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Thread: Tankless water heater, recirculation pump?

  1. #1
    DIY Senior Member Homeownerinburb's Avatar
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    Default Tankless water heater, recirculation pump?

    Hey guys.

    My neighbor got talked into replacing his big tank water heater with a tankless. Now he is unhappy with the lack of hot water in his master bedroom shower, which is as far from the tank as any other hot water tap. Strangely, the bathroom that is adjacent to it has no issues.

    I was wondering if there is a recirculation pump that could be installed, that with the push of a switch, would push the cold water back to the tankless and draw hot until the temp rises enough.

    Here is the thing. His bathrooms are just lousy with tile, and there is no cutting of walls. The house is on a slab and the supply is in the attic.

    Is it a violation to put the pump in the attic above his shower? I know there is an electrical junction box up there with ample ampacity to drive a pump.

    If anyone could point me to a particular pump or three, I'd certainly appreciate it.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    First, some tankless systems do not allow recirculation (and if used, voids the warranty), so that is the first hurdle. If allowed, then you must follow the specific setup dicated in their manual. Retrofit recirculation systems can use the cold water line with a crossover to get the hot to the end of the line. This means that the cold line is partially, if not fully, full of at least warm water. Some people find this particularly annoying, but without a dedicated return line, is one of the things that just is.

    A recirc system will have at least three parts, the pump, a crossover (if not using a dedicated return line), and a check valve. If you do a forum search, you'll find lots of discussions on them.

    The pump on most goes in the utility room near the heater. One retrofit unit (that I have in my townhouse) is made by RedyTemp. Lang, Chilipepper, OnDemand, and others have units.
    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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    DIY Senior Member Homeownerinburb's Avatar
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    Well, we are not looking for a unit that will recirculate just because the hot water in the pipe is cold. We want the person about to take a shower to have to activate the pump. Otherwise it seems to me that pump is running frequently and the whole concept of the "heat it only when you need it" tankless is lost.

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    In the trades Dana's Avatar
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    Now he is unhappy with the lack of hot water in his master bedroom shower, which is as far from the tank as any other hot water tap. Strangely, the bathroom that is adjacent to it has no issues.


    It sounds like a symptom that would not be fixed with a recirculation pump. If the adjacent bathroom has no issues and is being fed by the same hot water distribution line, the problem is likely to lie with an interaction with the anti-scald valve in the shower mixer. A tankless water heater's heat exchanger has a substantial amount of "head" or backpressure at flow compared to a tank, and the anti-scald valves work off the difference in pressure between the cold & hot side of the feed. With a tankless there is now lower pressure on the hot side than with the previous hot water heater, and the anti-scald may be cutting back total flow to an undesirable or unstable level.

    If the tankless is set to 120F or higher, try backing it off to 110F, at which point there would be almost no flow on the cold side in the first place. This may be enough of a tweak to make it work, or maybe not. At 110F it's sufficiently hot to fill a cast-iron tub, and hotter than most people can actually stand in a shower (105F +/-3F is where most people like to shower.) Yes, this means slightly higher flow on the hot feed, for more of a pressure drop through the tankless, but since you've peeled off the cold side flow to near-zero, it often just works. If that doesn't cut it, try setting it to 105F so that you're doing 100% hot water at the shower, in which case.

    It's almost never necessary to crank up the output temp of a tankless to more than 110-115F in a residential application, yet there's a tendency for newbies to set them as high as the tank heater had been set. The reasons for setting tanks to the higher temp simply don't exist with a tankless- higher output temps don't increase capacity, and legionella etc isn't an issue since the entire water volume in a tankless is purged with every draw, and the water doesn't stagnate for days or even hours at high-growth temperatures. Running it at any higher temp than necessary makes the output less stable at low flow, delays the ignition cycles, puts somewhat higher thermal stress & increased the rate of lime deposition in the heat exchanger. As a rule they should be set to the lowest temperature deemed acceptable by the users.

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