I am getting mixed signals regarding auxiliary heating elements on geo thermal systems. I have seen people talk about how they are not necessary if you properly size your system to your house, and have seen others say that they are necessary during periods of extreme temperatures. How necessary would you say auxiliary heating elements are on a geo system? Are they more for peace of mind in case your compressor goes out?

Aux use during only extreme cold is probably a more right sized system. Sure you could put in a system big enough to never use aux, but you probably paid a lot more money then the cost of a couple hundred KWH's of aux use per year.

Also could end up over sized for cooling and having short cycle, humidity issues.

My 3T water to water has a buffer tank.  It is a larger HotWater tank with normal heating elements.  It is my Backup (in the event my HP fails) or to give some extra heat when the HP can't keep up.    It is set to kick in an outside temp of -20C.  The system was designed for -14C.    In 2012, it only kicked in 3 or 4 times x for about 6 hrs each time (so, perhaps if 1500 W for 24 hrs= 36 KwH @ .12.5 cents = just under $5.        So far in 2013 it kicked in for 1 night .     The heat pump still  continues to extract heat from my 2 drilled wells and the elements in the buffer top up the heat.    

My installer calculated the design temp.   Since putting in the HP 3 +years ago, I have continued to tidy up on some weather proofing the house.     I think that has helped me to allow for the Temp aux supplement  to be lowered.  The installer said I would have to keep that in mind as I learn how the system works.   Keep in mind when it has been >-12C, the system will stay on all the time.   However, my main level stays at +20C and does not stray much.

" I have seen people talk about how they are not necessary if you properly size your system to your house,"

Makes me grit my teeth.......some of the same morons suggest installing a unit big enough so you don't need second stage (cleverly inventing a single stage heat pump).

Look larger compressors in my AO use more juice. Period. So over sizing to avoid auxiliary use means you use a lot of extra electricity in the summer and bumper seasons.

Different design strategies depending on your location (not much use for auxiliary heat in Ja-ville FL) prevail, but in mid MI I have shown customers where the oversize guys cost them not only thousands extra on installation, but extra every year on operating cost.

Jax (Ja-Ville) response:

We do not install aux strips with water source heat pumps since cooling loads pretty much always exceed our heating loads, and of course we must size for the cooling load. Air source units lose capacity as outdoor temps fall, and the need for air source units to defrost themselves (switch to cooling mode for several minutes) means we still need strips for air source systems.

That said, we also sell as small a unit as possible for any number of reasons - dehumidification, efficiency, noise, comfort

Some northern codes want backup strips for safety, but it is safe to say no one is in real peril down here if heat dies

Thanks for the responses.

I am trying to get an overall picture of what my electric usage requirements are going to be. Obviously having aux strips will up the electric draw. I have seen an overall consensus that each ton a system has will require approximately 10 amps or so. So a 3 ton system would require approximately a 30 amp circuit.

Does the 10amp/ton ratio include aux strips? If not, on average how much does the ratio increase?

Auxiliary draw is based on KW of aux heater. loosely 5 amps/kw