@SS, when you say RH isn’t
@SS, when you say RH isn’t managed well, I assume you mean it sometimes gets too high. To address this issue, it’s important to first identify the moisture source(s) and verify the cooling system has adequate latent capacity (i.e., moisture removal capacity).

Common sources include excess ventilation air and/or infiltration, inadequate spot exhaust for showers & cooking, and poor roof and/or site drainage. Excess moisture in an vented crawl space or unconditioned basement will migrate into the home through leakage paths in the floor. And if the air handler and/or ducts are located in unconditioned space (including attic), any return-side leaks will draw in ambient moisture and overwhelm your A/C’s latent capacity. Home performance professionals rely on various diagnostic tools and methods to identify the sources for the moisture in your home.

Not knowing anything about your home or where you live, 1000 ft2 per ton suggests your RH issue is probably not the result of gross oversizing. But even a properly sized A/C can fall short on moisture removal if not set up properly. For example, continuous fan (as opposed to “auto”) tends to re-evaporate residual moisture on the coil at the end of each cycle. And the A/C’s latent capacity will be compromised if airflow is set too high or coil is too large for the condenser (requires independent evaluation by a qualified HVAC tech).

Supplemental dehumidification should only be considered as a last resort, after all sources of moisture have been minimized to the extent possible (‘source control’), and A/C latent performance optimized.

SS, you’re close but you

SS, you’re close but you missed two things in your calculation.  What you’re trying to end up with is square feet per ton, so you have to do the division the other way around.  Before that, though, you need to convert BTU/hr to tons.

34,048 BTU/hr ÷ 12,000 BTU/hr / ton = 2.84 tons

2,860 sf ÷ 2.84 tons = 1,008 sf/ton

@Heidi, it’s impossible to
@Heidi, it’s impossible to know what went wrong without auditing the load report, coupled with on-site assessments and diagnostics. But I can make the following observations:
1) manual overrides in the load calc software aren’t necessarily a bad thing
2) inability to satisfy the load could be caused by equipment problems and/or installation issues
3) critical construction details may not have been nailed down prior to hvac rough-in, or may have changed after rough-in
4) the HVAC contractor may not have done a proper Manual S calculation (equipment selection) based on the load report
5) even if 3 tons is enough capacity on paper, 4500+ ft2 seems too large to me for a single system (may be impossible to balance)

Bottom line: Proper design and installation involves a lot more than getting the load correct.

What does the HVAC company say about the problem? At a minimum, they should install a data logger (e.g., HOBO) at the thermostat location to document the temperature performance. With that, it should be obvious there’s a problem. If they ultimately agree to replace the AC, you should have an independent designer review their plan. For example, the furnace or air handler may also need to be replaced in order to get sufficient blower capacity. Also, as I said, I’m skeptical that a single system can handle a house that large.

Sorry, next-to-last sentence
Sorry, next-to-last sentence in my initial reply should read: “…dial in a higher SHR…”

Great question, Armando!
Great question, Armando! Naturally I can’t know how your mech contractor did the load without reviewing his reports, but I can tell you that the problem you describe is pervasive. As it turns out, MJ software requires the designer to stipulate system SHR so it can convert the sensible load to tons. This is WRONG as it ignores the Manual S procedure! Determining the size of the system is beyond the scope of MJ, thus in my opinion, the software shouldn’t need to know the equipment’s SHR, nor should it convert loads to ‘tons’.

In my experience, most contractors leave the SHR field at the default (0.75 in Right-J), probably because their MJ8 instructor didn’t correctly explain this setting. The problem is that (most) load calc programs were designed before Manual S was developed and before (most) manufacturers began publishing expanded performance data. But even if the designer determines the correct SHR and enters it into the MJ software, the resulting ‘tons’ shown on the report may still be incorrect. That’s because the software (at least in the case of Right-J) simply divides the sensible load by 12,000 to come up with tons. As explained in my previous reply, just because a system is rated 2 tons doesn’t mean it has 24,000 capacity at your design conditions.

For example, let’s assume the sensible load = 19,000, latent load = 2,000, and total load = 21,000, which is nominally 1.75 tons. A given 2-ton system might only have have 23,400 total capacity at your design conditions, which at first glance appears to be more than enough. Assume the expanded tables reveal a sensible ratio of .79 at your design conditions and design airflow. This works out to 18,486 sensible, not enough to satisfy the load!

As noted in my previous reply, the designer can select a different coil and airflow setting and perhaps satsify the load… But keep in mind, it may be counterproductive to try to match the load’s SHR (90% in your example, not unusual in the Southwest). In any case, there’s nearly always a difference between the load SHR and the equipment SHR.

Bottom line: I would not rule out that you may actually need a 2.5 ton system, but if the contractor set SHR = 0.7, my guess is that he’s simply using the SHR as a ‘fudge factor’ rather than following the Manual S procedure.

Hi David – Thank you for the
Hi David – Thank you for the detailed explanation. A builder I designed a house for and I are trying to work with a new HVAC company. Its promising, but we have some small “difference” of opinion in couple a points. One is the SHR. My MJ8 calcs tells me I need a 2 ton heat pump for the 3000sf/HERS42/1ACH50 designed house.The HVAC guys want to install a 3 ton HP (16SEER, 10HSPF), since his numbers with lower SHR were at 2.4 tons. His explanation is that with variable speed units, it didn’t matter. I know to many folks that’s small potatoes, but when we’re trying to build an efficient zero energy house, every bit of load counts… btw, the calculated SHR from the loads is .90, so why use .70? That’s my argument!