Super Insulated Hot Water Heater - Math Verification
I own an electric 40 gallon dual element hot water heater. Based on information received from the manufacturer here are the specifications:
47.25 inches height
20 inches diameter
24.98 sqft total surface area
2 inches of foam valued at R-16
My water temp is set for 120, ambient temps in the basement are 54F, though the floor can be cooler than this (48-52F). I used the conductive heat loss equations shown here (http://leaningpinesoftware.com/hwpipes/hot_water_heater_tank_insul.shtml) to calculate a loss of 264 kwh/year in standby losses (~$29/year at local electric rates).
The heater blankets (2 foil faces with a thin layer of fiberglass) typically found at the box stores retails for about $25. For $22 I purchased 49sqft of R-19 which was enough to wrap the tank and then some. I re-ran the numbers and came up with a loss of ~$13.3/year which seemed low (since I wasn't able to cover the bottom of the tank. The adjusted math factoring in the uncovered bottom of the tank adjusts the losses to $14.7.
My questions are:
1) Does the math for the initial calculation of passive loss through the tank look sound?
2) When calculating for ROI, do I have to factor in the exterior surface of the kraft batts? (e.g. The original tank un-modified is 20 inches in diameter, adding 6 inches of fiberglass adds an additional 12 inches to the diameter, doing so would mean the surface area goes from 24.98 sqft to 44.15 sqft which would make the modification a bust).
3) Would there be any benefit to wrapping the exterior of the batts with a radiant barrier of sorts (assume the barrier costs are 0 because I have excess from another project and I can maintain a sufficient air gap)
4) If yes to 3, do you know of any equations I can use to calculate potential savings from this added step
The tank is sitting on 5 inches worth of common masonry brick and a metal shims. I have the means to lift the tank in place without draining the water to replace the bricks with 5 inches of ridgid foam. Based on the answers to the original 4 questions I may continue down this road using scraps from the sheet I need to purchase to redo my attic hatch.
Posted Thu, 01/30/2014 - 16:57
Other Questions in Energy efficiency and durability