Posted by Harold Kestenholz - Hydronic Network on November 12, 2001 at 09:22:16:
In Reply to: indirect vs power vent water heater posted by jay darcy on November 11, 2001 at 12:50:11:
: I have an existing gas 150,000 btu gas boiler, with an AFUE of 83.
Is the energy factor and the AFUE
: comparable? That is can I calculate the difference between the
: direct-fired and the indirect fired by using a factor of .52/.83 ? In
: other words, can I assume that the annual operating costs of the
: indirect will be about 5/8 of the direct fired?
*** No, the methods of calculation are very different and not directly comparable; the boiler test is for heating season efficiency, the water heater for year-round operation. There will not be that much difference, but there will be savings using the Superstor (or other brand) from the increased insulation and flueless construction of the Superstor.
: We have some complaints about enough hot water. One reason for choosing
: the Super Stor is because we've been told that it's equivalent to a 75
: gallon direct. Is this correct?
*** The company states that and they mean it. 140 gallons of hot water in the first hour is far more than a 75 gallon tank can supply.
But one plumber warned us not to
: get an indirect, because it would cause the boiler run all year long,
: resulting in a lower life-time. He also said the boiler was much less
: efficient when it was only supplying heat to the Super Stor. This
: doesn't sound right to me. Is it ?
*** It is nonsense. A water heater ordinarily has 1/2 the lifetime of a boiler or less. There is no need to have two burners going in the winter when one will do much more efficiently with longer lifetime savings. In the summer the boiler will fire two or three times a day for less than an hour; it is better to observe and 'exercise' a boiler daily. Long-term losses will be larger on the water heater.
: In looking at indirects, I see a specification for "pressure drop"
: measured in feet. I assume this describes the pressure required for
: the boiler water to move through the heat exchanger coil. Therefore
: a lower pressure drop is better. Correct?
*** A separate circulator should be used to drive the water through the coil; this means that the pressure drop has no real bearing on the selection.