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Instant Hot Water, Heater Coil?

How does one know when the heater coil inside an Oil fired burner is going bad? I have a Weil Mclain Oil fired boiler that heats the home and provides hot water to the entire house. My problem is that after the initial burst of hot water, let's say approximately 2 gallons. The water cools for the next 2 to 4 gallons to at best luke warm, then it heats back up to the original hot water temperature, before dropping off again for good. This is most apparent in the showers but it can be seen at any sink. I spoke with my HVAC company and they seemed to think it was the coil inside the boiler. Before I take the my house's hot water supply off the boiler I wanted to make sure this is a sign of the coil going bad. I've done things like replacing the mixing valves at the showers. There does not appear to be a mixing valve on the boiler.



  • Jamie Hall
    Jamie Hall Member Posts: 21,094
    There's bad and bad. The first thing I would do is descale that coil on the domestic water side. There isn't much that can go wrong with a coil other than scale -- it is, after all, just a loop of pipe...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 13,427
    Tankless coils are not the best. But:

    Is your boiler Hot Water or Steam?

    Tankless coils are rated with 200 deg boiler water. You can't really run it that hot but you probably have to be in the 160-180 range on the boiler water at least

    Try this. Run your how water and check the boiler water temp and see if your 160-180. Also see if you run out of hot water what temp the boiler is and weather the burner fires before you run out of hot water .

    Pictures will help. You should always have a mixing valve on the tankless coil. Tankless is usually only good for two people. Low flow shower head and drawing hot water slower will help a little
  • newagedawn
    newagedawn Member Posts: 586
    if you have a tempering valve, those go bad also
    "The bitter taste of a poor install lasts far longer than the JOY of the lowest price"
    HVACNUT Member Posts: 5,023
    From my own situation, WGO-3, Riello F5 burning .68 GPH, triple aquastat, 180-160 with a tankless and thermostatic mixing valve. 2 adults, 2 teens. We can take consecutive showers with no trouble unless outside temps really drop. But don't think about running the DW or laundry during a shower.
    The WM came with the house. I did the Riello and mixing valve.
    That being said, for efficiency alone, i.e. cold start, ODR, TDR on the burner circuit, and plenty of hot water, an indirect is the only way to go.
    Fingers crossed I'm hoping to replace the system this summer.
    @SteveSchaffling, how old is your WM? Is it gold or blue? If gold (WGO) think about an indirect. If blue (66 or 68 series), think about a new, high efficiency system.
  • TheLeakyTub
    TheLeakyTub Member Posts: 10
    Posted this twice tonight. Just pulled mine cleaned the outside and decaled the inside. Leaked on reassembly, wrapped o ring in take and PTFE on face, got it working. Definitely hotter water now. But my high limit keeps tripping.