Combustion/Dilution air is required on all gas appliances; always include hot water tanks total BTUH input in the calculation. If hot water tank is the only system requiring combustion/dilution air then it is only required on over 50,000 BTUH units.

Fresh air (make up air) is required on every home with any type of furnace. Fresh air is connected more than 1 meter from the drop of the return air for the furnace then connected outside. If the fresh air is connected closer to the drop than 1 meter it will cause cold air to be brought into the room even without blower motor running.

High efficient furnaces do not use combustion air from the room as they have two pipes to the outside one exhaust air and one combustion air. Sometimes only one pipe (exhaust air) goes outside in this case the furnace will need combustion air brought in to operate properly.

These sizes are to be used at your discretion, as it may be different depending on local codes

Tight structure design

Combustion air inches round duct – total of all appliances BTUH inputs

3-inch – 25,000/50,000
4 inch – 75,000/100,000
5 inch – 125,000/150,000
6 inch – 170,000/225,000

Combustion air pots must be located within two feet horizontally and level to within one foot above the burner of the appliance with the largest BTUH input rating

If you find cold air being drawn in from either the combustion or the fresh air I recommend getting a Hoyme motorized damper that will only open on a call for heat. 
hoyme website
Shell Busey and Cliff Hoyme Explain the Motorized Damper

Combustion and fresh air outside terminations have a mesh that need cleaning annually. Check out picture of fresh air intake under deck

Click here to see my Post on protecting your home “Alarm for Home”

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