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Old 06-03-13, 09:41 AM   #1
AC_Hacker
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Default Heating Degree Days Revisited...

Heating Degree Days (AKA: 'HDD') are a measure of the difference in temperature between the outside temperature and the temperature inside your house.

[there is a complementary measure called Cooling Degree Days (AKA: 'CDD'), but this post concentrated only on heating. Cooling data is obtained in much the same way]

If, on some particular day, the outside temperature was 24 degrees F, and your house was at 65 degrees F, the HDD would be 65-24 or 41. If you maintain your house at 70 degrees, your HDD would be 70-24 or 46.

The inside temperature of a house is held pretty constant, but the outside temperature will vary during the day, so there is some averaging that is included in a HDD value.


There are detailed charts for HDD, that show HDD for various parts of the US that can be found HERE.

HDD information can be very useful in helping us to design how much heating we will require, and also how much insulation we will require.

The charts are also very useful when talking to someone who lives in another region, because it gives us a clue as to the heating and insulating problems they will be faced with. So it helps us understand their situation, and to communicate more relevant advice.

There is also a free service that will enable you to determine HDD more precisely, sometimes for your particular micro-climate. The service I have used degree day (dot) net) is located HERE.

You can use your zip code to locate previously existing data collection stations near you. For instance, in my city, Portland, Oregon, if I put in my zip code, I see that there are 41 different data collection sites from which I can choose. They are listed alphabetically, so the first one is rarely the best. I have found that some of the data collection stations have very complete data sets, while others are not very complete.

So, when I choose a data logging station that is very close to my house (1 mile away) and use all the defaults, I get the data set below, which is monthly data of the HDD for the last year:

2012-06-01,140
2012-07-01,63
2012-08-01,58
2012-09-01,118
2012-10-01,308
2012-11-01,486
2012-12-01,690
2013-01-01,826
2013-02-01,548
2013-03-01,467
2013-04-01,329
2013-05-01,189

If I add up all the HDD for the year, I see that my annual HDD is 4222 heating degree days.

and if I drop that data into my favorite free spreadsheet program (Gnumeric) I can get a plot like this:


Here I can see that just as I suspected, lots of heat is required in January, and not so much in August. Actually, none because of the solar gain of my house, and the insulation, too. But seeing a graphical representation of the data gives me a deeper understanding of heating needs throughout the year.

If I dig a bit deeper, I see that I can request daily data, which looks a lot like the monthly data set above, but is too long to show here... however, I can graph it out as shown below:


Here I see pretty much the same information, only now I can see the 'outlying' data, the unusual temperature swings. This is important, because winter weather doesn't arrive in monthly averaged packages, it's a day by day kind of thing. An outlying cold day can have you shivering if you're set up for average heating.

Next, I wanted to see what this same kind of plot looked like over a longer period. Unfortunately, my closest data collection station had not been functioning for very long, so I was forced to get another data station that was three miles away... Not really too bad.

From that data station, I was able to get three years of data. I had to do a bit of manipulation so that my spreadsheet would graph them on the same plot.

Here's what it looked like:


Not only more colorful, but a much richer plot of data. Also apparent are the small variations that a distance of two miles can make.

Degree Day data by itself is not enough to make a decision on a specific heating system, or a particular insulation strategy.

But when combined with heat load analysis, very well informed decisions can be made.

Best,

-AC

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Last edited by AC_Hacker; 06-03-13 at 10:36 AM..
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Old 06-04-13, 08:48 PM   #2
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AC,

You are correct that the information can be VERY helpful. I use this type of analysis all the time.

However, one correction. For heating, the baseline is at 65 degrees. Just a couple people in a house, normal activity, appliances, etc. all add in an effect so that the neutral point for heating is 65 F.

One flaw is wind - and this factors very strongly in a windy climate like Oklahoma. Therefore, I try to get my DD data on houses on a very cold, but still day (before the sun comes up) typically at 4-5 AM. This gives me the degree of insulation the house has. The same temperature, same time, but WITH wind allows me to know the degree of infiltration. Faster/cheaper than a blower door and I have an automated device that does this.

Basically, I just look at the run time of the heating unit and by knowing the BTU output at 100% run time, I can calculate the BTU input. I will get a graph of some data later and put it up showing how the degree day (DD) data vs. heater run time is a linear relationship.

Points taken with and without wind are very illustrative to homeowners.

Great observations.

Steve
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Old 06-06-13, 01:54 PM   #3
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Quote:
Originally Posted by stevehull View Post
...Therefore, I try to get my DD data on houses on a very cold, but still day (before the sun comes up) typically at 4-5 AM. This gives me the degree of insulation the house has. The same temperature, same time, but WITH wind allows me to know the degree of infiltration...
Great.

Please describe exactly what your procedure is...

-AC

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