National Oilheat Research Alliance 25
According to the chart, it should be fired at .68
gph for 83,000 Btuh, which is slightly over the
Btuh required to properly heat the building.
Universal Hydronics Formula
Once the boiler size has been determined
based on heat load calculations, it is necessary
to determine the right amount of heat that must
be delivered to each room. In doing this work,
examining piping sizes, pumps, circulators
and the heat transfer in each room is necessary
for this analysis. Key to this is the use of the
Universal Hydronics Formula.
The Universal Hydronics formula states
that Gallons per Minute (gpm) equals British
Thermal Units per Hour ( Btuh), divided by
Designed Temperature Drop (Delta T—also
written as ΔT) times 500.
gpm = Btuh/(ΔT x 500)
• gpm is gallons per minute—the flow
rate required to deliver a specific
amount of heat
• Btuh is the Btu per hour requirement
at a given point in time—the heating
load
• ΔT is the designed temperature drop
across the piping
circuit.
In a series loop, fin tube baseboard zone,
the design ΔT is 20—that’s the manufacturer’s
rating for the baseboard itself—meaning that if
the water enters the baseboard zone at 180°F, it
should return to the boiler at 160°F, 20 degrees
cooler.
In most residential radiant floor heating
systems, the design ΔT is usually 10 degrees—
meaning the water enters the radiant loop at
130° F and returns at 120° F. This 10 degree
ΔT is important in radiant because it ensures
an even, comfortable floor surface temperature
throughout a room. A wider ΔT might mean a
noticeable drop in floor surface temperature
near the end of the run.
The final element of the equation is 500—
that’s a shortcut representing the weight of one
gallon of water (8.33 pounds), times 60 minutes
in an hour, times the specific heat characteristic
of the fluid (1 for 100 percent water). It
takes 1 Btu to raise the temperature of 1 pound
of water 1 degree in 1 hour.
8.33 x 60 x 1 = 499.8 (500)
Virtually everything in hydronics—from pipe
sizing to circulator selection—stems from this
formula.
For example, if the need is to distribute
70,500 Btuh (assuming piping to a zone of
baseboard), the formula is:
gpm = Btuh/ ΔT X 500
gpm = 70,500 / 20 X 500
gpm = 70,500/10,000
gpm = 7.1
To properly heat this area, 7.1 gallons of hot
water per minute needs to be moved.
Pipe sizing guidelines
The “rules of thumb” are as follows:
Pipe sizing guidelines
• 2-4 gpm = ¾"
• 4-9 gpm = 1"
• 8-14 gpm = 1¼"
• 14-22 gpm = 1½"
One inch pipe will suffice for this application.
There are a number of charts available, from
the Hydronics Institute section of AHRI that
are shown at the end of this chapter on pages
30, 31 and 32.
Head
In hydronics, pressure is typically referred
to as “head pressure” or “head.” Head is the
total mechanical energy content of a fluid at a
given point in a piping system. In hydronics,
it is used to express pressure loss in the piping
system. When water flows through the system,