Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Tankless coil with circulator & storage tank temperature control

doughess Member Posts: 28
Have an old Weil McLain oil fired hydronic system with 3/8" tankless copper coil feeding 50 gallon storage tank. Use circulator activated by PID controller to maintain tank water temp. All piping is 3/4" copper with cold make up water fed thru check valve. System generally works well. Issue comes when under hot water hi flow more cold make up water is coming directly into tank than from 150F tank less coil resulting in lower temp water to taps. Is there some way to control the mix of from coil and cold water make up?


  • doughess
    doughess Member Posts: 28
    System has outdoor reset that varies temp from 150 to 180. The circulator pump turns on at 109F and off at 113F.

    Was hoping to use some type of "mixing valve" set up during high flow. Now it gets most of make-up water cold from meter. That causes some fluctuations in shower water temperature.

    The storage tank has oil burner that is run only during the summer when the main boiler is shut down.
  • doughess
    doughess Member Posts: 28
    In winter operation the 50 gal tank is for storage, getting heat only from the tankless coil. Winter boiler cycling is driven more by house heat load and passive loses than tap water load.

    The oil fired water heater manufacturer considers 630F to be normal. That puts efficiency in the low 70%. A boiler stack temperature in the low 300'sF puts efficiency in the mid 80% range.

    In the summer a delta T of 3F is used to keep the water heater stack in the 500F range. It does not reach steady state but running 2 or 3 minutes per cycle is more efficient than 5 to 6 minutes. Since it runs less than 50 hours in the summer I am not worried about lower system longevity from short cycling.

  • doughess
    doughess Member Posts: 28
    Passive loss on the boiler during the summer exceed the hot water heat load. It can cost more to keep the boiler warm than it does to heat tap water consumed.

    Steady state is not the most efficient operation. As you note it is the balance point. Look at any chart on stack temp vs efficiency. A unit is more efficient before it reaches steady state and actually goes down after reaching it.

    I use rise in stack temperature as gauge for when to clean boiler. No rise, no clean

    The positive image of steady state and negative one about short cycling are major misconceptions widely held in hvac.

  • doughess
    doughess Member Posts: 28
    From a cold start, heating 70F to 120F a high mass cast iron boiler with XX gallons of water in the dormant heating system to raise 50 gals in a separate water tank is silly and raises a number of other issues.

    The definition of steady state is not about efficiency. The stack temp charts are about efficiency. Someone is paying for the higher exhaust temp and wasted energy going up the chimney.

    If a system reaches steady state in 4 minutes at 400F then running 2 minutes twice to a stack of 300 F creates the same btu's but is more efficient. System longevity might be lower but fuel savings are much higher.

    This is what condensing systems are all about, low stack temps bring high efficiency. They are not factually incorrect!

    I am running a 60 year old Weil-McLain at 50% of the rated firing gpm. In the old days of oil heat, with frequent oil burner fires, high stack temp was considered good. Some of that legacy thinking is still out there.
  • Photojumper
    Photojumper Member Posts: 1
    Is there any place where I can order a rinnae "water flow sensor" TODAY so I can have it by Monday or drive 150 miles and pick it up today?
  • kcopp
    kcopp Member Posts: 4,092
    Where are you? Most supplyhouses open on Saturday close by noon....especially the day after Christmas.