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Floor radiant supply temp

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wrooper
wrooper Member Posts: 58
I have been having a discussion over in the DHW forum. Several posters have told me I can get 120F water from my mixing valve while firing my boiler at 180F

With a delta T of 20F , my return water is 160 F if my boiler is firing at 180F. How does mixing 180F supply and 160F return produce 120F supply for my radiant floor?

Just for giggles, I will ask my original question?

I fire my cold start conventional oil boiler at 140F [short direct vent/wall flue,condensing has not been an issue]. This allows me to mix my floor supply to 125F [approx] which produces 85F floor temps. Works well

I have a flat plate HX feeding my 40 gal DHW tank. It is piped just like a System 2000 DHW. I would like to expand my DHW supply and one fix I am considering is adding a second 40 gal tank. The new piping for said tank is my question.

The current setup has cold supply entering at the tank drain tap. Teed directly above it is my circulator feeding the HX. The output of the heat exchanger feeds a top tap of the DHW tank. The other top tap is the house HW supply. The circulator pulls water from the bottom of the tank, through the HX to the top of the tank [abbreviated dip tube]

My piping plan is to route the HX ouput to one of the top taps of the second[new] tank. The other top tap of the new tank will be routed to the top tap on the original tank vacated by moving the HX output to the new tank.


In effect the circulator will pull water from the bottom of the old tank, througgh the HX and push it into the top tap of the new tank. The other top tap in the new tank will transfer water to the top of the old tank. The old tank will keep the thermostat so the old tank cannot be satisfied until the new tank water is at thermostat temp.

I concluded I will have to move the cold supply to the drain tap of the new tank to force the water out of the second tank and into the house supply [in the original tank].

Any yeas or nays?

Comments

  • Zman
    Zman Member Posts: 7,572
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    This is what a properly installed radiant mixing valve looks like. It just recircs the same water around the loop, blending in hot as needed. You can have any temp you like. They work best when you install the outdoor sensor. The sensor on the boiler return prevents boiler condensation.

    How is yours piped?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Solid_Fuel_Man
  • wrooper
    wrooper Member Posts: 58
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    Zman, you still aren't convincing me you can make 125F water by mixing 180F supply and 160F return. Of course we are WAY off the original question. OTOH I am fascinated by the insistence you can make "any temperature water from a mixing valve" regardless of supply and delta T

    Mine is piped exactly as your diagram with less sensors etc. There is a bimetal valve that opens/closes the return contribution to the "mix"
    ZmanGroundUp
  • Jamie Hall
    Jamie Hall Member Posts: 23,415
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    The point which is being missed here, perhaps, is that in the radiant loop you are not mixing 180 with 160. You are mixing the 180 with whatever is coming back from the radiant -- say 80 or so. So it's no problem.

    But... the other thing which is being missed is that the radiant has its own circulation pump, and is connected to the boiler hot water circulation only by the two closely spaced Ts on the primary (boiler) loop. Under most conditions, the radiant will pull (through the mixing valve) very little water from that primary loop -- just enough to make the outlet temperature of the radiant loop what you want it to be. Most of the water in the radiant loop just goes round and round.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    GroundUpBillyOSolid_Fuel_Man
  • HVACNUT
    HVACNUT Member Posts: 5,855
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    You're not supplying 180° the the mix.

    You see the circulator is pumping away from the mixing valve.

    Once the water reaches the mix setting, the supply from the boiler basically shuts down. Now it's only recirculating from the cold side to the mix side. When the mix temp drops, the hot side will open to bring the temp back up. Lather, rinse, repeat.

    Don't fight with those guys about hydronics. You'll lose.
    Bigly.
  • hot_rod
    hot_rod Member Posts: 22,244
    edited April 2020
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    Unless you are supplying 180 to the floor there is no way you have 169 return from the floor
    And if you need 180 to the floor, you sure don’t need a mix valve?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • wrooper
    wrooper Member Posts: 58
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    I understand there is a perfect system out there and with major investment /repiping etc I could have it. I realize of course that I am closer to the subject and to expect these posts to be a conversation is asking too much.

    I really just want to increase my DHW capacity without having to run my boiler hotter. Other than this issue, my system has worked very well for 12 years

    I will report my experience with the piping/tank mods I have proposed
    ZmanHVACNUTGroundUp
  • GroundUp
    GroundUp Member Posts: 1,924
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    It's literally the cost of a mixing valve and a few fittings to do it right, maybe an hour of labor. The return temp from your floor is maybe 90-100 degrees as it sits, which mixes with your 140 degree supply water to create the existing 125 degree supply temp you are feeding to the floor. There must be a bypass valve to throttle that side, which is allowing you to have 125 degree supply water as it is now. If you crank the boiler to 180, you simply need to add more return water and use less supply water to maintain that 125. That's how mixing valves work. Seeing as you know everything and refuse to listen to the professionals here though, I'm curious to see how this works out for you
    KC_JonesSuperTech
  • wrooper
    wrooper Member Posts: 58
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    shoot, I posted my results in the wrong forum.

    Too long to type again, it is a qualified success in that the piping change delayed the boiler fire/circulator but it more than doubled my useful temp hot water while still firing my boiler around 140-150F

    GroundUp I apologize if I sound argumentative but yours is not the first post that insists I can use 180F water, my 20 F Delta T and somehow get 125F water by mixing supply /return. I am too dumb to understand how that happens. I do have boiler bypass through my DHW heat exchanger.
  • Jamie Hall
    Jamie Hall Member Posts: 23,415
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    If you put it like that, @wrooper , you can't, of course. That, however, is not what we are suggesting. You have -- or should have; if you don't, the system is set up wrong anyway -- a radiant flooring loop in which circulation is powered by a pump. There is a mixing valve. The hot side of the mixing valve connects to your 180 supply. The cold side of the mixing valve connects to the radiant loop, but there is also a line from the radiant loop return which connects back to your 180 degree boiler water loop (which has its own pump to do whatever).

    Now. If the hot side of the mixing valve is closed completely the circulation will be entirely within the radiant loop, and the water coming back from the loop to the cold side of the mixing valve will be whatever the floor is. Suppose now you want to warm that floor to -- oh let's suppose the floor is at 80 and you want 100 degree water at the beginning of the loop. Just for picking numbers. The mixing valve will open a little and allow a small amount of 180 degree water to mix with the 80 degree water. A bit of algebra and you will get 1 part 180 water to 4 parts 80 water, or looking at flow, you will have let's say 1 gpm 180 water and 4 gpm 80 water. Now there will also be 1 gpm of 80 degree return water going into the boiler loop.

    Now in a really nicely setup system, the mixing valve temperature is controlled by your outdoor reset (perhaps with a floor temperature trim) and the flow rate is set by your radiant circuit pump which is operating in delta T mode.

    In the meantime the boiler circuit is doing its thing with whatever temperatures are set by the boiler controls.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    ZmanSuperTech
  • wrooper
    wrooper Member Posts: 58
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    UPDATE for future folks in the same situation

    I was worried about the long delay [18 minutes] before the boiler fired when a hot water use started would make for a slow recovery and potentially let days go by before the boiler fired while in the third? day my hot water supply would be low.

    Took me a day to realize I could have both tank thermostats on the same TT terminal in the control. As The EK rep suggested I set the tank that recieves cold supply at 125 F and left the "second" tank at 135F.

    Now my boiler fires/recovery starts at 2 1/2 Minutes
    Zman