No, a pressure reducing valve is not a back flow preventer, and neither
one can be considered a check valve. These are all different beasts
that serve different purposes.
(See PS about mixing hydronic water with potable water below:)
My understanding is that there's no actual "check valve" per se built
into the pressure reducing valve, but the low pressure of the hot water
heating system (typically about 12 to 16 psig) prevents back flow of
water into the water supply piping (typically 40 to 80 psig). That is,
water won't flow against a pressure gradient any more than it will flow
Where I live, in Winnipeg, Manitoba, there's a city by-law requiring a
"back flow preventer" on all commercial boilers (which includes hot
water heating boilers in apartment blocks). The concern is that if the
pressure in the water supply piping drops to 0 psig, or event drops
below atmospheric pressure, then the low pressure or partial vaccuum in
the supply piping could suck the hydronic water out of the boiler
through the pressure reducing valve. A back flow preventer senses that
drop in pressure and allows air to be sucked in to the water supply
piping instead of the hydronic water. In that respect, it differs from
a check valve.
PS: Drinking hydronic water. It's water from the same water supply
piping that supplies both your kitchen sink and your boiler with water.
So, where's the concern about mixing potable water with hydronic water?
I never understood that concern.
In a hot water heating system, the new water that goes into the heating
system has both dissolved oxygen and hardness ions in it. With a few
days of heating that water, the hardness ions precipitate to form
"scale" in the hottest parts of the heating system (the boiler). Also,
the dissolved oxygen reacts with the iron in the system to form a black
form of iron oxide (Fe3O4, I think) which causes the hydronic water to
gradually turn black. And, rust isn't poisonous. It's not as healthy
to drink as a V8, but from what I know, it's no more harmful than
drinking rusty water from rusted cast iron piping before it clears.
Once the water in the heating system is both oxygen depleted and
ionically dead, no further changes occur to it except that it repeatedly
gets hot in the boiler and cools down in the radiators.
So, unless someone is using corrosion inhibitors in their hydronic
water, I really can't see why drinking hydronic water would be
And, in fact, I know a girl who spent time in Russia prior to the Soviet
Union collapsing in the early 1990's, and it was common for people
living in apartments there to open the air vents on their heating
radiators to get hot water out of those radiators for cooking and
cleaning and making tea. The reason why was because electricity was
unreliable, and so if you had an electric stove, you couldn't count on
there to be the electricity needed to heat the water in your tea kettle
24 hrs a day like we've come to expect here in the West. So, Moskovites
would use the water out of the building's heating system (which was
heated with gas or heating oil) to make tea and for cooking supper.
And, the building maintenance people would intentionally not use
corrosion inhibitors in the heating systems because they knew people
were drinking that water and using it for cooking.
I'm not encouraging people to sample the water from their heating
systems for taste. I'm just disagreeing with the premise in the
original post that "I realize that mixing of the forced hot water
heating system water, and
the home water (washing and consumption) supply is a real no-no, as it
should be." I just don't see where the no-no comes from if nothing is
added to the water after it's entered the heating system.