Will Water Heater Wiring Overheat with External Insulation?

Discussion in 'Water Heater Forum, Tanks' started by TJanak, Mar 2, 2012.

  1. TJanak

    TJanak New Member

    Messages:
    154
    Location:
    South TX
    Was reading an article in my local electric co-op magazine. They recommend not setting an electric water heater over 130* if wrapped with an external insulation blanket to avoid overheating the wiring. Is this really a concern? My water heater is set at about 150* and wrapped with a blanket on the sides and top.

    Thanks
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    22,003
    Location:
    New England
    Guess it would depend on the type of wire that is used...on the jacket of the wiring there is normally a rating. A call to the manufacturer might be in order to see what's internal - it's fairly easy to check the external wiring. And, yes, there is a temperature limit, not so much on the copper in the wire, but on the insulation.
  3. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    It would have to be a really p*** poor design for the insulation to break down at 130 or 150 F. Those are well within the limits of what any competent engineer would select for the service.
  4. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    I advise not to cover the external thermostat/element access panels with an external insulation blanket.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,003
    Location:
    New England
    Wiring can be purchased with heat ratings from as low as 50C on up. One would hope that the thing has sufficient grade of wire, but the thing was designed and tested for NO added insulation, and to save costs, the wire they used may not support the added internal heat if you add insulation. It should be easy to check, as it should be marked on the wire. Then, you'd know. Most WH seem to tell you to not add external insulation. This could be the reason.
  6. TJanak

    TJanak New Member

    Messages:
    154
    Location:
    South TX
    I wonder if they meant the internal wiring on the heater or the wire from the panel to the unit. I looked the other day and it is NM (no B) so technically that is probably under-rated. But I would hope it's not 150* at the top where the splice is made...

    Hackney, why? Adjustment/service issue?
  7. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Since the article if focusing on water set point differences and the impact on the wiring, one can assume the concern is at the panels themselves (which are normally not covered, providing a heat sink.) Wrapping the water heater, but leaving a cut out for the panels uncovered would seem to accomplish the goal while allowing higher temp operation of the water heater. There is also a possibility that parts of the terminal block can't handle higher temps, but this seems less likely.

    When you do enough process safety reviews you learn how to identify design cases. You do these with reasonable variations in mind (and some not so reasonable), plus some allowance for likely failure scenarios. Since the wires are connected to a heating element that you can be certain will burn out one day (and the burnouts that I've seen were all scaled up/in effect insulated from the water) and you have the possibility of insulation being added, it would be completely irresponsible to use wire that could not withstand these easily foreseen conditions. Even without insulation some water heaters are installed in attic spaces or the like where the ambient temp can be well over 120 F...way over that in some cases...which would constitute design cases.

    Doing cut outs for the panels on electric water heaters is the norm. Certainly you want cut outs for servicing it. I have the packaging from my water heater insulation blanket and the install only shows gas although the packaging says for gas and electric. Interestingly, it is not clear from the DOE site if they recommend leaving the cut outs for electric panels off or not. The diagram arrows imply that they be cut and put back into place! http://www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=13070

    Covering the panel with insulation will indeed allow higher temperatures for the wiring than one would have normally because it is losing a heat sink to the ambient air. The resultant local temps will be close to that of the water in the tank, but elevated above that due to conduction from the hot resistance element (worse when it is fouled) as well as electrical resistance heating of the wiring/terminal connections when operating. Since wiring for oven elements work with even more challenging conditions, it would be hard to defend a water heater wiring design that can withstand insulation cover at 130 F, but not at 150 F. Still, this could be the straw that breaks the camel's back for some specific design case: e.g. 140 F ambient air temp in an attic with insulation over the panels and the element scaled up.
  8. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    The blankets I have installed say do not cover the access covers on an electric heater because it can cause control wire insulation to deteriorate which may result in electric shock.;)
  9. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
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    The reason some manufacturers give for NOT putting any blanket around an electric water heater is that they feel it will affect the thermostat's operation and result in overheated water. As a practical matter, unless the outside of your water heater is "hot", or even warm, to the touch a blanket is not going to give any measurable benefits.
  10. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Insulation shouldn't have any measurable impact on the operation of the thermostat since that is an immersion probe in the fluid. If it measured wall temps it would be another matter. For comparison the one time I had trouble with a thermowell set up inside a reactor I was getting about a 10 C offset in a thick thermowell operating at above 1000 psig and 280+ C, gas phase. This had happened in multiple reactors as they were put in service and careful checks of the TC and their placement in the wells didn't yield any results...they had worked properly before. The walls on these reactors were very thick and they had several layers of different metals and a thick teflon liner to protect them from small amounts of highly corrosive liquids. I suspected that we were somehow getting wall effects transmitted to the tip a foot or more inside the reactor, but the question was why? When we had our next reactor shut down on one of the problem vessels I went into it and immediately found the problem: the mechanical shop had inexplicably relocated the seal ring for the wall teflon at the thermowells' near the tip, rather than at the wall. They had in effect insulated most of the well with teflon so that only the tip was exposed to the process gas while the walls and thick thermowell were serving as massive heat sinks. I had them put it back to the way it was before (and was drawn up) and the problem was gone.

    The benefits of insulating water heaters are measurable and can be calculated. My insulating blanket paid off in 2 years because the tank only had 1" of foam originally. The better (thicker) the original foam insulation is on the water heater, the less the potential energy savings are. There is about half as much potential with 2" of manufacturer foam as there is with 1". The surface doesn't have to be warm to be worth doing--and the colder the ambient is around the water heater, the greater the benefit.
  11. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    The typical electric water heater has surface mounted thermostats that are pressed against the outside of the tank. Its dry.
  12. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Then I stand corrected with respect to electric. Where exactly are they placing it? Is it on the wall underneath the service panel. Because if they do a calibration based on an unscaled wall, and a given insulation thickness, it's going to be off anyway with any scaling--which will make it read low, and therefore it will heat the water in the tank so that it runs higher than desired set point. However, if one added insulation outside, the thermostat should read high (because the inside wall temp will be higher), meaning the actual water temperature will be less than desired. It's the reverse of the reason given, but the manufacturers might be saying that...
    Last edited: Mar 8, 2012
  13. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    The thermostats are located above each element and are held tight to the tank with a metal bracket that provides tension.
  14. Runs with bison

    Runs with bison Member

    Messages:
    892
    Location:
    Midwest
    Thanks, I vaguely recall the bracket now. I haven't had to mess with them much since I left the farm long ago. Back then I tended to the water heater scale and elements in our families homes and barn.
  15. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    I worked on a Rheem 50 gal electric for my neighbor last week that was 21 or 22 years old. It needed a lower thermostat. The upper thermostat is still working fine. I replaced the elements a couple years ago. The anode has never been touched.

    5 people lived in that house for the majority of those 22 years. I cant believe its lasted that long under those conditions. Theres not much insulation in those old heaters thats for sure so adding one is a good idea. The wiring looked fine,and I checked.
  16. ballvalve

    ballvalve General Engineering Contractor

    Messages:
    3,261
    Location:
    northfork, california
    The odd thing about water heater internal wiring is that it has no markings on it, on the ones I look at. We might assume that it is 190'F or more insulation, but only the MFG seems to know, and maybe UL.

    Which would mean that only THHN wire should terminate in the heater from the feed. But I see no code on that.
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