Page 148 - Boiler_Operators_Handbook,_Second_Edition

Special Systems 133

do to the water to prevent scale formation or corrosion water heater has to be able to produce the quantity of
so the equipment has to be made for the service and you hot water drawn between 7 and 8 in the morning but is
will have to operate and maintain it properly to provide required to produce a fraction of that load for the rest of
continued operation. the day.
Service water heaters usually have much lower Note that the chart is based on gallons per hour
rates of heat transfer than steam and heating boilers to re- and does not show instantaneous flows that could eas-
duce scale formation. They are also fabricated for the ap- ily exceed the values shown. In my home I can draw
plication, some of them are glass lined with glass coated water at the rate of 920 gallons per hour, about eight
heating surfaces. We can’t treat the water to make it non- times the maximum rate shown on the chart. However,
corrosive so we have to protect the heater from corrosion. since my bathtub has a capacity limit of approximately
The equipment sold in your area is usually suitable 200 gallons I would draw hot water at that rate for no
for service water heating of water used in the area. Don’t more than a few minutes. An instantaneous heater with
do like a friend of mine that thought the hot water heater a capacity of at least 920 gallons per hour would be re-
prices were too high in his new neighborhood and trans- quired to ensure a continuous supply of heated water.
ported one from his old neighborhood in another state. However, with a 200-gallon storage tank I am able to fill
He saved a lot on the heater, but it didn’t last a year. the tub and satisfy other household requirements with
Small electric and gas or oil fired service water a heater that can heat water at the rate of 10 gallons per
heaters require more attention in a commercial applica- hour. Unless, of course, I intend to fill the tub more fre-
tion than the ones in your home because they get more quently than once a day.
use. You should have a schedule for blowing them down Since most of us do not take 200-gallon baths that
on a regular basis to remove any mud, scale, or other de- example is improbable. It does, however, do well to ex-
bris that may accumulate. Regular checking and record- plain the difference between instantaneous and storage
ing of the stack temperature is also a must for the fired water heating. The best system will always consist of a
heaters because that can indicate problems with scaling; proper mix of water heater and storage that handles the
as scale forms it insulates the heating surfaces requiring load without excessive cycling of the water heater. See
higher flue gas temperatures to do the heating. There’s the discussion on cycling boilers for reasons why exces-
also some checking and adjustment required for storage sive cycling is a problem.
water heaters to keep everything working right. When your hot water loads are large and variable
Since entering semi-retirement I’ve encountered a a modulating burner on an instantaneous hot water
fair number of projects involving problems with stor- heater will reduce cycling or eliminate it. Instantaneous
age water heating. I’ve also encountered many instal- heaters with modulating burners can only eliminate
lations where someone felt they solved the problem by cycling if the burner’s turndown capability exceeds the
installing instantaneous water heaters. If you think an variation in hot water usage. As you can see from the
instantaneous water heater is an appropriate solution to figure, that would require a burner with a turndown
any problems you may be having with your hot water better than 20 to 1. Such burners are very expensive so
system I urge you to reconsider.
Instantaneous hot water heaters do just what they
say they’ll do, heat water quickly, primarily as it is used.
Except for facilities where the instantaneous hot water
heating load is less than about 25% of the lowest plant
loads those heaters can be a real problem for smooth
and reliable boiler operation. It’s also hard to believe an
instantaneous heater is anywhere near efficient because
they’re capable of heating more water than is normally
heated so they only operate a fraction of the time allow-
ing considerable off cycle losses.
The amount of hot water used is a function of the
activities of the occupants of the buildings. The curve
4
in Figure 4-8 is based on ASHRAE data indicating
the typical hot water consumption for a family over a
24-hour period. It’s obvious that an instantaneous hot Figure 4-8. Daily hot water consumption curve