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Extend capacity of tankless coil with tank

Many moons ago, I saved a post (by Patchogue Phil) describing how to increase the capacity of a tankless boiler coil by adding an inexpensive water heater tank and a pump, and circulating water through the coil and tank. This was proposed as an "inexpensive" alternative to an indirect tank set-up.

I've finally decided my tankless coil does not provide enough hot water for the family (Weil Mclain oil-fired boiler), so I am looking into options. Is there any reason not to consider Phil's idea as an alternative to a true indirect?

(I can post his directions if it helps...)


  • Robert O'BrienRobert O'Brien Posts: 3,096Member
    Aquabooster. Very common in the past, somewhat cheaper than an indirect but generally less efficient since the old oil boiler still maintains temperature.
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  • HVACNUTHVACNUT Posts: 2,047Member
    Yeah an indirect is more efficient and you can add outdoor reset.
    But for what's it's worth, here's the John Wood specs.
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,634Member
    You can use an electric water heater and do the same thing. Use electric HWH thermostats to run the pump and maintain temp.
  • CanuckerCanucker Posts: 527Member
    If you look under @icesailor posts, he shows how to set it up
    You can have it good, fast or cheap. Pick two
  • bob eckbob eck Posts: 849Member
    Why not just put a 50 gallon electric water heater in and connect the water heater elements as a live water heater.
    Cold water into the coil then dump the hot water into the 50 gallon electric water heater. The elements will keep the water in the water heater hot. So when you need hot water the complete tank will be hot. As you use hot water the water going into the water heater will be hot from the coil in the boiler.

    If you put in a indirect water heater you can save 100 to 200 gallons of oil per year by letting the boiler be a cold start boiler.

    You can put a storage tank in with circulator pump but that pump needs to be a Stainless Steel or bronze pump. This type of pump is not cheap.

    How old is the Weil Mclain oil boiler with coil?

    Do you have hard water? If so the coil may need to be pumped out (cleaning the inside of the coil from lime build up)

    Find a good contractor and get pricing on all three options.
  • SuperTechSuperTech Posts: 938Member
    I'd say it's not worth considering. You still are using a tankless coil and maintaining a hot boiler. A tankless coil is the least efficient way to heat your water, an aquabooster is only a slight improvement.

    I wouldn't hesitate to install a indirect tank. If the cost is the issue an electric water heater is a good alternative.
  • WalnutFarmerWalnutFarmer Posts: 37Member
    Thanks all. I think I will focus on the indirect option. The booster approach seemed like a clever short cut, but I did not appreciate the down sides....
  • WalnutFarmerWalnutFarmer Posts: 37Member
    So... I'm back. I started reading old posts by @icesailor, @gennady and others, and I am re-intrigued by the simplicity of the coil/booster tank for my existing coil. I found instructions and diagrams, so feel I could give a good contractor solid info.

    A couple considerations/questions come to mind:

    1. My weil-mclain (wm68) is 25 years old and working fine. But... If I go with an indirect, it would be cold-start operating during the non-heating months for the first time in its life. Is leaking when it goes cold a real concern for a boiler of this style/vintage? (I think I'd still lean to indirect but I am concerned about this...)

    2. Should I worry about the state of the coil at this age? Scale, gunk build up etc?

    3. I wonder if the efficiency savings from an indirect over coil/booster will ever cover the extra investment to install the indirect. (I don't have estimates for either yet, so this is a big unknown)

    4. Capacity: We are mostly making due with coil alone now, so I am not too concerned about the extra HW capacity an indirect would provide over coil/booster. But, at the same time, I can foresee removing shower head flow restrictors, not worrying about running washer while showering, etc. once we have new capacity of any kind. And if I go with am 80 gal tank vs 40, the savings of going the coil/booster route start to diminish.

    Any more thoughts wise wall people?
  • EBEBRATT-EdEBEBRATT-Ed Posts: 5,634Member
    If you are "almost getting by" with the coil now than buy a 30 or 40 gallon electric water heater and yes you need a bronze pump and hook it up.

    It's not going to cost you much. Your boiler is 25 years old so who knows??? You may get 5 or 10 out of it if properly maintained. If the boiler has been hot for 25 years I wouldn't change it's operation now

    Then get a price on a new boiler with an indirect and start budgeting for it's replacement.

    Only you can decide your $$$$ situation.................sooner or later your going to replace it.
  • SuperTechSuperTech Posts: 938Member
    @WalnutFarmer brings up a very important point. Sometimes older cast iron boilers can begin to leak when they cool off. Usually I can judge whether its a good candidate for cold start conversion by keeping the boiler off and allowing it to cool during cleaning or repair. Sometimes it's just boiler drains, packing nuts and air vents leaking, no problem. Sometimes it's a leaking tankless coil gasket/rotten flange or leaking sections.
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