Up until the 1960s our house was heated using just the fireplaces in the front room, living room, master bedroom, and the original wood-burning stove in the kitchen. With no insulation in the house to speak of it would have been a huge job to keep the house warm. In the 1960s it was decided that a forced-air oil furnace would be installed in the crawlspace in a horizontal position, and the only way they were able to get it down there was by cutting a 30″ x 30″ hole right in the middle of the floor of the front room and dropping it down. I cringe at the idea of cutting a hole that size through 20+ foot long quarter-sawn fir boards, but it was different times back then and I’m sure it was a fairly utilitarian decision. Once the furnace was installed the hole was patched with plywood, flush with the finished floor, and a carpet was installed over top to hide the evidence until I came along 50 years later. The ductwork running from the furnace was uninsulated, and ran throughout the uninsulated crawlspace to each room, with one or two register holes cut into the floor of each room. Two cold air returns were also added in the floor of the living room and hallway. It would have been an incredibly inefficient system! By the time we bought the house the furnace and ductwork were all still there, but in such bad shape that one of the first things I did at the house was to rip it all out. All the holes in the floor still remained however, so in August I set out to patch them all up using original boards that I salvaged from the kitchen and bathroom.
The method I used to patch the floors came straight from the March 2014 edition of Fine Homebuilding. Click here to see the article. The subfloor is rebuilt by closing off the hole with plywood flush to the existing subfloor; the surrounding boards are all cut back in a staggered fashion so that the patch is not as apparent; and the replacement boards are cut to length and installed groove-to-tongue.
In total there were six 4″ x 10″ register holes, two 6″ x 14″ cold air return holes, and the big 30″ x 30″ hole to patch up. It was a lot of work, but I think it was well worth it. The photos below show the patch on the big hole in the front room just before the hallway: