check valve for tankless water heater

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PaulS

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Sep 14, 2007, 9:11:51 AM9/14/07
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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.
Thanks,
Paul


CWatters

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Sep 14, 2007, 7:07:55 PM9/14/07
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"PaulS" <seme...@ix.netcom.com> wrote in message
news:13el253...@corp.supernews.com...

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.


PaulS

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Sep 18, 2007, 11:05:00 AM9/18/07
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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?
Thanks,
Paul
"CWatters" <colin....@turnersoak.plus.com> wrote in message
news:13em50c...@corp.supernews.com...

Robert Allison

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Sep 18, 2007, 11:31:15 AM9/18/07
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PaulS wrote:
> 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?
> Thanks,
> Paul

Paul,

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
instances.

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.

> "CWatters" <colin....@turnersoak.plus.com> wrote in message
> news:13em50c...@corp.supernews.com...
>
>>"PaulS" <seme...@ix.netcom.com> wrote in message
>>news:13el253...@corp.supernews.com...
>>
>>>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.
>>>Thanks,
>>>Paul
>>
>>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.


--
Robert Allison
Rimshot, Inc.
Georgetown, TX

Erik Dillenkofer

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Sep 18, 2007, 6:02:16 PM9/18/07
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Without the check valve, if there's a break and pressure is lost the unit
could empty. If the unit is empty and fires up it will burn out.

"Robert Allison" <rims...@spamless.net> wrote in message
news:7bSHi.6186$A72.24@trnddc08...

PaulS

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Sep 19, 2007, 1:20:14 PM9/19/07
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"Robert Allison" <rims...@spamless.net> wrote in message
news:7bSHi.6186$A72.24@trnddc08...
> PaulS wrote:
>> 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?
>> Thanks,
>> Paul
>
> Paul,
>
> 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 instances.
>
> 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.

Thanks, got it.
paul


Dennis

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Sep 20, 2007, 7:56:33 PM9/20/07
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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.)

"PaulS" <seme...@ix.netcom.com> wrote in message
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Message has been deleted

Dennis

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Sep 25, 2007, 7:53:44 PM9/25/07
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> 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.

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
operation).

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.

> 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

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.)


Jack R. Hansen

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Jan 13, 2018, 7:14:05 PM1/13/18
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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.

--
for full context, visit https://www.homeownershub.com/construction/check-valve-for-tankless-water-heater-10769-.htm


DJO

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Jan 31, 2018, 3:44:04 PM1/31/18
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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.

Jasongriff

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Apr 24, 2018, 5:44:04 PM4/24/18
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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.

Donald

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Sep 24, 2018, 2:44:08 PM9/24/18
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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.

Ray Dahl

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Jan 16, 2019, 9:14:03 AM1/16/19
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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. ???

UFGator121222

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Jun 8, 2019, 10:44:03 PM6/8/19
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replying to Jasongriff, UFGator121222 wrote:
A check valve is not going to do anything to fix your water heater. A check
valve does on thing and one thing only which is let water flow one way not
both. The only time you actually need a check valve on any water heater is for
a recirculating pump. All the talk from years ago about them being for
contaminated water and whatnot is way off also. The water meter on a house has
its own check valve so there is no reason to use one anywhere for that reason.
The most common correct use for check valves are on pumps to keep them from
losing prime.

Pockeyway

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Oct 30, 2022, 11:15:04 PM10/30/22
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Install the check valve before the cold inlet on the water heater. I recently came across this dilemma (tankless heater fan will start up for a few seconds after toilet flush. Water heater won't fire up or anything, no flame light on remote panel, but heater fan will turn on just for a couple seconds. The issue is with siphoning effect being created by toilet flushing. Water in cold line to heater is being pulled back when toilet is filling up tank. When cold gets pulled back from heater, so does it's hot in this instance. When that happens, heater will begin process of pressurizing the lost hot line again. That is 100% the issue & check valve installation is 100% the remedy. Btw, id probably be one of those wrong answers on this thread as well, if I did not have first hand knowledge dealing with this issue in the past. :)

--
For full context, visit https://www.homeownershub.com/construction/check-valve-for-tankless-water-heater-10769-.htm

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