Sizing solar thermal for a dorm: Here’s an example

The question I am most often asked is how to size a solar hot water system: How many collectors, size of tank and pump, etc. Here’s one of the recent examples. I’ve reprinted here my email exchange with a wholesaler’s rep. This example is for a college dorm, but the calculations can be used in any number of commercial situations. If you have trouble following the calculations, just shoot me an email.

Mike: Can you provide me with panels needed and tank sizing for a dorm building?

Rod: What are the required GPD (gallons per day) and GPH (gallons per hour)?

Mike: I am using a rule of thumb, but I think around 330 gallons.  They want 50% of their heating provided by solar.

Rod: 330 gallons a day? OK. Let’s move back a step. Tell me more about the dorm: What type is it? What season will it be used? Does it have a cafeteria or laundry? How many students per dorm room? Does each dorm room have a kitchen? I might guess it’s one of two types: 1) Most common has 6-8 students in a dorm with its own kitchen and then a central laundry elsewhere in the building; or, 2) no kitchens in dorm rooms and a central cafeteria. How many beds? Is it new or existing?

Mike: 16 students. 2 washing machines. Unknown on kitchen.

Rod: I’m going to focus on a drainback system because in the summer there will be more heat than the tank can accept. A drainback system provides overheat protection to deal with excess solar energy without requiring a larger storage tank. The tank I’ll recommend is sized for wintertime solar production at 100%.

Here are the calculations:

1. 16 students x 20 gpd (ASHRAE estimate is 12-26 GPD per person) = 320 gallons of total hot water needed

New condo development in Portland, OR, by Gen-Con Solar

New condo development in Portland, OR, by Gen-Con Solar

2. 320 gallons x 8.3 (weight of 1 gallon of water in pounds) x 70 (increase in water temp required) = 185,920 Btu required per day

(NOTE FOR THE FOLLOWING: The HTP 4’x’8’ flat-plate solar collector, FP32-SC, will deliver about 32,000 Btu per day in the summer and about 22,000 average in the winter).

3. 185,920 ÷ 32,000 = 5.8 panels for summer performance

185,920 ÷ 22,000 = 8.5 panels for winter performance

If this is a normal school season, winter performance is the probable target. If the customer wants to offset the fuel cost by 50%, four panels would be the best choice. Following is the parts list I would recommend:

Parts list for a drainback system for small dormatory with 50% of heat provided by solar:

  • 1 ea. PH199-119S Phoenix gas-fired water heater
  • 4 ea. 4’ x 8’ flat plate solar panels
  • 4 ea. Mounting feet
  • 2 ea. Rear legs mounting for a flat roof
  • 1 ea. 15 gal. drainback tank
  • 1 ea. Drainback solar controller
  • 1 ea. Taco pump sized for 6 gpm at the lift from the drainback tank to top of solar panels + 5 feet

How does this sound to you?

Mike: Sounds great!  Thank you for the help.

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