# HVAC off vs turned down



## house92 (Dec 26, 2013)

Whenever I am to be gone for 8 hours or so in winter, is it more efficient to turn the thermostat down, or just turn it off; this is assuming it won't get below freezing in the house.  Even if it got down to 40F and had to heat back to 70, is that better than just turning it down to 55 or 60?

If it is cold outside and I turn the thermostat down to 55 while away, once the temperature falls to 55, is it really going to cycle on and off a lot less that if it were on 70?

I'm also still wondering about the best span setting.  Is it more efficient for a unit to cycle on more frequently, but run for shorter periods or to cycle less frequently, but run for longer periods when on?


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## havasu (Dec 26, 2013)

Most friends and family I know who live in a snow area only reduce the heat and never turn it off...but I know this may not be an expert opinion.


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## house92 (Dec 26, 2013)

havasu said:


> Most friends and family I know who live in a snow area only reduce the heat and never turn it off...but I know this may not be an expert opinion.



Sure, I would guess that turning it off is not common.  I just wondered, however, if I didn't mind coming home to a cold house, if it was more efficient to just turn it off.  I use propane, which is really warm, so it doesn't take long to heat things back up.


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## Wuzzat? (Dec 26, 2013)

You need the heating degree days for your target month for your area, your house tightness in BTU per (square foot x HDD) with 6 BTU being average and 2 being tight.  My leaky 1964 house comes in at about 11.

All this info is available online and from your fuel bill.  It's a tedious calc. but not complex - no exponents or other nonsense.  
You may want to practice this 
http://learnlab.hfcc.edu/sites/learnlab.hfcc.edu/files/dimanalsci.pdf
first.

Or, if the weather holds constant, run some tests.  Thermal inertia and wind conditions determine how fast your house cools and this is shown on your thermostat.
One winter without power for a while I think it took two or three days for our house temp to level off at 45F.  And that temp vs time curve *is* exponential. 
With gas/forced air back on the temp maybe went up 1F per hour.


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## dipstick (Dec 27, 2013)

Your home loses heat at a slower rate as the inside  temp gets closer to the outside temp; the cooler you allow the home to get the the less heat you lose.  So as long as it is above freezing turning the heat off will save you money.


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## Wuzzat? (Dec 27, 2013)

dipstick said:


> So as long as it is above freezing turning the heat off will save you money.


There is a minimum energy cost setting for your thermostat involving nighttime setback, and daytime setback while you're gone.  
Exact analysis for this is not worth it for residential use but with experimentation you can come close.
This is a science problem.

There is also a discomfort factor in coming home to a cold house and waiting for it to warm up.
This is a psychology problem.

This factor can probably be converted into dollars by asking yourself: "How much would you pay each day to come home to warm house?"
Ask each family member because each has a different 'utility function' for money and for comfort.
This converts the psychology problem into one of economics.

There is also radiative heat loss which means a 72F air temp may feel cold depending on the temp of the walls, which depends on inside and outside factors.  High end  thermostats sense outside temps and correct the inside temps.


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## house92 (Dec 28, 2013)

What about my span question?  Is it best to have a 3 degree span where it runs longer but doesn't cycle as often, or a 1 degree span where it runs for a shorter period but cycles more often?

Also, if I turn my Th down to 60, for example, once the temperature falls to 60 in the house, does it cycle less that if it were on 70, or is it roughly the same?


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## Wuzzat? (Dec 28, 2013)

house92 said:


> Also, if I turn my Th down to 60, for example, once the temperature falls to 60 in the house, does it cycle less that if it were on 70, or is it roughly the same?


The duty cycle should be less because on average the furnace is supplying less heat.

My furnace is oversized and so according to my heating bill it is only one 1/3 of the time in the winter.  This could be 5 minutes on/10 minutes off which is probably considered short cycling, or the on/off cycle could have a longer period.


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## nealtw (Dec 30, 2013)

You might find this link interesting. http://www.homehdw.com/tips/thermostats.html
Years ago we would here arguement about the cost to reheat the house and expansion and contraction but a few years before that we got up in the morning and light fire in the wood stove to heat the house and make breakfast. Keeping the fire going all night and when away would have tripled the wood bill. We are spoiled with automatic heat systems.:


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## house92 (Jan 2, 2014)

I just did a little calculation.  I know the temperature is never exactly the same, so this is based on average conditions.

I've noticed my HVAC usually runs three times per hour around 7 minutes each.  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.

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 and have to turn on around 40 times.  Those minutes include an hour for re-heating, which is more than it requires, but I'll call it an hour.

With these figures, is there any way that turning it off could not be more efficient?


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## bud16415 (Jan 2, 2014)

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&#8217;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&#8217;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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## Wuzzat? (Jan 2, 2014)

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.


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