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Thread: Tankless or Boiler for Radiators and DHW?

  1. #1
    DIY Junior Member
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    Default Tankless or Boiler for Radiators and DHW?

    Hi, I have an older home, 2300 sf with lp boiler feeding cast rads and lp DHW heater. Both need replacing.

    I'm considering the benefits of going tankless or HE boiler/indirect hot water tank system, maybe adding some solar at some point.

    I have small demands for hot water. One or two showers daily, DW twice a week, almost never wash clothes in hot. Only one zone for heating.

    I live in Central VA.

    I'd like to do this effectively but I'm also pinching pennies here...

    Any thoughts?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    I think a boiler with an indirect WH would work out best. Some of the companies make indirect WH's with dual coils, one for the boiler, and a second for your solar input. A boiler with a variable fire rate could match your low heating requirements, but still ramp up when called for to provide DHW. You probably only need one with a small output. A condensing, modulating boiler costs more, and you might find the break-even point doesn't exist, but as energy is only likely to rise in cost, that's hard to predict. Personally, I had a Buderus unit put in that has proven to work well.

    Most tankless are not really designed for space heating requirements. Not to say that you can't make it work.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
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    Eliminating the solar aspect of the plan, I am really considering the possibility of tankless only. No holding tank at all.

    If I had a good two manifold tankless unit, it seems to me I might be able to pipe one side for rads and one for DHW.

    A modern con boiler holds very little water and is, by comparison to my old boiler, practically tankless. I just wonder why that wouldn't work. Save me the cost of a fairly expensive indirect tank.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Carefully read the installation instructions...only some tankless systems are designed for space heating. Also, with the heat exchanger needed, I think you'd lose some efficiency since you don't want to use potable water in your heating loops. You heat exchanger would need to be something safe for potable water, and you'd need logic to make DHW the priority. A good boiler can work for many years with minimum maintenance - it is designed to run continuously. Double-check the recommended duty cycle of any tankless. You want it to be able to run 24/7 if you get a long cold spell. A boiler can do that, not sure about a tankless. A boiler is designed to produce some quite hot water - much hotter than a tankless. Depending on the radiators being used, you may never achieve the required delta T to keep the house warm. While lower water temps is more efficient, unless the radiators and system were designed for that, you won't be able to keep the house warm.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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