Onix Tubing Installation Manual
PV = T
(Pressure x Volume = Temperature)
With a fixed system volume, if the ini-
tial water temperature is 50¡ and it is
raised to 180¡, the internal pressure
will increase, since the volume can not
change. By quadrupling the internal
temperature, the internal pressure will
also quadruple, changing it from an
initial 15 to 60 psi. This can damage
system components and/or cause relief
valves to discharge.
In order to keep the internal pressure
roughly the same, the system volume
has to change. The question is by how
much? What tank size would be
required if the temperature changed
from 50¡ to 180¡ and the fluid volume
was 20 gallons (approximately 2400’
of radiant tubing). Since we are deal-
ing with an incompressible fluid, ele-
vation will factor into the total expan-
sion rate of the system. A step-by-step
form can be found in this section,
along with other useful charts for
determining component volumes.
Step 1:
Determine the initial volume of the
system. To do this, calculate the vol-
ume of fluid in the tubing, supply-
return piping, and all other mechanical
components.
Step 2:
Determine the static pressure of the
system. The static pressure is the force
exerted on the system from the weight
of the water above the mechanical
room. The relative elevation change of
the system will dictate how much stat-
ic pressure is in the system.
Step 3:
Determine the fill pressure of the sys-
tem. This is the static pressure plus a
factor of safety. In our case, 3 psi is
more than enough to account for
minor piping variations within a floor.
Step 4:
Find the allowable volume increase,
which is a percentage, in the system.
This percentage will be determined by
maximum pressure rating for the sys-
tem, which is usually dictated by the
pressure relief valve.
Step 5:
Find what the actual volume increase
will be for the fluid. This is done by
multiplying the initial volume by the
corresponding temperature factor. The
higher the temperature, the more
expansion the fluid will undergo.
Step 6:
Find the expansion tank volume by
dividing the actual volume increase by
the percentage.
There are several educational books on
the market that describe primary/sec-
ondary piping arrangements.
Expansion Tank Sizing form
Step 1: System Volume
Determine the amount of fluid in the radiant tubing, supply and return lines and near
boiler piping (include boiler and other accessories).
Pipe Length
× Volume/Foot
* = Fluid Volume
Radiant Piping
Supply/Return Piping
Boiler Volume
(see boiler manual)
Total Volume (TV):
Total System Volume Includes:
Boiler (see manufacturer s specifications) Optional Buffer Tank
Fancoils Radiators
Radiant Supply/Return Lines Additional Hydronic Components
Tank Size =
Fill Pressure = (No. of floors
× 3.87 + 7 psi) + 14.7 =
Expansion Volume (EV) = Total Volume (TV)
× Expansion Factor
EV =
AF = =
Tank Size = =
* See system volume table on the following page.
See table of expansion factors on the following page.
Expansion Volume (EV)
Acceptance Factor (AF)
(Relief Pressure + 14.7) — Fill Pressure
(Relief Pressure + 14.7)
(EV)
(AF)
Appendix
Watts Radiant: Onix Installation Manual page 71