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SWEATING (CONDENSATION) on PIPES, TANKS
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VENTILATION in BUILDINGS
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Bathroom exhaust fan duct length specifications: here we provide specifications for recommended minimum & maximum duct lengths for bathroom vent fans. A bath vent fan duct that is too short may violate the manufacturer's installation instructions, may not work properly, or may be drafty; a bath vent fan duct that is too long may restrict air flow such that the fan is not functional.
This bath vent fan installation, troubleshooting, repair article series explains why bathroom vent fans are needed and describes good bath vent fan choices, necessary fan capacity, and good bath vent fan and vent-duct installation details.
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You note for the best bath vent fan duct termination to route the duct: horizontally through the building wall at a building gable-end wall (first choice), or if the gable end is too distant ... . what is a distance the would be within you recommendation?
Reply: Bathroom Vent Fan Vent Ducting Installation Advice Details from InspectAPedia
Our bath vent fan duct installation photo (above-left, courtesy Galow Homes) shows use of solid metal 4" ductwork conducting the fan exhaust to an outdoor soffit under a cathedral ceiling roof.
Following the sloping roof/ceiling and exiting into the soffit, this fan vent duct slopes down away from the vent fan which you can just see in the photo upper right corner. The ceiling cavity was later filled with solid foam insulation (second photo below).
Any condensate in the ductwork drips to outside - a possible winter icing worry, but because of the solid foam insulation there should be little condensation in the fan duct. Also we insulated the full outdoor soffit bay where the duct exits the building to avoid condensate icing.
Take a look at the vent fan installation manual for the particular bathroom vent you are installing. Because bath and kitchen vent fans vary in their power, the length of the vent ducting could provide enough airflow resistance to a small vent fan that it would not work properly.
In our photographs above, courtesy of Galow Homes, three long, un insulated bathroom exhaust fan ducts cross the cold attic floor of a New York home and then join together at a common sidewall exit of the same diameter. There is little chance that these bath vents will work effectively and a good chance of pooling condensate in the low spots of the duct run.
The bath vent fan installation manuals we reviewed did not specify a maximum exhaust duct length, they just say route the duct to the outdoors. If you contact your bath vent fan manufacturer directly and speak with a technical support contact, s/he should be able to give you a maximum recommended exhaust duct length for their fan model.
We haven't yet found a bath vent fan manufacturer's spec nor building code or ASHRAE specification limiting the horizontal run length for bathroom vent fan ducting, perhaps because the normal size of residential buildings means that the duct run length will be within the operating capacity of currently-sold bath vent fans. But we do have some specific suggestions for a good bath fan vent duct installation and some duct length specifics as well:
Maximum & Minimum Bath Exhaust Fan Duct Length: As we introduced at BATHROOM VENTILATION, installers should make the bath vent fan duct run through the attic (or any other space) as short as possible. To avoid unnecessary reductions in air movement through the bath vent exhaust system, avoid elbows and bends as much as possible.
Question: recommended minimum distance required between bath vent fan exhaust and outside main gas line and meter
Above we quoted
I haven't seen a gas code specification for the required clearance between a bathroom exhaust vent fan outlet and an incoming gas line or pipe at the building, perhaps because an exhaust vent opening should be venting only one-way: from the building interior to the outdoors. Second, gas piping should not be leaking, and if it is, you should smell the gas leak and repair it immediately.
Question: recommended maximum distance for horizontal bath vent duct routing
You note the best practice is to terminate the bath vent duct: horizontally through the building wall at a building gable-end wall (first choice), or if the gable end is too distant. what is a distance the would be within you recommendation. - DGB family
Good question. Naturally we want to keep the fan duct run as short as possible. I reviewed various industry sources and did not see a maximum allowable distance, but typically we see it's 10 - 12 feet or less.
Longer duct runs provide more air resistance, a problem you can mitigate by using solid metal ducting or metal flex duct rather than the more sinuous plastic and wire flexduct often found in these installations. The bath vent exhaust duct shown in our photo (above left) continued to snake across the building attic - too long, too many turns, too much up and down variation in slope - it was an ineffective installation that collected condensate inside the ductwork.
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Frequently Asked Questions (FAQs)
Reader Question: is it OK to vent a bath vent fan straight-up, vertically out through the roof? Is it ok to vent the bath vent fan through a larger duct size than the fan's outlet diameter?
I am going to install a new bath fan, I am having a new roof put on the house and decided now would be a good time to put the vent on the roof. My question is I got a vent for 6" ducting, I will need a reducer at the fan end to 4" Would this be a good size duct for the fan.? Also I an using metal ducting and it's about six feet from the fan to the roof, Should I angle the duct a little or would it be ok to go straight up.? D.K. 10/19/2013
You've raised several key topics, and your question helps us realize where we need to work on making our text more clear or more complete. A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. For example on site I might notice something about your attic and roof structure, ease of routing venting, placement of insulation, and even very basic stuff like - where the heck is your home? Bath ventilation worries may be a bit different in a cold climate than in a warm dry one and different again in a wet humid climate. That said I offer these comments:
Local Climate Affects Good Bath Vent Fan Designs: freezing vs hot and humid
For freezing climates we don't want to risk ice accumulation in the vent system - ice can collect from freezing condensate that arrives inside the bath vent duct during hot steamy showers;
For hot humid climates we don't want to have condensate accumulation in air conditioning systems and A/C ductwork, but a bath vent run through a hot attic is less likely to raise that same issue.
Bath vent routing vertically up through roof - not my first choice
I prefer to run a bath vent to outdoors via a horizontal line that goes across an attic and out through a gable-end wall or one that vents down and outside through a roof overhang or soffit. The vent run needs to be designed to drain any condensate outside not back into the bathroom ceiling; in a freezing climate I'd insulate such a vent line as well; If we run a bath vent vertically up through a roof we have two risks I'd prefer to avoid:
The vertical run guarantees that any condensation runs back down into the fan (risking damaging the wiring or fan motor) and back into the bath or bath ceiling.
The vertical run also means another roof penetration. I prefer to minimize the number of roof penetrations on any building since every penetration is a potential leak point, more so if the penetration flashing is not installed correctly.
Bath vent diameters & vent duct materials
The vent fan manufacturers installation instructions typically give maximum run lengths and recommended vent diameters for their products; long vent runs and vents that use plastic dryer-type flex-duct (not your case) cut the effectiveness of the fan by adding airflow resistance and thus increase the risk of accumulated moisture too. Metal duct work (your case) is in my opinion always a better installation: smooth interior means better airflow. Metal fan vent ducting also reduces the risk of duct crush or collapse.
I am guessing that for a very short bath vent duct run, going to a larger duct size is fine - it'd make no difference but you're probably not gaining a thing on a short run by using a 6-inch duct to vent a fan that expects to vent through a 4-inch duct.
Bath vent fan capacity
In my experience inspecting and troubleshooting buildings, I've seen many bath vent fans that seemed ineffective. A fan that nobody uses because it's too noisy means a bathroom that is rarely vented adequately (risking mold, smells, even wet insulation). A fan that is under-powered means even if the fan is used it doesn't do anything.
The fan capacity you need depends on the size of the bathroom being vented - usually calculated in cubic feet. That figure is matched against the fan manufacturer's recommendations for fan capacity measured in cubic feet per minute (CFM). The CFM rating of the fan in turn presumes that the vent routing, diameter, length, and number of obstructing turns and bends is within the company's specifications. In the article above we explain how to calculate the required bathroom vent fan capacity. Also, for bathrooms over 100 sq ft, the HVI recommends a ventilation rate based on the number and type of fixtures as shown in Table 6-12 - data discussed in more detail at BATHROOM VENTILATION DESIGN
Sorry that these notes are a bit long on arm-waving and short on more specific details, but as we've got no information about your particular installation except what's in your original note, I have to stop here.
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