# WH blanket

Ripped this off another forum. Found it of value so it’s being passed on.

“Peter G. Engle, PE” <Pete@ALMOSTHOME.COM> 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 for 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 “pathways” to escape. And since these pathways increase in a geometric way while the thickness only increases in a linear way, there is a crossover point where the heat loss actually begins to increase with thickness. It turns out that for any given set of inside and outside temperatures, insulation factors, and cylinder geometry, there’s an optimum insulation thickness where heat loss is minimized. If you apply any more or less, the heat loss increases. It really works this way. We worked through the math, and it really does happen. Go figure.

What’s pretty cool is that the water heater manufacturers aren’t dumb – they study basic thermodynamics, too. They already insulate water heaters to the optimum level for the type of insulation used. To get more insulating value in modern water heaters, they haven’t increased the thickness, they’ve changed the insulation. Insulation with a higher R-value gives you more insulation in less thickness, and the magic crossover point moves to a lower total heat loss level, even with reduced insulation thickness. The ideal insulation would have infinite R-value, so the thickness could approach zero, and still have zero heat loss.

But I digress. The bottom line is that with water heaters, if you go slapping on a loosely fitting blanket of fiberglass, chances are that you will actually increase the heat loss. Pretty neat, huh?

Peter G. Engle, PE
Almost Home, Inc.
www.almosthome.com

Replies:

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.

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 should be insulated not the control valve.

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.

Well I never…

Good article. Thanks.

If only the US Dept of Energy aggreed with the aforementioned PE.

If only the US Dept of Energy aggreed with the aforementioned PE.

Actually, the U.S. Department of Energy does agree with the aforementioned P.E., and the aforementioned P.E. is correct.

But a “tank that’s warm to the touch” is an oversimplification. It would depend on where the heat is coming from that causes the tank to be warm to the touch. Here in sunny San Diego over the past week, with temperatures in the 90s and 100s, probably every tank was warm to the touch, especially those in exterior utility closets.

What we need is a tank on a conveyor belt so that when it’s cold, the tank gets moved into the insulated utility closet, and when it’s hot, the tank gets moved outside to soak up that solar heat and lower utility costs.

However, water heater manufacturers probably did their calculations at their recommended temperatures, so if one runs the water heater at the hottest setting, the insulation probably lets some heat through, causing the tank to be warm to the touch.

So there could be many different factors that go into determining if the water heater needs an insulation blanket. If one buys a modern water heater (one made after 1994, I believe it is) and keeps the thermostat at the manufacturer’s recommended setting, and installs the water heater in a protected location (exterior utility closet, garage, interior utility closet), an insulation blanket is probably a waste of money that could better be spent on margaritas :margarit: .

One reallly never knows who is right or wrong when polar opposite information exists. I’m sticking with the P.E. looks like he’s done the math correctly.

I really wanted to stess the issues about the seals leaking at the gas valves and impingement to combustion and draft air when the insulation is improperly installed at a gas WH.

Food for thought when inspecting jacketed units. Just like the pipe insulation needs to be cut back 6" away from draft hood and flue.

I have a insulative blanket on my electric hotwater heater. The formula provided by the engineer maybe correct but the curveatuer of a 60 gallon tank would probably not really effect the equation in any great manner. When I put my hand in between the metal jacket and fibreglass jacket you can feel the trapped heat. Heat that otherwise would be disapating to the surronding air.

I do not buy the engineers principle in this. If you effectively reduce the conductive surface of the water heater you will increase its efficiency.

I would put some good money on that if set up two water heaters under the exact same conditions next to each other and apply the exact same amount of energy to heat the water and maintain that heat that the one with more insulation will be more efficient.

This would be a pretty easy experiment to set up and I am sure they have.

This is n 8 year old thread.

Very little is to be gained by adding an insulation blanket to a modern water heater.

As you add more and more insulation your return on investment decreases.

It’s simply not cost effective to continue to add insulation beyond a certain level.

Do the math.

Well, I couldn’t let this go without doing some research on my own. It turns out increasing insulation on a cylinder shape will increase the rate of heat loss up to a point call the Critical Radius (Rcr). But once the outer radius of the insulation passes the Critical Radius the rate of heat loss goes down. The critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h. The lowest value of h encountered in practice is about 5 W/m2 °C for the case of natural convection of air, and that the thermal conductivity of common insulating materials is about 0.05 W/m2 °C. Doing the math the largest value of the critical radius we are likely to encounter is 1 cm. So the problem with heat loss from an insulated cylinder is only an issue for very small cylinders, such as heat loss from an insulated electrical wire. The size of a water pipe or water heater in a building is way beyond the Critical Radius and adding more insulation does reduce heat loss. As others have said, its economics that drives the cost/benefit of adding insulation to a water heater, which is no different than adding insulation to the attic. See the attached pages from a thermodynamics book for more detailed explanation.