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Duct Boosting - How to increase air flow in ducts.

What is the problem ?
First of all let's talk about the problem of inadequate airflow (we can assume that's why you are reading this page). For one reason or another many homes have been built with undersized ductwork. The problem usually shows up in remote rooms that are just plain too hot or cold. What's going on here is that pressurized air (from your furnace, air handler) will take the path of least resistance and not travel to those distant rooms.

What can you do ?
First rule: Cheap things first! Most furnaces or air handlers have multiple speed connections (taps). Perhaps you can raise fan speed on the unit. If you do have too much airflow on the shorter duct runs, you will need to throttle back that air. Dampers located on your grilles or registers are not very useful for this purpose and can cause a lot of noise if used. We do recommend balancing dampers (see link here). Obviously this approach may not work for you and does require some labor.
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Booster fans.
We have several varieties and brands of booster fans. How do you know which one will work for you. As you would expect, there are quality and performance improvements which come with higher prices (also said as .."you get what you pay for")

  Suncourt Booster Fans
(Less Expensive)
Fantech Booster Fans
(More Expensive)

Performance requirements.
Remember that you are looking for a fan to add pressure and flow to the air flowing in your duct (windmills are not considered a success). Without bringing in a team of technicians and engineers, we can still do much better than just a guess.

For the more technically minded we will talk about pressure drop (resistance to airflow). This pressure drop (measure in inches of water column) is proportional to distance and also rises with the square of the velocity. This means that if your airflow in a duct doubles the pressure drop will go up by a factor of 4.

How much airflow
The thorough and proper (and expensive) way to figure airflow is to provide an HVAC engineer with all the parameters of your house (dimensions, insulation, window type, ....). And while you're at it, why not rebuild the whole house :-) We're going to estimate the required airflow by a couple of methods.

Method 1: Multiply the room area by 1 to get approximate CFM (cubic feet per minute) required.

Method 2: Multiply the furnace airflow (you can guess at 400 CFM per ton of AC) x % of house floor area served by the duct.

How much pressure to add?
Here are some scenarios

Raise airflow by xx% static press. required (" WG)
20% 0.09
40% 0.19
60% 0.31
80% 0.45
100% 0.6
120% 0.77
140% 0.95
150% 1.05

Data above is based on an initial duct pressure loss of 0.2" w.c. As you can see, if you need to increase your airflow by a large margin, then we need to go to some big fans!

OK, now you are ready to pick a fan. Here are our suggestions:

Suncourt Booster Fans
- for low to medium boosting
- inexpensive

Fantech Booster Fans
- for medium to high boosting
- 3-5 year warranty
- full range of fan performance
- German made motorized impeller design

S&P Booster Fans
- for medium to high boosting
- 3-5 year warranty
- full range of fan performance
- motorized impeller design
- very cost effective

Automatic operation of fan.
Booster fans are typically connected back to the furnace/air handler and interconnected (with the help of a relay) so that the booster fan runs when the main fan runs. This is called hard wiring.
Some other options:

  Suncourt DS100 Booster Thermostat
(NOTE: cannot be installed in an attic/crawlspace)
Fantech DB10 Booster Pressure Switch

Don't forget.... the Accessories:
- speed controls
- duct tape
- flex duct
- silencers

As always, if you would like to ask more questions about this or other technical topics give us a call at 877-711-4822 - 8-5 PST. Live people for technical and sales support - an Internet first!

Advice and information given through our representatives is checked for usability and conformance with most codes. You as the user, however, have the responsibility for verifying conformance with local codes and standards and ensuring a safe installation. Although HVACQuick.com may employ and use professionals, their advice is not meant as substitute for professional engineering services. It is the customer's responsibility to hire and retain suitable engineering and architectural professionals as required.