factors for degee days

Doug_7

Tim - Not sure exactly what you want to accomplish using degree days.

Fuel use per degree day is a very useful measurement for comparing building efficiency between different buildings or for showing the results of one buildings changes - such as installing a new boiler.

I have recorded actual daily fuel use vs. actual heating degree days and it is almost a straight line relationship as would be expected. I can easily see the difference between the old inefficient boiler and the new condensing boiler (30% improvement).

See attached graph. Each data point is one day and is the actual daily fuel use vs. the actual heating degree day for that day. Degree days are published by the local gas utility (ATCO Gas). The lines intersect the gas use axis at about 2 GJ of gas use - which is the gas use due to domestic hot water and other miscellaneous gas use.

Black line = Old boiler, Red line = new boiler, blue dots = recheck on new boiler two years later.

There is some scatter to the data but the confidence level of the straight lines is 90 to 95%.. Microsoft Excel plots the graphs and calculated the correlation coefficient. I think the scatter is mostly due to solar radiation or the lack of solar radiation.

So degree days can be a useful tool - depending on exactly what you want to accomplish.

Outdoor reset is a very good thing to do, but the results may be hard to measure using degree days.

Doug

Tim_41

I am doing some degree day calcs on a couple of the systems where I installed outdoor reset controls. Is there a factor that can be used to take into account the efficency that you gain from the outdoor reset control? Thanks

Mike T., Swampeast MO

VERY difficult.

Why?

The "degree day" calculation has an artificial basis based on any structure requiring space heating when the outdoor temp is above some fixed (generally 65F) temperature.

This can work reasonably when the local climate is fairly predictable and the structure is assumed to be rather massive with the insulation on the outside.

For much of North America however, such assumptions cannot be fairly made!

Not only is the construction VASTLY different, but the climate is "normally" far more variable. This introduces the very sort of inefficiency that the finest European engineers cannot comprehend based on their experience.

Tim_41

DD

Thanks for the graph Doug. I wanted to try and show some customers the difference that a Outdoor reset control would make on thier boilers. But, as everyone has pointed out, it is tough to use DD. I still am quite new to DD's so my learning curve is still quite curved. Thanks for everyone's posts.
TimS

jp_2

biggest problem you face

is determining the actual efficiency the current customers system runs at.

without knowing that, you can not make assumptions on efficiency gains. something you do not want to put in writing.