Reworking Radiant Heating Zones. Does this look right to you?

Discussion in 'HVAC Heating & Cooling' started by JamesNY, Sep 30, 2010.

  1. JamesNY

    JamesNY New Member

    Sep 30, 2010
    Long Island, NY
    Thanks for taking a look. Please see the PDF drawings, and tell me if I have overlooked something important.

    The existing design has two heating zones with excessively long 600' loops of pex. Not much heat is being transferred from the pex to the floor because it is hung between basement joists with those plastic Wirsbo pex-clips. The tubing sits in a 3" airspace covered with only R13 batting. I believe this style of radiant floor was meant as a warmer, and not intended to provide heating. The major flaw I see is the continuous loops which should be no more than 300' long for 1/2" pex. It is either exceeding the minimum return temp for the boiler, or else not putting enough heat into the zones.

    My proposed changes would replace those plastic clips with aluminum Joist-Trak, and split the long loops into three 200' loops on manifolds. The tubing would be covered with R19 instead, filling the joist cavities. To save cost, I'd like to use the Mr. Pex manifolds with flowmeters and dials for balancing the flow by hand. After running the heatloss calculations with Uponor's ADS software, I've determined that the loops will call for a max of 97 degrees. So, my plan is to use one of the existing circulators as a primary pump, and feed two secondaries from it, all with 1" copper. The temperatue of each secondary loop would be controlled by a hand-set thermostatic mixing valve.

    So, naturally, each thermostat will control its respective secondary loop circulator. But either one will also trigger the primary loop circulator and fire the boiler.

    Existing Design (PDF)
    Proposed Changes (PDF)

    Apart from any mistakes or oversights I may have made, my questions are:

    1) Is the location of primary loop circulator acceptable downstream from the secondary branches as shown?

    2) Could this type of setup lead to short cycling the boiler?

    3) To control the secondary loops as described, can I use a spare two-zone Taco control box? It would recieve the signals from the two thermostats, and switch on the secondary loop circulators. The relay output from that 2nd box (which is normally used to fire a boiler) would instead provide signals to the existing Taco box, controlling the primary pump and boiler. Seems like it would work, but am I missing something?

    I understand that without all the fancy electronic zone valves and motorized mixing valves that I might lose some efficency, especially from the lack of automatic seasonal reset.
  2. kwieg1

    kwieg1 New Member

    Nov 17, 2010
    sandy, Utah
    You are exactly right about what is causing your lack or heat. If you shorten your loops to 200 with 1/2 pex and supply that manifold with 3'4 pex you will have no problem heating that space to your desired temp.
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    The circulator position is fine relative to the primary/secondary loop structure, but wrong relative to the location of the expansion tank. Wtih the expansion tank on the boiler, it'll air-purge easier with the pump on the other side, pumping away.

    The cycle time of the boiler is dependent on the size of the heat load & thermal mass of the zone calling for heat relative to the BTU output of the boiler, it's mass, and it's high/low limit hystesis. Any time you break it up in to smaller sub-zones those cycle times will run shorter & shorter. But if you install either a Beckett Heat Manager or(better) Intellicon HW+ (typically under $200 for the hardware, and a reasonable DIY) it will manage the hysteresis & mass of the boiler to optimize burn times, and purge heat from the boiler as it senses the end of a zone call nearing so as not to abandon heat in the boiler at 200F or something. Look them up on line- the installation & operation manuals are downloadble from multiple places. Outdoor reset would be useless with the water temps you're talking, since the return water to the boiler needs to be 130F or above at all times to prevent condenstation damage to the boiler or flue. Your highest efficiency you'll get from it without damage will be from exersizing it's thermal mass with an economizer. Set the high-limit of the boiler to max, set the low limit of the economizer to 130-135F, and let IT decide based on it's temp feedback when to cut the flame and start purging heat from the boiler, before the call for heat is up. With a continous call for heat it'll run it from low-limit to high, until it' senses (from learned system behavior) that the system's load is nearly done, then cools the boiler down near the low limit, hopefully just before the thermostats are satisfied.

    With extruded plates in place your water temp requirements for delivering design-day heat will likely be over the 97F calculated, but still well under 140F (whereas suspended-tube it probably wasn't cutting it at 180F or 200F.) If it's truly 100F or less even on design-day this project SCREAMS for a condensing boiler.
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