Last month we installed a Lochinvar NKC199N, using LP Gas. Attached is a picture here for the reference. We have 4 zones, but let's forget about the other 3 zones as I am having issue with one zone that we use to heat the radiant concrete slab.

The radiant concrete slab

We set the outlet water temperature from boiler to 120F, it is sent into the slab and comes out at around ~100F which is perfect, but when the return water get back into the boiler it raises up to ~116F as seen in the LCD display of Lochinvar.

I don't understand why 116F water returns to the boiler? So we only get 4F delta instead of 20F delta from the actual radiant loop. And when I touch the boiler inlet, it is as hot as the boiler outlet. So it makes me think the boiler inlet is actually pulling hot water from boiler outlet because of they are connected through that manifold.

So my questions are:

  • Why we need that mixing manifold?
  • Why does boiler inlet pull hot water from outlet? Do I need a strong pump at the supply side?

Note: since I've mentioned early that I have 4 zones, when all the zones running at the same time, I did see the return water temperature closely matches with boiler inlet, made me think the more pumps running actually force the water back into boiler inlet with the expect temperature. Is my radiant floor circulation pump too weak to push water back? (They're all brand new pumps)

enter image description here

  • IMHO, the person who plumbed your setup made a major mistake by mixing the return water with the outlet water. However, I'm not a plumbing/boiler/HVAC expert, so there may be a valid reason for doing so. I'm looking forward to being educated if there is a good reason.
    – FreeMan
    Commented Feb 8, 2022 at 16:43
  • 1
    There are valid reasons to want return water to be be above a minimum temperature, having to do with the oddities of building boilers from steel or cast iron rather than materials less subject to corrosion. However, the typical means for that is a thermostatic mixing valve (so if 140F water leaves the boiler, and 120F return is minimum, and the return loop water is 100F, a part of the 140F water is mixed with the 100F return water to raise the return temperature. You still have a 20F delta in your floor loop. I do find the whopping 2 hunks of pipe insulation a bit underwhelming.
    – Ecnerwal
    Commented Feb 8, 2022 at 17:05
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    Likewise, there are valid reasons not to want the water in a floor loop to be too hot - again, usually addressed by mixing valves. But I don't claim to understand the logic by which this particular arrangement is set up as I can't spot the "usual parts" I'd expect to see for those purposes.
    – Ecnerwal
    Commented Feb 8, 2022 at 17:21
  • Either a licensed home inspector or a decent HVAC rep should be able to "debug" the flow logic for you. Commented Feb 9, 2022 at 18:18
  • So I've reached out to Lochinvar, and I've learned couple things here. That mixing manifold is "hydraulic separation": The ability of two or more circulators within the same piping system to simultaneously operate without interfering with each other. They explained to me that the reason my inlet pulls hot water from the outlet because my pump circulation pump (Taco 0007-e) is not too strong, they recommend to upgrade the high velocity pumps ( Grundfos UPS 26-150F).
    – ysok
    Commented Feb 16, 2022 at 3:48


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