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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8675
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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please advise of return air grille size for Lennox G61V gas

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please advise of return air grille size for Lennox G61V gas heater and add on coil .?
Welcome to Just Answer!.

I need the rest of the lennox unit model number in order to tell how much air it moves CLICK HERE FOR THE MANUAL. Notice page 3

These systems require 400 to 500 cubic feet per minute, for every 12,000 btu's of cooling coil rating...
face velocity of the return air grill should run between 500 and 600 feet per minute.

Background data:
12,000 btu/hr is one ton of cooling capacity... the average 1500 to 1800 square foot home uses a 3 ton rated system... or close to it... 2.5 tons for smaller homes, and 3 tons for larger homes depending on the region of the country and how well the house is insulated.

The return air grill should be sized for 500 to 600 feet per minute face velocity... so that a 3 ton unit, will move on average 450 cubic feet of air per minute.. 2.5 to 3 square feet of return air grill will be required...

If the grill is very close to the furnace you can go a bit smaller because there is less pressure lost in the return air duct.

Those are the principles, The furnaces have adjustable speed motors for more air flow or less air flow depending the humidity conditions and need to remove moist air .......and...... the size of the evaporator coil relative to the condensing unit...

it can get quite complex.

However if you know the tonnage rating of your system you can size the return grill for 1 square foot per ton of cooling capacity and be close enough if not slightly over sized. Oversized is better than undersized.

I hold questions open after positive ratings to allow for unlimited follow up. Please be patient though. The research & typing take time. I am going to bed shortly and will follow up with you in the morning.

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