With the non-sense comment,
With the non-sense comment, it seems the point that Dan was making wasn’t grasped. I believe what he is saying is that in a home below BAS require mechanical ventilation so with passive (or more passive/Energy Star) houses you are still required to hit 70% BAS, so you can’t avoid the mechanical ventilation; therefore, you can’t avoid bringing in the moisture from outside even if you can avoid bringing in the sensible heat. Furthermore, if your house is cooling and shaded the moisture content from the warmer air outside produces a higher RH in your shaded, naturally cool house.

Dan is right on point, the
Dan is right on point, the evaporator coil will actually shift from greater latent removal to greater sensible removal depending on the humidity of the conditioned space.

Hi Jessica, long time no hear
Hi Jessica, long time no hear! I would be surprised if internal loads are causing your living room problem. As Allison said, today’s electronics (especially TV’s) produce much less heat than when the MJ internal loads.

What you describe sounds like an air balance problem. As you know, very few residential contractors do air balancing. You can adjust your bedroom diffusers to push more air to the living room, but you need to monitor the impact on the static pressure at the air handler to make sure you have adequate airflow across the coil (I know you have the tools and know-how to do this, but feel free to contact off-line if you want to discuss). Depending on duct layout, it may be necessary to add another supply the the living room, e.g., if living room supply duct is undersized or compromised. To the extent that your living room windows have direct sun exposure, be sure to use window coverings to block direct sun into the room.

(As an aside… Jessica trained me on REM/Rate back in 2006. She was our lead HERS Rater/QA designee.)

Jessica, you’re right.  I

Jessica, you’re right.  I just looked back at the most recent article I wrote about load calculations (How to Read Manual J Load Calculation Reports) and saw that I didn’t mention appliance loads there at all.  To address your concern, though, yes, we do factor in the internal loads from lights and appliances.  ACCA’s Manual J uses 1,200 BTU/hour as the default load for kitchens, 500 BTU/hr for laundry rooms, and 900 BTU/hr for living rooms.  When we’re doing load calculations here at Energy Vanguard, we’ll tweak things when necessary.  On Passive House projects, for example, we’ll go a little lower because they usually use more efficient appliances.  And on the other side, I added some extra load to a laundry room recently because the plans showed a freezer in addition to the washer and dryer. 

In your case, I don’t know what is the reason behind the nonuniform cooling in your house.  It could be failing to factor in appliance loads, as you wonder, or it could be a duct problem.  Was the system designed using the ACCA protocols?  My guess is they didn’t, instead using the old rules of thumb that result in the kind of problems you describe.

In addition based on all of Allison’s articles I read about ACHnat and how the blower door only tells us about the size of the holes but not about the real-life daily pressure differentials (due to outside conditions like wind but also variations caused by hood ranges, bath exhaust fans, etc.), it appears that modeling the infiltration rate correctly in Manual J is close to impossible even if we had the possibility to base it on blower door results.

Therefore, when choosing equipment without a blower door test how do you guys deal with these latent load swings? Just select “Tight” and go with whatever the outcome is? (this particular project is a stick frame, ZIP home with BIBS insulation + plugging all the “big holes” and the top plates sealed to drywall)

My dad used to say that band
My dad used to say that band-aids were for sissies. So I’m not one to promote any band-aids if they are not needed. But in this case, the band-aid solution might not even be enough if it is a low-cost $250 portable dehumidifier. If you have a gusher of a load, like ventilating in a humid climate, you will probably need to go to the ER for stitches for better control of the excessive flow. When the doctor/designer feels this is needed, that would be a whole house dehumidifier.
So it boils down to how much load. David, you are right, “If you keep the moisture out to begin with, a properly sized AC can handle the load”. If you could really do that.
But some type and amount of ventilation and infiltration is more realistic. Now do some dew point weather analysis and quantify the seasonal latent loads. Now compare that to the run times of your perfectly sized, sensible load activated AC in an energy efficient home. You’ll be needing several Steri-Strips on that gusher, son.

I should add… To the extent
I should add… To the extent that you’re talking about existing homes, I would agree that many homes are defective beyond reasonable repair (leaky envelope in particular) and thus may need supplemental dehumidification. But I would also argue that a lot of these homes don’t need to be ventilated in the first place, other than spot ventilation.

Also, controls can be added to to enhance an air conditioner’s latent capacity on demand.

My main point is to refute the premise (that I often hear) that low load homes necessarily need dehumidifiers. In my opinion, mechanical DH in most cases is a band-aid.

@Dan, the idea that low load
@Dan, the idea that low load homes necessarily require supplemental ventilation is nonsense.

The key to humidity control is a tight envelope, an enthalpy hx to minimize the additional latent load, and equally important, avoid over-ventilation. I consider anything beyond that to be a design and/or installation flaw.

If you keep the moisture out to begin with, a properly sized AC can handle the load. Source control always trumps remediation. During those occasional periods when there’s no sensible load and outdoor dew point is above, say, 60F, you always have the option to shut down the ventilation system. But as long as RH stays under about 60%, the building and occupants should remain happy.