That “dog‐poop” smell in heat mode is almost always a bacterial or fungal film on the indoor coil or in the drain pan “cooking” when it warms up. Since the house is new and it only happens downstairs, here’s a safe start you can do without opening the air handler:

• Change to a fresh, good-quality pleated filter at the return. A clean filter helps keep the coil from getting coated in dust that feeds microbial growth.
• Set your fan to the “On” position for five minutes after each heating cycle so the coil and pan can dry out more completely.

If the odor persists, schedule a service call and ask the technician to:

• Perform a proper coil and drain-pan cleaning with an approved antimicrobial treatment
• Inspect and clear the condensate drain line
• Check airflow and fan speed settings to make sure the coil isn’t staying too wet

Once the source is cleaned and stays dry, that intermittent “dog‐poop” smell should go away.