Backpressure backflow is the reversal of normal
flow in a system due to an increase in the downstream pressure
with in the system of the property, above that of the supply
pressure.
Back pressure-backflow is created whenever the downstream
pressure exceeds the mains water supply pressure, which is
possible in heating systems, elevated tanks, and pressure-producing
systems.
Many central heating systems in the UK today are
pressurized or sealed systems, this means that the water is
fed into the system direct from the water mains via a connection
and then sealed, this differs from the open fed systems which
requires a small water tank usually located in the attic.
An example would be a combination central heating boiler
operating at approximately 1 Bar of pressure combined with
a reduction of the mains water supply below such pressure
(or higher in most commercial boilers). In this example, the
central heating system water complete with its chemicals,
black sludge, rust etc would flow in the direction of least
resistance, a backpressure-backflow condition would be created
and the contaminated boiler water would flow into the mains
water supply.
Backflow can be extremely dangerous and can occur
in many different guises. Hundreds of people were evacuated
from their homes in New England after fires were reported in
two homes as a result of propane gas entering the water mains.
A work crew purging propane from a large propane tank that was
in need of repair never checked the pressure in the propane
tank, which was greater than the pressure in the water line
that fed it. As a result propane vapour entered the water lines
through backpressure forcing 500 people to be evacuated and
contamination of the towns water supply.
