temp of hot water in Base Board HW

Discussion in 'HVAC Heating & Cooling' started by JOE1934, Jun 9, 2007.

  1. JOE1934

    JOE1934 New Member

    Messages:
    11
    I'm installing BBHW (slant-fin 30).I will have to connect low temp radiate floor heat to system to a new boiler.Could I have temp set at 150 degrees,yes I know I would have to have more feet of slant-fin. I was wondering if I use 150 instead of 180 the return should be around 130 or so.This temp would then be low enough to pipe to the low temp radiate heat.I am also thinking of using a lot of PEX and some copper.Any problem with this way of thinking.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    New England
    Get a mixing valve, and set the boiler for the baseboard stuff, and let the mixing valve temper the water for the in-floor radiant to the desired temp. The problem with your idea is that on a mild day, the return temp would be much warmer than a really frigid day, and may not be hot enough for the in-floor stuff to do much or too hot and you ruin your floor. By supplying them both, and adjusting, you can have one on without the other, too.

    You can go really hi or low tech with mixing valves from computer controlled motorized valves to simple bi-metalic spring controlled mechanical tempering valves.
  3. JOE1934

    JOE1934 New Member

    Messages:
    11
    Yes I agree with a mixing valve,you make a very good point.My problem is that if I put BBHW on a wall I have to fill the whole wall (length-boss says so,won't look right-HA HA). The cost of dummy slant-fin and heat ones isn't that much different.So the question is will the S/F work if I bring the temp down to say 150 degrees.I would still use a mixing valve like you suggest.Plus the PEX would be well with in the 200 maximum temp.
  4. hj

    hj Moderator & Master Plumber Staff Member

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    heat

    If you use the mixing valve with two zones, then it doesn't make any difference what the boiler temperature is. So if 150 does not heat properly, raise it to 160 or whatever does work. But if you send that temperature water through the radiant system the floor will get super hot and any resilient tile or laminate flooring will get soft and develop permanent indentations where you walk or place furniture.
  5. Bob NH

    Bob NH In the Trades

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    Location:
    New Hampshire
    Mixing Valve?

    What would you mix with the water to make it cooler? You would need cooler source of water and you can't be adding water willy-nilly to your boiler system.

    The only way you will be able to keep the temperature down is to have a separate radiant circulation loop that circulates water in that loop WITHOUT circulating back to the boiler. Then you need a separate temperature control that allows a FRACTION of that circulation to go back to the boiler and admits an equal amount of hot water from the boiler to keep the circulating radiant loop at the correct temperature.

    Let's say you want the maximum temperature at the inlet of the radiant loop to be 140 F, the return is 120 F, and the boiler temp is 180 F. For each 3 GPM of flow in the loop, you must return 1 GPM of 120 F water to the boiler and add 1 GPM of 180 F water from the boiler to the loop. The 2 GPM remaining in the loop will be raised 20 F from 120 to 140 and the 1 GPM coming into the loop will be reduced from 180 to 140.

    You will not actually be worrying about boiler temperature or return temperature or GPM because your temperature control valve will control the temperature at the beginning of the loop to whatever you want it to be. The set point is the temperature at the beginning of the loop.

    You might be able to do it with a temperature regulating valve if the circulating pump pressures are correct. It will need a bit of engineering to be sure it works the first time.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    22,319
    Location:
    New England
    That's exactly how the mixing valves designed for radiant heating work...you connect the return from the radiant floor loop to one port of the mixing valve, the boiler is on another, and the output goes to the beginning of the loop. they mix the return water from the loop to the boiler inlet to the loop, adjusting the mix to produce the set temperature.

    You can get fancier with a motorized, computer controlled mixing valve and obtain a variable output depending on conditions (say by monitoring the return along with the outside temperature), but that may not be the most cost effective for a home.
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