I have designed a 4920 sf house in Dallas with a sealed crawl space. The crawl space walls are concrete and a have 2x10 floor joist system on top of it. The crawl space walls are insulated with 1” Polyiso and the rim joist is insulated with 5.5 open cell foam. The walls above are R23 BIBs in cavity + R5 exterior rigid foam over sheathing, and 5.5 open cell foam on the floor truss rim. The attic is ventilated with R50 BIF. The builder in previous houses has gotten air leakage of ≤1ACH50, so we know he does a good job there. The HVAC contractor doing the MJ says that he needs to do a “SEMI-TIGHT” construction because of the crawl space AND semi-tight fireplace installation when we are installing 2 sealed combustion fireplaces. I say the construction and fireplace should be label tight. It all boils down to add an additional 1 ton of AC form 5 to 6. I know is not much of an issue for most folks, but when I’m trying to design maximum efficiency (HERS 55), 1 ton extra is a lot, specially when we know all MJ are 15%-25% oversized from the get go. Any thoughts here? Thank you.

 

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All equipment can be tested for capacity once it's installed, testing removes all opinions and produces facts.

With such a low cooling load I question if oversized 2 stage units are even worth the investment, you may be better off just getting the smaller single stage units. Consider the outputs of the units at the design temperature, rated tonnage is typically done for 95 degree outside conditions, output decreases as temperature goes up.

Also consider what happens if you do happen to slightly undersize? Your house might be slightly warmer than the thermostat setting on design days for a few hours in the afternoon. This of course will be accompanied by full power AC running so there will be a cool draft blowing from the AC vents which can make a room feel cooler than the number on the thermostat. It's not the end of the world if you are slightly undersized.

Manual J does NOT account for thermal storage in your house, which could very well be enough to get through the peak temps of the day. Have you ever noticed a well insulated building is almost always cooler than the outside on a hot afternoon, even when there is no A/C running? That's thermal storage in action. When you did Manual J did you account for appliance/lighting loads that could simply be turned off when it's hot outside? Are there other Manual J variables that could be altered on the hot afternoon to keep temps down in your house?

A lot of contractors wrongly think multi stage provides justification to oversize. Even if the duct work is designed to handle the extra airflow (usually not) this is unfortunate misorientation. A catastrophic misunderstanding of how to deliver comfort and efficiency.

Oversizing removes the value added of 2 stage units, which is the ability to run without satisfying. Initially this may be counterintuitive, but give it some thought. The biggest advantage is having a system that doesn't shut off when the house is experiencing load. This provides much better latent control and much more even distribution to areas that might otherwise be imbalanced. Multi stage provides this advantage if properly sized. It does not if oversized.

If we reorient thinking, explain that hvac doesn't heat or cool, it replaces heat or cool. The best, most efficient, most comfortable way to do this is matching loss. Think about hvac as maintaining temperature rather than "heating up" or "cooling down," and the way people think about how to accomplish home performance changes.

If you hear multi stage being used as justification for oversizing, your antenna should go up. Ask whose interests the justifier is concerned about, certainly its not the homeowners.

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