"...when I look online the calculations usually say a ton per 500 ft.² which will mean that a 3 ton will be enough "

1) That's not a calculation, its a rule of thumb.

2) You say both 1,700 and 1,800 sqft. Neither would be a 3 ton unit under the rule of thumb anyway.

3) It would pay you back well to get a proper load calculation for the house, and also to consider your future plans. If you intend to improve insulation, you should probably do that before replacing the a/c, or you risk over-sizing the a/c for the improved house.

4) Over-sizing is bad because the the a/c doesn't run long enough to remove humidity while it cools. This results in an uncomfortable house.

5) Two units, totaling only 5 tonnes, seems a silly idea. Since household a/c comes in up to a 5 ton size, why buy double the equipment and maintenance? In case one fails, some of the house is cool?

I’m sorry , I’m right under 1700 . And the rule of thumb I keep seeing is 600/Sqft .

Skip the rule of thumb, get the calculation.

Sizing can be less of an issue when you have multiple units. Because each unit is controlled independently, provided of course the unit itself is not over sized for the area in which it serves resulting in excessive short cycling.

In your case 3 ton plus 2 ton isn't necessarily equal to 5 tons of cooling... unless you drive those thermostats really low.

By having two systems with total capacity of 5 tons (which, most of the time this would be oversized for a 1700 - 1800 sq ft structure) has a few advantages.

1. You have the ability to use one system only during low heat days or set the opposing unit higher. The unit set higher only runs if load conditions warrant.... meaning if it's not hot, the thermostat controlling the unit does not call... the unit can not run without a call...

2. Utility bills can be much less... because you rarely use both systems at the same time. You run the bedroom system when you sleep, the other when you're awake. If you have a home office in one the bedrooms you may need to rethink things a bit.

3. Extra capacity for when it's hot out. When it's hot you have the capacity to cool the home. (even while entertaining guests --- additional people add heat) Remember heat waves come, heat waves go... a so call 'properly sized' AC system is essentially properly sized roughly 10% of the time... especially if that system is a single speed AC system.

Alot of people I come across like to sleep cool. So for that reason I doubt I would change the configuration of the space. Once you realize what you have, the choices you make are mostly budget type concerns.

Essentially a set up like this runs like a 2 stage system. This kind of subject is above the pay grade of many... choose your contractor wisely.

I service the Katy, Texas area.

Some questions for you:

• When the current AC systems were functioning properly, did they cycle on and off for short periods of time?
• Do you monitor the humidity level in your house?
• What temperature do you typically set on your thermostat?
• How much insulation is installed in the attic?
• How do you heat the house?

I assume the reason you are replacing your current duct work is because it is in poor condition. If it is in poor condition or undersized, then you are not getting 5 tons of cooling. Therefore if you keep the the same 5 ton sizing with much better air flow from the new duct work, you are likely to get short cycling and high humidity.

The correct way is to do a load calculation both Manual J and Manual D. It is cheaper to pay someone to do a load calculation then just guess and pay for over sized equipment that consumes a lot of energy.

If it were me I would look into a 2-stage 3 ton AC or heat pump, and use the money you didn't spend on the 2 ton unit to upgrade the attic insulation and possibly the windows. This assumes you find a competent HVAC contractor who knows how to install duct work and HVAC equipment.

Thank you ! That makes sense . Since I’m having a new duct system put in I have the choice to configure it the way I want to so I was thinking on switching the units around and have the two time servicing the bedrooms and maybe the hallway and having the 3 ton unit service the rest of the house. What do you think?

By the way I Currently have the 3 ton and The 2 ton unit And they are pretty new that’s why I am reluctant to get rid of them in order to install a single 4 ton unit for the whole house.

You can do that if you wish. Florida has high humidity with moderate temperatures. You want to avoid over sizing equipment. The house will be cool, but not necessarily comfortable.

Your house is a ranch with equipment at either end. Your suggestion may necessitate much longer duct runs (expensive) in an attic (presumption).

Ha! He spammed his own spam!!!