check valve for tankless water heater

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
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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.
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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

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PaulS wrote:

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.

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Robert Allison
Rimshot, Inc.
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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.

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Thanks, got it. paul
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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.)

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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.
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.

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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.

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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