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Question DetailsAsked on 7/3/2017

Poor AC performance felt Secondary to too little size and # of Grille returns

Recently moved into a 3250 sqft home with a 5ton HVAC. Has 1-20x24" return grille in hallway. AC runs constantly with therm set at 78.
Live in NW Florida (Emerald coast). New construction.
Feel home needs multiple returns to run AC efficiently. Also would like to add second 2 ton unit to split the AC demand especially during sleep. Recommendations on # of grilles required and if extra AC would also help.

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Voted Best Answer

An HVAC system (assuming you mean central air through heating system ducts, not a mini-split with only a couple of evaporator/blower units up on the walls in a couple of rooms) with only one return would be gross incompetence or cheating you in a new build (or any home built since about 1950). Generally, each room with a supply register should also have a return grille - and typically in a well-designed system only small storage areas like typical sized closets and hallways do not have a supply. (Plus basements and garages and such which are "outside" the conditioned space).

Adding a second unit would likely result in the two units fighting each other, and almost certainly reduce the operating efficiency (not to mention adding to capital and maintenance costs). Generally, until the runs get too long, one larger unit is more efficient than two smaller ones, and if using two units (and your home is maybe getting into the size range where that would be considered in design, particularly if a one-story ranch or hacienda layout) each should serve only a portion of the house - typically with the only overlap being maybe the ability to open a door to allow leakage from one side to the other in the event one unit dies.

I would demand they provide the ACCA Manual J, S and D calculations showing the energy/cooling demand of your house, the furnace/AC capacity or sizing, and ducting/distribution needs of your house respectively - then find a different and long-time very good reputation Heating and A/C contractor (for probably $200-250 range) to come to your house and assess their assumptions and check the calcualations to see if they are reasonable - because I VERY seriously doubt this system meets building code. Both from the aspect of distributon/ducting - and also because a 5 ton unit in upper Florida would normally need about a ton per 500SF of house - so more likely about 6-8 ton of cooling (which is where you were heading with the additional 2 ton unit. Certainly if it is running full time to hold 78 degrees, unless you are in the mdist of a major heat wave, shows the unit is undersized for the house - and will also cause premature failure of your system. It should generally, in normal outside temp, operate not more than 80% of the time, and 100% only when at the design high ambient temperature, which for your area is likely 92-94 degrees (table here FYI) for the 2.5% exceedance temperature, meaning your unit should run continuously only about 2.5% of the air conditioning days -

If you had your own architect on the job, instead of an independent HVAC contractor review I would have him review this (unless he designed it, in which case I would say time to be looking into a redesign and system change at HIS cost).

It would probably also be a good idea to talk with the HVAC building inspector about your unit size and single return - because he might reinspect and fail the system, giving you more ammunition to make the contractor fix it. Of course, if you have not made final payment or there is still a punchlist open on the job, I would not close those out till this is remedied, because based on what you say I think you got cheated or someone did a grossly inadequate design - which you should not pay for the correction of.

IF you do go with an additional unit, the system design would have to consider demand areas and where you want the coolest air - it might well be that a secondary unit, using a mini-split unit or a secondary distribution system, could provide nighttime cooling in bedrooms only for instance, or even a switchable system for the major hot living rooms in the daytime and bedrooms at night, or maybe running a setback program so it only operates when people are home to drop below 78 when desired.

Answered 3 years ago by LCD

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