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dhw tank running wild occassionally

a while back i submitted a discussion on a hwr that was cooling down the dhw temp. i found the problem and
corrected it. (installed new s.s. spring check)
now i have dhw temps that can exceed the stat setpt. by over 30deg.f (stats set @ 120degf.)!! at other times the
dhw temp will actually cool to 110-115degf. this temp. problem showed up some time after the hwr was corrected.
this is getting to be maddening as i've replaced EVERYTHING inside the tank including 2 new stats! sometimes i
have to set the hw control on the shower to FULL HOT, to keep from freezing. this problem is experienced in both
showers, but only occassionally. the hw temp will restore itself to stat setpt. eventually.
hwr still tee's into tank cw make-up about 4ft. above tank. i saw no reason to replumb it. it has always been tee'd there,
years before this new problem arose.

is it possible for the stats to get "fooled" and allow very hw to leave the tank?
this is an electric AO SMITH 52gal.tank with reliance electric stats. the shower controls are newport brass single
lever controls with anti-scald. the controls are about 15yrs. old. i took the guts out to clean every thing and found
little evidence of calcium or hardness fouling in either shower control. proportioning appears normal.

difficult to duplicate the problem as it only occurs intermittently. it is not load dependent as it sometimes occurs
when the shower is first turned on. hw discharge being measured at tank with reliable immersed thermometer.


  • Larry Weingarten
    Larry Weingarten Member Posts: 3,247
    Hello, Have you checked the system for cross connection?

    Yours, Larry
  • bill_brooks
    bill_brooks Member Posts: 50
    yes. i did that during the hwr escapade before this current problem started.
  • bill_brooks
    bill_brooks Member Posts: 50
    p.s. just for arguments sake and curiosity, how could cross-connection cause the dhw stats to allow the setpt. to be exceeded? isn't that the purpose of the stat to prevent over heating?
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,247
    Hi, Cross connection wouldn't cause overheating, but it would create cooling. Another possible problem is modern thermostats are often not close to setpoint because they can have a large deadband. Some heater controls have a 30 degree deadband, and that could be part of the problem here.
    I'm just trying to get a clearer picture in my head of the entire system. Do you have any photos of the plumbing around the heater? You said "hwr still tee's into tank cw make-up about 4ft. above tank". I'd like to see that plumbing. One more thing; are the new thermostats in firm contact with the tank and do they have insulation over them, so they 'see' tank temp only?

    Yours, Larry
  • bill_brooks
    bill_brooks Member Posts: 50
    thanks larry. i don't own a digital camera. we'll see if my gf can
    take pix with her cell phone.
  • bill_brooks
    bill_brooks Member Posts: 50
    hi larry,
    we took pix of my dhw plumbing. unfortunately we had to send to this pc and (firefox?) won't allow me to forward to "the wall".
    in lieu of those pix, could i offer a clearer description of the plumbing arrangement? its pretty straighforward.
    i'm sure its my computer ineptitude preventing me from sending. i'd rather just describe the plumbing arrangement and explain it in clearer format.

  • pecmsg
    pecmsg Member Posts: 4,696
    Your HW set point needs to be at 140°F minimum to prevent legionnaires bacteria from growing. Then reduce the supply temp with a mixing valve to below 120°F for point of use.
  • bill_brooks
    bill_brooks Member Posts: 50
    in chasing this problem i considered adding an "apollo tank booster" which i purchased to end this hassle. i guess now is
    a good time to retro fit it.
    however, i'd still like to know what the source of the hassle is
    being caused by.
    thanks gents!
  • Zman
    Zman Member Posts: 7,549
    I would take a hard look at how the t stats making contact with the tank. Ideally some thermal grease would be used to assure good contact.
    It is also possible that the top tstat is miswired. When the t-stat closes does the correct element energize?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein