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Load calculations for new home Geothermal
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Posted by fish7577 (My Page) on Thu, May 28, 09 at 0:38
| Please help. I'm builing a new home (or at least planning it). I have 2 quotes for a geothermal system to do forced air on main 2 floors and radiant for the basement + garage. The 2 load calculations, supposedly both using a manual J calculation, differ slightly. One contractor suggests a 4 ton system, the other a 5 ton. I'll list their calculated heating load, cooling load, gains, set points, and delta T. Obviously, I don't want to undersize or oversize the system. Price isn't much different for the 2 systems despite being different sizes. How do I compare these? I assume the gains must be factored in to decide a size? I don't want to be running auxilliary heat with an electric coil unless it's VERY cold. I'm in Pennsylvania, so I shouldn't need it too often.
QUOTE 1: Recommended 5 ton system
Heat load- 62672
Cool load- 36884
Gains- 11704
Heat set point- 72
Cool set point- 75
Delta T 68 and 17
QUOTE 2: Recommended 4 ton system
Heat load- 56617
Cool load- 42000
Gains- 12626
Heat set point- 70
Cool set point- 75
Delta T 70 and 20
Thanks! |
Follow-Up Postings:
RE: Load calculations for new home Geothermal
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| Sounds like two diff houses to me! Too much variance. I would review them for any human errors. How many sq ft of conditioned space? |
RE: Load calculations for new home Geothermal
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| 3300 sq ft on 1st + 2nd floors, plus 1600 basement and 600 garage. (total is 5500) Garage is only served by the radiant system. The company for Quote 2 "doesn't release" the load calculations, but they may be the more reliable contractor. For the loads listed, have they recommended the right size units? Is quote 2 pushing the limits of a 4 ton system? |
RE: Load calculations for new home Geothermal
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| It’s hard to evaluate your situation over the web, so take this advice with a ‘grain of salt’… I would go for a 5-ton triple function GSHP. Worst case you’ll be a bit over sized. That may not be any big deal, with a 2-speed compressor & variable speed blower. The system will just run more at the lower speed at higher efficiency with less aux backup in winter. The down side is slightly greater cost and a larger duct system to accommodate the 5-ton system’s higher CFM at high speed. I would be just as concerned with the ground loop they’re planning; you haven’t mentioned a thing about that!! SR |
RE: Load calculations for new home Geothermal
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| Quote #2 suggested 4 ground loops, each 150 feet for the 4 ton system. Plan is 5 feet deep. Contractor #1 usually does a vertical loop, and suggested 3 wells each 250 feet deep. He can do horizontal, but I didn't get a length from him. I'm still stuck wondering which contractor calculated the load right. I'm awaiting a reply from each about their load calcs and how close they feel the system is to being over or undersized. Both quotes are for a Waterfurnace Synergy unit (triple function). If I make adjustments and use ground loops for both, the quotes are about equal in price (despite different size systems). |
RE: Load calculations for new home Geothermal
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| OK, so everyone is using the 150-feet per ton ‘rule of thumb’. This is the minimum depth for a vertical ground loop. Next questions. How do they plan to connect these ground loops, in series or in parallel (Parallel? Great – then what?)? Boreholes spaced how many feet apart? What is the diameter HDPE pipe? What type of grout and what procedures they’re planning to use? What type of anti-freeze? All the above questions have a DIRECT effect on the power required to run your ground loop. Power = Efficiency = $$$ over the life of this system. While we’re at it, how are they planning to fuse these pipes? I just want to raise these questions and awareness about ground loops just as we, and Mr. udarrall in particular, calls our attention to, and rightly so, distributions systems and static pressure in forced air systems. SR |
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