Question about Rheem Tankless Hot Water Heater Rtg-74pvp-2 7.4 Gpm

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When hot water is on & toilet flushed the temp drops

This just started last month, before you could have the shower & a faucet going & flush the toilet and the temp would not dip.

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  • Doug James
    Doug James Jan 06, 2009

    The pressure does not drop the temp does. It is like the heater just stopped working but once the toilet stops or other faucet it turned off the temp comes back up. the unit is about two years old.

    I have a filter on the supply from the street & a pump to keep the pressure up you can have about 4 things going before the pressure starts to drop.



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Check strainers and screens into the sink, at the spout and into the hot water heater. Sediment buildup could be limiting the flow which woul make the system more susceptible to pressure drop when the toilet is flushed.

Posted on Jan 05, 2009

  • Tripp REa Feb 10, 2009

    Its not the temp from the water heater. Its an imbalance between the hot and cold pressure going through the shower valve which is resulting in more cold than hot getting through the valve. No way is the water temp from the heater arbitrarily dropping in correlation with the toilet flush.



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We have a problem with our Rinnai 24 hot water service. Hot water goes hot and cold. Does not keep to 38 degrees temperature when showering.

Do You know if there is an anti scald valve in the shower valve? If so it may be scald up with corrosion and need replaced! To see if this could be the problem, flush the toilet next to the shower while the shower is running. If temperature changes in shower I would say that is your problem! If You need to replace it, it would be cheaper to do it yourself! They should be videos on Youtube on how to do it! Good Luck! Regards!

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I have a Richmond RCN 0595A12328, MODEL 5V40-7 40 gallon water heater. Can you tell me if there is some kind of an assy. I can use on it so when a toilet if flushed someone in the shower does not get...

The water heater has nothing to do with it. You need a shower control valve that won't let the water pressure change when the toilet is flushed. Moen makes just such a valve called a pressure balancing valve. Moen L82683 Positemp Tub/Shower Valve.

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Increase your flow, sounds like you are right at your activation rate.
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I am not sure what you are asking, But i am going to assume that you want to know about changing the Temp on the heater for inside the house.
If the floor heat was installed properly than it should have a temperture manifold or adjustable thermostat for seperate heating.
You need to find the thermostat or manifold for the garage heat and set it at what ever temp you want the garage to stay at.
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I recommend using a candy thermometer or digital thermometer to check the faucet and shower Temp prior to using them and make sure that you inform everyone in the house after you change the temp so that no one gets scalded.
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1 Answer

Water does not stay hot it is uneven in shower, from cold to hot then hot to cold

Ok that sounds more life a faucet that is bad. In today world, the newer (20 years and less) they have pressure balance faucets. Have the faucet serviced. NOW lets look at an other area check out your home to see if you have a cross connection. A cross connetion is a place where hot and clod water meet. I know that everywhere but the toilet!

Yes thats true. But we buy all kinds of things that we attach to a fauects or tub spouts. Lets say you attached a hose to the sink in the garage to wash the dog with warm water. if you turned the water off at the handle of the hose and you left the water on at the faucet. BINGO

the hot and cold water mix and then up and down water temp while taking a shower.

So start looking first before spending any money. if ANYTHING is attached to any faucet take it OFF and then take a shower.

Be safe


Tom ( how did we do ?) :)

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I have a noritz 069 M and what has been happening For the last few months is that while taking a shower the hot water will get super hot then shut off. The only way for it to come back on is to turn...

well it is time to get the the two way valves to both side, I would use only a product the noritz says to use.

Now if you cant get it you can get other products that break up lime and scale from you local store that sells plumbing product. I you can see I dont list a store or a product I am here to help you repair your heater, Not have you buy from one store or an other.

With that being said you can vinegar but dont use acid. If yuo "burn" the inside of the heat exhanger you jusy might be buying a new unit. also Please flush out the system with cold water for at least 10 mins. with the 3 way valves that should be very easy to do.

You also might have a lime build uo in the faucet also which will affect how they work. You may want to replaced and cleanout the cartridge and see if you have a lime build up. If that is the case you might want to think about a low priced water filter or softener.

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Our water heater runs out of hot water after a couple showers. Does it need to be replaced? Is there something on it that I can check? Does increasing the temperature setting help?

when was the last time that you have "opened the flush valve (faucet) at the bottom of the tank to clear out the sediment? if it has been more then 4 years then you will need to do this depending where you are located this should be done once a year just to keep the tank clean. attach your garden hose to the valve and run it over to the sink open the valve for a few moments to clear out the tanks bottom (HOT WATER ALERT!) after doing this and if you have seen dirt come out then your tank should be clear. check to see what the tempature is every 1/4 inch is 10-30* increase hope this helps

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This is typical for an installation without a balanced valve/anti-scald valve on the shower.

If you are using manual on/off valves on the shower, about the only way to fix this is to install the device mentioned in the previous paragraph.

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