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Energy consumption for augmenting heat

colinsmith | Posted in Energy Efficiency and Durability on

I have a 75 square foot Mudroom/Laundry room off my garage. It is cold. The space does not have a heat source and relies on air movement from adjoining space for heating. It is connected via a regular 30 inch door. In winter (zone 5) I use  a table fan to blow air out of cool space into the warm space.  It helps,  but it is still a little cool.

My question is around energy consumption for a small electric heater to augment the current situation  

Would the energy consumed be equal to the amount of btu required for the temperature differential? I.e. I could calculate BTU required to heat 75 sq feet from 60 to 70 (10F)? Online calculators indicate roughly 1500 btu, 450 watts. 

Is the calculation that simple, or does the in-room heat source somehow reduce the heat gained from the adjoining room such that the electric heater is providing more than the 10 degree differential?

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  1. Expert Member
    BILL WICHERS | | #1

    Putting in heat is the same as putting in energy, thermal energy in this case. You have to put in enough thermal energy to overcome whatever thermal losses there are through the structure. If you put in the same energy that is being lost, you maintain a constant temperature. If you put in more energy than is being lost, you raise the temperature in the room.

    In your specific case, where you are already maintaining a temperature near your target, you’re probably correct (or at least pretty close) with your assumption as to how much energy you’ll need to put into your space to raise the temperature to what you want. Since space heaters typically have thermostats, if your heater puts out more energy than is needed the thermostat will just turn it off and cycle it to maintain the temperature setpoint. In this way the heater is putting in less total energy over time by way of cycling on and off, so that it can adjust the total energy input to the space to maintain a specific temperature.


  2. Expert Member

    As Bill said, the heater just has t0 supply the additional energy necessary to raise the heat for where it is to where you want it.

    What can change is how much of the heat in that room is supplied by the portable heater. As Jon has several times pointed out, If it is set at the temperature you desire, over time that heater will end up supplying almost 100% of the heat. That of course offsets the heat you were using from the rest of the house, but if your main heat source was more efficient, the energy for the mudroom will be paid for at the rate the portable heater costs.

    1. colinsmith | | #4

      “ over time that heater will end up supplying almost 100% of the heat.“

      Thanks. That is exactly what I was wondering about.

  3. Jon_R | | #3

    > If it is set at the temperature you desire
    I prefer to say "if set at the same temperature as the rest of the house then 100%...".

    > energy consumption for a small electric heater
    Nothing is as accurate as trying it (with a thermostatically controlled heater and a Kill-A-Watt meter). Set it to 5F below the rest of the house and it may not be too expensive to run.

    If you work at air sealing and insulating, I'm sure you could get a windowless room that size down to a heating load that would be heated to within 5F with just an open door.

    1. colinsmith | | #5

      Thanks for the reply.

      Some insulation upgrades is part of the plan. It will be very easy to add insulation on the garage side of the shared wall, weatherstripping on the door, and a proper closing vent on the dryer exhaust.

  4. tommay | | #6

    Leave the door open to the heated space, insulate outside walls. Dryer vent electric....or ? vent inside....

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