Nights 65 F and below
Days pushing towards 90F.  In the  afternoons with the windows open, the house starts getting hot hot hot.  Almost 80 in the house at peak.

Should we be closing the windows in the morning to keep the cool in, turn on the A/C (1 mini split only , downstairs) and the HRV and leave it on until night time when it cools down?

I generally leave the windows open all day until it is too hot... maybe not the best strategy.

In our old but partially upgraded home, we've been closing windows and shades until the sun goes down or it cools off, whichever comes first, then opening them for the night. (If it's warner inside than out). It's now 4PM, 90o outside and 71o inside, with the mini splits turned OFF, as they have been all week. Actually 71o is the warmest it's been all week in here, been mostly in the 60's. Hard to be outside, but great in here!

Yes while nighttime temp/humidity is tolerable. Often it makes sense to use a whole-house fan to help move night air through the windows to more effectively cool down your thermal mass (small temp differentials needs lots of CFM).

Posted By jonr on 06 Jul 2016 06:48 PM
Yes while nighttime temp/humidity is tolerable. Often it makes sense to use a whole-house fan to help move night air through the windows to more effectively cool down your thermal mass.

"Often" if the dew point is low. This is a rare occurrence for most of us not living in a high desert plane or low desert. Most of the time, for most of us, controlling humidity is the first order of business and cooling the second, though this order is usually reversed for lack of experience and familiarity with the psychometric chart. http://www.coolerado.com/pdfs/Psychrmtrcs/0000Psych11x17US_SI.pdf

Sounds like you answered your own question Unless you have humidity problems, yes I would keep the windows closed to preserve the cooler air in the house during the day, and open them when the outside temp goes below the house temp. This is what we do when it gets uncomfortably hot outside. Usually leave them open all night and sometimes use some forced ventilation to get the house temp down near the cooler outside temp by morning. Then I close things up when I get up to go to work...

The strategy works well enough for me on 80% of summer nights in lower Michigan. No use of living space AC or dehumidification yet this year (the sealed off basement gets dehumidification).

On the other hand, I have neighbors who never open their windows. Depends what you want to optimize for: perfect comfort with no hassles? Lower bills?

With an inverter mini-split, you don't want to wait until it gets too hot/humid and then run it hard - this is less comfortable and uses more energy (lower COP).

I tried to leave it sealed up but doesn't help much for the 2nd floor, where it is 80F now.

I'm tempted to try exhausting hot air from the 2nd floor hallway down to the first floor to try to pull cool air from downstairs up, but not sure if it would really be effective.

The Fujitsu 15 RLSH3H I'm sure has way more than enough cooling to cool the house , it's just not getting upstairs by itself.

Theoretically, exhausting the hot air outside would force replacement cooler air upstairs, but you'd probably need a large fan in a tight enclosure, which would have to be airtight for the winter. Best method by far would be to install a second unit. If you didn't need it for heat, it could be a smaller unit.

I was thinking of pushing the hot, 2nd story air downstairs (not outside, which I think would just bring in more hot air through cracks) with a ducted fan, or alternatively ducting cold downstairs up.
for example, one of these in the wall upstairs: http://business.panasonic.com/FV-10VS3E.html

2nd mini-split unit would be ok but there is no easy spot for it except in one of the bedrooms. 

Can you duct the hot upstairs air down to feed the minisplit downstairs?

Maybe something like a 24" whole house attic fan moving air from floor to floor (with the stairwell as the return). 100 CFM is way too small.

Those are my guesses, let's look at figures. Say you want to keep the upstairs within 3F of the downstairs and the upstairs needs 10,000 BTU of worst case sensible cooling.

BTUs = CFM x ∆t x 1.08

10000 = CFM * 3F * 1.08, so you need 3100 CFM. Yep, about what an 24" attic fan (installed between floors) on low will push. Plus the air movement will help with comfort.

Hmm
wouldn't the volume of air to be displaced upstairs be a consideration (7344 c.f.)?

Would the sensible cooling requirement be the same BTU as the heat gain. In this case the manual J gave us 3000 btu/hour on a 85F day.

The building volume doesn't matter except for the comfort effect of moving air/skin evaporation. Best to plan for close to worst case loads (might double or triple the 3000 btu, but I have no idea what your climate is).

Want to try to make lower CFM work - try a blower that picks up cooler air right at the output of the mini-split.

This is a common issue with high-R houses with most of the mini-split on the first floor- summertime overheating.

Curt Kinder's suggestion in response #18 of this thread was a credible solution to a 2 story house heated & cooled with one mini-split:

I'll bet that the subject Greenfield house could be kept comfortable on both floors during the few torrid days in Mass., particularly at night - maintaining reasonable sleeping comfort on the second floor, by using a small portable 3 speed blower (Lowes ~$70) as an "air lance" as follows:

Set the Mitsu at 68-72F cooling setpoint at night. Place the small blower (NOT a fan - idea here is low CFM, long throw). on the floor of the lower floor Direct the blower discharge vertically up through the opening pictured for hanging laundry. It'll entrain significant first floor air and send it up, also breaking the stratified air upstairs The second floor ceiling fan, described as useful for quickening clothes drying in humid weather, would distribute the air driven up from the lower floor well enough, I think, for comfy sleeping upstairs if bedroom doors are open.

So true, the best cooling strategy is highly dependent on your local climate humidity and temp variation. Where we live in southern OR, our daytime Summer highs are often in the mid 90s and occasionally reach 110F. However, our nighttime Summer lows are often in the 40s and always in the 50s. And most importantly, our humidity is always very low... As such, we keep the windows closed (and run HRV unit on low setting) during the daytime and we either open the windows in the evenings or set the HRV to run on a higher setting whenever the outdoor temp becomes lower than the indoor temp whenever the indoor temp is above 60F, but no longer than sunrise. Our large diurnal temp variation and low humidity climate in combination with our high R-value passive solar home (which is therefore well-shaded in Summer and has a relatively large interior thermal mass) eliminates the need for any air conditioning whatsoever.

I don’t think anyone chases thru a psychrometrics charts these days as we now have computers and can run much more accurate software. Here’s our psychrometrics software which is based on ASHRAE methodology:

Borst Psychrometrics Software

I'm going to try pushing some air upstairs via a ducted fan. Looks like I should be able to fit 3.25x10, 12 or 14" in between an internal 2x4 wall .
Not sure about what fan to use yet.
Panasonics bigger CFM fans are loud (390 CFM = 3 sones).