Questioning the Savings from Grocery Rehab

Questioning the Savings from Grocery Rehab

This is a great tale, but I don’t believe it. The article [

Vol. 5, No. 4, pp. 10-11] implies that insulation and airsealing dropped 30 tons (106 kW) of peak load from this 17,000 ft² (1,600 m²) building. Nor do I believe the peak load of only 20 tons (70 kW) for this building. Indiana has hot summers: if it has occupants, lighting, equipment, windows, and any ventilation air, it is hard to conceive how they get to almost 1000 ft² (93 m²) per ton. Just based on simple hand calculations of what the infiltration reduction and the U-value improvement will buy in cooling savings—I’ll bet it isn’t more than ¹⁄₃ of the purported 30-ton savings. My guess is that the infiltration control cut 4-5 tons (14–18 kW), and the insulation perhaps the same, depending on orientation.

National numbers for food stores show that most energy is for refrigeration, then lights. I believe these numbers are not telling us the real story, which I believe is this:

•To start with, engineers oversized the system to come up with 50 tons (176 kW).

•Much of the cooling comes from open refrigerated/frozen cases, and the engineers didn’t count it. (In which case, the best strategy would seem to be to stop the freezers from cooling the store.)

Marc Rosenbaum, P.E.

Meriden, New Hampshire

The source of the data replies:

Glad to respond—the quickness and quality of the comments is more evidence of the level of readership enjoyed by

EBN.

I thought the results were pretty amazing, too, and double checked everything, since I was using a patchwork of inputs from BBH and another engineering consultant (I never had the opportunity to visit the site). Always risky. I assisted with this review, but I did

not to design the package, spec equipment or provide design inputs; I was asked to estimate the savings based on changes that were made to an unbelievably inefficient building.

The building shell was essentially

not thermally protected and very leaky. The equipment was pretty old and inefficient (and was replaced), and the ducts were leaky and repaired under the project, as I understand. No specifics were provided about internal loads, so I used the California Energy Commission small commercial schedule (T-24 sched #5). This schedule may under-estimate if there are large refrigeration loads.

Perhaps the article also understates the work that was done a bit, and the initial very inefficient condition of the building. Obviously the issue of lighting and refrigeration should be addressed, but this building was indeed envelope-“poor” and I fundamentally support the Harwoods’ beginning with correcting serious envelope problems, and upgrading the basic heating and air-conditioning system and ductwork.

This is a pretty simple building taken from a very inefficient status to a much-improved level of efficiency. Let’s make a point to check back with the owners in one year, to compare the before and after billing data with the very simple computer study I performed. For now, I am going to stick with the results provided. The best advice I can give to small commercial building owners is to obtain a good quality energy study (engineering audit or “energy rating” if available), and then use the priority measures list to install the most cost-effective measures first. Armed with the measures list, the earlier measures installed often help pay for later measures.

Bion Howard, Howard Associates

Upper Marlboro, Maryland