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Tankless Electric water heater in series with NG Boiler possible?

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  • Tankless Electric water heater in series with NG Boiler possible?

    Hello, We just had a new PV system installed. We live in the NE so in the cold season the electric usage goes down and the natural gas usage goes up considerably. In addition, the NG rates go up too. One of the main reason is obviously space heating. The Thermostats keep calling the natural Gas boiler to continue feeding the hot water to the circulatory pumps, etc. I wanted to ask if somehow I could interject that with a tankless water heater in series, would that reduce the usage of the boiler some how? I understand that tankless heater may not be enough to heat a house of our size by itself, but in combination with a gas boiler it could hold up. At least in theory, maximum energy is used in bringing DHW from cold to hot (which will be done by the main boiler) and then once it has reached its hot state and is circulating between the heating coil and the boiler, then this tankless heater, could help maintain this temperature, until it cant keep up and the boiler will kick off again. Well, I tried to search the web and did not find anything on this. So looks like I am clearly missing something here, otherwise a lot of people would be doing this.

    Does anyone has any comments on if this can be done? And if it would be efficient enough to save any money? Would love to hear thoughts from folks who might have a similar type of system.

  • #2
    You may include other types of water-heaters in line with your furnace to share the burden of heating your home.

    Our primary heat source is a wood stove that heats water, which circulates to a thermal bank, which then circulates through our radiant floor. We are installing a solar thermal system to also provide heat tot he same water.
    4400w, Midnite Classic 150 charge-controller.

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    • #3
      Why not just replace your old boiler with a high efficiency 95+ efficiency boiler such as a Burnam alpine or equivalent?

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      • #4
        I'd agree w/Lucman and upgrade the boiler, but I'd first make sure the existing equipment is in good working order while simultaneously reducing the heat load by tightening up the dwelling and adding insulation.

        Bolt-ons to a heating system seems like a lot of potential hassle and expense for what may not be much, if any, gain in efficiency, for a lot of $$ spent, especially using tankless technology.

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        • #5
          Here in IL PV production is way down in the heating season, but with grid tie the summer surplus
          reserve is available in winter. If you have considerable surplus, it could be injected as heat to
          reduce NG use, esp at peak rate times. A simple air heater unit would work as efficiently as a
          complex water system. Use of a mini split heat pump with very low outside temp capability
          would multiply the efficiency several times.

          Its not likely straight electric heating can compete with NG. But here the price of propane has
          been seen (at 5X) to reach twice the cost of electric resistance heating. Bruce Roe

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          • #6
            Thank you for your responses. Yes I already have Triangle Tube prestige Solo (high efficiency boiler). The question was how to use the PV surplus and offset some of the heating costs by using electric heating mechanism. I was under the impression that I could potentially use electric tankless just to maintain the hot water temprature in circulation. From what I have read, they can handle heating preheated water well as opposed to heating cold water from outside. So in my simplistic mind my rationale was that the main high efficiency boiler will heat the water and then the tankless will maintain the temp required by the heating coils without kicking off the boiler too much. So that is what I was trying to say in my post above. Is that worth it?

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            • #7
              Originally posted by bonbon View Post
              Thank you for your responses. Yes I already have Triangle Tube prestige Solo (high efficiency boiler). The question was how to use the PV surplus and offset some of the heating costs by using electric heating mechanism. I was under the impression that I could potentially use electric tankless just to maintain the hot water temprature in circulation. From what I have read, they can handle heating preheated water well as opposed to heating cold water from outside. So in my simplistic mind my rationale was that the main high efficiency boiler will heat the water and then the tankless will maintain the temp required by the heating coils without kicking off the boiler too much. So that is what I was trying to say in my post above. Is that worth it?
              Heating water via resistance electricity is like cutting butter with a chain saw. When you wrote of tankless, I assumed you were referring to nat. gas fired tankless. No matter really though, still a lousy idea..

              Leave the boiler alone and let it work as it was designed to work. I doubt if I could improve on that design after the fact as you seem to want to do, and I designed heat transfer equipment for a living. You won't improve on that design by fooling around with it.

              Find another use for any surplus electricity you have other than wasting it generating heat.

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              • #8


                1KW = 3413 BTU Just how much excess solar PV do you have? Unlike a water tank which needs about 70W just to maintain temperature, you are doing massive heating. You can do the math yourself on this web site.

                http://waterheatertimer.org/Kwh-temp.html

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                • #9
                  Instead of heating water with your excess PV consider heating ceramic bricks. But as JPM points out this is very inefficient when trying to heat your entire house. But if you use this resistance heat in a specific space then it makes much more sense. Instead of raising the temp in 1500 sqft you only need to make 300 sqft comfortable.

                  To do this you need a heat storage device that can be charged when the sun is up and producing excess PV power then release the heat as you require. There are a few products on the market that do this but after some research I use this:

                  http://www.steffes.com/electric-ther...ge/room-units/

                  Actually I have two units...the smaller one is in my wife's workshop upstairs which I charge from 10am to 1pm and the larger unit downstairs in the living room I charge from 1pm to 4pm. We discharge the stored heat at our convenience and makes our spaces cozy. The rest of the house stays at 68* or lower. Our overall heating costs have never been lower.

                  Edit: This approach and the timing required makes more sense if you are not on a net metering plan. With net metering cost of a kWh is the same any time of day. If you are on a net billing plan or some type of banked or other wholesale plan then it is more beneficial. In any case heating less space and being more comfortable is desirable.
                  Last edited by DanS26; 11-11-2017, 04:25 PM.

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                  • #10
                    Just how much excess MW's pv do you generate?
                    The most efficient use of electrical energy for heating purposes would be to use a high efficiency heatpump from Fujitsu or mitsubishi. These are air to air so they would not help with your hydronic system but would add BTU's to your living space at the lowest electrical energy consumption.

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                    • #11
                      Originally posted by LucMan View Post
                      Just how much excess MW's pv do you generate?
                      The most efficient use of electrical energy for heating purposes would be to use a high efficiency heatpump from Fujitsu or mitsubishi. These are air to air so they would not help with your hydronic system but would add BTU's to your living space at the lowest electrical energy consumption.
                      Depending on the net metering arrangement, seems like a less oversized PV system - if that's where the excess electricity is coming from - would have been a more cost effective alternative, but that's water over the dam now.

                      Heat pumps are better than resistance heating for sure, but if the OP has a nat. gas boiler and equipment and, depending on how cold the area is and how well insulated the OP's dwelling may be, an adjunct heat pump system may not be as effective a heating source or as cost effective as other means of meeting the load(s). Point or area resistance heating of specific or small areas may be an option.

                      As you know, and as a general but not universal statement, where available, nat. gas is hard to beat as a heat source in cold climates with respect to installed cost of equipment, fuel costs and heating comfort.

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