Question about Stiebel Eltron DHC-E 8 Tankless Electric Water Heater

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I installed a dhc tankless heater, but when I open only the hot water everything is ok, but when I open the cold water at the same time the heater turns off.

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Hi and welcome to FixYa, I am Kelly.

What is happening is that your cold water flow is greater in volume (gals per min) than your hot water. So when you open the cold water it is mixing with the hot water inside the faucett or shower plumbing and actually reverse flowing back into the hot water heater plumbing. The reverse flow slows the water flow sensing turbine inside the heater and it is thinking that there is not any water flow. (There isn't actually due to the equalization of water pressure when you open the cold water faucett) Since this is a safety feature of the tankless heater to keep it from overheating, it shuts off.
Somewhere in the hot water plumbing there is a major restriction. It may very well be the internal plumbing of the tankless hot water heater or the flow detector design.

There are 2 ways to correct this. The easy way would be to restrict the available cold water flow. You can do this by installing another shut off valve inline with the cold water plumbing and just not open the valve all the way. You would have to adjust the setting so that the hot and cold water can be on at the same time and the hot water heater stays on. The hard way is to locate the hot water flow restriction and eliminate it.
It is possible.... if you have a flow restrictor in a shower head or a screen in a faucett areator that they are partially clogged. Clean any flow restrictors and screens then see how the hot water heater performs before you start modifying plumbing.

Thanks for choosing FixYa,
Kelly

Posted on Apr 10, 2011

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Check your manual, or add a comment with specific model number and maybe I can find the information.

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List of causes MIGHT include one of following:
Bad household circuit breaker that powers electronics, or loose wire inside outlet where tankless plugs in.
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Tankless inlet filter clogged.
Household aerators clogged at faucets.
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Faulty household check valve, including check valve on re-circulation system if that system is installed.
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Unequal pressure in household hot and cold lines, caused by restriction in hot line. When single-handled faucet turns on, the cold water pressure is higher so hot does not flow and tankless will not turn on.
Loose wire connection inside tankless.
Unit temperature set too high.
Unit temperature set too low.
Water temperature is very hot coming out of tap, so more cold is added, but that reduces amount of hot water flow, so tankless turns off because flow rate is too low.
On some tankless, if water is preheated by solar or other indirect source, the burner will not activate.
High mineral content in water > delime tankless yearly
Misaligned temperature sensor
Low gas pressure. Wrong type of gas. Undersized gas line.
Too many faucets on at same time.
Faulty tankless water valve. Water valve stuck.
Fluctuating household water pressure, for example MUD and PUD district water pressure problems or water well pressure.
Faulty household pressure reducing/regulating valve if installed.
Faulty household pressure tank, if installed -or- pressure tank needs to be installed.

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Circuit can test for electricity if just one leg of 240V circuit is on, but the other leg is not.
http://waterheatertimer.org/240-v-water-heater-circuit.html#120-240

All costs are energy.
Find a plumber trained in that specific brand of tankless.
Read manual completely several times, and make sure maintenance is done on yearly basis.
Read troubleshoot section completely.
Manual should have phone number of manufacturer.
Call manufacturer for name of trained service technician in your area.

Before installing tankless, make sure trained person is available in area.
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Tankless should be de-limed each year.
Tankless cannot be exposed to more than 11 grains hardness in the water.
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Problems can be simple or complicated.

Install tank type heater as back up for those days when tankless is not working.
http://waterheatertimer.org/How-to-install-gas-water-heater.html
http://waterheatertimer.org/images/Basic-240-Volt-water-heater-circuit.jpg
http://waterheatertimer.org/Tempering-tank.html

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.

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The water heater "Stiebel Eltron


I am a free answerer, and I specialize in giving you resources for free
The paid answerers may have a person who knows your product and the problems associated with your model.
I believe they offer a refund if the 'fix' doesn't work.
The manual might also have troubleshooting.

Each day I see 20-25 unanswered questions about tankless water heaters.
And the day before, there were same number of unanswered questions about tankless products.
The number of questions shows the complexity of the machines, the brand-specific-ness of the problems, and the inability of the manufactures to take care of their customers.
If I understand correctly, the manufacturer will refer you to the installer.
There are not many ways a do-it-yourself-er can fix a tankless.

http://www.thetankatwaterheaterrescue.com/forums/forum3/

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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 :)

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