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Community and Q&A

Super Insulated Hot Water Heater – Math Verification

Cardnyl | Posted in Energy Efficiency and Durability on

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 ( 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

Side Note:
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.

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  1. wjrobinson | | #1

    Fun with math... another way to play with your math. If you worked for pay the amount of time you have put into postulating...

    If you started to wash your hands always in cold water, rinse dishes in cold. laundry in cold... the savings would eclipse your blanket wraps...

    And as to your insulation... just get at it and start saving... more time spent say widdling wooden art sculptors to sell for a source of green just like insulating the tank...

  2. davidmeiland | | #2

    Christopher, I think that math is oversimplified, but maybe it gets you in the ballpark. In reality, water in the tank is stratified with the hottest at the top. So is the air in the room. The piping on the top and side of the tank siphons heat out. A highly accurate calculation would be more complex and would probably involve a time simulation.

    In my opinion, extra fiberglass will make somewhat of a difference, as will the radiant barrier. The shiny side should go out, so not sure what you mean by an air gap. Setting the unit on foam insulation is a major improvement.

  3. mackstann | | #3

    Fun bit of trivia: Foam insulation pads are required in Washington state now.

    I made a stand from PT wood that puts 7 inches of Roxul under most of the water heater. I'm surrounding the water heater with another 7 inches of roxul but I need to get around to seismic bracing before I complete that. One particular benefit is that the roxul is so thick and relatively rigid that I can carve out a niche around the TPR valve (normally an uninsulated thermal leak) to safely operate, while still insulating over it.

    Heat trap loops on the pipes, and generous pipe insulation close to the heater will also help reduce standby losses. I used big pieces of roxul on the pipes near the WH as well, although I reverted to regular pipe insulation past the heat trap loops.

    I can't really help with the math, although I can say I did it at one point and decided it was probably worthwhile in the long run (the roxul cost more than fiberglass, too). It's definitely getting into diminishing returns territory, but it's also a cheap and easy task you can do while saving up for bigger improvements.

  4. PAUL KUENN | | #4

    Proof is in the pudding. I wrapped my three tanks (domestic 50gal and two 70s for floor heating) in use R30 fiberglass with one side foil and put 4" of EPS underneath. The tanks stay MUCH warmer all day and night now. They certainly did NOT beforehand.

  5. Cardnyl | | #5

    AJ: I was on my lunch break at work while I posted that question so technically I was being paid to postulate =). That aside we already do all of the other things you mentioned, but theres always more to be done to maximize savings.
    David: I was afraid of that. I know theres savings to be had just curious how long until the project pays for itself. The an air gap comment was because radiant won't work without one.
    Nick: We had a bunch of 3/8" wall thickness polyethylene pipe insulation given to us so I went ahead and insulated all of the copper leaving the tank and a few feet of cold line which felt warm. I'm pretty sure the tank has built in anti-siphon nipples but they still felt warm; I guess they don't work perfectly so they got some insulating as well. I had enough left over fiberglass to cover the top of the tank with 18 inches of fiberglass cut with a hole saw to fit around the pipes.
    Paul: I think I will end up tanking the plunge and remove the bricks and replace it with ridgid foam.

    Thanks everyone for taking time out to respond.

  6. wjrobinson | | #6

    Chris, have you started to wash hands and laundry with cold? Have you a no water toilet? There is quite a bit you can do along with insulating your water system. Try out some habit changes that can really cut down on water and energy and time and materials and maintenance.

    Doing less and you will be doing more...

  7. Cardnyl | | #7


    Yes and Yes. In fact our washer explicitly has a setting for "Tap cold" as opposed to cold because cold is actually tempered with warm water for washing performance on some machines. I don't have a "no water toilet" but I'm not really concerned with any sort of water saving/reclamation techniques (using grey water recovery for toilet use, rain water, etc.) because water is so cheap in upstate NY.

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