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check valve for tankless water heater PaulS 09-14-2007
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Posted by PaulS on September 19, 2007, 1:20 pm

> 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



Posted by Dennis on September 20, 2007, 7:56 pm
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.)

>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
>
>



Posted by PeterD on September 21, 2007, 9:47 am

>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

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.

>(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.)
>
>>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
>>
>>
>

Posted by Dennis on September 25, 2007, 7:53 pm
> 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.)



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