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Question DetailsAsked on 5/18/2018

Does size of ductwork affect use of variable speed blower motor?

Replacing HVAC older home. One contractor states my ductwork is too small for a variable speed blower motor. Three others are quoting using variable speed saying it should work fine. Who do I believe?

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1 Answer


If the ducting is undersized you may need a higher pressure rated blower unit (most manufacturers have that as an option, at least on units which arenot their bottom-of-the-line) to move the required amount of air through the ducts at full operating speed. But if suitable at full blower power, then the lesser speeds (with multi-speed or variable speed blower) should work fine. Not as energy efficient or as quite-reacting as one with proper sized ducts and may mean the unit runs a bit longer each cycle, but should work.

However - with the factory standard blower they are designed for only a certain maximum airflow resistance - use too small a ducting or too many long runs and it can have airflow which is less than the unit is designed for - which results in lower operating efficiency (less efficient removal of the heat from the heat exchanger or electric heating elements so more of the heat is escaping out the flue) and in extreme cases can result in overheating of the heat exchanger or flue which can cause safety shutdowns.

So - your ducting may not be suitable for certain blower units or certain BTU rated stock furnace, but undersized ducting would arbitrarily not rule out multi-speed or continuously variable speed blower. Perhaps check back with that one provider on this and ask, if the unit he is quoting will not move enough air, if he can quote a different unit which can and still provide the required BTU output, or an upgraded blower unit on the unit he proposed.

It is also possible, in extreme cases, to put in a commercial continuous-duty rated unit (so designed to run up to 100% of the time in cold weather rather than only 50-60% of the time max) with lesser maximum BTU output rating, running it a greater percentage of the time to move the BTU's per hour you need. Though that can increase your electric bill maybe on the order of typically $10-20/month in cold months, due to the blower running a lot more - and also makies for slower household temp recovery in going from a lower to a higher temperature setting period if using a time-of-day variable temperature thermostat. Of course, if you have A/C installed in the duct (central air system) you may not be able to do this because it will reduce your A/C capacity and extend its run times too - shortening its life.

Another solution in some cases is to put a booster fan in the ducting - to increase the air movement in the ducting - basically the Tim Taylor solution - MORE POWER - to overcome the increased airflow friction caused by the small ducting. Sometimes this is cheaper than putting a larger capacity blower in the furnace. If this booster is required to meet the airflow requirements for the furnace to operate safely, then additional safety sensor(s) would be required to prevent the unit from operating at full heat output without that booster fan operating - not hard to do, but not all techs know how to wire that. Manufacturer should be able to help with that if needed.

Answered 2 years ago by LCD

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