08-05-2007, 03:31 AM
Part two of my post.
On-Demand Water Heaters:
On-Demand Water Heaters are more efficient because there is only a tiny amount of stored hot water that can leak energy when there is no demand. The more expensive units also have efficient copper (expensive) heat exchangers. The more efficient (and expensive) units have power vents that force the burn products through a laberynth type heat exchanger to extract more heat. The exhaust air is used to pre-heat the incoming cold water for instance. Some units go so far as to use co-axial intake/exhaust tube where the inside tube that exhausts the burn waste is surrounded by an outer coaxial tube that pulls in combustion air that is pre-heated by the inner tube.
Installation considerations are of course venting, electricity for power venting or electronic control units, adequate gas plumbing and sometimes place to route a condensate tube. The high burner on a stove might put out 15,000 BTU but the burner on a on-demand water heater may have a 100,000 BTU burner. Such a burner will probably require 3/4 inch plumbing from the tank all the way to the heater. If the tank is more than 50' from the heater, you may need 1". The manual for the water heater will tell you what the plumbing requirements are. The manufacturer web sites will generally have manuals that you can download and study before you purchase.
Larry
On-Demand Water Heaters:
On-Demand Water Heaters are more efficient because there is only a tiny amount of stored hot water that can leak energy when there is no demand. The more expensive units also have efficient copper (expensive) heat exchangers. The more efficient (and expensive) units have power vents that force the burn products through a laberynth type heat exchanger to extract more heat. The exhaust air is used to pre-heat the incoming cold water for instance. Some units go so far as to use co-axial intake/exhaust tube where the inside tube that exhausts the burn waste is surrounded by an outer coaxial tube that pulls in combustion air that is pre-heated by the inner tube.
Installation considerations are of course venting, electricity for power venting or electronic control units, adequate gas plumbing and sometimes place to route a condensate tube. The high burner on a stove might put out 15,000 BTU but the burner on a on-demand water heater may have a 100,000 BTU burner. Such a burner will probably require 3/4 inch plumbing from the tank all the way to the heater. If the tank is more than 50' from the heater, you may need 1". The manual for the water heater will tell you what the plumbing requirements are. The manufacturer web sites will generally have manuals that you can download and study before you purchase.
Larry
Larry