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Manual J results are in!

Retired1 | Posted in Energy, Heating & Insulation on April 5, 2006 10:14am

Still on the path to enlightenment. To determine the correct sizing of HVAC equip in this 100 yr old home [ off grade, 2-story, 12/11ft ceilings, lots of big single pane windows, 4900sf (2580 down/2360 up), basement/stand-up crawl, no insulation except fiberglass batts R-38 in attic floor/upstairs ceiling joists, hot humid gulf coast], I had the Manual J run. (Scenario #1)

 Also ran the Manual J with adding an unventilated, icynene foam R-21 attic with removal of the fiberglass batts (Scenario #2), adding R-19 to the first floor joists (Scenario #3), and doing both (Scenario #4). The other energy efficiency options are adding storm windows with low-E and UV blocking film, Sealing and insulating the basement/crawl space, but that was not calculated.

The results were interesting:

Scenario  Heating load   Cooling load   Req equip capacity in tons

#1  down    69,052 btuh    60,375            5.0

       up         64,923            60,543            5.0

#2                70,759            61,460            5.1

                     65,426            56,616           4.7

#3                 61,038            56,395           4.7

                      64,923            60,543           5.0

#4                 62,752             57,491           4.7

                      65,426             56,616           4.7

I guess 4.7 means a 5-ton unit is required still. I would think the Manual J might not tell the whole story. With limited funds, and needing new HVAC (currently 5 ton split a/c upstairs, 5 ton gaspack down, and a functional massive old boiler/radiators for back-up), my question is bang for the buck. Replacing with high eff heat pumps with icynene unventid attic was the plan, with insulating the floor to come later. Geothermal open loop (underground river with good water) is an option, but not with the icynene due to budget.

Any thoughts or comments would be greatly appreciated. Thanks 

 

 

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  1. RayMoore2G | Apr 06, 2006 03:33pm | #1

    It is difficult to get the proper credit for the unvented attic design with current manual J. I have 3400 feet of living space and 800 feet of full height conditioned attic. Manual J would have required 7 tons of AC. I used 5 tons of AC. In 100+ degree heat, the home actually requires less than 3 tons of AC. The efficiency is not due to the unvented attic design alone as the walls use a speciallized airtight design, but manual J does not give proper credit there either.

    The 5 tons works well for us. (3 ton and 2 ton) During large parties we never lack adequate cooling. In low load conditions we have a DEC dehumidifier to maintain humidity levels. We also use the DEC unit to provide filtered mechanical ventilation. Last month we used 763 KWh and we average 1000 KWh over a twelve month period.

    Your windows are a killer I'm sure and all you can do is tighten them up as much as you can with glazing and sealants.

    For new construction, it is easy to justify any additional expense up front if you judge it on cashflow. Many cost analysis are done by figuring how many years out the payback is. I instead would look at what it adds to the mortgage payment compared to what it saves on the energy bill. If I can save you 100 dollars on your energy bill then you can afford a higher mortgage payment. 100 dollars at 6% for 30 years will buy you an additional 16,800 dollars up front. That amount is far more than is needed to implement the system that we use. Viewed from that perspective, the payback is immediate. Two other factors to consider are the tax advantage to a higher mortgage and the effect of rising energy cost over the period of the loan. Both those factors make investing in an efficient home even more attractive.

    I'm off your topic so I'll stop here; although, it is all one topic to me.

  2. FrankDuVal | Apr 08, 2006 04:30pm | #2

    Where did you get a copy of the manual J program to run on your computer?

    Frank DuVal

    You can never make something foolproof because fools are so ingenious.

    1. Retired1 | Apr 08, 2006 05:19pm | #3

      Frank, I had the local utility Gulf Power do it for me as part of an energy audit. They have the good software I'm told. If VEPCO doesn't provide this service, $49 gets you a copy of  HVAC-Calc Residential 4.0. There's a link to a promo movie below. There's also a professional/commercial version from the same company  as above.  I'm sure there are other programs. In my readings on the subject, the thermodynamics are straightforward, and the calculations are simple but very cumbersome and time-consuming (boring). Changing any variable will cause you to repeat the entire process. It appears the process is decades old and sound.

      Maybe one of the pros on this forum has a better way to go and insights, history, and practical considerations. 

      http://206.191.48.133/hvac/watchme/WatchMeSl_viewlet.html

      1. danski0224 | Apr 08, 2006 08:50pm | #4

        HVAC Computer is a good program.

        Remember, you need to pay attention to the sensible and latent loads. A 5 ton condenser (60k BTU) WILL NOT give you 60k BTU of sensible capacity. The sensible capacity is much, much less- especially on 5 ton units.

        Equipment ARI ratings are also based on an 80* indoor temp, and the sensible capacity of the equipment must be derated if you desire a lower indoor temperature.

        If you really need 60k BTU's of sensible cooling capacity, you can't do that on one system. 

        I would redo the numbers with a heat gain limiting film applied to your single pane windows.  

        Manual J just gives you the numbers. You must know how to use them.

        Edited 4/8/2006 1:52 pm ET by danski0224

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