That scenario is unlikely if a MERV 8 or greater air filter is installed at the inlet to the air handling unit. Supplying ventilation air via HRV/ERV's or other separate supply fans into the return side of the central system ducts can also be risky because air still can flow through the central air handling unit while it is off. Therefore, it is not recommended to inject ventilation air from HRV/ERV’s or other separate ventilation supply fans into the central supply ducts during humid outdoor conditions. If that occurs over a sufficient length of time, mold will grow. Those are conditions for condensation inside the central supply ducts. The supply duct surfaces are about 50 to 55 ☏, and the summertime ventilation air dew point is usually above 65 ☏ in humid climates. Then think of humid ventilation air flowing through those ducts from a single-point supply ventilation fan or an HRV. Think of the inside surface of your central air conditioning system supply ducts at the end of a cooling cycle like the surface of the soda can. In wintertime, if room temperature is 68 ☏ and the room relative humidity is less than 30%, then the soda can temperature is slightly higher than the room dew point and condensation will not occur. Those conditions will cause condensation to form on the soda can. In summertime in air conditioned buildings, the room air dew point temperature is usually 55 ☏ or higher. To be more specific, the temperature of the surface has to be at or below the dew point temperature of the air for condensation to occur.Ī soda can that you take out of the refrigerator is about 36 ☏. The best grilles have a means to close down the air flow and have curved diffuser-type surfaces to spread the air out evenly with little noise.Ĭondensation occurs when air is humid and surfaces are cold. Many good plastic grilles are available in sizes suitable for ventilation ducting. Inline balancing dampers do a better job of adjusting air flow. Grilles with a means to adjust the volume of air flow can help with balancing, but there are limitations. Grilles with adjustable curved blades to turn the flow to suit the situation are worth the investment. Aluminum rather than steel should be used where moisture will be present. Stamped metal grilles are the least expensive, but offer no adjustability and are the least efficient. General conventions for branching round ducts are:Įxhaust fans in the same dwelling can share a common discharge duct, but each fan must have a back-draft damper to prevent movement of air from one fan back through another. Throw and efficiency have to do with how well the air is injected into the room so that it mixes well with room air. This will help keep static pressure and noise down, while keeping throw and efficiency up. If you are designing the duct system, when laying out duct runs and sizes, plan for air velocity of:ħ50 ft/min or less for exhaust ducts after the fanģ50 ft/min or less for exhaust ducts before the fan (also called pickups) While they are less commonly stocked, they are available, and can make achieving the right air flow much easier. Don’t ignore the odd sizes of 5 inch and 7 inch diameter duct. Smooth increaser/reducer fittings to change duct sizes are commonly available and inexpensive. Remote fans usually have 4, 6, or 8 inch diameter inlet or outlet connections. The outlet connection for standard bathroom exhaust fans is usually 3 inch diameter. Some UL 181 listed tapes can also work well on clean surfaces, but do not use cloth-backed tape. Seal all joints with long-lasting material. Size the wall cap or roof jack to match the final duct size. Go up 2 inches in duct size if the duct run is long or there are many fittings. Use that size if the duct length is fairly short (less than 10 ft.) Increase the duct size 1 inch if the duct run is not long (less than 25 ft.) and there are few fittings (less than 3). Start with the size of the fan outlet/inlet connection. Pull the inner liner of flex duct to its full extent to avoid the spiral “accordion” effect which causes a lot of flow resistance, or use smooth metal duct. Make changes in airflow direction as infrequently and as smoothly as possible. Following are a few simple, common sense rules that will serve you well for most situations: However, that level of detail is usually not needed for residential ventilation systems. The best known of these procedures is the Air Conditioning Contractors of America (ACCA) Manual D. Following some basic, uncomplicated design guidelines can go a long way to avoiding most trouble spots.Ī lot of time and effort can be spent following engineering procedures to design the ducts and fittings for a ventilation system. This Insight is an excerpt from Armin Rudd's " Ventilation Guide." This publication can be ordered online from Experience is a great teacher, but much bad experience can be avoided through education.
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