Dielectricunion water heater

No flow + bad valve + power failure = big fire, ergo none of the electronic controls can compensate for one failed control valve, unless there is a mechanical device that physically blocks the gas flow to, or from, the main valve.

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I'm short on time this morning but here goes. This only applies to the units I am most familiar with. Noritz, Rennai, Takagi. Any one of these things will shut off the unit if there is a problem. Safety #1: No flow no fire. Period. Nothing happens if the flow sensor does not sense flow. It is like a paddle wheel or impeller and can't "spin" without flow. Safety #2: The computer. It acts like a thermostat on a tank heater, and them some. Monitoring everything and controlling everything. It uses thermistors all through the machine to monitor temperatures at multiple points. Safety #3 Low gas pressure switch. Shuts down unit in the vent of low gas pressure Safety #4 Flame rod. Will shut off the unit if there is a flame failure Safety #5 Combustion sensor. Will shut off the unit if there is an abnormality of combustion. Safety #6 High limit. Shuts off unit in event of overheating if the combustion sensor does not do it first. Ultimate safety! Fusible link on the HX melts at about 500* shutting down the unit if everything else failed to do it fist. Unlike a tank water heaters, these can’t “Run Wild”. Their safeties are more like those found on advanced boilers. Boy, that seems like a lot of stuff to break? (It took me a long time to get to where I’m going next and I expect some of you to not believe it. That’s ok. You’ll see.) Some might think so, but solid state components are very reliable. If you ask the guys that sell these things about parts, none are stocking them. Guess why! They don’t need to. They can get them. But they won’t spend thousands on parts to sit around for years. If you need a part its going to be because you broke something. (No back-up wrench?) - - - - - - - - - - - - - Give your kids a great start on the future! Encourage them learn a trade. Even if they go on to do other things, it's always nice to know that they have something to fall back on. Call your local technical training center or trade school to learn more.

WattsDielectricUnion

That sounds like a variation of the "electric water heaters are as safe as a light bulb, (supposedly because of the ECO switch), so they don't need T&P valves", that was the mantra of the Southern codes during the 50's.

That Webstone isolation valve kit is a must for installing the tankless water heaters,but it does not include a pressure relief valve.

Scott, You mentioned that a tankless WH is considered an 'open' system and that T & P valves are not required in other places and possibly in the U.S. down the road. Let's say if the unit ran wild because of a malfunction or repair error, wouldn't it be wise to have a T & P valve in place for safety measures? e-plumber ----------------------------------------------------------------------------------------------------------------------------------- "The society which scorns excellence in Plumbing as a humble activity and tolerates shoddiness in philosophy because it is an exalted activity will have neither good Plumbing nor good philosophy: neither its pipes nor its theories will hold water." - John William Gardner 10/8/1912 - 2/16/2002 Repair your leaking Plumbing fixtures ASAP [www.theplumber.com] This slow drip will waste 7+ gallons of water per day.

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Dielectricunion for gas piping

There is a small pressure relief that is more commonly used on internal tankless coils of oil boilers. Cant recall the numbers though. [lib.store.yahoo.net] Edited 1 times.

You can use a typical t&p valve if you want, but I believe that the tankless are being allowed to be installed with just a pressure relief valve (a bad idea IMO). If you want to use the conventional style, the probe must be installed so that it is directly and completely in the flow of water. This can be accomplished by using a tee with a female adapter on the run. So you would have (flow in) X (t+p valve) X (flow out) on the tee (if that makes sense!) Good luck. Jason Jason

Thanks for the reply Scott. I still say that a T & P valve cannot hurt on any type of domestic water heating apparatus. Any device that is capable of producing boiling water should have some sort of way to safely release excessive temperature and pressure. The likelihood of the unit 'running wild', (with ALL those components that you listed) is probably slim to none, but I would think definitely not impossible. Would you agree? e-plumber ----------------------------------------------------------------------------------------------------------------------------------- "The society which scorns excellence in Plumbing as a humble activity and tolerates shoddiness in philosophy because it is an exalted activity will have neither good Plumbing nor good philosophy: neither its pipes nor its theories will hold water." - John William Gardner 10/8/1912 - 2/16/2002 Repair your leaking Plumbing fixtures ASAP [www.theplumber.com] This slow drip will waste 7+ gallons of water per day.

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e-plumber, i would agree except for 1 thing. if all of those devices fail at one time, there is a much bigger problem that even having a t&p wont help. just the first safety by itself pretty much elimates the need. no flow no fire. to get fire something has to be open to get flow which means no pressure build up. these things are as close as you can get for safety with hot water systems - - - - - - - PLUMBERS "Protecting The Health Of The Nation"

The tankless water heater is my first major home plumbing project so this is one of the gaps for me... My manual calls for a pressure (only) relief valve with a discharge capacity of 140,000 BTU/H. Looking for the lower price option, the Watts 3L seemed like a good option for pressure but it does not list a discharge capacity. Since the unit is rated at 140,000 BTU's (Takagi Tk-jr), that makes intuitive sense to me. I am inferring that discharge capacity means how much heat the valve can disipate (please tell me if I have this wrong). Should I be looking at something like a Watts 3L or something that has a rated listing? Are their other manufacturers I should consider (other than webstone)? Since I have water ready to go except for hanging a pressure relief valve, it seems like I should pass on the webstone or did I misunderstand the product?

Buy one of these. You can find them at most plumbing supply stores and they will come with the proper relief valve. [www.webstonevalves.com] - - - - - - - - - - - - - Give your kids a great start on the future! Encourage them learn a trade. Even if they go on to do other things, it's always nice to know that they have something to fall back on. Call your local technical training center or trade school to learn more.

I will be installing this tankless water heater for a month. Got the electical done, I'm working on the gas run and then I'll cut in the water. The unit calls for a pressure relief valve on the hot water out line. All of the prvs in the big box stores have temperature sensing probes on them and are not made to go in-line. Can I just place a tee in the hot water line and use a conventional temp/pres relief valve or is there a prv that is made to go in line?

All of the ones in stock in VA do. The RV is a Watts 3L pressure only. Temperature (The "T" part of a T&P )is not required because tankless heaters have many built in safeties that tank water heaters do not. In most of the world, even the pressure releif valve is not required because they are designed to operate as an open system. No flow no fire, even with the electrics. Our Codes simply are behind the curve. They'll catch up in time. The XTP valve set gives you more servicability. - - - - - - - - - - - - - Give your kids a great start on the future! Encourage them learn a trade. Even if they go on to do other things, it's always nice to know that they have something to fall back on. Call your local technical training center or trade school to learn more.