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Supplement DHW with boiler?

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Notaplumber
Notaplumber Member Posts: 7
in Radiant Heating
Hello all. I am new here and have a question. I have a forced hot water/radiant baseboard system for heat. I also have an electric domestic water heater. The boiler has a tankless water heater that is not used. My question is, can I cycle the tankless water heater on the boiler to run through the water heater to gain free heat in the cold season when the boiler is running anyway?

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  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
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    Nothing is FREE...

    The cost of natural gas is significantly less than that of electricity except at the base of a hydroelectric dam.



    There may be some expectation issues with the gas boiler, but you CAN incorporate the electric storage tank (with elements turned off) to increase the hourly dump load availability.



    ME

    There was an error rendering this rich post.

  • icesailor
    icesailor Member Posts: 7,265
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    Tankless Coil:

    Why do you have a unused tank-less coil in a gas boiler? Did it start its life as a oil boiler? Did you convert it to gas with a power gas burner?

    There is a way to use the electric water heater as a storage tank if you do have a boiler indirect coil.



    Post a photo of your boiler and set-up.

    Nothing is free. Just some things cost less than others.
  • Notaplumber
    Notaplumber Member Posts: 7
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    Specifics

    I'm sorry, let me try to be a little more specific. I have an oil fired boiler being used to run a hydronic baseboard heating system. The boiler has a tankless hot water heating section to it. The tankless is currently not being used. I do not want to get my hot water from here. There is an electric water heater for DHW. I am interested in plumbing the tankless water heater in the boiler to circulate the water in the electric water heater. I live in a fairly cold region (New England). My hope was that as the boiler is running anyway to provide heat to the house, that I would be able to circulate the to the electric heater, thereby keeping the water hot and not needing the electric water heater to kick on (as often). I would not be using an aquastat, I would not want the boiler to come on just to heat the DHW. I would just like to supplement the water heater while the boiler is running. In other words, I would continue to heat the water with the electric heater, when needed. I was thinking of wiring and plumbing a circulator pump to the electric water heater so that whenever the boiler comes on for heat, it would circulate hot water into the water heater at the same time. Does this make any sense?
  • Notaplumber
    Notaplumber Member Posts: 7
    edited January 2014
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    Nothing is free

    Mark - I agree nothing is free. So let me re-phrase that portion. I am not looking to get hot water for free, But as you will see from my last post, currently I am heating a tankless water heater for no reason. I am more looking to recover heat that is already being generated and going to waste.
  • Zman
    Zman Member Posts: 7,569
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    Indirect coil

    As Mark has said, Natural gas costs less than electricity so heating your hot water with natural gas is a good idea. You could turn off the electric element off and circulate the water from the  indirect coil in the boiler through the water heater. In the summer turn the circulator off and the electric element on and you are all set.

    As for the "free heat" or "generated heat being wasted", your reasoning is flawed.

    You are not presently circulating cold water through the coil, so very little energy is being "wasted". The boiler is just heating a small piece of metal. Once you start moving cold water through the coil, the boiler temp will drop causing the boiler to fire. This will use more energy than was used before the cold water was introduced.

    You are on the right track. Go for it.

    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Notaplumber
    Notaplumber Member Posts: 7
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    close, but...

    This is an OIL fired burner. I do not want to hook up an aquastat, therefore, the cool water would not cause the boiler to kick on. What I am saying is to leave the electric element on as the primary heat source in the electric water heater. But just to supplement it with the tankless system in the boiler. I am thinking that if I do this, when the boiler kicks on for home heat, it will also heat the water in the water heater tank, thereby causing the electric element to come on less often. The circulator would only come on to exchange the water in the water heater when the boiler is already running. The water heater would never get its primary heat from the boiler. Does it make any sense to do this?
  • Weezbo
    Weezbo Member Posts: 6,232
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    sure . it is done everyday ,

    bring cold into the coil and out to the cold water side of the water heater .



    thats the easiest way to use the coil .



    recirc systems use the bottom thermostat to control the circ pump if you want to recirculate .

    the piping arrangement is different as then you want to bring another line into a T off a nipple where the water heater drain valve is located ..



    you then have a circ that takes the back to the cold of the coil and has a check valve in it . so that the coils supply cold water does not find a path to the bottom of the water heater thru the recirc line.



    you can add a temp sensor to the pipe at the threaded brass nipple at the old water heater drain valve port , that turns off and on at a specific temp if you want to leave the wiring as is on the water heater for use year round. or the summer . add a means to turn the circ off and on in line with the temp switch .



    do you see it in your mind ? it is that easy.
  • Notaplumber
    Notaplumber Member Posts: 7
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    Thank you!

    That is exactly what I was asking. My only question is, instead of using a temp sensor at the bottom pipe, will it be ok if I just have the recirc motor come on whenever the boiler is on, regardless of the temp in the water heater? Or am I over-simplifying things here? And yes, I can see it in my mind, but the picture is getting clearer now.
  • Notaplumber
    Notaplumber Member Posts: 7
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    One more thing...

    Just to be clear, I would run the water out of the drain valve (with a tee), through the boiler coil and back into the cold side of the water heater (with the appropriate check valves, of course), correct?
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    edited January 2014
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    Energy costs...

    What are your energy costs per unit? ($/gallon of oil and $/KWH)?



    Also, I **** u me d you had a gas boiler. My bad.



    ME

    There was an error rendering this rich post.

  • Notaplumber
    Notaplumber Member Posts: 7
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    Well,

    I really don't know what the energy costs are because this is a new house that I am buying next week. The thieves got into the basement and stole all of the copper. As I am forced to re-pipe everything, I was looking for ways to be more fuel-efficient. However, I wouldn't think that the unit cost would really come into play here, as I am leaving the system essentially the same. I will be using an oil fired boiler to heat the home, and an electric water heater for DHW. I don't really have a choice there. I was just trying to maximize the efficiency of the electric water heater. propane would only be an option if I were to purchase a new unit and direct vent it to the outside, which would be cost-prohibitive.
  • Zman
    Zman Member Posts: 7,569
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    Oops

    I also assumed natural gas.

    Oil or propane are a different story.

    You may be better off with the electric. Unless you boiler is grossly oversized and the additional load would improve performance, a BTU is a BTU and you pay for it no matter how it is generated.

    Check out this slick calculator and make an informed decision.

    www.eia.gov/neic/experts/heatcalc.xls



    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein

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