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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Thank you all for your replies, those are really good comments. Doug, I appreciate all you info. I have written to Huntsman for a copy of your white paper. The HVAC Company has done a good job in all aspects; however, we just have a difference of opinion on the tightness of the construction on the MJ and the size of 1 AC unit. I know for most people I should be happy with what I got, which is better that almost all HVAC companies; but my issue is to get the maximum efficiency I can get in my projects, and knowing that we are doing the best that we can for our clients.

You could leave one unit rounded up and one unit rounded down. Make a judgement call on the internal gains at peak conditions. 


Are we talking AC here, because to my knowledge air leakage does not affect AC sizing too much. Is it showing an increase in latent load from the air leaks? I would be willing to bet $100 there are other aspects of the Man J entries that should be tweaked for what happens in the real world. Kind of like Man J just estimates the load idea.


To further this point, after doing many, many Man J and HVAC performance installs, I started intentionally guessing at risky sizing. Crazy huh? Then I would do the Man J afterwards. From doing this I learned a lot about how to do better job at Man J. I only had one system that would not get below 82 when it was 115 outside in Las Vegas. I tweaked the TXV and straightened the ducts, and got it down to 78. That was a house with typical 15 year old construction, and nary an energy upgrade to the shell. I sized it at 1 ton per 700 sq ft at a summer design temp of 106. All with the permission of the homeowner. We are going to bury half of the ducts in insulation soon. 


Anyhow getting back to the point. Sometimes you just have to take a leap of faith. This is what my mentor taught me.


On this one I say go with the smaller of both. Especially if it is only a debate on the air tightness. Smaller is better for the latent loads as well. Fix the air tightness, not the AC sizing.

I'm with Chris, although he's actually seen a unicorn and I'm still waiting.  (if 82 at 115 indicates undersized...)


If you are too small you can always improve the house.  If you are too big you have to start again.  


The savings I'm calculating from downsizing equipment is much greater than the "equipment efficiency" improvement would suggest.  Unfortunately TREAT software actually calculates smaller equipment as using MORE energy for some reason.  So there are impediments at every step, and until results are transparent, contractor incentive remains avoiding complaints not banging down consumption. 

Although 'rule of thumbs' are not particularly good, recent comments on a RESNET Linked In board indicate that you would start at no more than 5 tons and maybe less.
Thanks for your replies Chris & Danny. It trully shows that folks that are experienced in TRUE high performing houses do follow the selection of smaller equipment. I guess its goint to take time to educate the masses.
Why doesn't the contractor doing the Manual J use the 1ACH50 value or the equivalent cfm50 expected blower door value? I am pretty sure wrightsoft lets you do this. I am sure most other Manual J programs allow you to use the expected (or measured in the case of existing) whole building air leakage value. This should make the labels used for the type of construction and the fireplaces irrelevant- the program will use the whole building air leakage value.

If he is pushing on the leakage point, what is he using for the other inputs?  I find lots of high variations in temperature inputs.  Also what about the Manual D?

Open cell in a crawl space is not as effective as closed cell.  I would question that call. There is more margin for the SPF installer on open cell V closed cell.  Some contractors in this area do not install closed cell.  So one inch of CC on the crawl wall and 2 inches on the Rim Joist would give you higher R Values then the rigid polyiso and OC.

As to the Man J and the leakage inputs;  ask the guy doing the Manual J to provide the written requirement from ACCA to use Semi-Leaky --  or  -- not leaky.

Thank you all for your comments. The HVAC contractor has redone the MJ with tight construction and with results of 2.1 & 2.3 tons. Of course that means they still want to install the 3 & 3 ton units since they are 2-stage units and “oversizing AC units doesn’t matter in 2-stage equipment” theory. I really have taken step back, the builder has all the information on both sides of the issue, I’ve sent him the link to this thread, and now it is up to him what equipments he gets installed.

Carrier has 2 ton 2 stage heat pumps.  Pretty sure Goodman and American Standard do as well.  I say heat pump because I only recommend, and currently have only sold heat pumps.  (Once I started digging into the numbers it became apparent to me I needed to recommend heat pumps.  If you can't make the horse drink, that's another story.  But you need to show him the drinking hole.)  


When you start selling the high end high efficiency stuff, selling just AC is akin to grand theft, malpractice. Particularly when you get to the higher end equipment the incremental cost is negligible, and energy and comfort benefits start to get really big.  Educating the consumer about this opportunity, if it becomes a missed opportunity, takes the onus off of your shoulders.


Carrier also has a 2 ton inverter driven unit (Greenspeed) that drops to 40%, so that ranges down to below 1 ton.  

True about the sizing limitations of the nicer stuff. True about the staging capabilities of the better equipment. Just be sure that the control can handle the staging correctly, and not just turn on 2nd stage after a pitiful 8 minutes of run time. I use a thermostat that has a droop setting - so if the temperature droops out of range, then the second stage comes on then and only then, and resumes low stage when the temperature goes back into range. "Passing gear" if you will.


There is great future for inverter technology. Just a prediction. 


"redone the MJ with tight construction and with results of 2.1 & 2.3 tons. Of course that means they still want to install the 3 & 3 ton unit" 


Ohh yeah. Forgot about that tactic.


Remind them that per Man S you can round down by 5%. So the 2.1 will allow them to install a 2 ton. Haha!


I haven't had a good debate with a contractor or engineer regarding AC sizing for about a half a year. I forgot how much it is like playing chess.

Isn't that the slip you are looking for, the loss of credibility.  If they can re-calculate with "tight construction" and the recommended total ac needed goes up, then the software is bad or the HVAC contractor is fudging the numbers.  Either way the builder should now see that this guy is not capable of giving an honest competent answer.  This HVAC guy is no longer concerned about properly sizing this system, but in justifying his position, at all costs.




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