Refrigerant line size and air returns
We are looking to replace 5-ton AC unit and 30 y/0 furnace. We are trying to qualify for local utility rebates and need to have functional 15 SEER, variable stage, and manual J completed. I have quotes for several products from several well-known contractors. I believe that it's between Lennox XC-21 and Amana 16ASX (contractor tells me we'll get 15 SEER -- I don't have that written estimate yet).
Questions: Lennox quote says to replace current 3/8 inch in-slab copper set (30 years old) with 1 1/8 and 1/2 inch lines (thru attic and down exterior wall, about 25 feet). In my last post that size was questioned as being tooooo large. Amana contractor says to leave, flush and use current lines. I've read previous threads and know that people have varying opinions regarding this. How do I ask each contractor, without insulting them, what is required for the product. If they don't install right and there is a problem, who's pocket does the repair money come out of (if it is still under warranty)? If it isn't put in according to specifications, is it my problem, or the contractor's problem?
Q2. We have 2700 SF and will need a 5 ton unit. We have 3 - 12x12 inch returns in the house. Amana and another contractor says we need another, yet another contractor said he'd put the system in and THEN see if the air return was adequate. Lennox contractor said nothing about the air return being inadequate. Do we need another return? Thanks, my dad was a HVAC man and I truly miss the ability to call and ask him these questions. Y'all helped a great deal in my last post.
Its not just the size of the return grille, its also the size of the duct hooked to it, and the size of the duct they get connected to.
A simple static test will tell them if the amount of return needs increased or not.
As for the line sets.
Each manufacturer has their own requirements to get their SEER rating.
5-Ton Airflow & Return Sizing
If the unit is delivering 60,000-BTUH, @ 2,000-cfm, i would like close to a 500-sq.in. Return Duct area, no less than 452-sq.ins.
The velocity @ 452-sq.ins., one 90-ell, is 637-fpm, @ a 100' duct Friction Rate of 0.02". The filter area needs to be ultra large to slow the velocity way down & thus reduce all the filters' pressure drops.
Air Filter Rack Sizing
I realize this will never happen if you use the furnace filter size; however, Air Filter Rack Sizing for efficient operation - Size Gross Return Air filter grille area for 200-sq. ins. per ton. For a 5-Ton system, that would mean Two filter racks 25X20's each, I would go with Two racks & 25X25's each.
Keep air velocities through the filter(s) as low as possible. It is a good idea to Size the Filter rack(s) for a Free Air Area with a Velocity of 350-FPM or less.
For ultra efficient operation, there probably ought to be oversized filter racks in each of the 3, or if four, - all four Returns.
If duct SA registers & RA duct & filter rack sizing is inadequate, it can be difficult to get the ESP in the prescribed range for 5-ton of airflow, without over taxing the vs blower motor.