diff --git a/bpeng/ecm/__init__.py b/bpeng/ecm/__init__.py
new file mode 100644
index 0000000000000000000000000000000000000000..e69de29bb2d1d6434b8b29ae775ad8c2e48c5391
diff --git a/bpeng/ecm/low_flow.py b/bpeng/ecm/low_flow.py
index c06d296d2b58e70788bc6614b1e41751460e3e78..7fcd6f4187478d15572af6a00d8c2afed2814346 100644
--- a/bpeng/ecm/low_flow.py
+++ b/bpeng/ecm/low_flow.py
@@ -1,4 +1,80 @@
 """
-    This module calculates the water usage and energy savings for low flow fixtures (toilets, faucets, and showerheads)
+This module calculates the water usage and energy savings for
+low flow fixtures (toilets, faucets, and showerheads). The
+algorithm is adapted from the EPA WaterSense calculation
+method.
+
+https://www.epa.gov/watersense/how-watersense-calculator-works
 
 """
+DAYS_PER_YEAR = 365             # number of days in a year
+HEAT_CAP_H2O = 1                # [BTU] energy required to raise one pound of water 1 degree F
+RHO_H2O = 8.34                  # [lb/US gal] density of water, assumed at 50F
+
+
+def toilet_usage_save(old_flush, new_flush, resident_count=2.6, flush_count=5.05, day_span=365):
+    """
+    Calculate annual savings on water usage from low-flow toilet replacement
+
+    Args:
+        old_flush (float): gallons of water per flush of existing toilet
+        new_flush (float): gallons of water per flush of new toilet
+        resident_count (int): number of individuals in the building, defaults to 2.6 (EPA avg)
+        flush_count (float): number of flushes per day, defaults to 5.05 (EPA avg)
+        day_span (int): number of days in savings period, default gives annual
+
+    Returns:
+        float: annual water savings [gallons]
+    """
+    gal_save = (old_flush - new_flush) * flush_count * resident_count * day_span
+
+    return gal_save
+
+
+def heated_fix_save(old_flow, new_flow, resident_count, run_time):
+    """
+    Calculate savings on water usage from fixtures that use hot water, such
+    as showerheads and faucets.
+
+    Args:
+        old_flow (float): flow rate [gal/min] of existing fixture
+        new_flow (float): flow rate [gal/min] of new fixture
+        resident_count (int): number of individuals in the building
+        run_time (float): time [min/person] fixture is dispensing water
+
+    Returns:
+        float: water savings [gallons]
+    """
+    gal_save = (new_flow - old_flow) * run_time * resident_count
+
+    return gal_save
+
+
+def heat_fix_energy(gallons, supply_temp=120, fresh_temp=55, hot_ratio=.73):
+    """
+    Calculate energy savings from fixtures that use hot water, such
+    as showerheads and faucets.
+
+    Args:
+
+        gallons (float)
+            gallons of water used
+
+        supply_temp (int, optional)
+            temperature of heated water supplied to building, typically 120F.
+
+        fresh_temp (int, optional)
+            temperature of fresh water to be heated, defaults to 55F
+
+        hot_ratio (float, optional)
+            fraction of fixture water usage assumed to be heated, defaults to .73
+
+    Returns:
+
+        float: energy [MMBTU] used to heat water
+
+    """
+
+    heat_used = (supply_temp - fresh_temp) * HEAT_CAP_H2O * RHO_H2O * gallons * hot_ratio * 0.000001
+
+    return heat_used