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in floor radiant problem

SamF Member Posts: 2
have an open direct in-floor system.  4 zones each with own pump.   One zone takes forever to heat,  several hours, causing the pump to run & run.     Other zones take 15-20 min. before satisfying thermostat and shutting off.   suspect air in the system but isolated it and ran domestic hot water with no positive result.   other ideas out there?


  • Snowmelt
    Snowmelt Member Posts: 1,416
    Water temp

    What's the water temp, what's the length of the loop, what's the btu valve of that specific loop.

    What's the spacing on that zone.

    After all that, I would need to know what the water temp of the water going into that loop and water coming back, that shouldn't be no higher then a 12 degree difference. Seams like a lot but need them factors, ohhh last factor would be how much tubing, linear feet of tubing do you have, it could be as simple as not enough pump for that zone.

    Last question is, is there any insulation under the installed tubing?

  • SamF
    SamF Member Posts: 2
    water temp

    Not sure off hand of the length of tubing but have 3 loops in this zone, pretty sure it's the smallest of the four.  other zones have 4 or 5 loops with same size pump. 

    when it kicks on incoming water temps climb to 110-115,  water coming out of zone climbs slowly to 70-74.

    this zone is on the first floor with a basement underneath, very well insulated from below. 
  • Snowmelt
    Snowmelt Member Posts: 1,416

    That's one of the problems that I see right from the start you won't heat up anything with 80 degree water or the 40 degree delta tee. Do you think that zone maybe the coldest zone out of the 4. In other words does that zone need the most btu's.

    Do you know how to size a pump out ? Do you know how to get the right gpm for that pump. gpm = btu / D. T. x 500

    Then you have to get head loss

    More I am thinking of it the more I'm thinking you need to get the right size pump.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    One zone takes several hours to heat,

    I am a homeowner, not a heating professional. But it is fine with me if my radiant slab zone (5 loops, not one the same size as another) takes 12 to 18 hours to heat. And when it is quite cold outside, it does. It does not matter to me how long it takes because for that zone; my mod-con boiler just modulates down to the supply temperature needed. If I were attempting setbacks, that would not work, but it makes no sense to me to use setbacks on a radiant slab at grade heating zone anyway.

    I am not sure what you mean by an open direct system. Do you mean one that is not pressurized? And by direct, do you mean that there is no heat exchanger other than the one in the boiler?

    If your other zones are also radiant slab zones, I find it impossible to believe you can make significant temperature changes to a zone in 15 to 20 minutes unless you run extremely hot water in them, and if you did that, you could burn your feet walking barefoot, and you would get enormous overshoot after your thermostat was satisfied.

    In my system I can run 76F supply water into my slab and satisfy my thermostat provided it is over 50F outside. At 14F outside (design temperature), I need 114F supply temperature, and at 0F, I need 128F supply. With 76F supply, it looks like the delta-T is zero, but since it heats, it must be a little more than that, but it sure is less than 1F. With higher supply temperatures (when it is colder out), the delta-T is greater of course.
  • Snowmelt
    Snowmelt Member Posts: 1,416
    Delta t

    Your delta t shouldn't change you should always try to has a set point delta tee, rule of thumb is

    Ten degree for a radiant floor

    20 degree for baseboard

    25 degree for comercial baseboard like a dance hall

    30 for a snow melt application

    Open system I believe he is using his water heater to do domestic and space heat if you go to radiant floor.com they have samples of how to run on open system, it's really not recommended.

    Set backs on radiant will save even more money, taco have what they call the I-valve with set back built right in the I-valve for outdoor reset. So that will be set back on the t-stat, set back on the boiler, set back on the circulating pump, ( a delta tee pump) and set back on the three ways mixing valve. Before you say over kill, what your doing is doing a nice set back on boiler and pump is actually protecting the boiler with longer run time so it doesn't short cycle.