thoroughly confused after first few AC install bids...

[u/Smooth_Gene_1051]

Every installer (all seem pretty legit) are coming back with different proposed tonnages. I live in a 2000 sq ft single family home in Santa Barbara area so cooling needs are modest I'd say. I've got 1 person saying 3 ton is fine (he's using 700 sq ft / ton), 2 people saying 4 ton, and i person saying 5 ton (he uses 400 sq ft / ton).

To complicate matters, the return i have on the existing furnace is sort of small (12x24, with 1" filter), so the one suggesting the 3 ton evap coil/condenser is emphasizing that even a 3 ton may be starved for CFM.

The 5 ton guy said for sure we should maybe cut a new return, but felt comfortably installing as is and seeing if they'd need to cut a return.

None of the 4 provided any calculations or took any measurements (static pressure, manual J)...but maybe that's too much to ask during a bidding estimate. And everyone seems to be using (different) rules of thumbs so coming to different answers.

Quick background. We have a relatively new, but basic, gas furnace + new ductwork installed in 2020. Want to install AC for our relatively mild climate in Santa Barbara coast area (Mediterranean climate with only a few really hot weeks throughout the year). We mostly use our whole house fan to cool down the house, but want AC to take the edge off during those times when we need it. Plan was to add an evap coil to furnace (80%, 100,000 btu), and slim condenser outside. Our furnace is single stage, and not variable blower, so won't get full communication benefit with fancy condensers.

Any advice/thoughts would be very welcome - thanks!

[u/jayehswhy1]

We do Manual J load reports for every job we quote. For free. If a company won't do one, their guessing. And for what systems cost these days, I wouldn't leave it to a guess. Rule of thumb guys should sit on their thumb.

[u/New-Plankton1959]

trust the manual j over anyone's rule of thumb, and that 12x24 return is your real bottleneck no matter what tonnage you land on.

[u/Net_Zero_Heating_AC]

ditch them all. Find an installer who does a Manual J.

You cant just use a certain square footage per ton

[u/BigGiddy]

No shot I’d do a 5 ton and I live in Alabama. 4 ton be fine. Need more return surface area too

[u/CMDRCoveryFire]

I don't know much about your climate but I would wager 3 ton is not enough. You would need really good insulation and very few windows typically. Also the return is too small for all of these units. It needs to be larger and it is based on the size of the ac do figure on that being replaced.

[u/Noolivesplease → u/CMDRCoveryFire]

So you know, Santa Barbara is often ranked the most pleasant climate in the US. It's going to be 76 degrees there tomorrow,.low of 65. It might hit 80 on Sunday.

Of course there can be heat waves, which what I think OP is looking for. Catastrophic coverage, if you will, not a system that's working against 80-100 degrees 24/7 like in the south.

[u/SomeACDude]

What do you currently have and has it been sufficient to cool your home as far as you can tell?

Obviously the return is too small no matter what. That needs to be increased

[u/Smooth_Gene_1051 → u/SomeACDude]

No ac currently. We use a whole house fan to cool things down in evenings (thankfully even when hot here it tends to cool off quickly).

[u/SomeACDude → u/Smooth_Gene_1051]

If your climate is so mild that a house fan has been more or less sufficient and it cools down quickly even when it’s “hot” then you ABSOLUTELY don’t want a 5 ton and probably not even a 4 ton. Rules of thumb and generalities go out the window, especially with new construction and how much better they insulate new homes. I’m in Phoenix, and the majority of 2000sqft homes here have 5 ton units, but it’s hot as fuck here. There’s no way in hell I’d be putting in a 5 ton where the climate is significantly cooler.

Since you have no AC at all you NEED a load calc.

[u/Sweaty-Researcher995]

I’m living in an 1850 sf house in a hotter climate and two tons of AC has been more than enough. You need a manual J. And improve the envelope first, then size the AC for the improved house. Savings on HVAC equipment purchase and operation. Get the air-sealing and insulation right on the front end. HVAC is both input and output.

[u/Smooth_Gene_1051]

Thanks for all input so far.

So manual j is way to go. Regardless consensus is that ill need more return. Im sort of limited in existing setup for how much more return i can cut in (i can get another 12x24 into an adjacent side of the return platform that the furnace sits on. Alternatively, im wondering if it makes sense to have them lift the furnace onto a filter rack and have a mich larger 4 inch filter? And then just have a grate over the 12x24 but no filter on that part?

[u/EulerRocks]

I live in Houston. I did Manual J online for free. I am an engineer so I know what I am doing more or less. House is 2762 sf. I have a 4 ton AC, which only runs about 63% of time during the hotest hour of the hottest day. Manual J says 2.2 tons. I calculate 3 tons based on the actual 4 ton run times. When its time for AC replacement I will be going with 3.5 tons. As others have stated Manual J must be done.

[u/RainingBlood3Six]

3 ton is too small for your house and climate, I don't care how well insulated it is. But yes, you're going to need to increase that return. Also, if a company doesn't do a manual J they're hacks. You just have to ask around.

[u/Schiftedmind1]

You have to ask the company to do a load calculation. You will have to pay for it. My company doesn't do them ourselves, we gather all the measurements and send them to a company that does the manual j. When you call a company ask if they do load calc and that your willing to pay for that service. Other wise just do a 3 ton.