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how to Sidearm or plate on Marathon heater?

I would like to install a sidearm, or plate exchanger but am unsure on how or what would work best for my marathon tank. It is the all plastic, and the pressure/vacuum relief is between the inlet and outlet on the top of the tank. I thought the best method for the sidearm was to have it connect only to the side of the tank and not to the top? So would it be best to install a plate and use a circulation pump to push the DHW through the exchanger? 4 people in the house.


  • Larry Weingarten
    Larry Weingarten Member Posts: 2,082
    Hi, Can you give us a system overview? Are you using the heat exchanger to take heat from the Marathon? Are you looking at having two pumps? The more info, the better B)

    Yours, Larry
  • mustang_gt_350
    mustang_gt_350 Member Posts: 25
    Sorry for the lack of detail. I was heading to bed and had thoughts scattered about in my head. I'm wanting to use my outdoor furnace to heat my DHW.

    I wanted to use a sidearm exchanger and use convection for lack of better terms on my part to avoid using a pump to circulate the DHW through the exchanger. Both for simplicity and to not have the expense of the pump and electricity to run the pump. I found 2 videos that show what i'm wanting to do. In the first for the sidearm he explains my situation about the ports being on the top of the tank.

    in the 2nd video for installing the plate exchanger I dont understand how that works since it can't allow the water to circulate around the tank. To me the only time that the plate exchanger would heat water is if hot water was being used and the cold water coming into the tank would be passing through it. I don't see it being able to hold the tank to the desired level. Maybe i am wrong, thats why i came here to ask questions :smile: My plan was to install the exchanger like shown on video 2 only use a circulation pump and a aquastat to control the water temp. So i would T into the hot outlet run into the pump, through the exchanger and T back into the Cold inlet and dump the heated water into the cold inlet.

    Again, i'm not claming to know all, i just didn't see how the video 2 install can heat the water and hold the tank to temp? I would much rather prefer to keep it simple and avoild all of the extra costs with the pump and aquastat. I was hoping someone here could confirm that the install as shown on video 2 does work, or confirm that i was right and needed to have a way to circulate it.

    Video 1 on the sidearm. This option is out for me


  • Larry Weingarten
    Larry Weingarten Member Posts: 2,082
    Hi, I watched the second video and it sure looks like magical thinking to me. I have seen long heat exchangers designed to go beside the tank, tying in low and high. That side shouldn't need a pump, but the other side will need one. I'd consider using a thermostatic control located on the tank, so heat is no longer put into the tank when it gets up to temp. That make sense?

    Yours, Larry
  • mustang_gt_350
    mustang_gt_350 Member Posts: 25
    edited October 2019
    That a what I was thinking about the 2nd video, I just don't see it working as stated. Heating it as it comes into the tank yes, but not maintaining the tank to temp.
    That is the sidearm heater I was talking about that you sent in the link that I wasn't sure if I could do since my water would have to flow down to get back into the tank since I don't have 2 ports on the side of my tank to connect into

    I was either going to control it with a thermostat or put a mixing valve on it incase it did overtemp so no one got burnt.
  • hot_rod
    hot_rod Member Posts: 14,552
    thermo siphon is a slow somewhat un-predictable way to transfer heat energy. Heat transfer is driven by delta T and large pipe size to allow near zero pressure drop.

    The old gravity heat buildings need balance valves on the top floors as the heat would shoot to the top quickly leaving the lower floors with inadequate flow.

    Pumped counterflow is the best way to use a plate HX as they have some pressure drop.

    Copper "long tube" heat exchangers work well on wood fired boiler installations as DHW exchangers they have high temperature to work with and low pressure drop in the tube. Although you greatly increase their performance if you pump both sides :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • mustang_gt_350
    mustang_gt_350 Member Posts: 25
    Thanks for the reply hotrod. I was hoping you would venture into this section and see it.

    Looks like I will be pumping both aides,it will be more of an initial investment but at least I know I'm getting more out of it per time vs wondering how much flow I am acutaly getting if I had went with thermosiphon.

    Thanks again, time to plan out a parts list.
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