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Indirect DHW tank Delta T?

In a primary/secondary boiler system a Taco 0010 is serving a Slant Fin WH-40 D indirect DHW tank. The Boiler is Trinity TFT 110(99,000 DOE). The manual for the WH-40 D says that is has 8 feet of head at 6 gpm and will give a 65 deg Delta T at 115 deg. Total head for the DHW loop is about 8.4 feet. At 8.4' of head the 0010 should move about 11 gpm. This is no doubt lowering the Delta T dramatically.

So here's the questions; what is the effect of a lower Delta T on the DHW Tank? How will if affect the 1st hour rating and how will it affect efficiency?




  • Jean-David BeyerJean-David Beyer Posts: 2,641Member

    On my system, that is piped primary-secondary with the boiler on the primary circuit and the two heating zones on the secondary circuit, the indirect is across the supply and return in the primary circuit.  Because of this, I must use the same size circulator to run the indirect as I do in the primary loop in order to get the same flow through the boiler's heat exchanger. In my case, these are Taco 007 circulators.

    The boiler has two sets of thermal detectors to measure delta T. One is across the heat exchanger, and one is in the secondary loop a little outside the closely spaced Ts. When running the indirect, the temperature sensors measure the delta-T across the heat exchanger, and when heating the house, it measures the supply and return temperatures of the heating load.

    For the indirect, the delta-T runs around 20F. But this is not a steady state measurement because some of the time, the boiler is heating up, some of the time it runs fairly steady state, and some of the time, the boiler is cooling down. There is very little steady state time as it takes only between 5 and 10 minutes to reheat the tank. If I wanted a lower delta T across the indirect, I would need some other means of running this, probably with a bypass valve of some sort in order to keep the flow through the boiler in specification. And as long as it is flowing, it might as well go through the indirect, rather than having some of it bypassed around it. So I do not do this.

    Basically, I let it run and do not concern myself with flow rate or delta T.  I could increase the delta T by increasing the temperature of the water I supply to heat the indirect. So I would get more rapid transfer of heat to the indirect. But unless I got a major increase of delta T, I would lose because the boiler would be in condensing mode, such as it is, even less of the time. I had not thought it would run in condensing mode heating the indirect since the supply temperature is nominally 175F (default is 190F), but it does condense as the boiler is warming up, which takes about 5 minutes. And that counts for something.
  • barefootbarefoot Posts: 18Member
    Delta T issue

    Thanks for your interesting post.

    I'm still trying to wrap my head around how heat is transferred inside of a DHW tank. Will I get more heat transfer, thus a faster recovery/ higher 1st hr rating/higher continuous flow rating, with more flow (ie, lower Delta T), assuming the same temp when comparing one flow rate to another? I'm curious to know if the tank mfg recommends 65 Delta T for efficiency purposes rather than pure performance specs.
  • SWEISWEI Posts: 7,356Member
    deltas T

    There are two different numbers at work here.  The first, which you have described, is the difference between water leaving and returning to the boiler.  The second, which the manufacturer quotes as 65F, is the difference between the incoming cold domestic water and the desired hot water temp or tank limit.  The greater this difference, the faster heat will transfer from the boiler to the DHW, but the water still has to get up to the target temp, which will take longer with colder incoming water, assuming the boiler water stays at a fixed temperature (which of course it doesn't.)  The nominal boiler water temperature is an average of supply and return temps.  If the circulator runs slower, this average will drop, but that can be remedied by increasing the supply temp (assuming the boiler can still keep up.)

    Does your indirect have a mixing valve at its output?  Do you have hard water?  How long has it been since the boiler was inspected and cleaned?   Assuming you want more hot water, these are the areas I'd be looking at first.
  • barefootbarefoot Posts: 18Member
    Delta T


    Thanks for chiming in, you've been helping me with my Delta T issue in the Radiant Heating section of this forum. The Boiler is a brand new Trinity TFT 110. There is no mixing valve on the DHW loop. The current pump on the DHW loop is a Taco 0010. It's providing a lot of flow. I installed a Grundfos 15-58FC on the primary loop (replaced a Taco 0011) yesterday and the Delta T at the boiler increased from 4 to about 10 with just that change (pump on lowest setting). This is with a call for DHW. I'm thinking that if I put a smaller circ pump on the DHW loop that I would increase the delta T at both the tank and at the boiler and get the boiler in condensing mode. Currently, it modulates to the lowest setting during DHW calls.

    I haven't checked to see what happens to the Delta T at the boiler with the 15-58 when there is a CH call. That may require moving to the mid pump setting on the 15-58. If I do that I will really need a smaller pump on the DHW loop to get any decent Delta T at the boiler during a DHW call.

    While I really appreciate the help attempting to solve my issues, what I really need right now is for someone to explain a little hydronic theory. So, the questions that I posed are of a more theoretical nature; how is heat transfer affected if there is an increase or decrease of flow through the DHW tank? Does a higher flow provide more or less heat transfer? Assuming the same temp of inlet water.

    Why does the tank mfg recommend a 65 deg Delta T at the tank? Is this done for efficiency of the system (ie, lowest energy usage) or is there a performance reason also?

    again, I really appreciate the generous help provided on this forum.

    all the best,

  • SWEISWEI Posts: 7,356Member
    heat flux will give you the engineering school version.  Across a heat exchanger, heat flow is proportional to surface area, conductivity of the HX material, and the difference in temperature.  The first two don't change appreciably, so it really comes down to the difference in temperatures.  As ∆T increases, more heat will move during each unit of time.  The hotter the average boiler water temperature and the colder the incoming water, the faster heat will move into the DHW.  As the DHW gets close to the setpoint, the rate of heat transfer slows down.  A smart boiler control will "see" this coming using a thermistor in the DHW tank, and stop before the tank overshoots too much.  Keeping the tank at 140F prevents legionella growth but requires a thermostatic mixing valve at the tank output to protect occupants from scald injury.  Raising the tank temp to 150F or 160F has the effect of making the tank "larger" from a thermal capacity standpoint.  Speeding up or slowing down the DHW circulator without also changing the DHW target temp will simply move the average boiler water temp up or down a bit.  This will affect DHW recovery rate, but not anywhere near as much as changing the tank setpoint, boiler water target temp, undershoot and overshoot temps, etc.
  • icesailoricesailor Posts: 7,265Member
    Delta "T"s:

    Somehow, I'm nut understanding your goal of Delta T.

    The only heating systems I have been able to gauge Delta T is one where I put Tridicator gauges on the supply and the return, between the circulator. With the high limit set to 180 degrees and the differential set at 20 degrees, I could watch the water flow through the boiler and zone and measure the head pressure. The difference in pressure going out and coming back. Then, I watch the temperature going out (Supply) and the return. The boiler would shut off around 189 degrees but keep climbing as the return was dropping and rising. When the supply started temperature started dropping, the return would be rising. The return would start to fall after the burner came back on around the temperature that gave somewhere near the 20 degree mark. But the spread between the two was always near 20 degrees. To me, that is a Delta T.

    When the manufacturer of a indirect calls for a 65 degree Delta T, aren't they figuring 55 degree incoming and heating to 120 degrees? 65 Degree Delta T? That's when the cold water is flowing. If and when the water flow/demand stops, the circulator is pumping whatever temperature the water is  that is in the tank and how much you are trying to raise it. The closer you get to the limit set, the closer you get to the Delta T. So, how are you measuring it?

    Some seem to swap circulators like little kids swapping toys in the toy box. Who's paying for that?

    Only God and engineers work in a perfect world. I'm not either. I work with what I get.

    If you had a 100,000 BTU boiler with 5 series loops at 20,000 each, you designed for a 20 degree Delta T, with a 180 degree design temperature, and you turned on all five zones, and you got your 20 degrees, good on you. Have 4 zones satisfied, and you have 100,000 BTU's going into a 20,000 circuit with the same circulator. 20 degree Delta T?

    I had a call, cold. There was not a big drop between the supply and return. After I sucked out all the animal hair and dander and dust from the top of the fin tube, and moved stuff from in front of the baseboard, the return temperature went way down.
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