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HERS Index Rating Initial Evaluation Results

by Chad Ludeman on April 7, 2008 · 1 comment

in Building Science,envelope,Green Programs,HVAC,LEED

We received the initial HERS Index scores from the Energy Star evaluations that MaGrann performed on our initial specs about two weeks ago. I have been crazy busy lately and forgot to post on this at the time. If you recall from our LEED Design Charrette, we need to hit a HERS Index of 50 or below in order to have the best chance of hitting LEED Platinum as well as getting the Federal Tax Credit that will add another $2K to our budget.

Well, the bottom line is we can’t come close to the 50 point score we are seeking with an all electric house. We have been concerned about this from the time we decided to pursue an all electric home, but now have the confirmation in black and white. Our score came back at 80 with an electric boiler, which gets us the Energy Star Certification but not anywhere near the desired rating for LEED. We also never intended to give our homeowners a house that only exceeds the energy efficiency of a standard home by 20%. The long term energy saving potential of the home is just as important, if not more so, than the low purchase price.

Thankfully, MaGrann offered a few options based on what they are typically doing for other LEED and Energy Star homes in the region. Here is a blurb I cut and pasted from their email that gives some more info on the three choices they evaluated:

  • The first set – 100k house proposed is the equipment on the spec form; 33k mini split and electric boiler for radiant floor. [HERS Index = 80]
  • The second set – 100k house gas boiler gas instant is 33k mini split, with a 92% 40k boiler and a separate 82% instant H20 heater. This is the only combination that meets the tax credit. [HERS Index < 50]
  • The third set – 100k house gas instant radiant and H20 is 33k mini split with a Takagi T-K3 serving both the radiant floor and H20. [HERS Index = 57]

The document for the second option was actually not included in the email on accident so I’m not sure of the exact HERS Index that it would come in at, but it is obviously less than 50. We are not crazy about using two different water heaters for the home as we have discussed previously due to complexity, budget and loss of flexibility for a solar thermal system. I would much rather pursue the Takagi tankless unit to heat all of the hot water for the home (both domestic and radiant) and try to make up the other seven points somewhere else. The Takagi unit would allow us to install an insulated tank in line with it that could be hooked up to a solar thermal system (either now or in the future) that would preheat the water for the entire home.

I am not sure solar thermal is the way to make up these points but I will save the details of why for a future, much juicier post I am working on. Below are some more of the key inputs to the HERS calculation based on our initial specs provided to MaGrann:

Conditioned Area (sq ft): 1,296

Conditioned Volume (cubic ft): 11,664

Insulated Shell Area (sq ft): 2,499

Foundation Type: Slab

Vaulted Ceiling: R-40 SIP, U=0.023

Above Grade Walls: R-17 SIP, U=0.051

Slab Floors: R10P, R10U, 24W, Rad U=0.043

Window/Wall Ratio: 0.16

Window Type: U=0.34, SHGC=0.31

Infiltration: Htg=1296, Clg= 1296 CFM50

Measured Duct Leakage: 0.00 CFM25

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HERS Evaluation from Magrann is on Target |
July 12, 2008 at 6:31 am

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