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Heat Dissipation Calculation for Water Heaters

Heat Dissipation Calculation for Water Heaters

Hi engineers.

Recently I was asked this question which I have no knowledge of. Hope to get some ideas and knowledge from here to start off.

For a commercial Hot Water Heater with storage tank type, is there a fixed formula to calculate what is the heat dissipation of the heater.

Figures given; 54kW, 480V/3P/60Hz, 65A. The tank is a stainless steel 500 liters capacity. There is 50mm thick rock-wool insulation and 0.4mm thick stainless steel 304 external cladding.


RE: Heat Dissipation Calculation for Water Heaters

Is this a 'constructed' tank of some sort and you're trying to figure out the standby losses?

Or, what these losses contribute to the space the heater is in?

What the capacity of the heater is? (54kW)

If you're trying to figure out the standby losses you need the side surface area, bottom surface area, top surface area, and knowledge about pipes coming to and from the tank.

Perhaps you should tell us exactly why you're needing this 'result' as then we can help you more effectively.

Keith Cress
kcress -

RE: Heat Dissipation Calculation for Water Heaters

It seems like the question is about the "standby" heat loss through the insulation.

You could start here. You'll need to estimate the "R-Value" of the insulation. You'll obviously also need to know the water and ambient temperatures. There can be other losses, such as back through the cold water pipe.

For my domestic hot water heater, I bought a tank with "high tech" (high R-value) insulation, put it on a slab of blue insulation board, added an external blanket kit, plumbed the pipe with 'anti-siphon' bends. I was trying to minimize standby heat loss to save money.

RE: Heat Dissipation Calculation for Water Heaters

My calculations suggest that a well insulated water heater is only losing about 200W to start with, with a surface temperature of only about 28C. You'd need about 2 inches of Styrofoam to cut that in half.

Nevertheless, payback appears to be under a year.

I can do absolutely anything. I'm an expert!
FAQ731-376: Forum Policies forum1529: Translation Assistance for Engineers

RE: Heat Dissipation Calculation for Water Heaters

I have a 30 gallon tank with rock wool and an external fiberglass blanket. I was doing some night time 12 hour measurements with it with no water usage and calculated 3KWH a day just to maintain temperature.

RE: Heat Dissipation Calculation for Water Heaters

Now you guys have me wondering if I should (could?) install some form of timer on my propane water heater. Can't say I've seen a timer for gas-controlled appliances, but it's worth a look...

Dan - Owner

RE: Heat Dissipation Calculation for Water Heaters

"3KWH a day"

That'd be 125 watts of heat loss. $165/year at $0.15/kwh. Sounds about right for a fairly well insulated installation. Because of the NPV of $165/year, it'd be pretty good ROI to make more investments towards reducing that further.

My next project would be a tempering line to naturally (free, at least in summer) raise the temperature of the very cold well water before it enters the HW tank. I expect that'll give the best ROI for a cheap project.

RE: Heat Dissipation Calculation for Water Heaters

Remeber that the "loss" is not really a loss during the heating season.
On the other hand, it is "double jeopardy during the AC season.
Insulation may be more cosy effective than timers.

"Why not the best?"
Jimmy Carter

RE: Heat Dissipation Calculation for Water Heaters

The problem with gas water heaters is that the central flue through the tank is always open and uninsulated. But gas is usually a far cheaper source of energy than electricity. I have a 1000 gallon propane tank but use an electric water heater. Since I use very little hot water I believe that the convenience and lower stand-by losses for electric heat are an advantage.

RE: Heat Dissipation Calculation for Water Heaters

Why rockwool? Isn't that crap insulation compared to about anything else, fiberglass or polyurethane? I'd only use rockwool against fire hazard when using electric heat that is in direct contact with the insulation.

Keith Cress
kcress -

RE: Heat Dissipation Calculation for Water Heaters

"Insulation may be more cost effective than timers."


Schedule timers in a heating context are funny...

Cutting power doesn't directly stop the thermal losses. It's the subsequent temperature drop (e.g. of the water) that reduces the temperature gradient (water to ambient), and that's what eventually reduces the heat loss. How fast that happens is a measure of how poor the insulation is.

In other words, if a timer saves you lots of money, then your insulation is inadequate. Conversely, if your insulation is really good, then a timer isn't going to save you much money (because the water temperature would hardly drop over eight hours). There might be a middle ground in between.

The conclusion is that a timer isn't a good thing. Certainly nothing to be proud of.

A 'timer' (controller) in the context of Time Of Use metering is different of course.

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