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Help Solving this Question.

willie t
willie t Member Posts: 4
Will someone please explain the following problem to me. It's not making any sense to me.
You have two 75 gallon indirect water heaters in two different basements. The boilers, pumps, lenghs of piping, etc, are the same. Everything is the same except in tank # 1 you have a 20 degree temperture drop across the indirect and in tank # 2 you have only a 10 degree temperture drop across the indirect. If both tanks are filled with 40 degree domestic water and the boilers and pumps turned on at same time, which tank will reach the set point temp of 140 degrees first? #1 with a 20 delta T or tank # 2 with a 10 degree delta T ?
A major boiler company is trying to tell me the tank that has a 10 degree temp drop will reach set point first.
I can't buy it.
Can someone explain in english?

Comments

  • I don't buy it either

    because you're dropping off more BTU's into the indirect with the larger delta T.

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  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928


    Supply Temp = 200°

    Tank 1: 20° temp drop = 190° average coil temp

    Tank 2: 10° temp drop = 195° average coil temp

    Tank 2 heats faster because the coil is hotter...

    BUT, in reality for this to happen with otherwise identical setups, tank 2 would REQUIRE a higher flow to achieve the lower delta-t.

    OR (given everything else identical), the coil in tank 2 would have to be smaller in which case the tanks would heat at the same rate...

    If everything were truly identical the delta-t would be the same in both as welll <;)
  • Sweet_2
    Sweet_2 Member Posts: 143
    At first glance

    the indirect with 10* delta t gets my vote like Mike T. said the coil is hotter. Again like Mike T. said if both pumps are indentical they would be the same. In a perfect world. When your taking Delta T, I assume ther is no demand on either one? Heating or domestic?
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    A Tale of Two Baths

    Have told this story before, but it might be worth repeating.

    Two baths of nearly identical floor area (40 sq.ft) and cubic volume. Nearly identical heat loss characteristics through the ceiling, both over conditioned space.

    One bath fully internal; the other has two medium-sized (for an old house) double-hung windows and 8' of exposed wall.

    Identical amount of heated floor area (20 sq.ft) and identical floor construction. 3/8" copper in Thermofin under the floor.

    Both floor loops identical in length and even the same number of bends. They both connect in reverse-return fashion to 1/2" copper risers that themselves connect to the mains of an old gravity conversion system under constant circulation.

    Ball valves at the 1/2" riser connections to the mains for initial flow balancing. Valves adjusted for 20° delta-t with outside temp in low 30s. VERY low flow--estimated around 2-3 ounces per minute.

    Bath with exposure and greater heat loss shows a 30° temp drop; bath with no exposure and extremely little heat loss shows a 10° temp drop. BUT, the average floor temp in both is extremely close (the unexposed is typically 1° warmer)AND the space temperature produced is so close as to be identical!

    While the temp measured near the center of each heated floor area is very close, the unexposed floor has very little variance from end-to-end. The floor in the exposed bath varies by about 5° from end-to-end.

    In this instance there MUST be more heat from the panel with the higher delta-t. The only rational explanation I can offer is that the exposed wall draws more radiative heat from the panel (and thus more conductive heat from the water) as it attempts to cool. The unexposed bath, lacking this exposed wall, conducts less heat through an otherwise identical construction....
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