I have an installation guide for a tankless hot water heater. It says to put
a check valve between the water heater and water shutoff valve. Why? I
know what the valve does since I have one, but am don't see the point. Any
help would be appreciated.
It's probably a water supply co regulation to stop any possibility of back
flow into the mains causing contamination.
If you have a tank the fill pipe is usually above the water level which
stops back flow so no need for a valve in that case.
Thanks for the help. I'm still confused. Mine is a tankless unit in the
basement and the input/output lines are on the bottom. Wouldn't the water
pressure in the input line before the water heater prevent the water from
reversing direction? Can you explain a little more?
Please don't top post.
The check valve (or as some call them; backflow preventers)
are only there to stop what could be a problem in only certain
If you were gone for a month or so, the water sitting in your
pipes could become stagnant. If there were to be a main break
in a water line somewhere near your neighborhood, they might
have to shut off the water supply in your area. When the
water supply is shut off to YOUR main, the level in the main
drops a bit from people turning on their water, leaks, etc.
If the main that supplies your house is lower than any point
in your home system, then water may flow back into the main
from your home. Thus putting stagnant or even contaminated
water into the main. Thus, the check valve.
Miniscule chance of any problems, but it has happened.
I don't believe that it's a question of contamination of the city mains for
so long as a water system is intact, it is considered potable (water doesn't
"go bad" unless it comes in contact with the air or other source of
contamination.) I was unable to find any code requirement for the check
valve (unless the water heater is also being used as a boiler). I'm not
certain is the purpose is to prevent burn-out as a full-flow globe valve is
mandated for all water heaters (means to isolate the heater) and there is
*no* similar requirement for tank-type water heaters. Another code official
I asked thought that it might be a manufacturer's requirement (it doesn't
seem to be universally required.)
Buzz... Wrong answer. Since there is no such thing as 'intact' (after
all there is a faucet, and whatever else that is attached) this
concept doesn't apply.
What happens is related to a number of pre-existing problems that
occured some years ago. The biggest one was where some pesticide and
fertilizer companies, and companies making things like car washers
that all mix chemicals with water in the device, suffered from
problems of back flow. As previously mentioned when the water main
pressure drops, there *will* be backflow. Hence the requirement for a
check valve. (Most often they are required at the meter.)
So why is the check valve required at the water heater? As water is
heated, it expands. This pushes hot water back into the intake,
reducing efficiency, and sometimes cycling the heater element.
Well, your arguements are pretty much nonsense. Faucets, urinals, water
closets, sinks, drinking fountains, washing machines, ice makers, etc. all
have backflow prevention built-in in the form of an air gap. Backflow
prevention is present at all openings into the system (where ever a source
of polution can occur).
The real danger in back-siphoning is the common garden hose lying in a pool
polluted water. Many older homes do not have back-flow protection on their
hose bibs, and should a condition of low or loss of pressure such as drawing
water from a hydrant by a fire engine, can drop water pressure low enough to
siphon water from down stream (this has actually happened near major fires
when several pumpers were connected.) Anyway, so long as a system is air
tight, water does not go stagnant (remember that municipal supplies use
ozone or chlorine to maintain sterilization until comming into contact with
the outside air.)
Note that the code does not require the use of backflow prevention in the
average home supply. It's common practice to allow water heated in a
domestic water heater to expand back into the public supply (city mains).
If a *local ordinance* requires a backflow prevention device, due to a
private supply, or the supply serves a boiler, or if the residence lies in
an area subject to flooding, or for fire suppression equipment (double check
vales would be required here), or for any other reason, then it's required
to install an expansion tank after the check valve.
Installing a check valve in a supply main can be dangerous; when a tank-type
water heater is installed. If water cannot expland (due to a check valve in
the supply) dangerous high pressure can develop (under certain conditions)
whch can damage the piping or water heater. It's for this reason that a
expansion tank is required whenever a check valve is installed). (Private
supplies alreay have an expansion tank as a necessary part of the well/pump
Check valves in the main supply are not a requirement of the model codes (at
least that I'm aware of; IRC and UPC). If you know of a reference to any
requirement mandating the installation of a backflow prevention device (in
the form of a check valve) in a supply from a municipal main, please post it
here as I would certainly be interested in reading it.
I agree that ground water contamination is always a problem. In my area we
have had water supply pollution due to leaking underground gasoline storage
tanks. In another example, a nearby village was forced to shut down
supplying water for several days due to farm pesticide runoff contaminating
the ground water in a local pumping station. (He had an open, abandoned
well that extended down to the depth of the municipal level.)
replying to Dennis, Jack R. Hansen wrote:
I believe all the above posts are incorrect. I have two of these water heaters.
If you do not have a check valve every time someone opens a valve (Cold Water)
the heater will fire up for a few seconds until the pressure equalizes. I buy
the above reasons if the water heater might be used for closed loop radiant heat
but not for domestic hot water heating.
replying to Jack R. Hansen, DJO wrote:
I have a Bosch Aquastar tankless heater. It has a flow sensor that is used to
turn the heater on, so it does not do anything when the cold water is run in the
house. I removed the check valves and the expansion tank when I removed the
tank heater. A tankless heater is just ten feet of pipe, so it makes absolutely
no sense to put in check valves and an expansion tank. The expansion tank
bladder is the least reliable thing in your house. I have never seen a truthful
need for one with a tankless system. Plumbing contractors got it added to the
law simply to boost required work and profits.
replying to Jack R. Hansen, Jasongriff wrote:
I have a tankless water heater that is doing that exact thing. I was thinking
of putting a check valve in to try to fix this issue. That is how i found the
discussion. Thank you.
replying to Jasongriff, Donald wrote:
I am also experiencing the same problem. I just built a detached garage/apt
with a tankless water heater. The plumbing for the building was tied into the
house. I started to notice that while in the garage alone, the water heater
would fire randomly for a short period. Again, I'm the only one in there at the
time, and know no hot water fixtures were opened. We seem to have associated it
with water being turned on/run in the house, whether through a fixture or toilet
flush or what have you. My guess is that the pressure change/drop is causing a
momentary back flow from the unit, and the unit fires upon sensing flow without
really discriminating which direction the flow is going. I'm told some units
have back flow prevention built in, but per the Mfr, mine doesn't.
replying to Donald, Ray Dahl wrote:
I just installed a tankless water heater and ran short of a "T" to install an
expansion tank so I bypassed for now just so I could fire it up. Worked great,
turn on hot and it fires up...turn off hot and instantly shuts down. After I
installed an expansion tank it was delayed in starting up and also shutting down
AND also would fire up at random once in awhile. Not really sure whats going on
but seems to work perfect without an expansion tank. ???
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