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Circulator Pump 1/2" Pex Slab w/ 5 loop Manifold

Have a 0015-MSF3-IFC hooked up to a manifold with 5 connections that is supplied with a 1" pex line that seems to reduce down to 3/4" and is run with 1/2" pex in concrete slab. There are 2 taco zone valves with one feeding the kitchen and the other 4 areas making up the living room, dining room and 2 hallways. The largest area I'm guesstimating is the dining area with about 210ft to 250ft tops 1/2" pex, the kitchen which has its own taco zone valve is about 160ft pex run. Is this pump able to produce adequate head? When the 4 line zone valve calls for heat the 4 1/2" pex supply manifold lines heat up well, but the returns stay the same basement room temperature, and the pump runs for hours. Is the pump no good or is there just not enough head produced by this pump? I can test the capacitor on the pump, have a tester, not sure if those fail and the pump underperforms or if its maybe debris in the pump. I activated only the kitchen thermostat since its on its own in the manifold with the taco zone valve and it ran for about 2hrs, the supply water was hot at around 110F-115F, the return was room temp still after all that time.


  • Jamie Hall
    Jamie Hall Member Posts: 21,112
    Need a bit more information here. Do I presume that the heat source -- the boiler? -- is running continuously all the time? And that the output temperature of the boiler is pretty much as desired? And that the return temperatures stay low?

    If so, the boiler is creating heat -- and that heat is going somewhere, so that means that circulation is occurring. In fact, you could use the good old hydronic formula to determine roughly what the flow rate is: BTUh = 500 times delta T times gpm.

    Now. The next question is just how big and heavy is this slab you are dealing with? I presume the starting temperature of the slab is more or less the basement temperature which you mention? If this is a heavy concrete slab, and the heat output of your boiler is relatively low, it may take many hours or even days to heat that slab up.

    So it is quite possible that nothing is amiss at all -- it may be that you are expecting things to happen far more rapidly than they are able to.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • CPDRobert
    CPDRobert Member Posts: 3
    The other 3 zones serving the rest of the house ground floor turn off in under 1hr when they call for heat, they're same setup with floor slab, their return lines comes back elevated in about 30min or less, not as hot as the supply of course. I don't know much, its new construction, wasn't around when they poured the concrete unfortunately, but it was poured on top of the subfloor. It boggles my mind though that the kitchen which is on it's own thermostat and zone control valve on that 5 line manifold loop doesn't even have hot return, I would expect that one to show some signs of heat on the return since its on its own. All other zones have elevated heat on their return lines in about 30min or maybe less. The other zones when calling for heat stay on for under 1hr, its the areas served by the 5 loop manifold which runs for hours.
    EBEBRATT-Ed Member Posts: 13,450

    Air bound or the pump is too small? If you knew the tubing size, length the head could be calculated and compared to the pump curve.

    If you don't know then install pressure gauges on the inlet and outlet of the pump and calculate the head off that.

    It would help if you knew the gpm the system was designed for.
  • psb75
    psb75 Member Posts: 701
    Here's another important variable: how well are the slabs insulated--IF they are insulated?
  • Robert_25
    Robert_25 Member Posts: 474
    edited November 2021
    Your circulator has 3 speeds. What setting are you using?

    Is there a mixing valve for the radiant zones?
  • CPDRobert
    CPDRobert Member Posts: 3
    Air bound pump a possibility, since the system pressure was around 7-8psi last heating season while the contractor was working, set it to 14psi when I noticed, after that the air scoop was working and system pressure dropped a little bit more, refilled it slightly, have it steady 14psi now. Pump runs smooth, but then at random makes that low gurgling very light clicking type noise. There's a 3/4" hose valve at the end of the manifold on the 1" pipe, guess I can push the air out of the pump if it is indeed air bound? Never had to purge a circulator pump.
    The longest run is 1/2" pex, guessing about 210-250ft
    High speed and yes there is a mixing valve.
    Basement exterior walls are insulated, it's a finished basement, subfloor in basement seems to be insulated somehow too, but there's no actual insulation running in the basement ceiling joists.
  • Robert_25
    Robert_25 Member Posts: 474
    Mixing valves usually have a very fine screen on the hot and cold ports.  If nothing else helps I would remove the valve and see if the screens are plugged.  
  • Dave H_2
    Dave H_2 Member Posts: 526
    I noticed something in the post and need clarification.

    You have radiant in a slab, when I hear slab, i think of a 4" concrete slab on grade or below grade. If that is the case with the area in question, then yes, it will take a long time to get the water warm on the return. The concrete slab is great at sucking those btu's out of the pex.

    The other area that you said is fine you mention the pour was done on top of the subfloor, typically that tells me that there is a 1" to 1-1/2" thick pour. It pulls out the heat well, but there is not as much mass to really cool down the loop.

    And as someone else mentioned, are the areas insulated below?

    The 0015 is more than enough circ for your application.

    Dave H.
    Dave H