Choosing the right design temps

[JLSinPDX]

Hi, all. Another question as I try to sort out a lot of contradictory info before making a buying decision:

The recommended outside design temps for Portland, OR, are apparently 23 degrees Winter and 85 degrees summer, but dealers here seem to use 18 or 20 for Winter and more like 95 for summer. The higher summer temperature, at least, makes sense to me: When it's only 85 outside here, air conditioning isn't really necessary, because you cool the house off easily in the morning and evening with a window fan, and humidity in the summer is generally low. When you really need the A/C is when the temperature gets above 90.

Problem is, when I changed the recommended design temps in HVAC-calc, the Heat Gain jumped from about 34k to 43K, which bumps the size of the AC needed by at least a ton and also boosts the size of the furnace, because I need a furnace with a five-ton blower.

But my question: It does make sense to boost the summer design temperature if what we're really after is comfort on those 90 plus days, right? Or is this one of those fallacies that causes people to end up with over-sized systems?

Thanks -- Jon

[Junkers]

HVAC-calc is clear about adding 20-25% to the Heating BTU requirement to take care of the extra 2.5% cold days that the standard city design temps do not cover. However it is not clear about in its recommendations is how much if any you should add to the cooling requirement on those days when the temp exceeds the design temp. I believe it is up to you to decide how much extra cooling you might want to add taking into account the fact that under those circumstances you would probably have fulfilled the latent heat requirement assuming you would have already turned the AC on and fairly well brought the humidity mostly under control except for infiltration.

What made sense for me was understanding at what upper temps does the cooling requirement shift from one tonnage to the next. Which can easily done by seeing the cooling requirement at different outdoor design temps. Then you can "estimate" or guess on how many or few days the AC might not keep up because you did not put in enough cooling for those rare days and what the cost of those days might be. Also unless you get a two stage AC or variable speed blower , oversizing the unit will adversely affect the ability to control humidity. Finding the right sweet spot is the art.