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Thread: Water heater insulation

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    DIY Junior Member leeelson's Avatar
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    Default Water heater insulation

    I have a gas water heater for which I can't find a water heater blanket that will fit. Can I just use faced fiberglass attic insulation as long as I avoid the vents and top of the heater? Is there a fire hazard? Seems not to me.

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    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by leeelson View Post
    I have a gas water heater for which I can't find a water heater blanket that will fit. Can I just use faced fiberglass attic insulation as long as I avoid the vents and top of the heater? Is there a fire hazard? Seems not to me.
    Not if the insulation would be considered a combustible material. Your local area will have a min. distance your HWT needs to be from combustible materials.... usually 3'.

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    Moderator & Master Plumber hj's Avatar
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    Default blanket

    1. Many water heater manufacturers specifically say NOT to use a blanket.
    2. Most heat loss in a gas heater is through the center flue which a blanket will not help.
    3. Unless your heater's jacket feels hot, or warm, to the touch a blanket is not going to save enough money in a year to buy a cup of Dunkin' Donuts coffee.

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    Homeowner Thatguy's Avatar
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    Back in the 70s, my elec. WH lost 75w thru the insulation, then 50w when I insulated it. I used an elec. analog clock modified to run on 240v to see how many minutes per 10 hours the element was on.

    I guess you could measure it.

    Heat the water, turn off the heater, wait a while and measure the water temp from the bottom drain. Then wait a few hours and measure it again.
    400# of water dropping 1 F must have lost 400 BTU (over how long)?
    50w is ~170 BTUs/hr.
    Last edited by Thatguy; 12-12-2009 at 02:32 PM.

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    DIY Junior Member leeelson's Avatar
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    Quote Originally Posted by Doherty Plumbing View Post
    Not if the insulation would be considered a combustible material. Your local area will have a min. distance your HWT needs to be from combustible materials.... usually 3'.
    Well, I guess that's my question. I don't think the fiberglass is any different from the fiberglass in a water heater jacket. Is fiberglass a "combustible material"? Does the facing change things?

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    Homeowner Thatguy's Avatar
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    Quote Originally Posted by leeelson View Post
    Is fiberglass a "combustible material"?
    Mineral wool melts at 650C, FG at 1000 to 1400C.

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    Plumber jimbo's Avatar
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    Water heaters today are insulated with blown in foam, which is why they have pretty minimal heat loss through the insulation. Yes, the paper on the insulation would be a fire hazard it seems to me. Front usually requires 4 " clearance to combustibles.

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    DIY Junior Member leeelson's Avatar
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    Quote Originally Posted by hj View Post
    1. Many water heater manufacturers specifically say NOT to use a blanket.
    2. Most heat loss in a gas heater is through the center flue which a blanket will not help.
    3. Unless your heater's jacket feels hot, or warm, to the touch a blanket is not going to save enough money in a year to buy a cup of Dunkin' Donuts coffee.
    As a quick test, I wrapped part of the heater. Sticking my hand between the fiberglass and the outside of the tank results in a significant warmth. Much warmer than the un-wrapped surface of the tank.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by leeelson View Post
    I have a gas water heater for which I can't find a water heater blanket that will fit. Can I just use faced fiberglass attic insulation as long as I avoid the vents and top of the heater? Is there a fire hazard? Seems not to me.
    Out of curiosity, what are the dimensions of the insulated sections of the tank (diameter and length?)

    Quote Originally Posted by hj
    1. Many water heater manufacturers specifically say NOT to use a blanket.
    This is untrue as was pointed out in a recent thread. Some manufacturers claim the blanket is unnecessary and for compliance with national standards they are correct. That in however, does not mean they are prohibited or actually cause any harm.

    2. Most heat loss in a gas heater is through the center flue which a blanket will not help.
    This is probably true or close enough to it. However, that does not in any way negate reducing other heat losses.

    3. Unless your heater's jacket feels hot, or warm, to the touch a blanket is not going to save enough money in a year to buy a cup of Dunkin' Donuts coffee.
    As pointed out before your reasoning on the touch test is completely wrong as are your wild guess at benefits. Even for a water heater in a utility space within the home the savings exceed that.

    Typical 1" foam insulation gives R8. With a blanket one can acheive up to about R18. By comparison some of the high efficiency water heaters (that you stated you've not sold?) tend to have 2" of insulation for R16. Looking at the State product line it appears that the average 1" insulated tank runs about 0.59 eff. and the 2" at 0.62. That 0.03 delta works out to about 12 therms a year. (Note that the efficiency factor change with increased insulation is inline with what I manually calculated for my tank.)

    Average cost around here is around $1/therm (although the Great Recession has made it cheaper for the past year.) Cost of an insulation blanket? I've seen them from about $16 - 24. So worst case you are looking at about 50% annual return on investment.

    Run the water heater at higher temps and/or put it in a garage in a northern climate and the actual benefits will be a multiple of the above.
    Last edited by Cass; 12-13-2009 at 07:44 AM.

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    DIY Junior Member leeelson's Avatar
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    Quote Originally Posted by Runs with bison View Post
    Out of curiosity, what are the dimensions of the insulated sections of the tank (diameter and length?)
    65" in diameter and 57" tall (50 gal). The local HD only carries 48" widths and carries *no* unfaced R13 attic insulation (thicker won't fit). So I'm back to looking (online) for a wider jacket. Any help there is appreciated.


    This is untrue as was pointed out in a recent thread. Some manufacturers claim the blanket is unnecessary and for compliance with national standards they are correct. That in however, does not mean they are prohibited or actually cause any harm.


    This is probably true or close enough to it. However, that does not in any way negate reducing other heat losses.



    As pointed out before your reasoning on the touch test is completely wrong as are your wild ass guess at benefits. Even for a water heater in a utility space within the home the savings exceed that.

    Typical 1" foam insulation gives R8. With a blanket one can acheive up to about R18. By comparison some of the high efficiency water heaters (that you stated you've not sold?) tend to have 2" of insulation for R16. Looking at the State product line it appears that the average 1" insulated tank runs about 0.59 eff. and the 2" at 0.62. That 0.03 delta works out to about 12 therms a year. (Note that the efficiency factor change with increased insulation is inline with what I manually calculated for my tank.)

    Average cost around here is around $1/therm (although the Great Recession has made it cheaper for the past year.) Cost of an insulation blanket? I've seen them from about $16 - 24. So worst case you are looking at about 50% annual return on investment.

    Run the water heater at higher temps and/or put it in a garage in a northern climate and the actual benefits will be a multiple of the above.
    This is the type of info I was looking for. Even though 12 therms isn't much, it's still about 5% of my total gas use. With some recent unusually cold weather (-3) seems like a good idea to save a few bucks/therms. Thanks for your input.

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    Plumber Cass's Avatar
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    Quote Originally Posted by leeelson View Post
    As a quick test, I wrapped part of the heater. Sticking my hand between the fiberglass and the outside of the tank results in a significant warmth. Much warmer than the un-wrapped surface of the tank.
    It will always do that as you are trapping the heat and slowing it...this does not equate to savings, all insulation does is slow the rate of heat loss...there is a point where reguardless of the amount of insulation you have your rate of loss is the same...

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by leeelson View Post
    65" in diameter and 57" tall (50 gal). The local HD only carries 48" widths and carries *no* unfaced R13 attic insulation (thicker won't fit). So I'm back to looking (online) for a wider jacket. Any help there is appreciated.
    The tank is actually about 20-21" in diameter; 65" is the circumference. And the 57" height includes the burner section and such which you don't insulate.

    You will find that the standard 48"x75" blanket works. I have one on mine and the dimensions sound identical to your tank. I'm not sure why they list the "width" of the blanket as 48 and the "length" as 75". It actually goes the other way. I wrapped mine starting in line with the T&P with the overlap being trapped under the T&P discharge line. This provides a compression fit so that I don't even need tape along the overlap.

    Follow the directions to cut out for the thermostat and T&P valve and don't cover any air intake grating (typically near the bottom.) You should be able to tape the uninsulated flap to the top of the tank. You don't want to put any insulation on the top of the tank as it could interfere with the flue.

    Before you do all of this, record all of the information on the tank label and attach that info to your manual. (Serial #, model #, etc.) That way you won't have to remove the insulation to find it.

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    DIY Junior Member leeelson's Avatar
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    Quote Originally Posted by Runs with bison View Post
    The tank is actually about 20-21" in diameter; 65" is the circumference. And the 57" height includes the burner section and such which you don't insulate.

    You will find that the standard 48"x75" blanket works. I have one on mine and the dimensions sound identical to your tank. I'm not sure why they list the "width" of the blanket as 48 and the "length" as 75". It actually goes the other way. I wrapped mine starting in line with the T&P with the overlap being trapped under the T&P discharge line. This provides a compression fit so that I don't even need tape along the overlap.

    Follow the directions to cut out for the thermostat and T&P valve and don't cover any air intake grating (typically near the bottom.) You should be able to tape the uninsulated flap to the top of the tank. You don't want to put any insulation on the top of the tank as it could interfere with the flue.

    Before you do all of this, record all of the information on the tank label and attach that info to your manual. (Serial #, model #, etc.) That way you won't have to remove the insulation to find it.
    You are absolutely right. The 48" x 75" should work OK. Time to think out of the box...

    I did think about covering the info on the tank. My solution was to take a digital photo of everything for future reference. Thanks for your help...

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    DIY Junior Member leeelson's Avatar
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    Quote Originally Posted by Cass View Post
    It will always do that as you are trapping the heat and slowing it...this does not equate to savings, all insulation does is slow the rate of heat loss...there is a point where reguardless of the amount of insulation you have your rate of loss is the same...
    Seems to me that this will equate to savings. Heat loss decreases as the temperature difference between inside and outside the tank decreases (assuming everything else stays constant). The purpose of the insulation is to trap heat between the insulation and the tank, raising the temperature on the outside of the tank and decreasing the temperature difference. Seems to me that this is how all insulation works (e.g. attic insulation).

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    Moderator & Master Plumber hj's Avatar
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    Default heat loss

    But then you have just moved the temperature differentiation to the insulation. Now the insulation has a higher temperature on one side than the other, so the heat will STILL gravitate out, just somewhat slower, (at least initially).

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