Exposed Duct High Temperature Limit

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I bought a crappy old house that was built in the 1920’s a few years into marriage and there were so many things wrong with it. Water intrusion, leaking pipes, roots in the sewer line… on and on… but it was ours and we loved it.

One issue was the water heater was tiny and it was in the attic with no room to make it larger. I couldn’t even make it through a shower without running out of hot water. So what did I do? I jacked up the thermostat!

My wife washed dishes the next day and was not amused at the blazing hot water that came gushing over her hands. I’m glad I listened to her and set it back down, because not only was it safety hazard it was also a waste of energy.

So much of what we do is about controlling the temperature of air, fluids and objects as well as the rate of transfer of heat from one to the other. We can impact this rate of transfer by changing the distance between the objects, changing their temperature differential or changing the resistance to energy flow or R-value.

In the case of the water heater, increasing the temperature of the water in the tank and in the pipes increases the rate of energy loss through the tank and pipe walls as well as being a safety hazard. This is why the Department of Energy suggests setting your water heater thermostat to 120ºF which is 20º lower than many manufacturers even set it.

In addition to changing temperature differential, we can insulate to reduce energy transfer. We insulate things for three primary reasons

  1. To reduce the rate of heat (thermal) transfer from hot to cold for efficiency or comfort
  2. To keep the temperature of a surface above dew point to prevent condensation and water damage
  3. To protect safety from scalding or frostbite

The IMC (International Mechanical Code) 2015 edition 604.2 surface temperature states that ducts that contain air of over 120º must have enough insulation so that the external surface doesn’t exceed 120º. This serves as a high limit for duct temperatures for safety reasons, but it also has practical energy application as while.

While locally adopted mechanical and energy conservation codes will generally require certain insulation R-value for ducts you can use this 120º surface temperature as a litmus test. On the other end of the spectrum, a duct surface temperature should never be allowed to fall below the dew point temperature of the air surrounding it. This can be quite tricky in humid climates where ducts are installed in unconditioned spaces but should be considered nevertheless.

So be safe, efficient and stay dry by keeping your ducts and water properly insulated.

— Bryan

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