New construction with sealed crawlspace in NC
I’m building 1950 SQFT single level home in NC. It’s on a 48″ high Sealed Crawl Space. I am trying to decide on best system for handle conditioning the crawl space. Have read several articles on this site and others. We have a block foundation with a newer latex based expandable foam injected into the block cavities. Should be R10 on all 8″ block and more in the 12″ sections. R-6 Zip 2″ x 4″ walls and all brick.
I have one HVAC contractor suggesting system that is a 2.5 ton heat pump and 3 ton AH with in essence three zones. One zone for Master suite, closet and bath. Then one for main footprint of home and a third that has a type of balloon diaphragm designed to open and close to keep positive pressure and conditioned air in crawl space. I have another contractor suggesting a two zone system for Master area and main living area. Then an inline dehumidifier for the crawls space. All capable of being set separately. This system would be a 2 ton Heat Pump unit and 2 ton AH. Any thoughts or suggestions would be appreciated.
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Darrin,
It doesn’t sound like you have ordered a Manual J. You should have one done by an independent engineer or RESNET rater. A good analysis will pay for itself by allowing you to right- size your system.
Here is another article with more information on the Manual J. https://www.greenbuildingadvisor.com/article/who-can-perform-my-load-calculations
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Hi Darrin.
I agree with Steve on the need of a manual J. You may find this helpful: A Beginner’s Guide to HVAC Design.
The mechanical designer should also be able to determine the dehumidification needs and determine if the A/C will be able to handle it. In many low-load homes, a dehumidifier, separate from the A/C, is required because the A/C doesn't need to run that much. That's a good thing as long as your mechanical engineer understands low-load homes.
Tomorrow morning there will be a BS* + Beer Show on our homepage about just this topic.
Thanks guys. Looking forward to that show... I did read that article already and many others. ANY IDEAS of how to find said PRO to help design at a reasonable rate?
I am doing a lot of the build myself and have contracted with a great Green Builder here to consult, guide and handle all the foundation and envelope. We are suing R-6 Zip walls and and R-40 Blown as I just could not afford or justify the expense of a totally sealed attic and foam insulation in rafters and such. He suggested hiring a PRO to do all the calculations and design. However that was about $5K just for the design work and then testing and adjusting after install. There was a plan to put in a air exchanger, stand alone dehumidifier and all rigid ducting... But that whole system and design was in the neighborhood of $30K. I just could not justify that extra expense in the area I'm building and ever expect to get the resale value back out of it if I ever did sale. I don't plan on it and actually plan for this to be the last house I ever live in but budget is an issue so I had to nix all that and opt for something I could afford. Not sure how thats going to shake out yet and I am definitely willing to hire a qualified pro to consult and advise. I guess I need to try to find one in the area...
Note that most latent load calculations are done just for design day. The typical result is poor dehumidification in most conditions - so use a dehumidifier.
If the HVAC equipment and duct work are in the crawl space and the installed like most it will be leaky the crawl space may condition itself.
You may find this article interesting even if it does not convince you to do an manual J it may convince you that your house is tighter and better insulated than most and your home could be closer to 1 ton than 2.5.
https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die
Walta
Thanks gentlemen. I’m definitely not against having and independent pro consult work it up for me. Just don’t have unlimited funds. 😁
I did Manual J calcs myself with Calcunow. It took a couple hours (though I have blueprints for the house, making takeoffs much easier). Definitely worth doing and educational, though you can't save the outputs, which is very frustrating. You can play with inputs to understand impacts of doing different things. Definitely highlights benefits of enclosed crawl.
Thanks Brad.
I did the Manual J on CalcuNow. Not sure I did it correct. Just not that up to speed on many of the terms... But it appears i may need about a 2 ton if I'm understanding it correctly. But still don't have a clue how that relates to drying and or conditioning the crawl space. Thanks everyones guidance and help. It's an adventure.