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Water not getting hot on main and 2nd floor

Hi everyone,

Approx 3 days ago while taking my early morning shower with water temp hot as normal (also stayed hot with prolonged use) the water went cold then got warm and fluctuated + - couple degrees never getting hot again. I checked all water faucets on 2nd and main floor and none of them get hot only warm. In my fully finished basement the water gets very hot as normal. Here are some details about my home:
-built 2002
-Private well water
-Main and upstairs living quarters approx 3775 sqft.
-4 bedroom 2 full bath upstairs
-5 rooms + powder room on main floor
-finished basement is approm 1750 sqaure feet with 1 bedroom with full bath, 3 additional room + half bath.
-Maintenance room is unfinished with oil tank, air handler for main floor of house, and boiler/furnace (dont know difference) I will be posting pictures unit.
-I was told by a friend to check the units temp guage with no water running then with water running. Read slightly under 170 then had my wife turn on upstairs bathtub on full hot. Watched the guage drop relatively quick. Unit kicked on at 159 degrees and took approx 6 min to reach 180 degrees and shut off. The temp difference is set at 170 high and 150 low w/ 10 degree variance.
I have a Beckett AFG model# EM-135 which is original from 2002.
I hope I provided enough details (not like original lost post) for you professionals to give me some ideas on where to start looking. I am pretty handy so let's get into this! Pictures are attached.
I appreciate any and all input and suggestions!! Thank you for your time and knowledge because this is one area I dont know anything about. If anyone needs particular pics please let me know and i will post.


  • psb75
    psb75 Member Posts: 756
    Have you ever had the domestic hot water coil in your boiler flushed/cleaned? Your well water is probably quite "hard." Is the tempering/antiscald valve working as it should?
  • Zman
    Zman Member Posts: 7,496
    You have a cross connect between hot and cold somewhere in the house. Have you installed a new shower head with a shutoff button on the head? Do you have a mop sink with a hose "Y" connection on it?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • HomerJSmith
    HomerJSmith Member Posts: 2,314
    edited October 2019
    You have a honeywell thermostatic mixing valve on your boiler and that's the problem.

    Well water can be in many locations mineralized which comes out of solution on hot metals. Your boiler's tankless DHW coil is probably also calcified. A cleaning with a descaler would be beneficial. You can clean the thermostatic mixing valve, but if it is more than 5yrs old replace it as the parts in it have deteriorated.

    Co-incidentally, I have one that I removed from my indirect W/H sitting on my desk that I replaced several day ago. My daughter was complaining about the same thing as your wife. Ya, Ya, I kept saying. It's a lot of work, I have to drain down the system, etc. It's a fact, it is always the plumbers house where the pipes leak and never seem to get fixed. So, I bit the bullet after I got tired of all the complaining and replaced it.

    I always take things apart to find out what created the problem. I knew what the problem was tho and I thought I would clean it. Because of the deterioration in the old valve, I replaced it. I keep a bunch of them on the shelf.

    I just bought 2 more valves from the site below. I didn't care that they were CPVC connections as I use the union nut on the original valve when I replace them. Just use new gaskets.


    I always put a temperature gauge on the mixed outlet when installing them. The new valve will have a temperature strip that you can paste on the outlet to set the temperature.

    You definitely have hard water. You should clean your system with a descaler and use de-ionized or distilled water in your heating system. Sometimes cleaning a badly calcified boiler can cause leaks, rare.

  • psb75
    psb75 Member Posts: 756
    Buy a Caleffi DHW mixing valve. It has an integral thermometer on the outlet.
  • HomerJSmith
    HomerJSmith Member Posts: 2,314
    Ya, Caleffi has a cool valve, but it might require re-piping where the honeywell will fit right in.