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What consumes more energy - heating or cooling?

Paul Anilprem
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    2
Just wondering if you want to keep the house temperature at 70F, when would your heat pump consume more energy - when the outside temperature is 90F or when the outside temperature is 50F?

This question popped because I was thinking of chopping off a big oak tree that shades the house equally during winter and summer. Without doing any calculations, I am guessing that heating is cheaper than cooling (assuming that both the operations are run off electricity) and so it is better to have shade in summer as well as winter rather than not have the shade in either season.

Any thoughts?


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Maneesh Godbole
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    8

You cannot generate cold. All you can do is take away the heat.
Thinking logically heating a certain area from x->y would be the same as cooling from y->x. In reality this would depend on the efficiency of the apparatus being used (e.g. heater and ac).

And do you know what consumes most energy? Getting out of bed in the morning on a cold day


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Maneesh Godbole
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    8

Paul Anilprem wrote:
This question popped because I was thinking of chopping off a big oak tree that shades the house equally during winter and summer. Without doing any calculations, I am guessing that heating is cheaper than cooling (assuming that both the operations are run off electricity) and so it is better to have shade in summer as well as winter rather than not have the shade in either season.

Instead of such drastic measures, have you considered some reflector kind of option mounted on the same tree which would sort of reflect the sunlight into your room during winter? If nothing else, you would save a tree and so many life forms dependent on it. Also it will be fun to figure out the optimal orientation for the reflectors. Yeah it should be 23.5 degrees, but in which direction
Paul Anilprem
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Maneesh Godbole wrote:You cannot generate cold. All you can do is take away the heat.
Thinking logically heating a certain area from x->y would be the same as cooling from y->x. In reality this would depend on the efficiency of the apparatus being used (e.g. heater and ac).


Well, here was my "logic" - Since no process is 100% efficient and since the loss of efficiency invariably translates into heat (not absence of it), so logically, cooling should require more energy that heating.

However, both of us wrong. As per this article:

To the surprise of many, air conditioners are more energy efficient than furnaces or boilers. Another way of stating this is that it takes less energy to cool down an interior space by one degree than to heat it up by one degree. This is the case, because (in layman's terms) it takes less energy to transfer heat (air conditioners) than to generate heat (furnaces and boilers).

Jayesh A Lalwani
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Joined: Jan 17, 2008
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  28

I think your calculations might be a little more complicated. If you cut down the tree, you will have to counteract the heating provided by the sun in the summer. If you don't cut down the tree, you will need to counteract the heat that your house loses in the winter. Remember that the tree is not cooling your house. Your house is cooling itself by either leaking warm air or basic radiation of heat.

Without knowing the specifics, and depending on your latitude, I am guessing that the sun will heat up your house a hell of lot more efficiently than the house will cool itself. Unless you have really bad windows. Also, taking away a large tree is going to be a much larger initial investment than upgrading your windows.

Save the tree. Just get better windows.
Paul Anilprem
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    2
Maneesh Godbole wrote:
Paul Anilprem wrote:
This question popped because I was thinking of chopping off a big oak tree that shades the house equally during winter and summer. Without doing any calculations, I am guessing that heating is cheaper than cooling (assuming that both the operations are run off electricity) and so it is better to have shade in summer as well as winter rather than not have the shade in either season.

Instead of such drastic measures, have you considered some reflector kind of option mounted on the same tree which would sort of reflect the sunlight into your room during winter? If nothing else, you would save a tree and so many life forms dependent on it. Also it will be fun to figure out the optimal orientation for the reflectors. Yeah it should be 23.5 degrees, but in which direction


Mounting something on top of a tree? Do you realize the risks you will be taking by doing that? No, thank you.
Paul Anilprem
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Jayesh A Lalwani wrote:
Remember that the tree is not cooling your house. Your house is cooling itself by either leaking warm air or basic radiation of heat.

Well, the tree is indeed cooling the house because of two factors - 1. Not letting the sun heat up the house and 2. By keeping the temperature under it lower because of evaporation effect of the leaves . Of course, in winter there are no leaves but the shade is still there (not as much as in summer though).


Without knowing the specifics, and depending on your latitude, I am guessing that the sun will heat up your house a hell of lot more efficiently than the house will cool itself. Unless you have really bad windows. Also, taking away a large tree is going to be a much larger initial investment than upgrading your windows.

Save the tree. Just get better windows.

Assume good windows already in place and ignore the cost of removing the tree. Now?
Maneesh Godbole
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Joined: Jul 26, 2007
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    8

Paul Anilprem wrote:
Mounting something on top of a tree? Do you realize the risks you will be taking by doing that? No, thank you.

Huh? Risks? What kind?
Paul Anilprem
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Maneesh Godbole wrote:
Paul Anilprem wrote:
Mounting something on top of a tree? Do you realize the risks you will be taking by doing that? No, thank you.

Huh? Risks? What kind?

Kind of obvious...what if it breaks loose and falls on top of someone? This is a no brainer. I would lose my shirt (and rightly so) if the person is hurt real bad.
Maneesh Godbole
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    8

I would have assumed if you can cut the tree, thats inside your property. If someone gets hurt, they would be trespassing. Or am I misunderstanding something here.
Paul Anilprem
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Maneesh Godbole wrote:I would have assumed if you can cut the tree, thats inside your property. If someone gets hurt, they would be trespassing. Or am I misunderstanding something here.

I imagine, for a tree mounted reflector to reflect any reasonable amount of sunlight, it has to be large, light weight, and mounted high and on the outer side of the tree. To be honest, I am having difficulty even imagining it. But anyway, during even moderate winds, it will sway quite a bit and if it breaks loose, it is not going to fall straight below. In stormy weather, it could be even more dangerous. So, no go.
Jayesh A Lalwani
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Joined: Jan 17, 2008
Posts: 2273
    
  28

Paul Anilprem wrote:
Jayesh A Lalwani wrote:
Remember that the tree is not cooling your house. Your house is cooling itself by either leaking warm air or basic radiation of heat.

Well, the tree is indeed cooling the house because of two factors - 1. Not letting the sun heat up the house and 2. By keeping the temperature under it lower because of evaporation effect of the leaves . Of course, in winter there are no leaves but the shade is still there (not as much as in summer though).


Without knowing the specifics, and depending on your latitude, I am guessing that the sun will heat up your house a hell of lot more efficiently than the house will cool itself. Unless you have really bad windows. Also, taking away a large tree is going to be a much larger initial investment than upgrading your windows.

Save the tree. Just get better windows.

Assume good windows already in place and ignore the cost of removing the tree. Now?


Look at it this way. The tree is blocking some energy from the sun, and letting some energy through. Out of the total energy absorbed, it uses some to grow, and some energy is lost to processes like evaporation. It doesn't matter what the processes are. THere is some finite amount of energy that is blocked from coming into your house.

TotalEnergysun = Energysun - EnergyBlockedtree

This is true for every season. The amounts will change from season to season, but the basic equation stays the same.

Now, if you had perfect windows then you wouldn't lose any heated/cooled air. But even though your windows are good, you lose air. So, according to this the total energy required to heat/cool a home depends on the volume of air that needs to be pushed through, which is calculated using

L = Q/(1.08(th-tt)) where Q is energy efficiency of the home. Assuming th-tt = 20 for summer and winter. So,
Energylost=Effeciency*Q/21.6

Remember that in the summer, you are fighting against sun's energy, but in winter sun's energy is helping you

So, if you keep the tree

Energysummer=Effeciencyac*Q/21.6 + EnergySun-summer - EnergyBlockedtree-summer
Energywinter=Effeciencyheater*Q/21.6 - EnergySun-winter + EnergyBlockedtree-winter
Energytotal = (Effeciencyac+Effeciencyheater)*Q/21.6 + EnergySun-summer - EnergySun-winter+ EnergyBlockedtree-winter - EnergyBlockedtree-summer

If you remove the tree

Energysummer=Effeciencyac*Q/21.6 + EnergySun-summer
Energywinter=Effeciencyheater*Q/21.6 - EnergySun-winter
Energytotal = (Effeciencyac+Effeciencyheater)*Q/21.6 + EnergySun-summer - EnergySun-winter

So to figure out how much energy you will spend by removing the tree, subtract the 2 total energy

Deltaenergy(notree-withtree) = EnergyBlockedtree-summer - EnergyBlockedtree-winter

Since, the tree will always block more energy in the summer than in winter, the above will always be positive. This means that without the tree you will always spend more energy than with the tree. It doesn't matter how efficient your ac/heater is. It doesn't matter whether the energy that it blocks is lost to evaporation. All those things are red herrings.

Putting it in simpler terms, the miracle of a deciduous tree is that it starts absorbing energy out of the sun exactly when you don't want that energy to go into the house, and goes dormant when you want the sun's energy. Don't remove it unless you have other good reasons to do so.

Paul Anilprem
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    2
Thank you for the detailed analysis, Jayesh. It looks very logical. The tree stays
fred rosenberger
lowercase baba
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  16

Don't forget that the tree also offsets some of the carbon released by both heating and cooling.


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Amit Ghorpade
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Joined: Jun 06, 2007
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    6

Paul Anilprem wrote:Thank you for the detailed analysis, Jayesh. It looks very logical. The tree stays

So the environment gets another tree (yes the tree had a rebirth) and Jayesh gets a cow.


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Frank Silbermann
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Joined: Jun 06, 2002
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To the surprise of many, air conditioners are more energy efficient than furnaces or boilers. Another way of stating this is that it takes less energy to cool down an interior space by one degree than to heat it up by one degree. This is the case, because (in layman's terms) it takes less energy to transfer heat (air conditioners) than to generate heat (furnaces and boilers).


This argument is irrelevant, because with a heat pump (as contrasted with, say, baseboard electric heaters) you are _transferring_ heat winter and summer way.

Also, the argument is relevant only if the magnitude of temperature change is the same, winter and summer. A heat pump loses efficiency as the magnitude increases.
Also, you have to consider how many days you would be heating versus air-conditioning.

Thus, in Florida it would be a big mistake to cut down the tree. Cutting the tree might make more sense if you lived in Maine. Of course, if you lived somewhere really cold it probably makes more sense to burn oil or natural gas in your house rather than burning fossil fuels at the power station, transferring the electricity over lines, and then using a heat pump.
Pat Farrell
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Joined: Aug 11, 2007
Posts: 4646
    
    5

The answer depends, are you counting the solar load? In general, heating and cooling loads are calculated with a very simple formula

btu = sqft * u-value * delta-T
where u-value is simply the reciprocal of r-value, the effective insulation of the surface.
delta-T is the difference in temperature in degrees.

So with a setpoint of 70 degrees, there is no difference in the amount of heating or cooling you want if its 90 degrees or 50 degrees. Both have a delta-T of 20 degrees.

Solar load is substantial and the above formula ignores it. So you have to add in the solar load in the daytime.

Modern A/C have an efficiency rating of about 12, so you put in one btu of electricity and get 12 btu of cooling.
Resistance heating with electricity has an efficiency rating of 3.41, you get 3.41 btus of heat for every btu of electricity you want.

Metric folks can apply all the usual conversion factors, this is just arithmetic.

 
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