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Thread: Understanding my tankless with recirc system

  1. #16
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    Quote Originally Posted by jadnashua View Post
    Yes, but do you make many 10's of gallons of tea every day?
    While the amount would matter its the temp that makes the biggest difference,while your heating more the container is much larger with the water heater. and temp much higher with the kettle.. Thats why circ systems limit the water to 140......circulating it even makes it worse. How much does a circ system circulate....

    ADD> 180 degree water is 4 x as corrosive as 140 degree water.
    Last edited by Hackney plumbing; 03-23-2012 at 05:35 PM.

  2. #17
    General Engineering Contractor ballvalve's Avatar
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    Corrosive is a product of water chemistry not temperature. Precipitation of minerals is another.

    At my elevation, I think boiling water is only about 200'F or less.

    In any case, there is no argument that the flow thru beasts get limed up - you can see the rigs designed to acid wash them, and read the MFG's instructions. With an electric rig, you can go decades between a bottom of tank vacuum job.

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    Quote Originally Posted by ballvalve View Post
    Corrosive is a product of water chemistry not temperature. Precipitation of minerals is another.

    At my elevation, I think boiling water is only about 200'F or less.

    In any case, there is no argument that the flow thru beasts get limed up - you can see the rigs designed to acid wash them, and read the MFG's instructions. With an electric rig, you can go decades between a bottom of tank vacuum job.
    Water of the same chemistry wil be 4 times corrosive at 180 compared to 140. The hotter the water gets the less dense it becomes allowing the minerals to precipitate.

  4. #19
    Retired Defense Industry Engineer jadnashua's Avatar
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    While the aggragate water temp doesn't get super hot, depending on the design, the flow, and how often it gets turned on, when the flow stops, there can be some very significant hot spots.

    All tankless heaters will eventually require demineralization. It depends on your water quality (hardness), temperature setting, use pattern, and total usage when that needs to happen. It might be once a year, it might be longer, or shorter time period, but it will be necessary, or you'll notice a gradual drop off in performance (or maybe not until it gets really bad!). The minerals act like an insulator. The water has a relatively short path through the heater, and to raise the temp, there's a lot of heat being applied. The minerals will precipitate out of solution to a degree.

    The hotter the water, the more reactive it is. That's one reason why there is a difference in the max flow rates for same sized pipes between hot and cold supplies - it's must remain slower for hot than cold at the max, or you'll eventually have problems.
    Jim DeBruycker
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  5. #20
    DIY Junior Member AaronHartwell's Avatar
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    Good debate on the degree to which temperature, flow, and mineral content all play a role in corrosion.

    I've been staring at my plumbing this weekend trying to figure out how it's set-up.

    If you look at the picture below you'll see that the cold water comes out of the wall and T's with one branch going to the inlet in the bottom of the water heater (bright flexible copper pipe) and the other branch going directly into the recirc pump.

    The water comes out of the recirc pump and goes into the wall. It doesn't appear to ever go through the heater!? What am I missing.


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    Aaron
    Last edited by AaronHartwell; 03-25-2012 at 11:02 AM.

  6. #21
    DIY Senior Member jastori's Avatar
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    Are you sure that the pump is not pumping to the right in the picture? Your "??? Water" pipe would typically be the return of the recirculating loop.

  7. #22
    Plumber jimbo's Avatar
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    Look for a direction of flow arrow on the pump.

  8. #23
    DIY Junior Member AaronHartwell's Avatar
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    Ewww… Good question. That would make more sense if the pump was pushing water to the right. It seems like that green valve (barely visible behind the yellow gas line in the photo) would need to be closed if that's the case.

    I'll check the pump tonight to see if it has any markings.

  9. #24
    DIY Junior Member AaronHartwell's Avatar
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    Ok, so there's an arrow on the back of the pump that would indicate the pump is indeed pumping left to right. So the ??? line is in fact the hot water return.

    Now I'm curious about the green valve. I wonder if that's cold water coming in with a one-way valve.

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    Looks to me your check valve is in the wrong location. It should be located just after the stop valve at the wall on the cold side. The direction of flow would be toward the heater.

  11. #26
    DIY Junior Member AaronHartwell's Avatar
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    Ok, so I just took the cover off the Takagi TK-3 water heater and pushed a button that's supposed to bring up any error codes. I threw a 3-2-1 code which maps to "Inlet Thermistor Fail" Assuming the inlet and outlet thermistors work together to control temperature and decide when to power the pump, having a bad one on the inlet seems like something that could cause the water to randomly go from hot to cold.

    I've searched on the web and can't find parts for this thing. Where would I go about getting a replacement thermistor?

    As for the check valves, I'll look at that Hackney. Given that things worked well for a long time and suddenly started failing I'm guessing something has gone bad vs. things are plumbed wrong but as I said in my original post, it wouldn't surprise me if things actually are plumbed incorrectly given the sub.

    Aaron

  12. #27
    DIY Senior Member jastori's Avatar
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    If I'm understanding correctly, the green valve is the supply of cold water to the heater. If you close that valve, you will have no hot water at all ! Keep in mind that the recirculating loop is important only when hot water is not being used - it keeps the water in the pipes hot for when needed. Once you start using hot water, the recirculating loop has very little effect on the system - then you need cold supply flowing through the heater to create the hot water.
    Last edited by jastori; 03-27-2012 at 06:39 AM.

  13. #28
    DIY Junior Member AaronHartwell's Avatar
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    Jastori - Thanks for the clarification. That makes sense and is consistent with what I decided last night after looking at the system a bit more.

    I'm pretty convinced the error code I mentioned above about a bad inlet thermistor is the culprit. Now I just need to figure out where one gets a thermistor for a Takagi TK-3. Anyone know?

  14. #29
    Retired Defense Industry Engineer jadnashua's Avatar
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    While a thermistor might fail, they're pretty robust SS devices...it might be the control board. Often, there's an easy test for a thermistor if you have a multimeter...the resistance at a specific temperature can easily be read. A pot with some water in it and maybe some ice, and you can check that it changes over temperature changes according to the spec sheet (which might be in the manual).

    As said earlier, some tankless systems/models void the warranty if you add recirculation, or don't do it per the instructions. Regardless, it does stress the thing more than without, at least in a typical home. Another reason why a commercial installation and residential tend to have different length warranties.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  15. #30
    General Engineering Contractor ballvalve's Avatar
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    Sell it all on ebaby and get a nice electric water heater without any circuit boards.

    And what Jad said: you get super hot spots on shut down and pauses in circ. - thats why they crud up so quick and you get a 30% efficient unit fast.

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