Water heater insulation

Discussion in 'Water Heater Forum, Tanks' started by leeelson, Dec 12, 2009.

  1. leeelson

    leeelson New Member

    Messages:
    23
    Location:
    Nevada
    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.
  2. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    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'.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,026
    Location:
    Cave Creek, Arizona
    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.
  4. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    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: Dec 12, 2009
  5. leeelson

    leeelson New Member

    Messages:
    23
    Location:
    Nevada
    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?
  6. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Mineral wool melts at 650C, FG at 1000 to 1400C.
  7. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    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.
  8. leeelson

    leeelson New Member

    Messages:
    23
    Location:
    Nevada
    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.
  9. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    Out of curiosity, what are the dimensions of the insulated sections of the tank (diameter and length?)

    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 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 a moderator: Dec 13, 2009
  10. leeelson

    leeelson New Member

    Messages:
    23
    Location:
    Nevada
    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 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.
  11. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    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...
  12. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    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.
  13. leeelson

    leeelson New Member

    Messages:
    23
    Location:
    Nevada
    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...
  14. leeelson

    leeelson New Member

    Messages:
    23
    Location:
    Nevada
    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).
  15. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,026
    Location:
    Cave Creek, Arizona
    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).
  16. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    The insulation has a loss rate...it slows the rate of heat loss..and as you approach the max loss rate the savings plummets and then the other part of the equation is the amount of use the heater gets...the more it is used the less savings there are...
    Last edited: Dec 14, 2009
  17. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    Additional insulation will always reduce the heat loss in that direction with everything else being equal. However, there is a point where the additional cost of the insulation will not be recovered by energy savings.

    In addition, there is a point where you will not see a benefit to the additional insulation as the losses in the other directions (bottom of tank, top of tank, conduction through the piping, etc.) will dominate the losses.

    Most modern tanks benefit little from the blankets compared to the older tanks. I know that the manual for my GE/Rheem (electric) specifically states that a blanket is not needed.

    Where is your tank located? Garage or in conditioned space? If inside, the dT is lower during the winter (compared to a garage/attic) and the heat loss does go into warming the house, so the heat that is "lost" isn't really lost.
  18. Dana

    Dana In the trades

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    Location:
    01609

    True-dat!

    Putting a blanket on the already-insulated tank if you haven't insulated the near-tank plumbing (including the temperature/pressure valve plumbing and the cold water feed) to within the first 6' of the tank to at least R4 (5/8" wall closed cell foam or similar).

    Temperature actuated flue dampers can at least keep most of the center-flue heat losses within the room (heating up the space around it), as opposed to sending the bulk of it up the flue (along with conditioned-space air 24/365, when you really only need/want the flue drafting to dispose of combustion exhaust at a MUCH lower duty cycle- much closer to 1% of the time than 100%.)
  19. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    While the manufacturers might claim there is no need for the blanket, they qualify that by saying it is not needed to comply with federal regs.

    There is in fact a substantial benefit to the blanket, at least when the tank insulation is only R8. The loss rate with and without blanket can be calculated. This is also why you see some tanks in a series listed as "high efficiency" with 2" vs. 1" of insulation. 1" of foam = R8, that's the normal baseline now for a 0.58 or 0.59 EF tank in the 40-50 gallon size although some other tanks have 2"--probably to help prop up what would otherwise be too low of an EF. The respective 2" foam tanks in 40-50 gallon sizes are about 0.62. It's not hard to see how a small price differential for the extra insulation quickly pays out.

    Now, if the tank already has 2" of foam on the sides, then the benefits of adding a blanket will be much less for the area covered and space considerations can predominate. Looking at this in terms of deltaU: (1/8 - 1/(8 + 10) = 0.069 vs. 1/16 - 1/(16 + 10) = 0.024, therefore the savings are a little over 1/3rd as much as in the base case. As a result the minimum return is likely to be about 17% annually...instead of around 50%. If the tank lasts the minimum 6 years the additional insulation worst case result is break even over the lifetime...better if the tank lasts longer and/or if the blanket is removed and reattached to the replacement tank.

    I agree about tank projections needing insulation, but you have to be very cautious near the flue. If you put flammable or meltable material on the top piping within 6" or so of it, you can create a serious hazard. (I need to go down to an industrial insulation supply house to get some proper material for this short section.) It's of course much easier to insulate the T&P and discharge line.

    The argument several are making is like saying improving attic insulation from R19 to R30 or R40+ won't help because other areas (such as windows or infiltration) loses/requires so much heat.

    The "at least initially" part is just plain wrong. The resistance added by the insulation continues indefinitely. There may be some degradation of the insulation over time, but it is not going to go anywhere near zero unless it is saturated with water.

    More interesting is "The insulation has a loss rate...it slows the rate of heat loss..and as you approach the max loss rate the savings plummets and then the other part of the equation is the amount of use the heater gets...the more it is used the less savings there are..." The first part isn't really clear. I think what was intended was to say that as the overall insulation value increases the total losses become less and less so that the incremental savings plummet. This much is true and is the same as effect as gas mileage savings over a fixed distance. Going from 12 to 24 mpg saves a lot of gas. Going from 24 to 48 mpg saves less gas even though mileage doubles in both cases.

    The second part is incorrect. The wall losses are essentially fixed: If you put twice as much water through the tank at the same setpoint the wall losses for the year are essentially the same (okay, there is a slight difference due to more time spent in cooler recovery mode.) The overall efficiency factor of the tank changes...but that is why EF's are reported for a single usage standard. The savings are still essentially the same for doubling the insulation value, even if you double or halve the water rate through the tank.

    Note: the above discussion about benefits applies to standard gas storage water heaters primarily. Electric tanks seem to have thicker insulation (R20 to R24.)
  20. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    You were likely just feeling the heat being released by your body getting trapped under the insulation. I wouldn't attribute this to the heat loss of the tank.

    You need to use a remote thermometer.
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