Author Topic: Mom says it wastes more propane to raise home temp by 15 F when I get home.  (Read 3413 times)

babysnowbyrd

  • Stubble
  • **
  • Posts: 222
  • Age: 33
    • My Journal
I keep my thermostat between 50-55 when I'm not home. I work a lot of OT so this is often 12+ hours. I turn it up to 68 when I'm there and doing things in the evening. I turn it to 60 when I sleep.

I talked to my mother about it and she said it wastes more propane to do it that way because it makes the heater "work twice as hard."

My mom says silly things sometimes I and I think she's wrong, but I thought I'd ask the mustache community. Would it use less propane for my heater to kick on a couple times an hour to keep my place at a certain (higher) temp, or let it get cold first, maintain that temp, and then work for a longer chunk of time to get the inside temp up higher once I'm back?

Exflyboy

  • Walrus Stache
  • *******
  • Posts: 6172
  • Age: 57
  • Location: Corvallis, Oregon
  • Expat Brit living in the New World..:)
Your Mother is incorrect.

You will use more propane the longer you keep the house at a higher temperature. This is because the rate at which you loose heat is dependant on the temperature difference between the inside and outside. Thus is you a 100 degree temperature difference (think 30 below outside and 70F inside) then the rate of heat flow thru the walls, roof, windows etc will be twice as much as a 50F temperature difference.

So if you you were loosing say 5 kW at 50F, then you would use 10kW at 100F.

A certain volume of propane will have a certain amount of units of energy (kwH).. The longer you run at a higher temperature the rate of high energy use will happen for a longer time.. thus more energy (and thus propane use).

Conversely if the temperature is held at a lower temperature difference the energy use will be much less.

The fact that you are changing from a low temp to a high temp is not relevant.. yes it will use energy to raise the temperature, but much less than if the internal temperature was held at a continuously higher temperature.

Frank (I'm a registered HVAC engineer in the State of Oregon BTW)

darkadams00

  • Bristles
  • ***
  • Posts: 381
I tend to follow two ideas in my finances: (1) simplify when possible and (2) conduct my own testing when possible.

(1) In your case, the simplest idea would be to set the thermostat at 60 when you're asleep/not at home and 68 when you're up and about at home (68 is the same temp I also use btw). I would suspect that the cost difference between 55 and 60 would be minimal for such short periods. Also, the time required to warm up the house 18 degrees from 50-68 after you're home following a 12-hour shift would be longer than most folks would want to wait (and not very comfortable in the meantime). My wife and I expect the house to be up to our comfort level in about 30 minutes or less after we get home for the evening, so that only allows us to tip the dial back a few degrees while we're gone.

(2) You could start today and run your current method during January and compare the degree-days with your bill. Then do it a different way for February. This comparison would be specific to your climate, your structure type/insulation levels, your physical heating comfort level, and your method of heating.

Within a couple winter billing cycles, you'll know an exact answer specific to your situation. FWIW, this would also work for an early season test during the summer air conditioning season.
 

babysnowbyrd

  • Stubble
  • **
  • Posts: 222
  • Age: 33
    • My Journal
Thanks Frank!

I think my mom just wants to feel important sometimes so she'll make up random bad advice. Glad to get your expertise.

darkadams00: I usually don't mind the wait until it gets warmer. I usually keep my coat on for a bit or I'll snuggle up in some blankets over one of the vents while I surf on the computer.

I could watch it, but I think there are too many variables for to track it accurately. For example, if I understood Frank right, the outside temperature has a lot to do with it. I'm in Wyoming and we are in the middle of a very cold front. High temps are in the teens. It hasn't been above freezing for two weeks now (though Tuesday is supposed to hit 36 supposedly.) If February is pretty mild with temps in the high 40s or so the heater won't have to work so hard, so that wouldn't really show me.

Plus I also use propane for my stove. I might cook more one month than another. Or be home earlier more, or go out at night more, etc etc. It was a great suggestion though, and if I was more scientific and detail-orietented I might track all of those!

Exflyboy

  • Walrus Stache
  • *******
  • Posts: 6172
  • Age: 57
  • Location: Corvallis, Oregon
  • Expat Brit living in the New World..:)
Exactly.. too many variables.

Darkadams my be correct in that the difference is likely to be small and may not be worth it to you, but you will have to control the variables carefully to test it.

Your right outside temperature will be have the biggest effect.

If you want to save money of course you could try dropping the temperature both when occupied and un-occupied and see anecdotally how that affects your bills.

Frank

GoodStash BadStache

  • 5 O'Clock Shadow
  • *
  • Posts: 27
Conceptually what your mother is arguing would be like saying that driving from point A to B to C to D, letting your car cool down at each stop would use less gas than driving from point A to D on the same route in a single trip.  Many systems don't instantly reach peak efficiency and will require some warm-up time before being their most efficient.  If your furnace is running at some frequency during the day to maintain 68 it will most likely not be running as efficiently compared with running for an extended period once you're home.

Depending on your house and local climate you may find that if you set your furnace low enough it may not even turn on while you're gone during the day.

babysnowbyrd

  • Stubble
  • **
  • Posts: 222
  • Age: 33
    • My Journal
Depending on your house and local climate you may find that if you set your furnace low enough it may not even turn on while you're gone during the day.

Lol. Earlier this week our "high" temp was -5 to 0 F. Tomorrow we're supposed to get up to 35 by mid-afternoon! Gonna have to break out my shorts if it gets even warmer!

marty998

  • Walrus Stache
  • *******
  • Posts: 6320
  • Location: Sydney, Oz

I think my mom just wants to feel important sometimes so she'll make up random bad advice. Glad to get your expertise.


This is where the phrase "old wives tale" originates from.

Bob W

  • Magnum Stache
  • ******
  • Posts: 2947
  • Age: 60
  • Location: Missouri
  • Live on minimum wage, earn on maximum
Mom is wrong.  Try wearing more clothes at home and setting at 60 awake and 50 asleep.   It is good for your health.