Electric Tankless Water Heater Woes

[Tommie Martin]

Hello everyone. Our primary home was hit with a tornado last week so we have had to relocate to our hunting cabin/house. We shut it down 2 years ago and there were no issues with electrical or plumbing. We entered the house on Saturday and turned on the water and main breaker. Everything came on with no issues. We tried to wash some dishes on Sunday and we noticed the water temp was really low. Checked the water temp and it was at about 100 degrees fahrenheit with an inlet water temp at 60 degrees. The electrical tankless water heater was set at 120 degress fahrenheit.

Today in the daylight I started checking things. The water heater is a 3 year old Rheem 11kw Tankless Water Heater wired with a 60 amp double pole breaker using #6 wire. I checked main house load center, breaker and water heater voltage and all are at 240+ vAC. The heater is located about 12 feet from the kitchen sink and 15 feet from the dish washer with all runs made with 1/2" PEX.

I then started checking settings. When set to 90 degrees, the water get to 90 degrees and on upwards until the setting is at 110 degrees. Then the water temp is anywhere from 102 to about 105 degrees fahrenheit. All connections were checked for tightness, water and electrical. Both sides of the power were checked and were getting the aforementioned 243 vAC. When the heater cycles on, both heating tubes are recieving 240+ vAC. The resistance on the heating tubes/elements was checked and found to be at 10.07 to 10.10 ohms. When the water is turned DOWN to about 1/2 volume/flow, the water then gets as hot as the heater setting up to 140 degrees fahrenheit. The more water flow introduced, the lower the temp until at full volume the temp drops back down to the average 100 degrees fahrenheit.

Thank you for any and all help you can provide.

 

[breplum]

The chart says you should be able to get about 1.65 GPM. So depending on what flow your kitchen faucet aerator flows, that may be normal.
https://files.myrheem.com/webpartners/ProductDocuments/2B28DBDA-FEE8-4A8B-954C-02E1A7552C14.pdf

 

[Tommie Martin]

The chart says you should be able to get about 1.65 GPM. So depending on what flow your kitchen faucet aerator flows, that may be normal.
https://files.myrheem.com/webpartners/ProductDocuments/2B28DBDA-FEE8-4A8B-954C-02E1A7552C14.pdf

Thank you for the reply. When this water heater was running a few years ago is was running about 128 degrees at full flow. We are in an area where the chart says we are pretty good..

I have read today where some say it might be scale in the heat tubes and then again it might be some kind of flow sensor and I do not have a clue how to test for a flow sensor or where I might get a replacement.

 

[Twowaxhack]

The heater sounds like it’s working just like it should. Reduce your flow.

Until you accurately measure the flow, incoming and outgoing temps it’s a big guess.

 

[GReynolds929]

A 60° rise on an 11KW heater is asking a heck of a lot from that little heater. Max flow at that rise is 1.2-1.3 gpm, so one lav faucet would max it out.

When you used it before at what was the incoming water temp? Summer or winter? It's also possible you have a bad element.

 

[chinarus]

I have a RHEEM 36 that I installed in 2018 that was working fine until late yesterday
Are the elements replaceable?
Outlet temp is lukewarm when set at 140F.
All 4 elements are getting voltage - that is as far as I have gotten with troubleshooting.

 

[GReynolds929]

Are the elements replaceable?

Yes

 

[chinarus]

Sharing progress and head scratching:

First element is open - others are about 7 to 9 OHMS.
Number on the element (GREEN top) = R0047070247s - 240V 6000Watts
Appears to be about 12 inches max or so.
RHEEM and Grainger part numbers aparently have a SP number like SP10552LL but
I haven't been able to cross reference the element R004 number.

https://parts.rheem.com/category/rh-tankless-water-heaters

I did determine that Grainger does have a 6000W element:

RHEEM Water Heater Element: LWD, 6,000 W, 208V/240V AC, Threaded Pipe, 19.27 in Insert Lg​

Item 54JG99
Mfr. Model SP10869Q
Item 54JG99
Mfr. Model SP10869QL

In order to avoid a divorce I bought a replacement unit at HD last night but would like to see if I can get the original working again for backup.

 

[GReynolds929]

You should call Rheem to get the appropriate part, if you have a 36kw heater the elements should be 9kw each. A good element should measure close to 6.4 ohms.