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Community and Q&A

Odd thermostat on an electric water heater?

MICHAEL CHANDLER | Posted in General Questions on

I often use electric water heaters for solar storage and tempering tanks and use the thermocouple aquastats to control pumps and 120-volt three-way valves. Yesterday I was troubleshooting an install and discovered that the upper thermostat on a two-element tank was set to turn the element off when the tank cooled and on when it got hot.

Is this a pass-through to the lower thermostat that would turn cut its power when the upper thermostat was running? Or just a defective thermostat? Just curious if anyone has run into this before.

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  1. Riversong | | #1

    Maybe if you turn the tank upside down?


    thanks dude, I'll head over again tomorrow w/ a spare t-stat and a multi-meter.

  3. wjrobinson | | #3

    Michael, upper tstat is on till upper temp is reached, then it sends power to lower tstat which is on till it reaches temp. They are set up that way so only one element is powered at a time and so that the top of the tank recovers faster for next use.

    Hope that helps

  4. David Meiland | | #4

    I was going to mention that, but I gotta think Michael knows...?


    Thanks Robert
    Great illustration I see where I have a wire in the wrong place and will fix it straight away. Always something.
    Homeowner complained that her system was making an odd "bumping sound" the pump was hooked to the lead going to the lower t-stat instead of to the upper element, so it never turned off. the Quietside DHW was doing fine holding the tank at 135 degrees and all was good except that it was running at the bottom of its BTU range and making a funny sound trying to light itself with the gas valve choked down all the way.

  6. Riversong | | #6

    Looks like quite the cob job.


    I seem to specialize in ugly systems. This is a sixty tube apricus array feeding the big tank that runs to the little tempering tank with DHW backup on a 120 v 3-way valve and one zone of radiant off the second flat plate heat exchanger. Works pretty well actually but yes its ugly.

  8. homedesign | | #8

    Looks like quite the cob job.

    Robert ....I will be the "Nice" policeman if no one else wants to bother

    And in case you did not notice the subtle hint.....J Chesnut's name only has one "t"

  9. Riversong | | #9


    We don't need any police here, Michael agrees with me and didn't take offense, and I don't know what "subtle hint" you're taking about.

  10. wjrobinson | | #10

    Michael, my first thought other than "a tad untidy" was you could have posted that pic on the "Is PEX OK?" thread.... seems like I see you favor all types of piping. Do I see PEX, colored and uncolored, PVC, and copper? Any iron or galvanized out of the frame?

  11. Riversong | | #11

    The upper T-stat turns the upper element on and cuts power to the lower T-stat until the upper temperature is satisfied, then the upper T-stat turns off the upper element and sends power to the lower T-stat.

    There's a good schematic and explanation here:

  12. MICHAEL CHANDLER | | #12

    Robert it did bug me a bit that you called it a "cob job", I work to minimize elbows to cut down on turbulence and I use PEX were appropriate for the temperature of the water in the pipes so we use a lot of 3/4" soft copper and 3/4" PEX.

    Much of the messiness you are seeing in there is due to the fact that I haven't tied down the molded-end cords we use for wiring up pumps etc, but using molded cords allows me to swap out pumps and power sections of the system up individually and use a stranded cord insert to allow use of an ammeter to check current draw. there is a method to this madness. The entire system sits in a large PVC shower pan with a 2" drain on a piece of 1/4" hardi backer so all condensate and pop-off drains can be accommodated without individual runs to daylight.

    It may look cobbed together but we do it this way for a reason. and i do agree it would be nicer to get in there and tie down all those cords but the piping would not be any better if it were all set out in polished hard copper with twice as many fittings and anchored to a big chunk of plywood on the wall.

    Actually that project was a bit roomy for what we usually do as we don't generally get a crawlspace to work in.

    This picture below is a more typical install for us, layered up 3-D. This photo has a re-fill hose and a trouble light cluttering up the view and the 3-way valve is temporarily disconnected from the solar thermal tank for testing but you get the idea of the small spaces we typically work in.

    Here we fit a zoned bypass HVAC, DHW, tempering tank, radiant floor system and four-panel drain back system with a 119 gallon solar tank in a 2'6" x 10'2" insulated closet.

    And yes AJ, we have colored PEX, hard and soft copper, threaded Brass for tank and pump mounts, cast iron and stainless for gas lines, PVC for condensate, DHW exhaust and pop-off drains, and even glass pipe for the sight tube on the drain back tank (mounted above the solar tank) and that corrugated plastic stuff for the DHW condensate drain to the 2" PVC in the shower pan and maybe even a little steel conduit for electrical feeds. Each has it's purpose.

  13. Riversong | | #13


    I thought I was being polite by using the term "cob job". I don't think I've ever seen anything like what's in these pictures that was done by a professional plumbing/heating contractor, only homeowner installations which are cobbed together.

    Granted, these are slightly more complicated installations than what happens in my homes, but the attached composite image is what I expect to see from a professional installer (and this is well pressure tank, radon vent, gas supply, boiler, indirect hot water, radiant distribution and domestic supply in one closet).

  14. MICHAEL CHANDLER | | #14

    That's very pretty work Robert and worth being proud of.

    I just don't see that pinning everything back to the walls adds value, and using so many fittings in hard pipe, opening it all up like that, while it makes it easier to service, takes more room than I really thing it needs and adds to the pipe turbulence.

    I grew up in New England and my father, grandfather, and great grandfather all worked with pipe in various capacities. I know the standard way of doing these arrays in on a piece of plywood as you have here with appropriate pipe insulation and all, but I don't think the "home owner style cob job" is necessarily any less effective than the pretty "professional" style. Yes I agree it looks ugly, but it does the job without eating up a lot of space.

    By the way that t-stat i originally asked about wouldn't kick the tank on even when turned up to 150, it was defective, not that that really matters, but that is what got me confused when I went to check why it wasn't working and could only get it to turn the lower element off, not the upper element on.

  15. Riversong | | #15


    Maybe you know something I don't, but I can't see how putting the weight and vibration of the circulators on the pipe, fittings and fixtures makes for a durable installation.

    And, while I've always believed that form follows function, that shouldn't be an excuse to create ugly things.

  16. MICHAEL CHANDLER | | #16

    Fair enough, I'll leave it at that.

  17. Kevin Dickson, MSME | | #17

    There are pros and cons to both schools of piping thought, and this is a good discussion.
    Random comments FWIW:

    1. Robert's method shown has made it impossible to economically insulate the piping (close elbows and tees, pipe straps and supports that will interfere with insulation, etc.)
    2. Some of Michael's swooping PEX runs would be easy to insulate and not bad looking (but you have to slip on the armaflex before crimping the fitting)
    3. Mounting a pump on a threaded fitting near a tank tapping is pretty robust.
    4. Line cords definitely are convenient and make troubleshooting easier. Suggestion: use really long colored cords, and tie them all together with the control wires with spiral wire wrap.
    5. PEX pipe and fittings are about 5 times quicker and cheaper, more reliable, and PEX lets you eliminate many elbows. This is compelling, but oh so hard to make it look good.
    6. Fewer elbows and smoother pipe means pump energy savings.
    7. Good plumbers have always preferred right angles everywhere, but this adds pipe and labor. Color-coded and brightly labeled pipe insulation could possibly restore the ease of troubleshooting that Robert's system has.

    Michael, why do you have any electricity heating hot water in a room with a natural gas water heater?

  18. MICHAEL CHANDLER | | #18


    Thanks for these comments. I agree that threaded fittings on brass 3/4" and 1" tank taps and heat exchanger stubs are sufficiently robust as mounts for the little Wilo circulators we use and i know that I really need to bundle and tie down my cords more elegantly.

    I don't have any electricity heating water here. i use the thermostats in the tanks to send 110 power to pumps and three way valves. The unit that started this thread was re-wired to send juice to a pump.

    the t-stat in the top of the small tempering tank energizes two outlets one of which leads to a pump that runs out of the bottom of that tank to a three-way valve. the other leads to the t-stat in the top of the 119 gallon solar tank and then to the cord feeding that three way valve.

    if the 119 gallon solar tank is hot the line leading to it from the three-way valve gets no electricity and the water flows through the solar tank and into the tempering tank until that t-stat is satisfied.

    If the solar tank is cold the juice flows through to the valve and diverts water through the demand water heater until the tempering tank is satisfied, any use of domestic hot water draws cold water into the bottom of the solar tank advancing solar heated water into the tempering tank.

    The roof arrays are generally hugely over-sized by propylene glycol DHW standards so we have to use drain-back systems to account for summer over-capacity. Were focusing on spring-fall gains as deep into sunny cold winter days as possible so we've worked with our local solar guru, Dan Gretch from Solar Hot in Apex NC, to work out a cool patch on the Steca solar controllers to get the best performance out of drain-back systems here in NC. We adjust the differential-on to 12 degrees and off to 5 so the pumps kick in as soon as the panel is 12 degrees hotter than the bottom of the solar tank (and off at 5) we can really draw out every bit of sun. To get the most out of our tank capacity we adjust the tank high limit to 170 and set hysteresis at 30 to hold the overheated system from kicking back on until the tank has dropped from 170 down to 140. We run the rooftop over-temp down to 212-off / 200-on to reduce flashing steam. (it comes factory set at 266!)

    It's a lot of fun though, aint it?

  19. homedesign | | #19

    Polite alert

  20. Riversong | | #20

    Hah! If you can't even spell "polite", how can you police it?

  21. GBA Editor
    Martin Holladay | | #21

    It's Dutch.

  22. Riversong | | #22

    It's Greek to me.

  23. Kevin Dickson | | #23

    Of course. Those thermostats make good but cheap aquastats. You guys are adding a touch of solar space heating at a very low cost. A high collector angle is key here.

    Sorry, is the place to be critiquing system design anyway. But they are having an ugly system contest:

  24. MICHAEL CHANDLER | | #24

    I've been aware of that contest for a while but I suspect that it's not quite in the spirit of the event to enter my own work. (insert grinning emoticon here)

  25. jartellabib | | #25


  26. wz6DMEmbAP | | #26


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