Jeff has a very long thread on this topic.
If you google "physical presense test ACA", you will find the exemption requirement. Below is one:
"Exemptions from the ACA requirements and IRS payments
Under certain circumstances, you won’t have to make the individual responsibility payment. This is called an “exemption.”
You qualify for an exemption if:
You are outside the USA for more than 330 days that year, or
You’re uninsured for less than 3 months of the year, or
You have Medicare, Medicaid, or some VA coverages, or
You have such low annual income that you are not required to file with the IRS: see the IRS Interactive Tax Assistant (ITA).
The second item (from the Healthcare.gov websites) seems to mean that you can go back to the USA for up to 3 months without getting health insurance, but that is overruled by:
You qualify for an exemption to ACA / Obamacare if you are outside the USA at least 330 days per calendar year. (330 day “physical presence” rule) irs.gov/pub/irs-pdf/p54.pdf"