Recently, a newsletter recommendation was made about the ventilation of the utility room. I have decided not to run the vent fan in that room unless the door is open and I have also decided to leave the utility room entry door open. I provide my reasoning here.
The room is about 350 cubic feet in size and contains three appliances which have the following natural gas input:
- Furnace 100,000 BTU/hr
- Hot water heater 40,000 BTU/hr
- Clothes dryer 20,000 BTU/hr
The Role of Combustion Air
All of these appliances require air to properly burn the natural gas fuel. That air is called “Combustion Air”. The purpose of the air is to properly burn the fuel, dilute the gases that are being exhausted, and to vent and cool the appliances.
There are specific requirements regarding the air required to burn natural gas. Approximately 10 cubic feet of air is needed for clean burning of one cubic foot of natural gas (at standard conditions), assuming one cubic feet of natural gas has 1000 BTUs. Air contains about 21% oxygen, 79% nitrogen, as well as carbon dioxide and other gases. The oxygen is necessary for combustion.
When there is sufficient air, or oxygen for complete combustion of the fuel, there are three products: heat, water vapor (H2O) and carbon dioxide (CO2). Improper combustion results in another gaseous by-product, which is carbon monoxide. Carbon monoxide or CO is a colorless, odorless gas and is deadly.
Sources of Combustion Air
If I close the utility room door, there are no other openings to provide combustion air for the hot water heater, furnace, etc., which will use any available oxygen in the room for combustion purposes. After the available air is used, the air pressure in the utility room will become lower than the pressure in the vent pipe and the room may become a vacuum, sucking some fresh air through the narrow opening beneath the door and also from the exhaust vent pipe. Because that vent pipe is shared with an adjacent unit, it is possible to suck the byproducts of combustion from my neighbor’s furnace or hot water heater into my utility room. If there is insufficient oxygen for combustion, then deadly carbon monoxide will be produced.
This also wastes energy. Heat is produced when the fuel burns in the presence of sufficient oxygen. The natural gas fuel is about 90% methane, which is CH4. Maximum heat is produced when the fuel which is comprised of carbon and hydrogen, is burned completely, that is, when the hydrogen unites with the oxygen in the air to produce H2O and the carbon unites with the oxygen to produce CO2. However, when there is a lack of oxygen, the combustion is incomplete and the hydrogen in the natural gas is burned, but some carbon does not unite with oxygen to form carbon dioxide CO2 but instead forms carbon monoxide CO and less heat is released.
Properly Ventilating the Utility Room
The utility room is a confined space with a closed, solid door, and walls and ceiling that create a barrier between the fuel burning appliances and the rest of the house. It has less than 50 cubic ft of air volume for every 1000 BTU/hr of appliance input.
My utility room uses gas fuel appliances which include a gravity warm-air furnace and hot water heater. Gravity warm-air or fan-assisted furnaces are defined as those that vent out through the roof, usually through a chimney, and do not bring in combustion air from the outside of the building.
How Much Combustion Air is Required?
As stated above, approximately 10 cubic feet of air is needed for complete burning of 1000 BTUs of natural gas. Since my furnace is rated 100,000 BTU/hr, it requires 1,000 cubic feet of air per hour, when it is in operation. How much air is that? The slot under the door is 1 inch by 31 inches, or about 0.2153 square feet. To move 1,000 cubic feet through that slot each hour, would mean that the air must enter at a speed of 4,644.6 feet per hour, which is about 0.9 MPH, a nice gentle breeze of air moving about 78 feet a minute.
If all the appliances are running at the same time, then the utility room would need more air and the breeze under the door would be about 1.5 MPH.
The first question to be asked is, what will induce the air to move through the slot under the door? The answer is, the vacuum created when the gas fired appliances are in operation. The second question to be asked is, are there other possible sources of air for the room? The answer is, the vent pipe leading to the roof and also connected to the adjacent unit’s gas appliances. The third question is, wouldn’t a running vent fan in the ceiling of the utility room compete with the gas appliances for fresh air, increasing the vacuum in the room? The fourth and final question is, how can I be assured that there is sufficient air entering the room for the gas fired appliances?
The Bottom Line
I could cut a hole in the door in accordance with the Uniform Mechanical Code. According to that code, in ordinary construction where gravity warm-air or fan-assist furnaces will be installed in a mechanical room, two required openings of 1 sq. inch per 1,000 BTUs of fuel input are required, one located 12 inches off the floor and the other within 12 inches of the ceiling. So in my case, that means two openings of 160 square inches, or about 12 x 18 inches with a 25% mesh. Or, I can simply leave the door open! That is my choice.
Good point, but the second floor units have adequate air intakes, (two vents in the ceiling, one of those being the vent with the fan, both of which will allow air to be drawn into the utility room from the attic).
ReplyDeleteI wouldn't call the area of the fan and vent "adequate". The fan grill is 6 inches in diameter and about 50% obstructed by plastic and of course the internal parts. The grill in the ceiling is 3 x 9 inches, and is about 33% obstructed. If I include the free area of both, that's about 32 square inches of ventilation area. that's about 10% of the recommendation of the Uniform Mechanical Code. My concern was running the vent fan with the door closed. The fan does increase the effective vent area but what is the source of the air vented? In my circumstances, if the door is left open there is ample air inlet. I did discuss this with a mechanical contractor.
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