I am in the process of building a new home. I am needing help sizing my A/C. The type of home is a geodesic dome. I am wondering if anyone can help me to size it and/or point me in the direction of a formula or program to size it? If anyone would like to take a crack at it, I can give you all the construction/climate details. Thanks in advance.
Enlosed is a link to some pictures of our home.
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Replies
There are a couple websites to look for help in this area. HeatingHealp.com and HVACTalk.com.
I hesitate to throw out rules of thumb, but here goes. In temperate climates and structures with 9 ft ceilings, 500 to 600 SF/ton of cooling is appropriate for residential construction (one ton of cooling equals 12,000 BTUH). As the climate conditons get colder, the numbers can go to 600 to 700 SF/ton. As the climate conditions get warmer, the numbers can go to 400 to 500 SF/ton. As the ceiling height increases, the numbers change, but ceiling fans can move warm air from floor to ceiling as the season dictates. Commercial construction in the temperate climates comes in at approx 33 BTUH/SF for cooling and approx. 35 BTUH/SF for heating. Residential construction can be very comfortable at 20 BTUH/SF for cooling and heating. The caution with using the rules of thumb are oversizing the cooling side of the system, thus having the compressor cycling too frequently and reducing it's ilfe.
More empirical (sp) approaches can be found through ASHRAE (American Society of Heating and Refrigeration Engineers - ASHRAE.org).
Good luck
Thanks for the reply. I have heard also from different sources about the issues of oversizing the a/c. And that is why I am trying to get a fairly close estimate of what I need. The tricky part about doing a geodesic dome is you have about 30% less roof and wall sq. footage for the same size square structure. Then you throw in that my house is all 2x8 construction and insulated with icynene insulation and you would think the cooling load would be fairly small. For example, we've got about 3600 sq. ft total counting loft, main floor, and basement and our heating load from our radiant company that we chose was estimated at 50,000 BTU's/hr.
Hmmm... In order to maximize comfort and economy, it's in the interest of everyone to do a room by room heat gain calculation, no? Rules of thumb are prone to biting people in the rear... We're cooling over 1,000 sq. ft. per ton our house, for example. While our heat gain is distributed fairly evenly, some homes have extreme variations, that only a room by room heat gain calculation will uncover.
You're dead on with the room-by-room calculations, no beter way to increase confidence in your equipment selections. Most people don't have access to the software to do the job properly, but can get fairly accurate with heat loss/gain calcs by way of envelope analysis (windows, roof, walls, slabs, infiltration, outdor air, people, equipment). What is your 10-20 with at 1,000 SF/Ton and what type of construction do you have?
Thanks for the kind words. However, I think the software is closer than ever to our hands, as long as we're willing to put the time and effort into learning how to use it and then applying it. For example, Slant/Fin makes the free hydronic explorer package, which may not be the worlds best heat gain/loss calculator for me, but it will get the job done, and some people swear by it.Conversely, HVAC-Calc offers some pretty powerful tools for a $50, 2-month homeowner license and ~$300 for a full homeowner package. I eventually bought the full license because the AC/envelope issues took more than 2 months to resolve. I considered it a good investment, even if Don Sleeth (the programmer and founder of HVAC-Calc) took a belligerent attitude towards me.I'm not sure what a 10-20 is, but our design conditions call for 74* indoors, 91* outdoors. That's warmer than some people like it but we figured that between the shading from external decidiuous trees and the padding in Manual-J, 7th Ed. that we could reduce indoor temps even further than that if we wanted to.This 1872 house has standard balloon framing in the front (4", with 3.5" of Corbond) and modern platform framing in the back with 5.5 inch studs+icynene. Original windows were remanufactured and got quality storms (Harvey Tru-Channel) retrofit while the remaining windows are Marvin ultimate double-hungs. Speaking of which, 2/3 of our heat gain is via the windows. The installer put in the air handlers a long time before we insulated, so each air handler/evap is oversized by 1 ton relative to the condenser it's on. The installer thought we'd aim for 70* indoor temps with a full house. I don't doubt we'll have a full house from time to time, but 70* indoors combined with 96* outdoors is overkill in Boston by a wide margin.No worries though, our pairing is a factory-approved application and it simply increases the sensible heat extraction, which is what we need more of anyway than the 70/30 sensible/latent mix the manufacturers aim for normally.
Edited 1/9/2006 10:55 am ET by Constantin
A room by room calculation is an excellent way to size the ducting for the system and to get the balancing close, but not necessary for the equipment selection.
I was looking for the software that you had reviewed on "The Wall" but had no luck in my quick search. Is the review still there?
I, too, cool over 1000 sf per ton. No ROT would ever approach the proper equipment selection for my home. Always a bad idea.
Would you recommend the software you used to the OP?
Well, the room-by-room approach gives you the granularity to get the ducting right and then allows you to review the system layouts/selections/etc. For example, we briefly considered installing one more evap/condenser combo to reduce ducting needs. It's also interesting for the folk who want to install ductless systems...
The review is still on the Wall in the On The Job section, a slightly updated version can be found here on my own web site. I think HVAC-Calc is a pretty intuitive program and a good choice for smaller shops that do not need the add-on modules like the folk at Elite or Wrightsoft offer. Homeowners with a technical bent are also well-served by HVAC-Calc.
http://www.hvaccomputer.com/hvac/sizer.asp
Gooooollllyyyyyy, Bill, thanks for the tip! Is that how you good ole' boys do er down there? Ya'll have a good day now! :)
http://www.acca.org Manuel J and manuel s