rewiring composting toilet

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bobdog
bobdog Solar Expert Posts: 192 ✭✭
in General Solar Power Topics #1
I am now the owner of a Craig's list, brand new, Sun-Mar AC composting toilet. For better or worse it was too cheap to pass up. So I will make it work and need some advice.

Does anyone know about the thermostat and wiring of the Excel AC unit? I got it home last night and ran some tests on AC draw for the heater and fan (again both AC). With both going, it pulled 300 watts to start and dropped down to ~260watts within 15-20 minutes. At this point the heater reached ~110 F and turned off. The fan stayed on and drew ~25 watts. Eventually, the thermostat kicked in at around 90 F and the heater turned back on. The draw shot back up to ~270 watts.

Then I hooked up a dimmer switch which was advised by a repondant to Mike's Mendocino survey. I "dimmed" it down to 200 watts starting and after ~20 minutes it shut off. No big deal i thought, except that the fan stopped too and it started humming. Thought I had blown the fan.

Well I started to disassemble the fan box and got side tracked. Came back in ~15 minutes, plugged it in and the whole thing turned on including the fan.

So, my thinking is that the switch inside the heater (thermostat) opened and that caused the fan to turn off for some reason at the lower AC voltage. Why? Any ideas? Do AC fans not like low voltage?

Also, has anyone else put a dimmer on AC appliances that worked at reducing the draw w/o having a similar issue? Could it be my dimmer?

Should I rewire and put a dimmable/low valtage DC fan in and use the dimmer on the heater only?

Thanks for the help.

Tim
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: rewiring composting toilet

    The typical home lighting dimmer control works by just turning off the power during part of each AC cycle. This makes it hard for induction motors to function correctly (they can overheat).

    Placing a dimmer on the heater will not do much in terms of saving energy---assuming you want it to run between 90 and 110 F -- there is still the same amount of energy going it... Whether the heater is on for 15 minutes and off 1 hour--or turned down and just on for 1 hour at a time.

    The dimmer will reduce peak power -- but will not help the inverter as the peak current (assuming 50% power setting) will still be the same. And the cut-up AC wave form could cause issues for the inverter too (creating a buzzing sound--if nothing else).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
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    Re: rewiring composting toilet

    You're correct that AC motors don't like reduced Voltage. Especially on start-up.

    Why not wire the fan separately so it gets full power and just run the dimmer to the heating element? If you want to change the fan speed, a DC fan would work better.

    I'm not too keen on running heating elements through dimmers either. They tend to be rated for 600 W, and 500 W is close to the limit. Maybe a step-down transformer to cut the voltage in half?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
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    Re: rewiring composting toilet

    And remember that cutting the voltage by 1/2 to the heater will cut its power to 1/4...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    #5
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    Re: rewiring composting toilet

    Our SunMar Excel is now a garden ornament. Little flies were driving me crazy.

    It was 16 years old when i de-commisioned it. After the first month I disconnected the heater (too cheap to use that much electricity), and just ran on the fan. I don't think the variable speed switch even affected the heater element.

    5 years ago went off grid and replaced the squirrell cage fan with a muffin fan at 12 watts load. A little cutting, a little epoxy and venting out the same exit...at 3inches. Worked ok, until the flies decided to set up house in the mix. They will be around when the planet is visited in 2 billion years by aliens that are still ooze in some faraway galaxy. Persistent little buggers.

    Replaced with a macerating toilet.

    Ralph
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #6
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    Re: rewiring composting toilet

    At my mom's house, which had I guess about 2000sq' of stables and about triple that of corrals, we used both wasps and nematodes for fly control - they work great:

    http://www.biconet.com/flies.html
    http://www.kunafin.com/Composting_News.php

    I'm not recommending these for composting toilets, because I know nothing about their use in that application, but who knows - they might work fine.
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  • bobdog
    bobdog Solar Expert Posts: 192 ✭✭
    #7
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    Re: rewiring composting toilet
    BB. wrote: »
    And remember that cutting the voltage by 1/2 to the heater will cut its power to 1/4...

    -Bill

    I know I should know this, but could you help me out Bill? Why to (or is it by) 1/4?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #8
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    Re: rewiring composting toilet

    It is the Math behind the V,I,R...

    V=I*R
    P=I*V=V^2/R=I^2/R

    So--if Resistance is the same between two voltages--then the difference between power will be:

    Pdiff=(V1^2 -V2^2)/R

    Example--10 ohm heater at 120 V...

    I=V/R=12 amps

    P=V^2/R=120vac^2/10 Ohms = I^2 * R = 12a^2 * 10 ohms=1,440 watts

    If V=60 V

    I=V/R=60/10= 6 amps

    P=V^2/R=60v^2/10 Ohms = I^2 * R = 6a^2 * 10 ohms=360 watt

    Of course, if the resistance changes (resistance of materials can change with temperature)--then the formula is not exact (R many times will drop as temperature drops--so cooler elements have lower resistance).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bobdog
    bobdog Solar Expert Posts: 192 ✭✭
    #9
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    Re: rewiring composting toilet

    Thanks Bill. I'll print that one off for the records.;)
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #10
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    Re: rewiring composting toilet

    You are very welcome Bob. :D

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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