check valve for tankless water heater (2023 Update)

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

Reply to
PaulS
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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.

Reply to
CWatters

Reply to
PaulS

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.

Reply to
Robert Allison

Reply to
Erik Dillenkofer

Thanks, got it. paul

Reply to
PaulS

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.)
Reply to
Dennis

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.

Reply to
PeterD

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.)

Reply to
Dennis

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.

Reply to
Jack R. Hansen

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.

Reply to
DJO

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.

Reply to
Jasongriff

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.

Reply to
Donald

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. ???

Reply to
Ray Dahl

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.

Reply to
UFGator121222

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. :)

Reply to
Pockeyway

Thank you boss I just bought a recirculating pump and the diagram showed a check valve under the pump. Was trying to make sure if I needed one and why. Thanks now I know.

Reply to
Who

@Pockeyway, May I ask which check valve you went with for your tankless? There seem to be various pressure ranges as regards the back flow activation pressure, ie: "0.863 psi", or closer to .50, or even over 1.00 psi, etc. I just installed a Rinnai and am having the same unit activation when turning on a garden hose (FWIW: It also occurs when luandry machine is turned on, even on cold cycle. I read the machine's manual online and it confirms they blend hot and cold water on a cold cycle to keep it within a certain temp range (~70deg)). Any recommendations on which valve you used would be much appreciated. Thanks

Reply to
Jules

May I ask which check valve you went with for your tankless? There seem to be various pressure ranges as regards the back flow activation pressure, ie: "0.863 psi", or closer to .50, or even over 1.00 psi, etc. I just installed a Rinnai and am having the same unit activation when turning on a garden hose (FWIW: It also occurs when luandry machine is turned on, even on cold cycle. I read the machine's manual online and it confirms they blend hot and cold water on a cold cycle to keep it within a certain temp range (~70deg)). Any recommendations on which valve you used would be much appreciated. Thanks

Reply to
Jules

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