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Old 01-02-2014, 02:24 PM  
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I think you raise a great question and I have been enjoying the replies you have been getting. You asked a pretty straight forward question I thought and even went as far as discounting comfort during the time you were reheating. Modern thermostats allow programing and the selling point of programmable has always been energy savings. So you turn them down or off while at work and have them set back up some period before you return home. They compensate taking into account the comfort factor even and save you money if properly used. All the factors explained above in detail can be ignored because they are a constant in any given house. Sure more insulation, tighter sealing, the thermal mass of your home all are important but they are also a constant. If you want to factor any of that in you should be turning your heat off actually before you leave. Say 68f is your happy place and 60f becomes your low limit point you could take a look at outdoor temp and maybe wind speed and say ok my house is 68 now and will take X amount of time today to hit 60 I’m leaving for work at 7:00 so I shut the heat off at X:XX reapply the formula before returning only in reverse and you will start having what some of the newest high-tech controllers do. They communicate with weather predictions along with sensors and also learn your habits.

The second part of your question is in some ways related to the first I think. Most heating systems work by being on or off full heat or no heat and then the thermostat sets a high and low limit and there are some hysteresis factored in. In your case the hysteresis is adjustable between 1 and 3 degrees without this range a thermostat would be constantly switching on and off. You only use energy when your furnace running and you only save when it’s off. A simple experiment would be to wire a relay such that whenever your furnace fires or gets a signal to fire you supply power to a 120v clock, start it at 12:00 and let the experiment go for 24 hours on another clock. That would tell you how many minutes total it ran in 24 hours at whatever settings you want to try.

To me the on and off range is just a mini example of your all day question. The less it runs the better and the fewer times it starts and stops is better.

I would prefer a system that modulated the heat output to match the temp loss and just stay on but at a lower output.

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Old 01-02-2014, 03:18 PM  
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I've noticed my HVAC usually runs three times per hour around 7 minutes each.
>duty cycle = 7/20 = .35, period = 1/3 hour.

If I leave my heat on at the same temperature 24 hours a day, it will run 504 minutes a day and have to turn on 72 times.
>translate 504 minutes to 504 'units' of fuel, so when on the furnace consumes 504/.35 = 1440 units in 24 hours = 60 units/hr. = 1 unit/min.

If I turn it completely off for the 10 hours I'm away during the day and turn it on when I come home, it will run 354 minutes in a 24 hour period. . .
to turn on around 40 times
>the 24 hours includes 14 hours at .35 duty cycle which means 4.9 x 60 u/hr = 294 units of fuel.
>the 10 hours off consume 354-294 = 60 units.
>the one hour reheat takes 60 units.
>this leaves nothing for the 40 times.
>Sumpin's wrong.

Last edited by Wuzzat?; 01-02-2014 at 03:26 PM.
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