By, Doug Carruthers 
LoopLink’s Fixed Length Mode is a deceptively simple tool that is doing a lot more work behind the scenes than you might expect. Practically, there is no direct way to answer the question of what minimum and maximum entering water temperatures (EWTs) will result given a specific amount of loopfield for a given building. The only way to determine the answer is to repeatedly guess EWTmin and EWTmax and solve for the resulting loopfield size at each guess.
LoopLink is pretty good at guessing, but to get within five feet of a targeted fixed length can require quite a bit of work which, on larger projects, means waiting. The waiting isn’t restricted to the ghex design pages either because, EWTs are such an integral part of heat pump operation that LoopLink must continually determine whether or not a change has been made that will alter your final lengths.
We have done our best to improve our fixed length method to make it as fast as possible but on larger projects, there are some tips/tricks you can employ that can improve performance and decrease your wait time.
Add Zones First
This may seem obvious but, whenever possible, input all of your zone information prior to adding a GHEX to your project. This will stop the system from finding the solution to a moving target and make selecting equipment much faster.
Get Close Then Turn On Fixed Length
Your design should be close before you turn on Fixed Length. If you set your EWTs to 30°F and 90°F and your GHEX is returning lengths of 500ft/bore, your design probably needs to be adjusted before you run with a fixed length of 200ft/bore. LoopLink will be worse at making temperature guesses as fixed lengths get excessively long or short.
Find EWTs Then Turn Off Fixed Length Mode
The best way to use fixed length mode is to find temperatures that result in lengths in the range of your target, save the ghex and then turn off fixed length mode. If you don’t make significant changes to your loads, you will be in the neighborhood of the targeted length.
Block Load Multi-Zone Projects
Every zone is another set of calculations, so minimizing the number of zones is a sure fire way to speed up fixed length mode. For an initial design, sum all of your zones peak heating and cooling loads and treat the space as one big zone (aka: a block load)
Using the block load:
- Add a ghex and enter your basic loopfield parameters.
- Turn on fixed length mode and let LoopLink churn over the single zone and determine EWTs.
- Save the GHEX.
- Turn off Fixed Length Mode.
- Split your project into zones as desired.
The final GHEX length will be recalculated based on the EWTs determined using the block load without iterating over each zone dozens of times. There will be some variation but your answer will definitely be close.