The Glitch
Overview: An unpressurized outdoor furnace supplies heat to a fin-tube
baseboard system in a house. The circulator is located well below the water
level in the furnace to ensure it’s always primed.
The system
heats the house well (and keeps the path between the house and furnace clear of
snow). However, every time the circulator shuts off, the owner hears banging
and gurgling sounds in the pipes. When the circulator starts up again, he hears
“water fall” sounds in some of the pipes.
Exercise: So what’s going on here?
The Fix
The static
water level (atmospheric pressure level) in the outdoor furnace is lower than
the top of the distribution system. This causes the pressure in the piping to go
below atmospheric pressure whenever the circulator is off.
High
temperature water in the piping might even flash to steam when the circulator
pressure is suddenly removed. This is responsible for the banging sounds.
Air will
attempt to enter the sub-atmospheric portion of the piping system any way it
can, including air vents, valve packings and even slightly imperfect threaded
joints. As air enters, the water level lowers toward the level in the boiler.
The next time the circulator starts, it shoves water through the piping and
drives the air back down in the distribution system. If the owner is lucky, it
might even drive this air back out to the furnace.
Corrections actions include:
1. Not installing any vent, valves, threaded connections or other
fittings that could possibly weep air into the piping when under
sub-atmospheric pressure.
2.
Installing a heat exchanger to isolate the unpressurized portion of the
system from the building distribution system (absolutely essential in my
opinion). If this is done, there should be no problems with vents, valves or
threaded fittings at the top of the distribution system (see revised
schematic).
3. Reducing water
temperature to prevent steam flash under lowered pressure.
