Customer called for service on filter for for tankless hot water heater. Service technician checked water heater operation. Heater only goes into high fire when water flow is over 7 gallons per minute. Checked hot water in master bedroom. Ran hot water heater with four different faucets and gallon per minutes was at 6, but water temp drops to below 99° and continues to fall. At colder outside temps the water heater may have a hard time maintaining a 140° temp due to cold street water temp. House has 5 full bathrooms. 2-1/2 baths. 2 laundry rooms and a kitchen. Problems seem to stem from water flow rates and design issues. The manufacturer of water heater is out of business. Recommend replacement of tankless hot water heater. Customer very satisfied with prompt courteous service. Customer will make decision on replacement. Customer will highly recommend Advanced Heating & Cooling.