Posted By tinoue on 17 Apr 2010 10:20 PM


Interesting. I ran some numbers based on your graphs and other data. Your house is very efficient indeed. Based on the numbers, you home only needs about 2.25 BTUs/HDD/SF, making it more efficient than any free-standing home I've measured. Even more interesting is that, based on the total heat delivered to your home on a 15F day, the heat load for your place is only 16kBTU/hr - incredible!

Of course, based on those numbers, your system is only running a maximum duty cycle of about 33%, so it's no wonder your ground temperatures barely waver.

I may have misinterpreted your graphs however, but it sure seems like those numbers are in the right ballpark.

Interesting indeed.   It is true that my heat pump doesn't run much more than 50% on the coldest days. 

It is probably not as efficient as your calculations show.  There is something wrong with  the outdoor sensor.  It doesn't   read the really low temps right.

Some facts about my house:

It is standard 2 x 4 construction with blown in cellulose in the walls and attic.
Vinyl double pane windows.

On the hottest days in the summer ,  the unit will run pretty much non stop.
Since Utah is a heating dominated climate,  I am not sure why this is so.

I didn't design my system.  It was my first geo job.  I did however install the loop.

by looby:
Some loop! That's 1/4 mile per ton, correct?

I guess so, that's a new way to look at it.

by looby:
Possibly due to incredible solar gain?

We do have some sunny days in the winter. Sometimes we don't see the sun for weeks at a time.

good metric for building comparisons although I would write it as:
BTU/HD/SF/Day
In either case, the btus needed to heat one square foot for one day when the temperature is one degree below 65F.

Blackflag, 
Wanted to address one thing. You mentioned you had hoped for concensus on size....you probably won't get one for two reasons. Lack of load calculation and differences in opinion on geo sizing.
More importantly you suggested that larger unit would avoid activation of auxiliary coil. You are influenced by a mind set that auxiliary = bad and I do not agree. I am stumped by a trend that encourages 2 stage systems but fails to see the advantage to a 3 stage design.
Someone asked once why if first stage is so efficient we don't install a unit big enough to avoid second stage.
I do not understand why folks don't get that a unit oversized enough to avoid 3rd stage is also not most efficient.
Joe