THE HOT-WATER HEATING PLANT
Of the various systems of heating dwellings that by hot-water is
considered by many to be the most satisfactory. On account of its high
specific heat, water at a temperature much below the boiling point
furnishes the heat necessary to keep the temperature of the house at
the desired degree. The temperature of the radiators is generally much
lower than those heated by steam but the amount of radiating surface
is greater than for steam heating plants of the same capacity. It is
because of the relatively low temperature at which the water is used,
that the greater amount of heating surface is required.
One objection to the use of hot water as a means of heating is, that
once the heat of the house is much reduced, the furnace is a long time
in raising the temperature to normal. This is due to the fact that the
temperature of the water of the entire system must be uniformly raised,
because of its continuous passage through the heater. On the other
hand, this uniformity of the temperature of the water prevents sudden
changes in the temperature of the house. Water-heating plants work with
perfect quiet and may be so regulated to suit the outside temperature
that the heat of the water will just supply the amount to suit the
prevailing conditions.
The care required in the management of the boiler is less than that
required in the steam plant because of the fewer appliances necessary
for its safe operation. Another advantage in the use of the hot-water
plant is its adaptability to the temperature conditions during the
chilly weather of early fall and late spring, when a very small amount
of heat is required. At such times the temperature of the radiators is
but a few degrees warmer than the outside air. The amount of attention
necessary for maintaining the proper furnace fire under such conditions
is less then for any other form of heating. The increasing use of the
hot-water plant for heating the average-sized dwelling attests to its
excellence in service.
=The Low-pressure Hot-water System.=--A hot-water system consists of a
heater, in which the water receives its supply of heat, the circulating
pipes for conducting the heated water to and from the radiators that
supply heat to the rooms, and the expansion tank that receives the
excess of water caused when the temperature is raised from normal to
the working degree. In addition to the parts named there are a number
of appliances to be described later, that are required to make the
system complete.
[Illustration: FIG. 32.--Diagram of a simple form of hot-water heating
plant.]
A hot-water plant of the simplest form is shown in Fig. 32. The
illustration presents each of the features mentioned above, as in a
working plant. The different parts are shown cut across through the
middle, the black portion representing water. Not only does the water
fill the entire system but appears in the expansion tank when the plant
is cold.
Hot-water heaters are quite generally in the form of internally fired
boilers. The fire-box occupies a place inside the boiler and is
surrounded, except at the bottom, by the water space. Commonly, these
boilers are made of cast iron and are constructed in sections, the
same as the steam boiler shown in Fig. 16. Manufacturers sell a single
style for either steam or hot-water heating. The boiler in Fig. 32 is
cylindrical in form. It is made of wrought iron and contains a large
number of vertical tubes through which the heat from the furnace must
pass on its way to the chimney.
As the water is heated it expands and rises to the top of the boiler
because of its decreased weight. Since the water in the radiator is
really a part of the same body of water, the heated portion rises
through the supply pipe to the top of the radiator. As the hot water
rises in the radiator, it displaces an equal amount of cold water,
which enters the boiler at the bottom. This displacement is constant
and produces a circulation that begins as soon as the fire is started
and varies with the difference in temperature between the hot water
leaving the boiler at the top and the cold water entering at the bottom.
As the water in the system is heated and expands, there must be some
provision made to receive the enlarging volume. In this arrangement a
pipe connects the bottom of the boiler with the expansion tank located
at a point above the radiator. Under the conditions represented in
the drawing the water does not circulate through the tank and as a
consequence the water it contains is always cold.
In raising its temperature, water absorbs more heat than any other
fluid and on cooling it gives up an equal amount. As a consequence
it furnishes an excellent vehicle for transmitting the heat of the
furnace to the rooms to be heated. Water, however, is a poor conductor
and receives its heat by coming directly into contact with the hot
surfaces of the furnace, and gives it up by direct contact with the
radiator walls. To transmit heat rapidly and maintain a high radiator
temperature, the circulation of the water in the system must be
the best possible. The connecting pipes between the boiler and the
radiators must be as direct as circumstances will permit and the amount
of radiating surface in each room must be sufficient to easily give up
an ample supply of heat. Even though the furnace is able to furnish a
plentiful supply of heat to warm the house, it cannot be transmitted to
the rooms unless there is sufficient radiating surface. A plant might
prove unsatisfactory either because of a furnace too small to furnish
the necessary heat or from an insufficient amount of radiating surface.
Yet another factor in the design of a plant is that of the conducting
pipes. Both the boiler and the radiators might be in the right
proportion to produce a good plant, but if the distributing pipes are
too small to carry the water required, or the circulation is retarded
by many turns and long runs, the plant may fail to give satisfaction.
Fig. 33 shows a complete hot-water plant adapted to a dwelling. It
is just such a plant as is commonly installed in the average-sized
house but without any of the appliances used for automatic control of
temperature. The regulation of the temperature is made entirely by
hand, in so governing the fire as to provide the required amount of
heat. In the drawing the supply and return pipes may be traced to the
radiators as in the case of the simple plant. The supply pipe from the
top of the boiler branches into two circuits to provide the water for
the two groups of radiators at the right and left side of the house.
To provide any radiator with hot water, a pipe is taken from the main
supply pipe and passing through the radiator it is brought back and
connected with the return pipe which conducts the water back to the
boiler.
[Illustration: FIG. 33.--The low-pressure hot-water heating system
applied to a small dwelling.]
The expansion tank is located in the bathroom near the ceiling. It is
connected with the circulating system by a single pipe which joins the
supply pipe as it enters the radiator located in the kitchen. Like the
expansion tank in Fig. 31 the water it contains is always cold. It is
provided with a gage-glass which shows the level of the water in the
tank and an overflow pipe which discharges into the bathtub, in case
of an overflow. An overflow pipe must always be provided to take care
of the surplus when the water in the system becomes overheated. This
does not often occur but the provision must be made for the emergency.
The overflow pipe is frequently connected directly with the sewer or
discharged at some convenient place in the basement.
=The High-pressure Hot-water System.=--In the hot-water plant described
the expansion tank is open to the air and the water in the system is
subjected to the pressure of the atmosphere alone. The heat of the
furnace may be sufficiently great to bring the entire volume of water
of the system to the boiling point and cause it to overflow but the
temperature of the water cannot rise much above the boiling point due
to the pressure of the atmosphere.
If the expansion tank is closed, the pressure generated by the
expanding water and the formation of steam will permit the water to
reach a much higher temperature. In the table of temperatures and
pressures of water on page 3, it will be seen that should the pressure
rise to 10 pounds, that is, 10 pounds above the pressure of the
atmosphere, the temperature of the water would be very nearly 240°F.
(239.4°F.). The difference in heating effect in hot-water heating
plants under the two conditions is very marked. In the low-pressure
system the temperature of the radiators cannot be above 212° but the
high-pressure system set for 10 pounds pressure will heat the radiators
to 240°, and a still higher pressure would give a correspondingly
higher temperature. The amount of heat radiated by a hot body is in
proportion to the difference in temperature between the body and
the surrounding air. If we consider the surrounding air at 60° the
difference in amount of heat-radiation capacity of the two radiators
would be as 180 is to 132. The advantage of the high-pressure system
lies in its ability to heat a given space with less radiating surface
than the low-pressure system.
In Fig. 34 is illustrated an application of a simple and efficient
valve arrangement that converts a low-pressure hot-water system into
a high-pressure system without changing in any way the piping or
radiators. The drawing shows the boiler and two radiators connected as
for a low-pressure system, but attached to the end of the pipe as it
enters the expansion tank is a safety valve _B_ and a check valve _A_,
as indicated in the enlarged figure of the valve. The safety valve is
intended to allow the water to escape into the expansion tank when the
pressure in the system reaches a certain point for which the valve is
set. The check valve _A_ permits the water to reënter the system from
the tank whenever the pressure is restored to its normal amount.
[Illustration: FIG. 34.--The high-pressure system of hot-water heating.]
Suppose that such a system is working as a low-pressure plant. The hot
water from the top of the boiler is flowing to the radiators through
the supply pipe and the displaced cooler water is returning to the
bottom of the boiler through the return pipe as in the other plants
described. It is now found that the radiators are not sufficiently
large to heat the rooms to the desired degree except when the furnace
is fired very heavily. It is always poor economy to keep a very hot
fire in any kind of a heater, because a hot fire sends most of its heat
up the chimney. If the radiators could be safely raised in temperature,
they would, of course, give out more heat and as a result the rooms
would be more quickly heated and kept at the required temperature with
less effort by the furnace. The difficulty in this case lies solely in
there being insufficient radiator surface to supply heat as fast as
required.
The increase in radiator temperature is accomplished by the pressure
regulating valve _B_, attached to the end of the pipe as it enters the
expansion tank. The expansion tank with the regulating valve is shown
enlarged at the left of the figure. The valve _B_ is kept closed by
a weight marked _W_, that is intended to hold back a pressure of say
10 pounds to the square inch. A pressure of 10 pounds will require a
temperature of practically 240°F. (see table on page 3). The check
valve _A_ is kept closed by the pressure from the inside of the system.
When the pressure of the water goes above 10 pounds--or the amount of
the weight is intended to hold back--the valve is lifted and an amount
of water escapes through the valve _B_ into the tank, sufficient to
relieve the pressure. Should enough water be forced out of the system
to fill the tank to the top of the overflow pipe, the overflow water is
discharged through this pipe into the sink in the basement.
When the house has become thoroughly warmed, the demand for a high
radiator temperature is reduced, the furnace drafts are closed, the
water in the system cools and as it shrinks the system will not be
completely filled. It is then necessary to take back from the tank the
water that has been forced out by excess pressure. It is here that the
check valve comes into use. So long as there is pressure on the pipes,
this valve is held shut and no water can escape, but as the inside
pressure is released by the cooling there will come a point where the
water in the tank will flow back through the valve _A_ and fill the
system.
This is the type of valve used by the Andrews Heating Co. and
designated a regurgitating valve. In practice it gives excellent
service. The only danger of excessive pressure in the use of this
device is the possibility of the valve becoming stuck to the seat
through disuse. Any possible danger from such an occurrence may be
eliminated by the occasional lifting of the valve by hand.
=Heating-plant Design.=--A heating plant should be designed by a
person of experience. No set of rules has yet been devised that will
meet every condition. Carpenter’s rules given on page 25 serve for hot
water as well as for steam as a means of determining the radiating
surface required for an ordinary building, but the rules do not take
into account the method of construction of the house and the consequent
extra radiation demanded for poorly constructed buildings. In many
cases the designer must rely on experience as a guide where the rules
will not apply. In the case usually encountered, however, the rules
given will meet the conditions.
What was said regarding the size of steam boilers required for
definite amounts of heating surfaces, applies with equal force to
hot-water boilers, because house-heating boilers are commonly used
for either steam or hot-water heating. There are no established rules
for determining the heating capacities of house-heating boilers.
Manufacturers’ ratings are usually low. There are some manufacturers
who make honest ratings for their boilers but they are in the minority.
When the heating capacity of a boiler is not known from experience, the
only safeguard against installing a boiler too small for the radiators
to be heated, is to require a guarantee that the plant will give
satisfaction when in operation and when considered necessary a certain
percentage of the contract price should be withheld until the plant
proves itself by actual trial.
=Overhead System of Hot-water Heating.=--In Fig. 35 is illustrated
another system of high-pressure hot-water heating that corresponds to
the overhead system of steam heating. It differs from the high-pressure
system already described in the method of distribution and in the
radiator connections.
The flow pipe is taken to the attic and there joined to the expansion
tank as a point of distribution. On the expansion tank is a safety
valve set at 10 or more pounds pressure. The flow of the water is all
downward toward the radiators. The circulation through the radiators is
also different from the other plants described. The supply pipe joins
directly to the return pipe and the connections to the radiators are
made at the top and bottom of the same end. The circulation through
the radiators in this case is due to the difference in gravitational
effect between the hot and colder water at the top and bottom of the
radiator. The system requires no air vents on the radiators as all
air that might collect in the system goes up to the expansion tank.
The safety valve on the expansion tank in this case is the common
lever type. The overflow should empty into the sewer and be pitched to
prevent any water being retained in the discharge pipe. If water should
be retained in this pipe and should freeze, the system would become
dangerous, because of the possibility of high pressures from a hot fire.
[Illustration: FIG. 35.--The overhead system of hot-water heating.]
=Expansion Tanks.=--Fig. 36 is a form of expansion tank in common use.
It may be used for either the high-or low-pressure system. The body of
the tank is made of galvanized iron and is made to stand a considerable
amount of pressure. The gage-glass is attached at _B_, and the overflow
at _O_. The pipe _E_ connects the tank with the circulating system and
_D_ connects with the cold-water supply as a convenience for filling
the system with water. The object in placing the stop-cock _D_ near
the expansion tank is to avoid overflowing the system in filling. The
overflow pipe, as stated above, is most conveniently connected with the
sewer, into which the water will run in case of an overflow, but the
other methods shown are commonly used. There should be no valve in this
pipe nor in the pipe _E_.
[Illustration: FIG. 36.--The expansion tank.]
[Illustration: FIG. 37.--When the expansion tank of a hot-water heating
system must be so located that it is apt to freeze, it must be piped as
a radiator.]
The expansion tank must be so located that there will be no danger of
freezing. Should it be necessary to place the tank in the attic or
where freezing is possible, the tank must be so connected as to become
a part of the circulating system. Such an arrangement is shown in Fig.
37. The expansion tank is connected with a supply and return pipe as a
radiator. This arrangement is sometimes used but it is not desirable.
It is wasteful of heat and there is always a possibility of freezing
in case the fire in the furnace is extinguished a sufficient time to
allow the water to grow cold.
Any possibility of danger from excessive pressures in either the
low-pressure or the high-pressure system must originate in the
expansion tank. It is, therefore, desired to again mention the
possible causes of danger. Any closed-tank system is liable to become
overheated. The expansive force of water is irresistible and unless
some means is taken to prevent excessive pressure some part of the
apparatus is apt to burst. _No closed-tank system should be used
without a safety valve._
The low-pressure or open-tank system requires no safety appliances. So
long as there is open communication between the tank and the boiler the
pressure cannot rise but slightly above that of the atmosphere. There
is only one cause that will lead to high pressure in such a system. If
the pipe connecting the expansion tank is stopped an excessive pressure
might generate. There is little or no danger of this happening.
In the closed-tank system the expansion tank should be of greater
capacity than for the open-tank system. Its size is commonly about
one-ninth of the volume of water used. The larger tank is necessary
to prevent too rapid rise of pressure as the temperature of the water
rises. The air in the tank acts as a cushion against which the pressure
of the expanding water is exerted.
The extended use of hot-water heating has led to the invention of many
appliances for the improvement of the circulation and heating effects.
Pulsation valves are used for retaining the water in the boiler until
a definite pressure has been attained that will lift the valve long
enough to dissipate the pressure. Many of these systems possess merit
and some of them are great improvements over the simple plant.
=Radiator Connection.=--The method of connecting the radiators to
the distributing pipes depends entirely on local conditions. In a
well-balanced system any of the methods shown in Figs. 38, 39 or 40
might be used with good heating effects. The method of attaching the
supply pipe to the radiator is, however, an important factor in case
of accumulation of air. In Fig. 41 is shown the form of connection
most commonly used. The drawing is intended to represent a cast-iron
radiator with the valve at _D_, and the air vent at _B_. Should air
collect in the radiator it will rise to the top and displace the water.
The water will continue to circulate and heat as much of the radiator
as is in contact with the water, but that part not in contact will
receive no heat from the water and will, therefore, fail to fulfill its
function. As soon as the air vent is opened the air will escape and
allow the water to entirely fill the space.
[Illustration: FIG. 38. FIG. 39. FIG. 40.
FIGS. 38 TO 40.--Various methods of attaching the supply and return
pipes to hot-water radiators.]
[Illustration: FIG. 41.--The effect of accumulation of air in a
hot-water radiator with bottom connections.]
[Illustration: FIG. 42.--With this method of connections, if the
air collects sufficiently to force the water down to the level L,
circulation will stop.]
In Fig. 42 a much different condition exists, when air accumulates. In
this mode of connection the water enters through the valve _V_, and
escapes at the bottom of the opposite end. When air fills the radiator
to the line _L_, the circulation is stopped and the radiator will grow
cold.
The position of the valve on these radiators is of little consequence.
The valve is intended merely to interrupt the flow of the water and may
occupy a place on either end of the radiator with the same result.
=Hot-water Radiators.=--Radiators for hot-water heating are most
commonly of cast iron and in appearance are the same as those used for
steam heating. The only difference in the two forms is in the openings
between the sections. Those intended for steam have an opening at the
bottom joining the sections; while those for hot water have openings at
both top and bottom to permit circulation of the water.
[Illustration: FIG. 43.--The hot-water radiator valve.]
[Illustration: FIG. 43_a_.--Details of construction of the hot-water
radiator valve.]
=Hot-water Radiator Valves.=--Valves for hot-water radiators differ
materially from those used on steam radiators. Figs. 43 and 43a show
the outside appearance and the mechanical arrangement of the parts
of the Ohio hot-water valve. The part _A_ in Fig. 43_a_ is a hollow
brass cylinder attached to the valve-stem, one side of which has been
removed. When it is desired to shut off the supply of heat the handle
of the valve is given one-quarter turn and the part _A_ covers the
opening to the inlet pipe. The supply of water being shut off, the
radiator gradually cools. When the valve is closed a small amount of
water is admitted to the radiator through a 1/8-inch hole in the piece
_A_ to prevent the possibility of freezing.
=Air Vents.=--In the use of the systems of hot-water heating described,
every radiator must be supplied with an air vent of some kind to take
away the trapped air which accumulates through use. Any kind of a valve
will serve as a vent for hand regulation and generally such a cock as
is shown in Fig. 10 is employed.
=Automatic Hot-water Air Vents.=--It is sometimes desired to use
automatic air vents on hot-water radiators. For such work a vent is
used that remains closed as long as water is present and will open when
the water is displaced by the accumulating air, but will again close
when the air is discharged. In such vents the valve is controlled by
a float, the buoyancy of the float when surrounded by water serving
to keep the valve closed. These vents are not so positive in their
action as automatic air vents for steam. The change in temperature
which controls the steam vent does not take place with hot water. The
automatic hot-water vents are not perfectly reliable. They may work
with entire satisfaction for a long time and then fail from very slight
cause. The failure of a hot-water vent is generally discovered by
finding a pool of water on the floor or a wet spot on the ceiling or
wall of the floor below.
[Illustration: FIG. 44.--Automatic air vent for hot-water radiators.]
One type of the automatic hot-water vent that has proven quite
successful is shown in Fig. 44. The threaded lug is screwed into the
radiator at the proper point. As the water enters the radiator the air
is discharged through the vent, escaping at the opening _C_. When the
water has risen to a sufficient height it enters the openings _G_ and
_H_ until enough is present to raise the float _A_. The pointed stem
attached closed the hole _C_ with sufficient force to make an air-tight
joint. The float _A_ is a very light copper cylinder. Its buoyancy
supplies the force to close the vent and its weight opens the vent when
the water is displaced by air. It will be readily seen that very slight
cause might prevent the performance of its duty.