Thermal Loop

Unknown

Can somone explain the theory of a thermal loop on a cold water feed to a domestic water heater? Heater manufacturers indicate that 27" is the magic number as to the vertical length of the loop. This supposedly is necessary to prevent the migration of hot water into the cold water system.

Why 27"?

Dan Peel

numerical magic

John Siegenthaler uses 18" as a minimum drop for thermal isolation. 27" may be realistic with DHW VS inlet differential temps. More drop will be less likely to induce thermal migration or ghost flow. Dan

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Terry

1psi

27" vertical height equates to 1+psi static pressure.

[Deleted User]

True but....

static pressure really has little to do with thermal migration and thermo siphon. The weight of the water bearing down on the heat source has just as much to do with the thermo syphon cycle as the lighter less dense water does. In this case, I think it is probably just another manufacturer taking a shot at eliminating thermosiphon.

For my money, if there is no circ return, 3/4" insulation on the first 5 feet of cold and hot water lines works best at eliminating conductive thermo losses from a hot storage tank. If there is a circulation return, the whole thing ought to be insulated IMHO.

Of course, then there is the potential of one pipe thermo siphon...but I'm pretty sure insulation will negate that potential too.

ME