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Water Heater Math


kurt
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This is a little bit of information gleaned from Pete Engle in some emails. It describes how putting one of those cheezey after market insulation blankets on a new water heater can actually increase the heat loss instead of reducing it. Read on; fascinating stuff from one of the brilliant minds in this biz......

"Peter G. Engle, PE" wrote:

Yeah, that one really got me screwed up when we did it in college thermodynamics. But here goes:

Insulation acts in a linear fashion fir heat transfer through the material. In math terms, H=kxT, or heat loss equals the thermal constant (U value) times the thickness of the insulation times the Temperature difference between inside and outside. More simply – if you double the thickness of the insulation, you cut the heat loss in half. This, we pretty much all know intuitively. But it’s only true for flat surfaces. There’s a lot of simplification that goes on to get to that simple formula.

When you start insulating cylinders, you can’t ignore the effect of curvature. If the insulation is thin, the effects of curvature are small, and the formula above works fine. But as the insulation gets thick, the surface area of the outside increases with the square of the thickness while the surface area of the inside stays constant. Effectively, the heat has more “pathwaysâ€

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"Clean up on isle 6"- my head just exploded.

Funny, I'd always heard that blankets weren't a good thing on water heaters, but my understanding was that the maunfacturers were concerned about the cylinder actually retaining too much heat. I've never had any evidence to back it up so I have stayed somewhat neutral on the subject.

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I had a guy from Rheem explain this to me about 28 years ago during the first energy crisis; he was pissed that folks were adding insulation to his heaters because "it doesn't do anything; it may even make it worse".

I didn't get it then, but I (sort of) get it now, due to Pete's downhome explanation.

I'm sure there are many who will dispute this, and there are probably many variables that can effect the outcome. It made sense to me though.

I also think that many mfg's. don't like the blankets for the reasons Mike described, and the fact that so many folks would probably just slap it on & cover up air intakes, vents, TPRV's, and everything else.

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Here in the Water Wonderland of Michigan, the utility companies actually paid people to go door to door and install water heater insulation blankets. As the blanket insulated and held heat on the gas control valve, the "rubber" seals got hotter than designed for and started leaking.

We have found gas leaks on abt 90% of insulated gas water heaters at the top function knob; on - off - pilot knob. I think the valve was designed to be a little above room temp and adding insulation too close raised that temp 20 degrees or so. As a former slum lord it was not uncommon for me to spray the knob with WD-40 and stop the leak, for a couple months.

I like Peter's explaination and it makes sense.

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Good Point Les,

It's never made sense to me to insulate a gas water heater unless it's a direct vent type and then only the tank - not the control - valve should be insulated.

In order for a gas water heater to work properly, you have to leave off enough insulation blanketing around the draft diverter to allow air in, you can't cover the control valve, the access cover for the burner plate or the air intakes on the underside of the tank or on the sides. Once you've left that much of the tank uninsulated, there's not much point in trying to wrap it anyway.

My point to homeowners is that the tank manufacturers have figured out how they can achieve maximum energy savings with the way they've insulated their tanks. Adding more insulation might give one a warm and fuzzy but it's probably money wasted.

OT - OF!!!

M.

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