Yes this house is pretty strange. Its an old farmhouse and when they put in the oil furnace (which we don't use) they just sort of slapped the registers here and there. We basically just stopped the cold air with batting and sealed it up with duck tape (neon pink). its already made quite a difference.

I think that is okay, at least it was in 1983 when our house was built. We have a filter at each return and none on the air handler itself which is in a very hard to get to crawlspace. One bonus of this arrangement is that I know when the filters need to be changed. I get a slight "boom" in the ducts because there was negative pressure in the return ducts and the ducts "oil can" back when the blower stops. Then it's time to change the filters. ;-) This post was edited by Dovetonsils on Mon, Dec 8, 14 at 16:40

There are new ducts in the 700sf addition, but the existing HVAC unit was over-sized for the original old house, so the installer said I don't need to replace it. There are replacement ducts for the old kitchen (will be a den), dining room and two upstairs bedrooms where I moved walls. Most of the supply ducts are on the interior walls, but the two old air returns in the floor are under windows. Everything in my renovation/addition is permitted by the city and inspections have all passed. They also inspect the "old" part as it isn't grandfathered in when doing a renovation of this size. I've replaced any electrical and plumbing that weren't up to today's code. The furnace is located in the basement that has brick walls and a concrete floor, but there is also 21 foot section that is chest high that is open to the dirt floor crawlspace. 6 mil plastic is being added to cover the dirt. R-15 insulation was added since I wrote this and it seems to be helping. All window sashes have been replaced with new energy-efficient wood sashes (had to be custom made to fit the old window openings). Thanks!

When we moved into a new house the inspection found that the old owners didn’t clean the electrostatic filter for the unit in the attic. It was a mess. We had to clean all the ducts and vents. They removed the old filter, used (real) duct tape to seal off the filter area Then I was able to use a filter in the return air. They said I should not use both or it would compromise the air flow.