Closed Bedroom Doors Can Cause Mold and High Carbon Monoxide

One warm summer day a few years ago, I got a phone call from a nice lady with a lot of worries. She and her husband, a preacher, lived in a fairly new home with their four teenage sons. The problems she reported were that most of the air conditioner ceiling grilles were growing a healthy crop of black mold, the house stunk, and one son had asthma which the mold seemed to aggravate. She also mentioned that the carbon monoxide (CO-an odorless, invisible, but deadly gas) alarms in the house went off now and again, but she didn't think that had anything to do with the mold. She had asked several a/c firms to make service calls and all had told her that the a/c systems were fine and not the cause of her problems. Then someone referred her to me with the usual, "If anyone can figure this house out, it's Doug."

I went over and found a very clean, well kept home with four bedrooms upstairs and the living quarters downstairs. While making my initial walk through of the house, I noted that the boys kept all three of their bedroom doors closed and that the master bedroom door was also closed. A few minutes later I came across the natural gas water heater in a closet off of the utility room downstairs next to the kitchen. The home had two a/c units and two return air grilles, one of each upstairs and downstairs. The homeowners had also set the a/c fan to the FAN ON setting to keep the air moving and improve comfort. Now, none of these factors are unusual in the least. Most two story homes have two units, most teenagers keep their bedroom doors closed when at home during the summer, lots of families keep the a/c fan in the ON mode, and many Texas homes have a gas water heater in them.

I had an idea about what was going on, but I needed to get some hard evidence, so I decided to test the pressure difference in the house. Being a building science geek, these facts came together to form a quick hypothesis (that's forensic investigation speak for an educated hunch). You see, when we close interior doors, it it will block the flow of air out of the room and keep it from returning to the air conditioner. It doesn't seem like a big deal, but the pressure imbalances that can result are powerful enough to cause the whole house to experience negative air pressure sufficient to suck carbon monoxide backwards, down a fireplace chimney or the flue of a water heater.

Another thing that closing interior doors can do is create so much negative pressure in the house that it sucks in large amounts (think hundreds and hundreds of cubic feet per minute) of hot, humid outside summer air or cold winter air into the house. This is like opening a window and placing a 20" fan in it so that it blows outside air into your home! This flow of very humid air can bring in so much moisture that the air conditioner can't remove it fast enough and the house becomes too humid. When this happens, the excess moisture will condense on the coldest surfaces in the house, the supply grilles of the air conditioner. That's when the grilles start to grow a healthy crop of mold.

We building science geeks use pressure measuring devices (digital micro-manometers) accurate to 0.0004" water column or 0.1 Pascals of pressure. How accurate is that? Well, it's about three decimals to the left of the accuracy of the air pressure devices used by any other type of contractor you've ever met. (We sometimes joke that it can measure a gnat fart).

I left the a/c fan in the ON setting so it would run continuously. I placed one plastic tube from the manometer outside of the house to measure the constant reference pressure of the great outdoors and left one pressure tap open in the house. I then measured a baseline pressure difference from inside to outside with all of the interior doors open. It was +0.5 Pascals. I then went upstairs and closed all four bedroom doors and read the house pressure under these conditions. It was -4.1 Pascals. Then I went upstairs and opened the doors again and measured +0.4 Pascals, closed them all and reaffirmed -4.3 Pascals. Building science research has shown that negative pressures in homes in excess of -3 Pascals can cause combustion appliances to back draft. This house was experiencing more than enough negative pressure to keep the water heater from safely drafting.

I needed to test if the water heater was drafting safely. With the bedroom doors still closed and the a/c fans ON, I was walking toward the kitchen when the CO alarm went off. The homeowner quickly turned it off apologizing for the noisy interruption. When I got to the water heater closet I saw that the water heater was operating, so I took my smoke puffer (a handy little device which when gently squeezed emits a puff of thick white smoke which will drift with any air currents making them visible) and placed it next to the draft diverter at the top of the water heater. I squeezed the bottle and the smoke blew back past my head. When a natural draft water heater is operating, there should be a positive draft up the flu which should suck the smoke in and up the flu and out of the house. When the negative pressure inside the house is so great that it overwhelms the flu draft, the CO produced by the appliance is sucked into the living space. This explained why the CO alarms were being triggered. When I placed my hand next to the draft diverter, I could even feel the outside air being sucked into the house!

I then looked at the sizing of their a/c return grilles, and they looked too small. I measured them and based on the number of tons of cooling they had on the house, I calculated that they were about one-half the face area that they needed to be to meet manufacturer specifications. This is unfortunately the norm. The U.S. Department of Energy did a big national study and found that the average American air conditioner is being strangled and only gets 62% of the air flow it requires to efficiently make the amount of cold air it's rated at. That's a big reason (pardon the size pun) why you find so many five ton a/c's only producing 2.5 tons of cooling. But, that's another story.

I also measured the temperature difference of the air entering the air conditioners and leaving the units. It was too big a difference with the supply air being too cold. This happens often when you don't have enough air going over the evaporator coil. The air temperature leaving the coil gets too cold. This means that the supply grilles get extra cold and they condense more water and grow mold better as a result.

So, the problems were:

  • Interior doors being closed caused imbalanced air flows leading to pressure imbalances.
  • The pressure imbalances caused lots of humid, outside air to be sucked in which raised indoor relative humidity levels increasing the condensation potential and making it harder for the a/c's to cool the house (funny how having bunches of hot outside air entering a home can do that).
  • The negative pressure imbalance in the house also sucked the CO down the water heater flu and into the house setting off the CO alarms.
  • The A/C return grilles were undersized causing low airflow resulting in the temperature of the supply air dropping too low, thus making the condensation on the ceiling grilles even worse and reducing the ability of the units to cool the house!
  • All of this made the house too humid and sticky to be comfortable, the mold and CO irritated the asthmatic kids health problems, and the electric bills more expensive than they should be.

The solutions were:

  • Connect the bedrooms to the hall with jumper ducts (simple short duct runs that allow the air to go from a closed bedroom back to the main body of the house - a detour air duct) so the doors could be closed for privacy without causing imbalanced air flow and negative pressures in the house.
  • This meant that the house was no longer sucking in hundreds of cubic feet (think of a cubic foot of air as being the amount of air in a big 12 inch diameter balloon or in this case hundreds of big balloons) of air per minute of hot, humid air.
  • This reduced the moisture load on the house so excess water no longer condensed on the ceiling grilles watering the mold crop.
  • The balanced pressure in the house also let the gas water heater draft properly and safely so no more CO alarms!
  • Install two new return air grilles so the a/c's could actually breathe freely and operate efficiently at full cooling capacity (they were also quieter), and the supply air temperature was in the correct range

The results were:

  • The pressures and air flows in the house were balanced.
  • This stopped the water heater from back drafting CO into the house.
  • The indoor humidity went down to normal so the mold quit growing on the ceiling supply grilles.
  • The asthmatic kid got better.
  • The inside relative humidity went down, so the family's comfort went up.
  • Without the mold the bad odor went away.
  • The family could still close their doors for privacy.
  • Last but not least, the electric bills went down, too.

...Just another day in the life of a forensic building science geek.

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