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AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AIR CONDITIONER COMPONENT PARTS
AIR CONDITIONER TYPES, ENERGY SOURCES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FLOW MEASUREMENT CFM
APPLIANCE EFFICIENCY RATINGS
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
CHINESE DRYWALL HAZARDS
CONDENSATION or SWEATING PIPES, TANKS
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DEW POINT TABLE - CONDENSATION POINT GUIDE
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DIAGNOSE & FIX HEATING PROBLEMS-BOILER
DIAGNOSE & FIX HEATING PROBLEMS-FURNACE
DUCTS - Asbestos
DUCT INSULATION, Asbestos Paper
DUCT INSULATION for SOUNDPROOFING
DUCT SYSTEM & DUCT DEFECTS
DUCT SYSTEM NOISES
DUCTS, Asbestos Transite Pipe
DUST, HVAC CONTAMINATION STUDY
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
FAN LIMIT SWITCH
GAS EXPOSURE EFFECTS, TOXIC
GAS DETECTION INSTRUMENTS
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HEATING SMALL LOADS
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
Continuous blower unit fan operation guide: this article explains how and why to set your air conditioning or heating system blower fan to continuous operation in order to improve indoor air quality by increased and continuous indoor air filtration. (Naturally if there is an IAQ problem source in the building it is essential to also find and correct that condition. This website answers almost any question you might ask about air filters for heating or air conditioning systems.
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For maximum indoor air qality (IAQ) improvement in buildings: for a new blower installation we selected a fan unit which had both the capability of delivering adequate CFM of air flow and a blower fan motor duty cycle which permits continuous operation if we wish to run the system that way.
Reasons for Keeping the Blower Fan Unit On
Running the blower continuously at low speed resulted in continual air scrubbing in the building. When the heating or cooling needs of the building require, the fan shifts automatically to high speed. Here are the details:
There are reasons to leave the fan in the ON position on a heating or cooling system, but we do not recommend that you do this without first asking for advice from your heating and service technician.
If your air conditioner or heating system fan blower unit was designed to permit the fan to run all of the time that's great and you can consider the benefits of continuous fan operation we list just below.
But if your fan motor was not intended for continuous duty and you run the fan that way, you may find the motor fails prematurely.
Most heating or cooling systems have one or perhaps two methods for turning the blower fan to continuous "on": at the room wall themostat and (for heating systems) at the fan limit switch - a safety control found inside the heating appliance.
If your air handler unit provides both heating and cooling (that is not just air conditioning) then there will be a limit swtich installed inside the air handler blower compartment or burner compartment.
To find this manual fan switch, see the articles, photos, and sketches in these articles FAN LIMIT SWITCH TROUBLESHOOTING and see FAN LIMIT SWITCH INSTALLATION found in our article series about the FAN LIMIT SWITCH .
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Frequently Asked Questions (FAQs) about Continuous HVAC Blower Fan Operation
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Question: what is the relationship between continuous blower fan operation and indoor humidity where central air is installed? Can condensate in the air handler raise indoor humidity?
A disadvantage I found running the AC fan continuously in my residence, was a noticeable increase in the humidity in the house. The fan would blow across the wet coils and drain pan while the compressor wasn't running and put the water it had removed from the air during the prior cooling cycle back into the living space. So I stopped doing that. - Edgardo
Reply: from DF
Edgardo, thanks for the comment, it's an opportunity to clear up some confusion.
You should not see standing water there ever. If you do, the drip tray is not draining and the tray or drain system need to be cleared and repaired. Details about condensate handling are in the article titled CONDENSATE HANDLING, A/C and details about target levels for indoor humidity are at HUMIDITY LEVEL TARGET.
Hi Dan The pan drains fine, and the backup secondary pan is dry. I am not seeing any unusual amount of water. The problem I mentioned is caused simply by evaporation back into the living space of the residual condensation left on the coils and in the pan when the compressor shuts off. The parts can't be instantly dry when the compressor shuts off. Anyway I do run the fan more continuously in the winter, as between cycles the air will settle in the house and can be close to 50° at the floor when it's 75° 5 feet off the floor. Keeping the fan on keeps the air mixed up and I can run the heat a degree or two lower.
Edgardo I'm surprised that the volume of water left on the cooling coil after the A/C system shuts off would provide a measurable quantity of total humidity in the living space. Consider the total volume of water that's resting on the coils (as you explain the drain pan is dry) - it has to be less than a quart. Evaporate a quart of water into the cubic feet of air in even a small home - it just would not be likely to explain an indoor humidity problem.
I mentioned before that the main pan drains fine, but it will still be wet until the residual evaporates, and the secondary pan is dry which is there in case the primary pan drain gets clogged. There should never be water in that secondary catch pan otherwise the main drain is clogged. Anyway, all that is normal. I have access to the condensate drain pipe, and yesterday afternoon I put it into a gallon jug and captured almost a half-gallon of condensate water in 1 hour.
Relative humidity in the house right now is 42%. Temperature is 78°. I have a 1953-built frame house that I think cost about $8,000 new when it was built. Needless to say it has lousy insulation and leaks air everywhere, and moisture up from the crawl space. So I think the AC does well all things considered. Oversized system? Don't think so. It can just keep up with the heat load, and cycle off once in a while when it's 100°+ outside. We have some pretty extreme summer weather and humidity conditions in Texas and the system handles it pretty well with reasonably low electricity usage.
Edgardo I agree that if your RH is down to 42% that's a suggestion that the A/C is both cooling and dehumidifying and is not oversized. Perhaps taking some RH measurements at regular intervals and when the system is cycling on and off will give a more clear picture of the indoor RH.
It just doesn't seem likely to me that the volume of moisture residual in a wet but otherwise drained condensate pan would explain a measurable variation in indoor RH in the rest of the building. That volume of water evaporated into the volume of air in a building could not explain high indoor RH. It may be more likely that if the home is poorly insulated and more, drafty, indoor relative humidity (RH) climbs along with temperature between cooling cycles.
Also if the RH was coming from the ductwork as a prime source when the system is off, you'd find higher RH at the duct openings as well as evidence of airflow out of the ducts when the cooling system was off. But be sure to measure RH at both the return and supply sides and registers so we're not confused by cycling higher RH house air with moisture coming from other sources or outdoors. Some measurements ought to clear that up.
Please also be sure to take a look at DEHUMIDIFICATION PROBLEMS where we describe some sources of and cures for high indoor humidity in buildings.
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