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Thread: Potable hot water recirculation pump.

  1. #1
    "Retired" HVACR contractor - currently university professor KI7OM's Avatar
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    Default Potable hot water recirculation pump.

    Though this is not a solar or really a geothermal application I thought I might get a better quality of answer on that forum than on the water heater forum, but I'll give this a shot.

    I live in the Salt Lake City area and am a retired HVACR contractor – now a university professor :-) . About 30 years ago I installed an ECU “Energy Conservation Unit” (actually a superheat recovery system) on my split system AC. Is use my 50 gal NG fired tank water heater as the heat sink and have enjoyed both reduced compressor head pressure (lower power bills) and some level of free hot water during the AC season for a number of years now. About three years ago I installed a 200,000 BTU direct vent Takagi tankless water heater in back (on the use side) of the tank and turned the tank temp down to about 55 to 65 degrees - but even in mid-winter it rarely fires. This means that the tank is serving both as a tempering tank and as a heat sink for the ECU in the AC season. During the summer the tankless rarely fires when hot water is demanded since the ECU is preheating the tank water to 130 to 140 degrees. I also get a good flow during the winter months when the water supplied to the house is between 40 and 45 degrees. The mechanical room is inside the house and once I upgrade the two furnaces in the mechanical room to high efficiency direct vent units I will disconnect the gas supply and B-Vent from the storage tank and the open louvered outside air supply to the room will be closed.

    I would like to finish a hot water recirculation system I started two years ago to reduce the wasted water while waiting for the hot water to reach the point of use. I have two return loops of 3/8" insulated PEX roughed in. The loops are both about 35’ in length. These return loops would ultimately feed either into the “old” tank water heater fill pipe or into the bottom drain of the tank. Of course the system will have the appropriate expansion tank and checkvalves. I am not interested in cold water return loop systems.

    All of this now said what pump manufacturer and model would anyone recommend?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The pump needs to be either bronze or SS. Since it won't have much head, and you don't need a lot of flow, most any small one should work. The return line doesn't need an expansion tank, but does require checkvalves. You may want to put individual valves on each branch so you can balance the flow.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    "Retired" HVACR contractor - currently university professor KI7OM's Avatar
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    Thanks Jim. This seems to be the direction I was headed but I needed a sanity check. Good idea on the individual loops valves. Right now I have ball valves on the far end of each loop but they are for isolation and not balancing. Would you suggest gate valves at the pump end of the loops for balancing?

    Actually the expansion tank would be installed between the feedline PRV (inside the house/foundation) and the old water heater tank input - not in the loop(s). I'm looking at a 2-1/2 gal which should be more than adequate. Some localities have it in their code - ours does not - but it is still a good idea. Hmmm? I just did some checking - expansion tank is not required if the PRV has an integral bypass and checking the specs mine has one so I'll pass on the expansion tank. See, sanity checks are good. :P

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A valve anywhere in the loop should be okay. The ball valves aren't the best at flow modulation, but will work.

    Without the expansion tank after the prv, each time the WH runs, the pressure will rise to just above the input value. The bypass only works when the internal pressure meets or exceeds the external pressure - it's not magic, it can't force lower pressure water into a high pressure line. So, if it is 100#, the internal will rise to 100# while the WH is running before it can then push back into the main. Install an expansion tank if you want the pressure to remain constant and not stress things.

    You only probably need a couple of gpm flow out of the pump, so a small one, designed for potable water is all you need. It has to have enough head to overcome the height gain and open the check valves, so head won't be very high either...in a loop, the falling water mostly conteracts the rising water, so it is almost a wash except for relative resistance. You don't want a high flow or you'll end up with turbulance and erosion of the pipes.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Engineer Furd's Avatar
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    Definitely do NOT use a gate valve for throttling service. You can use a high quality globe valve or (better) a stainless steel needle valve.

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    "Retired" HVACR contractor - currently university professor KI7OM's Avatar
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    I will consider globe values. Stainless steel needle valves would be nice but pricey ($50 to $125 each) and probably overkill. If this were an industrial process requiring more precise balancing I would go for it. I expect to see close to the same resistance on both loops. I may just simply install two 3/8" brass ball valves at the pump end and use them for isolation and let them double for any minor balancing that may be required. The higher resistance loop will normally be wide open with perhaps some throttling on the lower resistance loop. If I remember my physics it should not make any difference whether one is lower or higher in elevation except when water is being drawn from any of the faucets in the respective loop. The challenge in using a ball valve is once that throttled intermediate position is found then keeping it at that setting. http://machinedesign.com/BDE/FLUID/bdefp7/bdefp7_5.html

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    Engineer Furd's Avatar
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    Tightening the packing nut excessively (way past the point needed to seal the stem) on either a globe or ball valve will usually keep it in one position. Teflon string packing will hold tighter than graphited yarn for this service. Be sure to loosen the packing nut if you ever need to change the valve position.

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