Ventilation

Posted in: Ventilation
Fresh Air Intake Vents
By Martin Smith
Aug 21, 2008 - 1:24:16 PM

Question: My open living and kitchen area has a free-standing wood heater and a fireplace 15 feet from each other. There is a fresh air vent in the fireplace. But then in the past decade and a half, I have started a fire in the fireplace just twice. Both these times, it gave off a sooty smell that was pulled from the wood heater. But when I used the wood heater, amazingly there was no smell. I have been thinking of getting the fireplace installed with gas logs. Will the same sooty smell come pulled from the air that gas logs might pull from the wood heater?

Answer: I am willing to wager on my answer. The description that you just now provided is clearly that of back drafting. Every minute large volume of air is guzzled by the burning fireplace. This could very well be in the range of 250 to 350 cubic feet every minute. Hough there is the fresh air vent in the fireplace, it may either not be supplying the required fresh air fast enough or it may be blocked.

Nevertheless the additional air is required to be supplied to meet the needs of the fire. This additional air has to come from someplace. Obviously the air will come from where it can easily come from with the least possible resistance. This is what physics tells you. The flue of the wood heater in your case seems to be the easiest source for air to come into the room.
 
A major problem in ventilation in homes is back drafting. Homes have become more air tight and energy efficient and back drafting is a result. There is lots of equipment in homes that blast lots of air away from the living space. Do you know what happens when your kitchen cooking ventilation fans, bath fans and central vacuum cleaner are switched on? And when your clothes dryer is turned on? When these devices are turned on, the air is exhausted outdoors resulting in a partial vacuum being created in your home.
 
The air which is being exhausted will have to be replaced. If there is no air inlet to replace the exhausted air, the replenishment will have to be from the cracks and leaks that may be there in the doors, windows, water heater, furnace exhaust piping and chimneys. If the demand for replacement air is too much due to a number of appliances operating at the same time, there is the possibility that exhaust gases could be sucked backwards from the fuel burning appliances to your home.

Built-in pathways are provided for a number of water heaters and modern furnaces to supply air that is needed for the appliance. Residential building codes also require that the combustion air needed for fuel-burning appliances be provided. But an average home requires more air than are required for the appliances alone. This can be supplied only by providing make-up air or extra ventilation which is mandated by institutional and commercial building codes. The environment becomes healthier from this fresh air that enters the building and in the process also avoiding back drafts which could be a potential hazard. Make-up air or ventilation in your home is to take care of these and for your own good.

This is well recognized. There are a number of companies that make air exchange appliances that are sophisticated. All or some of the make-up air or ventilation required are provided by these devices. But whether you need such a device is something that I won’t be able to say. There might be easier and cheaper solutions to solve your problem. But still if you do make up your mind to go in for these devices, you should check out the details and also consider whether the investment is worth in terms of the returns.  

I have seen a very clever device. It looks just like a dryer exhaust hood but bigger. Two pipes of four-inch diameter substitutes the single pipe that supply to this hood and connects to the vent side-by-side. One four-inch pipe is connected to the cold air return duct of the forced air heating and cooling system. Fresh air is drawn into your home when the air conditioner or the furnace operates. The other pipe acts as the main or supplementary combustion air supply, which pipe ends adjacent to your fuel burning devices. In your instance it will be better if this pipe terminates quite near your fireplace so that the required additional combustion air will be there.

You may have the fear that your energy bills will increase when you introduce hot or cold air inside your home. But it is likely to remain the same or could even drop! The vacuum caused due to the operation of devices such as clothes dryers, fans, fireplaces, vacuums, and fuel burning appliances will be offset by the consumption of air by these appliances. The thermostat manages your air conditioner and furnace cycle lesser times a day with the air entering from one place rather than many paces by installation of a simple device.


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