Tankless water heater: temperature fluctuates


  #1  
Old 04-23-15, 07:45 PM
D
Member
Thread Starter
Join Date: Apr 2015
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
Tankless water heater: temperature fluctuates

We have a Rheem Tankless water heater, model no. RTG-74DP-1. It runs on LP. It has worked flawlessly for eight years.

Several months ago we noticed that the water temperature was fluctuating while in use—usually in the shower. The temperature would drop to lukewarm and then rise to near scalding. Adjusting the thermostat seemed to help, for a week or two. The only fault code, P1, indicated low flow, but we have noticed no difference in flow (we use both low-flow and standard showerheads). The intake filter is clean—no visible blockage.

The WH itself turns on with water flow, as it should. The exhaust fan in the unit accelerates to a high volume, then shuts off. The WH itself does not shut entirely off—you can barely hear it running if your ear is next to it. If water is left flowing, the WH will re-ignite in 5-10 seconds, starting the cycle over again. We think the high fan sounds correspond with the scalding water temps, and the shutoff with the lukewarm (not cold) water flow.

Thinking the P1 issue may be caused by calcium deposits, we flushed the WH with vinegar for three hours to remove them, and the only result we got was green vinegar after 2.5 hours—is this copper reacting to the vinegar? We haven't seen evidence of calcium anywhere else, but you'd think if there were calcium in the line the vinegar would be cloudy or white. Regardless, flushing the WH didn't affect the problem.

Rheem customer service is difficult to contact, and when I reached them (as I did twice), they seem unable to provide helpful information. Your input is welcome.
 
  #2  
Old 04-23-15, 08:08 PM
G
Member
Join Date: Oct 2002
Location: Hamilton County, Ohio
Posts: 3,927
Upvotes: 0
Received 2 Upvotes on 2 Posts
Welcome to the forums. The plumbing pros will chime in soon. You say this occurs mainly with the shower. I would be considering replacing the anti scald cartridge in the shower after checking other faucets with a thermometer.
 
  #3  
Old 04-23-15, 08:48 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
Received 39 Upvotes on 31 Posts
I have no familiarity with that heater but it sounds to me as if the primary thermostat has failed and the unit is working off the high limit thermostat. A high limit thermostat would commonly have a wide differential which would account for the wild temperature swings.
 
  #4  
Old 04-24-15, 10:50 AM
D
Member
Thread Starter
Join Date: Apr 2015
Posts: 2
Upvotes: 0
Received 0 Upvotes on 0 Posts
Thanks, Furd. I had not thought of checking the primary thermostat. Do you know how I might confirm that issue or check the thermostat?
 
  #5  
Old 04-24-15, 01:29 PM
F
Member
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 16,321
Received 39 Upvotes on 31 Posts
Do you know how I might confirm that issue or check the thermostat?
Unfortunately, I do not. As I stated I am not at all familiar with that specific heater.

Sorry.
 
  #6  
Old 04-24-15, 07:24 PM
CasualJoe's Avatar
Member
Join Date: Jan 2010
Location: United States
Posts: 9,883
Received 188 Upvotes on 168 Posts
The heater is 8 years old, what maintenance has it had? My first thought is the heater is severely scaled, including the sensing element of the thermostat, and that is is having trouble sensing the temperature of the incoming water as well as the hot water leaving the unit. This may also account for the low flow issue.

We haven't seen evidence of calcium anywhere else, but you'd think if there were calcium in the line the vinegar would be cloudy or white. Regardless, flushing the WH didn't affect the problem.
You probably need a stronger acid to clean the heater. At most, vinegar has a 5% acidity.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: