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Tankless water heater: cold sandwich effect. While showering, hot water stops for about 1 min and then runs again. Was fine for 5 years.

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Tankless water heaters are a great appliance with one small draw back they require a constant flow of water through them to maintain production of hot water if the water presure fluctuates they can cut out momentarily or if your water supply is less than great ie hard ,high iron,
sulpher ect.. then the heat exchanger can become plugged over time like say 5 years. Contamination of the flow sensor/presure switch could also be a culprit depending on how handy you are a through cleaning with a good kettle descaler will probably help.
Good Luck

Posted on Oct 07, 2009

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Posted on Jan 02, 2017

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I have a recently installed richmond water heater. When the shower is running and someone turns the faucet on in the kitchen the water gets drastically colder in the shower. We didnt have that problem with...


Which type Richmond water heater?
Is this a tankless water heater or tank-type?

Tankless are notorious for 'cold water sandwich' under exact conditions you describe. You might need tankless with larger BTU burner.
Don't install a tankless.
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

If you have tank-type water heater, then I suspect a clog in the hot water line, or single-handle faucet cartridge in show is going bad.
Same problems can contribute to tankless problems.
http://waterheatertimer.org/Low-hot-water-pressure.html

Aug 06, 2012 | Rheem Richmond

2 Answers

I have a Paloma Whole Home 7.4 GPM Water Heater, it is a propane heater. I bought this heater in 07 and I get the same problem ever year. When the weather gets cold and the water is cold the hot water...


If incoming water is too cold, tankless might not have enough time to heat the water as it flows by insince the heat exchanger.

1) You can passively preheat cold water using staging tank, or tampering tank.
http://waterheatertimer.org/Tempering-tank.html

2) Buy old water heater from plumber that is not rusted out.
Replace anode rod to stop further rust.
Rinse tank with water and hydrogen peroxide to kill bacteria that results from anode rod.
http://waterheatertimer.org/Replace-anode-rod.html
Clean sediment out of bottom:
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

3) Don't forget, a tankless heater needs to be de-limed each year, and cannot be exposed to more than 11 grains hard water.

4) Since all costs are energy, when you get tired of messing with tankless.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
http://waterheatertimer.org/images/Compare-water-heater-parts.jpg

Add a comment for more free help.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.

Jan 18, 2011 | Rheem Paloma PTG-74PVN / RTG-74PVN...

3 Answers

Water turns cold very quickly


I had the EXACT same problem (installed my tankless last winter and worked fine, now this summer it fluctuates, have to turn the shower completely off and then on again to get any hot water and then it only lasts for a couple of minutes!) Argh - it is frustrating!

Here's what I found: the demand for hot water is lower during the summer (hot) months because the "cold" water coming in is already quite warm. You don't need as much hot water to make the shower temp comfortable. SO, my tankless was shutting off due to low flow demand. I've removed the flow restrictor from my shower head and it helped alot but didn't completely solve the prob. Next step is what has been recommended to me by several people but is much more expensive - change the pressure valve on your shower to a temperature valve. Or just wait til winter and it should work ok again :)

Jul 06, 2010 | Bosch & Tankless LP Hot Water Heater

1 Answer

Heats up the rapidly decreases within in minutes.


Problem new or old? How long you have unit? Sounds like not enough flow to keep burner on. Tankless need at least .6 gpm to keep hot water flowing or else the burner fires then shuts fires then shuts and you get a sandwich effect...hot cold hot cold. If you have new diverter valve in shower -vs- old two handle type try turning hot water ALL the way ON and see if hot stays hot. If it does stay hot then slowly adjust to cool and see if you maintain a good MIX for proper temp. If it goes cold again try this......LOWER control pad setting to a comfortable 105 degrees (ladies) 100 degrees(men) and turn shower valve all the way to HOT. See if your temp remains consistant now and if it does then problem solved. Tankless mindset...set temp on control pad and stop trying to MIX at shower valve. Thats how these things work. Thats why I install a control pad in each bath and one in the kitchen for adjusting ash temp during dishwasher on time. ditto for wash machine. You should never need to turn on cold water if your unit is set to proper temp at controller. Hope this helps

May 17, 2010 | Rheem Tankless Hot Water Heater...

1 Answer

Tankless on demand water heater.


Tankless water heaters normally take a little longer than a regular heater to provide hot water because they have to start and get warmed up. I'm guessing the heater uses a pilot light and that the short amount of hot water when you turn on the tap is hot because when the water is sitting still in the heater the heat from the pilot light is enough to get it hot. The tub faucet will get hot a lot faster than the shower because it lets a lot more water through and empties out the pipes faster than the shower. You could probably speed things up by letting the tub faucet run on completely hot for like 10-15 seconds and then turning on the shower.

Oct 24, 2009 | RTO Takagi Jr. TK-JR-N Tankless Water...

1 Answer

Rheem tankless water heater error code C4 13??


check diaphrapm valve or micro/switch best of luck

Sep 13, 2009 | Rheem RTG-53PVN Indoor Natural Gas...

2 Answers

Bosch AquaStar 125b ng suddenly goes cold during shower


Does it happen to any other fixture? If not, it's most likely your shower valve and not heater.

Sep 01, 2009 | Bosch AquaStar 2400ES-NG

1 Answer

Heater stops heating water


you have to much volume of water going thru the unit, did you size it properly by haow many baths and showers you have or did you just by the cheapest one,, there is a scale to go by run just one shower and see if it changes temp, with nothing else running,, a shower has 2.5 to 4.5 gal per min,, then you can also check to make sure it has the right size gas line 3/4 from the meter to the unit is required at a min of 2lbs of gas

Jun 17, 2009 | Rheem Tankless Hot Water Heater...

2 Answers

Runs out of hot water


I wonder if you have this problem below caused by a drop in water pressure from someone flushing a toilet or a washing machine. I would call the company to remedy this issue.

17. What is a cold water sandwich effect? The term “cold water sandwich effect” is a term that is used to describe the introduction of cold water into the hot water supply line during frequent on/off operation of an instantaneous water heater. The cold water sandwich effect, when present, appears as a momentary drop in hot water temperature as it is discharged from a hot water supply outlet (i.e. shower, tub, or faucet). This phenomenon is present in the operation of all instantaneous, tankless style, water heaters, but is minimized with the high tech design of Rinnai water heaters.
The technology built into the Rinnai water heaters is designed to minimize the cold water sandwich effect. Rinnai water heaters are microprocessor controlled and when water flow through them ceases, they remain in a “ready to fire state” for approximately 1 minute. If water flow through a Rinnai water heater begins within the first minute following water flow stoppage, the water heater will fire back up within 1 to 2 seconds. This minimizes the cold water sandwich effect that would otherwise be experienced with a low tech tankless water heater. It should be noted that the cold water sandwich effect cannot be removed completely from tankless style water heaters. The safety standards developed to insure the safe operation of water heaters require a delay in the ignition sequence of all gas water heaters.
While the cold water sandwich effect cannot be completely eliminated from standard plumbing systems, it can be eliminated from plumbing systems that have a supply and return hot water circulating system. Rinnai has developed 2 methods to eliminate the cold water sandwich effect in residential hot water circulating systems.
The first (and preferred) method to remove the cold water sandwich from circulating systems utilizes a small electric tank water heater (powered up) that is used with a dual purpose. The small water heater acts as a mixing tank to eliminate the cold water sandwich effect from the Rinnai water heater and it uses its electric heating element to offset the heat losses from the hot water circulating system.
The second method to remove the cold water sandwich from circulating systems utilizes a small 2 to 6 gallon storage tank installed on the hot water outlet of the Rinnai water heater. This tank acts as a mixing tank to blend the cold water sandwich with hot water and eliminating its effect at fixtures.

Mar 11, 2009 | Rinnai Water Heaters

1 Answer

Hot and cold water from tankless heater


check your tigh/low water temperture setting. you may have them crossed.
check all of your wiring.

Jan 13, 2009 | Rheem RTG-53PVN Indoor Natural Gas...

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