Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Standby power

I have a question for you fellows.

  I am in the process of setting up a standby power supply to run my house in the event of electrical power failure here. It consists of a 12 volt powered inverter of 3000 watt capacity, capable of 6000 watts surge.  I have enough 12 volt capacity to power the inverter. No problem there. This is not a sine wave unit- it puts out modified sine wave- which some electrically operated items don't work well on. 

 I have a Munchkin 80M for heating and domestic hot water. The hot water is distributed thru my system via 4 circulator pumps. All small cartridge pumps. I can't tell you the specifics on them at this time.

 What I am up against is that when I connect the Munchkin to the inverter and attempt to power it up, it doesn't do its' normal routine of the combustion fan running for a few seconds and then shutting off- like it does with normal 115vac 60 cycle sine wave power.

  I think the problem is with the modified sine wave form this inverter puts out. I've been told that I could use a UPS- uninteruptable power supply- like they use with computers. They put out true sine wave 60 cycle power.

  What I need to determine is what current draw the Munchkin has. I have the owners/service manual and it doesn't give any information on current draw.

  Another thing worth noting is that none of the four circulating pumps would run when I connected them each directly to the inverter.


  So, I think I'll size the UPS to be able to carry the Munchkin and 4 circulators at the same time.

  I am also in the process of installing a Takagi TKjr tankless h.w.h. I may need to run it from the UPS also. I read that manual and it has no information about current draw. Right now I am not sure if it would operate on modified sine wave power or not, either.

  Can anyone help me here, or should I call a Munchkin or Takagi guru?


  • LarryC
    LarryC Member Posts: 331
    Current draw calculations.

    Typically current is related to Power by the formula:

    Power = Voltage x Current where Power is in Watts, Voltage is in Volts and Current is in Amps.  There are some other things if the load is not a perfect resistor.  But for ballparking this is close enough.


    The pumps will draw 5 - 6 times their normal running current when they first start.  That is why there are "surge" ratings on UPS's and generators.  ASSUMING the furnace is on its own circuit, a 3000 W UPS should be able to power up the entire heating system.  120 V x 15 A = 1800 VA (Watts). 


    UPS's are rated in  Watt Hours.  The UPS could put out 3000 Watts but for how long?  Those little UPS's usually only last for 30 minutes or less.  If you add batteries, the UPS may overheat because they were designed for short duty cycles.


    If the tankless water heater is electric, don't even waste your time trying to back it up.  30 A x 240 V = 7.2 KiloWatts.  Even at 100 % conversion efficiency, you are talking about 600 Amps at 12 Volts !
This discussion has been closed.