If you have to use a dehumidifier, your doing something wrong with the load and equipment type.
I would suggest seeking help from a contractor who deals with this problem. A system can be sized and installed that will control humidity, such as using a var speed indoor system and setting up the controls properly.
If your using the little load calculations provided on line that asks only basic questions- you will fail. A good load calculation is complicated and designed at a certain indoor wet bulb, using the sensible capacity of a system to get the results. Each brand has it's own sensible capacity per it's size and that info must be used. Just the total BTU's means nothing.

Where did the values for sensible/latent cooling - heating come from? Did you use an 'online' calculator without the missing values?

I responded to your first post and suggested use of a professional program available to homeowners for a nominal fee.

Once you have all of the data entered, you can experiment with different values for the unknown parameters and see what the difference would be.

I think it will enable you to come to a better decision.

V

Some background information. I have a degree in mechanical engineering, so I completely understand the theory.

The calculation of the loads were done as follows:

- conductive heat transfer, done as a careful U A ÎT analysis using HDD & CDD (I used +35F for the insolated roof temperature); the roof has little shading, but the east and west walls have a good bit of shading, and the roof has long eaves, and the solar elevation angle is very high in the summer, so I did not use this factor for the walls); I used an effective R-value of 4 per perimeter length for the slab (admittedly this is something I would like to get a more accurate determination, but it seems that for a warm winter, it is not a big deal)

- window insolation, done using the insolation constant and expected normal angle and time (because of the excellent eave shading, this is very small); there are some larger east windows, but that is in the morning when the other heat loads are small

- internally generated sensible and latent loads (only the appliance sensible load was large)

- infiltration, as an expected exchange factor times the volume of the conditioned space, with the sensible load as the mass times heat capacity time temperature differential, and the latent load as the mass of the air times the specific humidity at the design wet bulb times the heat the evaporation (the determination of this exact value of this exchange factor seems to be problematic)

I don't think I missed anything here. Like I said before, the big question mark is the infiltration factor, which has a large impact, especially on the latent load when the wet bulb temp is 81F! But in any case, if because of the good sealing from the spray foam insulation, my home is so well sealed that I need an air exchanger, then I should just do my calculations based on what this net air exchange would be (a good value seems to be about 0.4, which is indeed what I used here.)

I tend to agree with daddo. An HVAC unit should be able to handle the proper dehumidification itself at peak loads. But I have heard that when a unit is sized too large, that there is a problem with keeping the space at the proper humidity - and by definition, for any time that is NOT peak load, the unit would be sized too large.

I have never personally heard of anyone in my hot humid area that had a special dehumidifier - the standard HVAC unit always sufficed. But anything that would give me more control over the thermodynamic properties is always a plus.

A/C unit dehumidifies more when undersized so if humidity a problem, bias sizing to slightly smaller unit better. Unit that keeps up by running nearly constant on hottest days ideal from both efficiency and wear on unit.

Some of newest units have variable speed fans (need compatible furnace). Humidistat automatically cuts HVAC blower speed when too humid biasing unit to do more humidifying and less cooling. This is ideal approach with no separate dehumidifier needed. These are high efficiency systems that cost more, but worth looking at paying for this rather than paying for a separate dehumidifier which dumps waste heat back into house and requires much redundant equipment.

A humidistat only costs maybe $100 for the part or you get a thermostat that regulated humidity also. Check with your contractor to verify but with mine.....I set it to a certain humidity % and as long as it is wetter than that inside.. the unit will automatically run at slow speed to extend the runtime before reaching the desired thermostat temp setting. Need a variable speed blower to do this. Works good in the morning when the unit may not go on cause there's little heat load but humidity is 85% outside. You have to play with it, cause if you have it set too low, it'll never shift up to high speed. During the hot day you want it running high speed for max cooling, and it will cycle enough that the air will dry out.

according to Joe of building science a stand alone
dehumidifier will handle a 2,000 sq ft home.
(it took me a LONG time..3 days.. to get his input on this btw)
visit www.hvac-talk.com
good info there.
best of luck.

OK zver11, you are saying that if I get a variable speed unit, then I will be able to set the humidity as well, and the system will simply respond by doing more dehumidification than just cooling?

I am getting a heat pump, so how would that be affected by the variable speed functionality?

Basically, AFAIK, it responds by running the fan at a lower speed for a longer period.

And a HP should not care one way or the other.