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Lukewarm Water - Tankless SGO-4 w/ L4006A

oldsteamer Member Posts: 8
edited February 2018 in THE MAIN WALL
Hi. - I have a SGO-4 steam boiler with tankless coil rated at 8GPM, yet whenever showering (~2.5gpm) the water first comes out very hot but quickly turns and stays lukewarm.

My aquastat is a Honeywell L4006A1967, set at 200F (per SGO manual) with a 5 degree DIF. I suspect I should raise the dif, but as I understand it the L4006A is a high cutoff, so I'm unclear of the implications. Perhaps I should set the unit at 180F with a 20F diff?

Tepidly yours,


  • Jamie Hall
    Jamie Hall Member Posts: 23,426
    Obvious question -- does the boiler come on when the water starts to get cool? It probably should.

    Then the next question -- is there a mixing valve on the system somewhere (there should be) and is it working properly?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • TheLeakyTub
    TheLeakyTub Member Posts: 10
    edited February 2018
    I just posted a similar item. On the wtgo cold water can be deposits on the outside of the DHW coil or scale in the inside. I pulled the coil and ran CLR through the inside with a submersible pump in a 5 gallon bucket and scrubbed the outside of the coils. I got it back together but I read here the studs that hold the coil on the boiler are easy to break. Also my o ring leaked on re assembly. I wrapped the I ring in ptfe tape and put paste on the face outside of the o ring. After that it wept a tiny bit when cold and sealed up when hot. You should have a new set of studs and o ring handy at the least and probably a way to extract the studs if you break one. If I had know I probably would have bought a new coil. If the old one is ok then you have a spare. Don't believe anything I say though I'm not a pro. I have a mixing valve with a thermometer on it so I had the wife take a shower while I watched the needle. Less fun than watching her but I was able to set the temp that way. The other alternative is to listen for cold water screams or hot water screams.
  • SeymourCates
    SeymourCates Member Posts: 162

    The fact that you have instantaneous very hot water rules out a mixing valve.

    The fact that the water quickly becomes lukewarm and stays lukewarm rules out the boiler itself. If the boiler fails to start on the aquastat (unlikely), the water would turn progressively colder as the shower continued.

    The issue is the coil itself and the failure to transfer sufficient BTU's to the moving water within. It does fine when the water is stagnant (waiting for the next shower) but not very good when the shower calls. The coil develops lime scale on the inside and a coating of soot on the outside. Both need to be removed for proper function. Follow the advice above to remove and clean the coil. Do be quite careful with the studs or cap screws that hold the coil in the boiler. Breaking one will ruin your entire day.
    EBEBRATT-Ed Member Posts: 15,612
    1. Check the boiler water temperature with the hot water running and see if the burner comes on before the water gets cold.
    2. Possibly you have a bad mixing valve.

    3.The addition of a flow restrictor may help