Last winter I purchased a programmable thermostat. It turns down the heat during the day when no one’s at home, turns it back up just before we arrive, turns it back down after we’ve been asleep, turns it back up before we wake up. (never did set the A/C program for summer, though)
As a gadget man - way cool.
But how can I tell how much money I’m saving? Sure, I can look at my gas bill, but it’s difficult to compare because this year’s winter is much warmer than last year’s. Short of doing a 20 year study, 10 years using it and 10 years not, I don’t think the answer lies there.
Is there a way to derive a formula that determines, say, for every degree below 20°C I save 1¢/hour? What would be the parameters of this function… overall outside temperature? R-factor of my insulation? Average airspeed velocity of a laden swallow?
And do I have it set optimally? Whenever we’re in the house I have it set to 19°C and find we rarely have to override. During the day it drops to 17°C. At night it goes down to 15°C (we’re toasty under the blankets, but oh! those 3am dashes to the john). Does it take more energy to raise the house from 15° to 19° than it would have saved if it was set to 16° at night instead?
(now where did I put the manual to my programmable coffee maker…)
One way of asssessing its effectiveness is to compare your furnace’s “therm” consumption. This is measured in standard units, irrespective of price volatility.
One thing I will mention: the automatic thermostats I have installed seems too sensitive to temperature fluctuations. Sometimes the furnace or A/C will kick on for only 60 seconds. I don’t know if that effect efficiency, but I do wonder if it puts needless wear on the start-up components.
DISCLAIMER: I started working on this response prior to any other responses appearing. I haven’t checked the now-linked threads so this may already appear. I post this as I am actually sort of proud of the thought that I put into it…and the term I created (whoops, apparently I co-opted it from elementary particle physics).
Here are some of the variables in the equation to answer your question.
[ul]
[li]Average Temp outside[/li][li]Time your furnace is on[/ul][/li]I think all the other possible variables could safely be ignored since they are common to your home where you are trying to figure out the value.
[ul][li]Insulation (R-value), [/li][li]size of house (area, or maybe volume), [/li][li]number of rooms, [/li][li]efficiency of furnace, [/li][li]cost of fuel, [/li][li]mode of heat distribution (forced air gas/oil, radiant…)[/ul][/li]I tried asking this question last year, and the general response was that it depends on how long your furnace is on (since that is when the fuel is being used). The latter variables I listed will dictate what is needed for your house to warm up by 1 degree.
To find out the relative cost of each, you’ll have to time the period of heating when the furnace cycles on and off. If it takes 90 minutes for the house to warm up in the morning from 15 to 19 degrees, you would have to compare that to the total time the furnace is running when it is trying to maintain the 19 degree heat during the 12 hours you would normally have the temperature set back to 15 degrees.
For example, if it takes 90 minutes in the morning to heat the house from 15 to 19, but the furnace has not turned on all night, then it took 90 minutes of heating. If maintaining 19 takes 20 minutes of every 2 hours then multiply the 20 minutes by the number of times the furnace is run, you will get 120 minutes.
NOTE TO PROGRAMMABLE THERMOSTAT MANUFACTURERS: All of this is gaverned by the programmable thermostat. Would this not be a feature that would be used/demanded by the consumer to allow them to find the optimum balance for their own house? Enough people are trying to figure out the optimum daily setback temperatures for their own house. This seems like the ideal place to put it. Surely it can’t take much extra programming to set up the programmable thermostat to display ‘thermo-statistics’…well, maybe a bigger LCD.
I don’t know about the wear part, but frequent short runs of modern furnaces
are definately less efficient(heat output/energy input) than one long run of the
same total duration. Of course, a longer run means a greater swing in temperature.
Does your thermostat have a run time log? Mine does. It tells me, at the push of a button, how long the furnace has ran since midnight today, another push gives yesterday’s run time, and another push gives total run time since last clear.
After some playing around with temperatures and times and comfort I settled on 21 C when awake, and 18 at night.
Incidentally, I have been plotting the average run times monthly for 10 years now, and although individual years have varied by as much as 15% the 10 year trend line is flat as a pancake. Well, until this year when I finally installed a wood stove in the basement. My run time is down 80% thanks to one wood stove in a 1700 square foot bungalow, and I’m heating about 40% more living space now.
Thermostats are designed to turn on/off and not turn off/on until the temperature changes by at least a couple of degrees and computer thermostats usually have a setting for this that you can change.
Rapid cycling is often a result of not having enough of a temperature gap in the hysteresis setting.
The hysteresis is for the purpose of avoiding “short cycling” and yes, such behavior is not too good good the machinery.
Assuming the inside temperature never gets down to freezing or otherwise cold enough to damage things, then the way to save the most money with a gas or oil furnace is always set the house to be as cold as possible when you’re not there, as cold as you can stand when you are there, and have it begin heating up as late as possible (while still reaching desired temp at whatever time you need it).
It always takes more energy to keep a house warm for a period of time than it does to warm it up afterwards.
