into the house via the vents.
The house should not go negative unless some of the air is not making it back in. That is my understanding of the physics.....
This is the correct behavior.
Which way is the drop cloth diaphragm being pulled and which system is doing it?
If supply air (after cooling or heating) is being blown into the crawl space then you want to stop that, not add a return in the crawl space.
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.