Page 139 - Boiler_Operators_Handbook,_Second_Edition

124 Boiler Operator’s Handbook

HYDRONIC HEATING Hydronic heating systems must have provisions
for thermal expansion. When you heat water from a
Much of this book addresses the steam generating nominal building temperature of 65°F to an operating
boiler plant and, while much of what we cover applies temperature of 180°F each cubic foot of water in the
to water heating as well, there are many considerations system will swell by almost 3%. That’s not a lot percent-
in a water plant that are not a concern in a steam plant. age wise but when you consider the total volume of a
Hydronic is just a word we use to differentiate low heating system that can be several hundred gallons. A
pressure hot water heating systems from other types of plant that’s waterlogged (all elements full of water) can
boiler plants. I tend to use whichever label is selected by experience extreme swings in pressure associated with
the people I’m dealing with, hot water one minute and the expansion and contraction of the water. An expan-
hydronic the next but that’s simply to make the other sion tank is provided in a hydronic heating system to
people comfortable by using their label. reduce pressure swings to a tolerable range.
Unlike a steam plant a hydronic system can be shut The tank can be an open type, located above the
down without admitting air to prevent a vacuum. For highest point in the system at a height adequate to
that one reason hydronic systems should last at least maintain the desired system operating pressure. The top
twice as long as a steam system under otherwise equal of the tank is open to atmosphere and the gage pressure
operating conditions. How long is that? About 60 years. at any point in the system is a function of the height of
It’s the system of choice today for residential boiler the water. The tank has to be large enough to accept the
applications and most commercial buildings because expansion of the water in the system without a consid-
it doesn’t require as much attention as a steam system. erable change in level because the system pressure will
Properly maintained it will require a minimum of make- change about 1 psi for every 2.31 foot change in tank
up, almost nothing at all when new, and therefore need level.
little attention to chemical treatment. With all that said, Sometimes the tank is too small to handle full
there’s some reason to wonder why anyone even consid- expansion and the water overflows from the tank as it
ers having an operator in a hydronic heating plant but I expands. A float valve can be added to replenish the
think I answered that question already. water when the system cools. Open tanks are used infre-
You don’t have to admit air to a hydronic system quently and normally only in systems using ethylene or
like you do steam because the change in volume from propylene glycol and rust inhibitors for freeze and cor-
operating to idle is not significant. That doesn’t mean rosion protection. The only time I’ve encountered these
that changes in volume are no concern for the opera- tanks they’re on cheap systems in locations that con-
tor. The problem with most hydronic systems is due to tained glycol and received very little maintenance. The
changes in volume that aren’t accounted for in various principle problem with an open tank is it allows oxygen
stages of operation. Close off a section of steam system to get into the water with corrosion as the outcome.
and the steam will condense leaving a vacuum that Closed expansion tanks can be a simple pressure
might permit atmospheric air to crush some thinner vessel or be fitted with a neoprene or Buna-N bladder
walled vessels attached to the system, that’s all that that separates the water in the system from the air that
will happen. Of course one of those vessels could be a provides the expansion cushion. Pressure maintenance
$60,000 stainless steel heat exchanger! That happened. in systems with closed expansion tanks is established by
Hydronic systems will also produce a vacuum as controlling the air pressure over the liquid and/or the
the water cools so you should expect air in that piping amount of water in the system. Some systems use nitro-
if you isolate it. Hot water and steam piping is usually gen instead of air to eliminate the oxygen as a source of
strong enough that it can withstand the vacuum and corrosion of the tank and system. Tanks without blad-
nothing happens. Close off a section of chilled water ders are usually epoxy coated internally, that’s why they
piping in a building so that water is trapped and you have those “do not weld” stencils that someone painted
have another story. As the chilled water heats it expands over several years ago. (That was a another snicker gen-
to build up pressure rapidly. It will rupture the piping if erator, a comment that indicates what some people man-
it can’t leak out somewhere. Unlike steam and air water age do to destroy a plant, hopefully you’re much wiser)
isn’t compressible. The best thing to do is close only Most plants are served by an expansion tank that
enough valves to stop flow, not so many that the system can take the full swing of expansion from an idle con-
is completely isolated. When isolating for maintenance, dition to design operating temperature. A few plants,
open some vents as soon as the system is isolated. however, either due to space or price limitations, or as