Comparing HDDs to Coal Used

[Lightning]

Are you measuring coal and comparing to HDDs? What's your pounds per HDD?

[Lightning]

Lee, you’re going to have to give me a refresher course on that.

So basically, what you do is take an average (mean) temperature for the day and subtract that number from 65 (which is the base temperature).. For example, if the low is 10 and the high is 30, then the mean temp for that day is 20 degrees. Then you subtract the mean from 65 degrees. 65-20 would be 45.. so 45 of those degrees are "heating degrees". Suppose you burn 68 pounds for that day. You then divide 45 heated degrees into the 68 pounds you used. In this example you used about 1.5 pounds of coal for each heated degree. Which is stated as 1.5 pounds per HDD.

[Retro_Origin]

My understanding of said method is that it's pretty accurate but how do you get an average day temp? You getting it online or something? And what about wind and sun? It's not unusual to have a thirty degree swing here between daytime and night. This may be a very stupid question but in theory would it take the same amount of btus to keep a house at 70 when outdoor temp is 0 as it would to keep a house at 60 if outdoor temps were minus 10? Like is the math really that linear? It seems to me that the greater your delta is the btu demand grows exponentially or at least more than linearly.

[Lightning]

It's not completely linear but pretty close. Cold air infiltration does happen a little faster the colder it is outside. For the example you posted above, I would be inclined to say yes because the delta is the same.

[Sunny Boy]

What is the factor for the greater the difference between inside verses outside temps the faster the heat loss ?

At below zero temps my two stoves can barely maintain house temp. Get down to -20 and they can't keep up. Then when it gets above 40F and the kitchen range, which has half the firebed of the base heater, is enough to maintain house temp by itself.

Plus, unless there is an excellent vapor barrier in all the outer walls, houses suffer from wind chill much like we do.

Paul

[Lightning]

What is the factor for the greater the difference between inside verses outside temps the faster the heat loss ?

I can think of a few - thermal transfer via conduction, cold air infiltration, and there would be radiant heat loss thru windows. Cold air infiltration is usually the main component of heat loss which is driven by chimney effect. So the colder it gets, the faster the turnover is in the house.

[Rob R.]

My understanding of said method is that it's pretty accurate but how do you get an average day temp? You getting it online or something? And what about wind and sun?

The information is readily available online from www.degreedays.net and other sources. You are correct that the sun and wind can make quite a difference in coal burned vs. HDD.

[Hoytman]

Thanks.

So do we just check one day, a few days, or the entire winter?

Having lost focus of the topic, and not having read again Lee’s post, what does this accomplish? ( I feel like I’ve had one large brain fart )

[Rob R.]

Comparing how much fuel is used vs. heating degree days allows you to evaluate the impact of changes to the system or house.

[Hoytman]

Ok.

So it seems then that calculating for more than one day would give the best results, or is one day enough to get close?

So I just need to weigh my coal at next tending and get the high and low temperatures for tomorrow?

Man…I’m usually the on the ball with these type things, but tonight I feel like I’m dragging a ton of coal behind me. My brain is dragging…

[Lightning]

A length of time will give you a better average. I use the filling of 2 ash tubs as a period, which is 9-10 days this time of year.

The pic below is a period of data.

[Hoytman]

Ok…I’ve got to check my batteries in my scale because it was saying the hod weighed 15.3 lbs level full. I’m pretty sure that in the past it weighed about 30 lbs full, so I need to check things out.

At any rate…
I added 9.9 lbs (per the scale…still think it’s wrong) after a 24 hr burn this morning and 31F was the nights low. Should be around 45F today. We’ll see…

[Lightning]

You will want to make sure your scale is accurate before trying to calculate your coal pounds per HDD. Also, don't forget to subtract the weight of the container (hod).

[Hoytman]

You will want to make sure your scale is accurate before trying to calculate your coal pounds per HDD. Also, don't forget to subtract the weight of the container (hod).

Yep. Checked this morning. Scape is off…by a lot. Had me weighing less by about 146 lbs. I knew it was off.

I’ll have to get a battery and start over. Shucks!!!

Just put new batteries in my inside/outside thermometer too.

A full his is generally around 28-32 lbs for me depending on how full I fill it.

If I had to guess I’m only adding about 12-15lbs every 24 hours. I have more than half a hod left after refilling the stove.

[davidmcbeth3]

So basically, what you do is take an average (mean) temperature for the day and subtract that number from 65 (which is the base temperature).. For example, if the low is 10 and the high is 30, then the mean temp for that day is 20 degrees. Then you subtract the mean from 65 degrees. 65-20 would be 45.. so 45 of those degrees are "heating degrees". Suppose you burn 68 pounds for that day. You then divide 45 heated degrees into the 68 pounds you used. In this example you used about 1.5 pounds of coal for each heated degree. Which is stated as 1.5 pounds per HDD.

GIVEN

OAT, high = max temp outside for day
OAT, low = lowest temp outside for day
Pounds = pounds coal used 24 hr period

THEN

HDD = Pounds / [ 65 - ( OAT, high + OAT, low / 2) ]

Converting to a formula from Lightning's post .. folks can check and confirm