Oil furnace backdrafting problem solved

Last winter, we occasionally smelled diesel near the oil furnace in the basement. We first thought there was something wrong with the furnace. We had our furnace guy checked it out twice and a power vac company sucked out all the gunk in the furnace. The problem persisted. We finally got some clues to the cause of the problem from the chimney guy. He checked everything including the furnace flue which looks just fine. He then concluded that the issue might be backdrafting. That seems to make a lot of sense. The diesel smell appeared mostly on Sundays when we cook and do laundry at the same time. The downdraft in the kitchen and the dryer are sucking a lot of air out of the house. While I was looking for a solution, we opened the kitchen window whenever we turned on the downdraft.

By this fall, I felt like we really need to get to the bottom of this. I called a local HVAC company. They sent someone out for an inspection. After explaining the problem, the guy brought in a manometer to measure the air pressure in the house. Here are the results:

  • Baseline (all appliances and furnace are off): -1.3 Pa
  • Furnace blower: -0.3 Pa
  • Bathroom exhaust fan: -0.5 Pa
  • Dryer vent: -1.25 Pa
  • Downdraft: -3.0 Pa
  • Sum of all appliances would depressurize the house past -5.0 Pa.

I was told the safety limit for depressurization is -3.0 Pa. The house depressurizes on its own to almost half of that limit. The HVAC guy doesn’t know a simple solution besides adding a recovery ventilation unit (MRV / HRV) or switching to a natural gas furnace that draws outside air for combustion.

For a house that is over 65 years old, it’s quite hard to believe we could depressurize it until I saw all the numbers. We did a lot of work on it over the years. The first major improvement was new vinyl windows. That’s probably the first instance of tightening up. Even before we started all the major work, we never had to use a lot of oil to heat the house. We get the tank filled just once and it lasts more than a year. I suspect the five outer layers of the house probably keep everything pretty tight. Those five layers are: vinyl siding, foam board, cement board (?), wood siding, and wood sheathing. We figured out all the layers when we tried to install a vent cap on the side wall for the bathroom exhaust. It’s a pain to get through that many layers.

I first tried to look for solutions that would not involve making another hole on the side of the house but nothing really fits the bill. Makeup air from the outside is the ultimate solution but the how is a bit of a mystery. A powered makeup air unit would cost a huge bundle. A fan to supply makeup air or a motorized damper would require new wiring for electricity. I am hoping there is a passive system and I have found a few of them. After a lot of research and cooking with cold winter air entering the kitchen, I settled on the Skuttle 216 Make-Up Air Control. It is a damper to be installed on the return air plenum and it opens automatically when the furnace blower comes on.

The typical installation of such a damper would require cutting a hole in the return plenum but not in our case. For some unknown reason, there is an old register in the return plenum right above the furnace. Everyone who had serviced the furnace scratched their heads over that register. We always keep it closed and we were even told to do so. The register covered an opening of 14″ X 6″ which turned out to be perfect for the 6″ Skuttle unit. Instead of cutting a hole, I had to patch a hole. However, cutting a hole on the side of the house is inevitable.

We bought a hole saw when we installed a 4″ vent cap for the bathroom exhaust so I opted to order the 4″ option for the Skuttle unit. When the package arrived, I discovered the damper remains to be 6″ and it came with a reducer and metal vent cap for 4″ installation. The other parts I bought include a plastic vent cap with a short duct, 25′ insulated flexible duct, a 6″ ceiling collar (I found this at Home Depot by accident), and a piece of aluminum sheet metal. Once I got all the materials, I was ready to work which turned out to be a marathon session of cutting a hole on the side of the house. The inner most wood layer that is visible in the basement is made up of thick boards like the joists. Cutting through 1.5″ of solid wood burned my arms, the cordless drill and batteries. I think the entire ordeal of cutting the hole took more than 4 hours. The rest of the install wasn’t a piece of cake but it was manageable.

The damper works immediately in conjunction with the furnace blower. The real test, however, is the downdraft. After several simultaneous sessions of cooking, clothes washing and bathroom exhaust, we have not had any episodes of diesel smell. And I think the constant addition of outside air would have some sort of health benefit. However, the new challenge is to change the furnace filter more often than cleaning the reusable one just once a year!

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