Heat Required as Outside Temp Falls

Well, I was thinking again.. Imagine that

I've noticed many members who have voiced that when it gets under a particular temperature outside that it becomes increasingly difficult to keep a comfortable temperature inside. It's been a wonderment of mine if the heat needed to maintain temperature in the house is linear to the temperature outside.

Example 1) Lets define the variables.. If its 40 degrees outside it takes 35 pounds of coal per day to keep 70 in the house. In this example there is a 30 degree difference between inside and outside temperature.

Example 2) If its 10 degrees outside, shouldn't it take twice the amount of coal (70 pounds) to maintain 70 in the house since now there is a 60 degree difference between inside and outside temperatures?

Example 3) Suppose we get an arctic blast and suffer -20 degrees outside? Will it take 105 pounds of coal to keep 70 degrees in the house since its 3 times colder outside than in example 1?

I didn't see this to be the case and couldn't figure out why.. As it gets colder I have to use more coal than what the examples above dictate as temperatures fall, especially when it goes 10-20 degrees below zero.

I'm sure there is some heat transfer efficiency loss as you run the stove hotter since the transfer areas can't keep up with moving the heat from the stove to the medium, whether that medium is air or water. But also I think a bigger culprit is stack effect in the house along with air turn over rate. It seems to me that air infiltration and air turn over in the house would increase as temperature difference between inside and outside spread. This would increase the volume of infiltrated air that needs heated and in a double whammy sort of way, the said infiltrated air would be colder than it would be at warmer outside air temperatures such as in examples 1 and 2..

Anyone have any thoughts on this scenario??

Your reasoning seems sound to me.

Lightning wrote:Well, I was thinking again.. Imagine that

I've noticed many members who have voiced that when it gets under a particular temperature outside that it becomes increasingly difficult to keep a comfortable temperature inside. It's been a wonderment of mine if the heat needed to maintain temperature in the house is linear to the temperature outside.

Example 1) Lets define the variables.. If its 40 degrees outside it takes 35 pounds of coal per day to keep 70 in the house. In this example there is a 30 degree difference between inside and outside temperature.

For this example you are burning 1.4 lbs. per HDD.

Example 2) If its 10 degrees outside, shouldn't it take twice the amount of coal (70 pounds) to maintain 70 in the house since now there is a 60 degree difference between inside and outside temperatures?

For this example you are burning 1.27 lbs. per HDD (a bit better than above). To remain consistent, at 1.4 lbs. per HDD you should be burning 77 lbs. here.

Example 3) Suppose we get an arctic blast and suffer -20 degrees outside? Will it take 105 pounds of coal to keep 70 degrees in the house since its 3 times colder outside than in example 1?

For this example you are burning 1.24 lbs. per HDD (a bit better yet). To remain consistent, at 1.4 lbs. per HDD you should be burning 119 lbs. of coal here.

I didn't see this to be the case and couldn't figure out why.. As it gets colder I have to use more coal than what the examples above dictate as temperatures fall, especially when it goes 10-20 degrees below zero.

How much more than 77 lbs. and 119 lbs. is the key question here? And since I'm weighing everything against a mild 40 degrees outside to establish 1.4 lbs. per HDD, is this a highly reliable base for comparison?

Perhaps you should consider that you are most likely to be burning 1.4 lbs per HDD at the two higher temperatures, just as you have done for the lowest temperature in example 1 above.

I'm sure there is some heat transfer efficiency loss as you run the stove hotter since the transfer areas can't keep up with moving the heat from the stove to the medium, whether that medium is air or water. But also I think a bigger culprit is stack effect in the house along with air turn over rate. It seems to me that air infiltration and air turn over in the house would increase as temperature difference between inside and outside spread. This would increase the volume of infiltrated air that needs heated and in a double whammy sort of way, the said infiltrated air would be colder than it would be at warmer outside air temperatures such as in examples 1 and 2..

Anyone have any thoughts on this scenario??

I'm not convinced that air infiltration due to wind plays a large part in this. I've not proven it to be the case for my house at least. In my experience coal consumed is quite linear to HDD's. This is a different statement than to more simply assume it is arbitrarily linear to outside temperature vs. inside temperature (although it is, but with a 65 degree "benchmark" qualifier). If anything, at the colder temperatures it may be more difficult to retain the exact same level of draft that you had at the warmest temperature, and this fluctuation does change things.

Lightning wrote:Well, I was thinking again.. Imagine that

I've noticed many members who have voiced that when it gets under a particular temperature outside that it becomes increasingly difficult to keep a comfortable temperature inside. It's been a wonderment of mine if the heat needed to maintain temperature in the house is linear to the temperature outside.

Example 1) Lets define the variables.. If its 40 degrees outside it takes 35 pounds of coal per day to keep 70 in the house. In this example there is a 30 degree difference between inside and outside temperature.

Example 2) If its 10 degrees outside, shouldn't it take twice the amount of coal (70 pounds) to maintain 70 in the house since now there is a 60 degree difference between inside and outside temperatures?

Example 3) Suppose we get an arctic blast and suffer -20 degrees outside? Will it take 105 pounds of coal to keep 70 degrees in the house since its 3 times colder outside than in example 1?

I didn't see this to be the case and couldn't figure out why.. As it gets colder I have to use more coal than what the examples above dictate as temperatures fall, especially when it goes 10-20 degrees below zero.

I'm sure there is some heat transfer efficiency loss as you run the stove hotter since the transfer areas can't keep up with moving the heat from the stove to the medium, whether that medium is air or water. But also I think a bigger culprit is stack effect in the house along with air turn over rate. It seems to me that air infiltration and air turn over in the house would increase as temperature difference between inside and outside spread. This would increase the volume of infiltrated air that needs heated and in a double whammy sort of way, the said infiltrated air would be colder than it would be at warmer outside air temperatures such as in examples 1 and 2..

Anyone have any thoughts on this scenario??

Simple, buy a larger stove. Then it will not be increasingly difficult to keep a constant inside temp...lol!

There is also in your case the consideration of daily average coal consumed to provide DHW, which neither you nor myself have factored in.

Right Larry, it would be heavily influenced on how tight the house is that we are applying the scenario to. You likely have a tighter house than I do, and I suppose the tightest houses would have a more linear coal usage compared to temperature drop, as you see in your case.When I've compared HDDs in the past, it seemed as it got colder the coal use per HDD increased in my case. I think its safe to say that if there is an increase of coal per HDD as temperature falls then, the house needs tightened and/or the appliance is too small for the heat load?

The use of a 65 degree base with using HDDs kinda throws things out of perspective a bit since the examples I listed are based on 70 degrees in the house.

lsayre wrote:There is also in your case the consideration of daily average coal consumed to provide DHW, which neither you nor myself have factored in.

Right also.

scalabro wrote:Simple, buy a larger stove. Then it will not be increasingly difficult to keep a constant inside temp...lol!

Yes, this would be a definite solution..

I'm no expert, but I believe the loss can be attributed to thermodynamics and heat conduction, in which heat is lost through transference through the wall of the stove, the walls of the house, and the floor and/or ceiling. Combine those losses with a drop in temperature and the losses increase significantly due to the increased difference. Heat energy always travels from warm to cold areas, even to those cold areas we don't want to heat.

Found this website with formulas that may help you calculate it if you want to.

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo ... ss.html#c1

A stoker is better able to handle a tripling of burn rate while still retaining close to its best efficiency. The hand fired will depart more from its ideal firing rate.

In Laymen's terms....

The greater the difference between OAT & IAT, the greater the rate of transfer through the wall, ceiling etc.

Julie250 wrote:I'm no expert, but I believe the loss can be attributed to thermodynamics and heat conduction, in which heat is lost through transference through the wall of the stove, the walls of the house, and the floor and/or ceiling. Combine those losses with a drop in temperature and the losses increase significantly due to the increased difference. Heat energy always travels from warm to cold areas, even to those cold areas we don't want to heat.

Found this website with formulas that may help you calculate it if you want to.

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo ... ss.html#c1

This still leaves the question of why Larry's pounds per degree day remains constant regardless of outside temperature. Perhaps increase of efficiency at higher firing rate?

For houses with less than adequate insulation in the walls & or single pane windows or lousy double pane windows there has to be a temperature "scrubbing" effect. As cold & warm meet there is a natural movement,as the temps inside vs outside pull further apart from each other so will the "scrubbing " action increase. This is just my personal opinion based on my 57 yrs of age,i am no expert on this or any subject so your results may differ.

In the case of a stoker they might be more efficient because of less idle fire maintenance at the lower OAT due to longer full rate burn times ?

If you research the 65 degree base for HDD's you will find that it was carefully chosen for homes and offices kept at 70 degrees. It was found that no one heats a home to 70 degrees when it is a nominal 65 degrees outside, since there is no need to do so. Only at temperatures below a median of 65 degrees do people actually begin to heat their homes. So when you hear 65 degrees as the HDD base, remember that it is the base for a 70 degree home.

My home was built in 1964, and I wouldn't by any means classify it as tight. Several of our windows are still single pane, and I can see light around all of the ill fitting original doors (which means all of them).

I found this on the "coal burning efficiency" thread..

Julie250 wrote:

This still leaves the question of why Larry's pounds per degree day remains constant regardless of outside temperature. Perhaps increase of efficiency at higher firing rate?

I would speculate that he must have a really nicely insulated home. And someone else alluded to that also. We burn a lot more when the temperature drops. Our field stone basement has really lousy insulation.

Larry, maybe your appliance gains significant efficiency as it gets more heat demand.
That be a fine attribute.