Solar veteran Roger Perry gives his two penneth about on-demand water heaters as viable solar water heating back-up candidates.
CAN I — USE AN ON-DEMAND HEATER AS SOLAR BACK-UP?
All solar water heaters in Maryland, D.C., and the Mid-Atlantic provide a family of four with around 75% of their annual hot water load. Most customers use their existing electric or gas sources for the remaining 25% backup (usually needed dead winter.) On-demand water heaters have a couple of advantages over tank-type heaters, but they are somewhat mitigated with a solar system.
The touted advantages of on-demand heaters:
Endless hot water– In a solar system with a properly sized back-up you will, pretty much, have endless hot water in the summer and plenty of hot water the rest of the time.
Large savings because of no stand-by losses– The claims of “up to 40% savings” are a bit exaggerated in all but the oldest water heaters. Our solar tanks are extremely well insulated and their stand-by losses are minimal. Also, in a solar water heater, the tank is an integral part of the system so the small stand-by losses are a fact of life.
Then there are the disadvantages with on-demand heaters:
On-demand heaters can get very confused when fed water that is already hot– This can result in temperature fluctuations as the heater tries to adjust or, in extreme cases, the heater shutting down on high limit. One way we have gotten around this is to use a motorized 3-way valve that bypasses the on-demand heater. This works well but adds to the complexity of an alredy complex piece of equipment, which leads me to…
They are very complex pieces of equipment– A standard water heater, gas or electric, has a bimetallic spring that moves to make an electrical connection when it cools, and moves to break that connection when it heats up. It doesn’t get a lot simpler than that. An on-demand heater must sense the flow of water (demand), measure the outgoing water temperature, and adjust the amount of gas to feed the burners so as to maintain a constant temperature at the tap. It also has to do this very quickly so as to not deliver water that is too cold when another tap is turned on or too hot when another tap is turned off. After a couple of years, scale begins to build up in the water valve; the unit needs to be descaled or it will stop working. For safety reasons, the default for most problems in any of these systems is for the unit to shut down.
All of this said, believe it or not, I do like these systems and would recommend one to anyone who could not get a solar system for some reason. I just don’t think they are worth the added headaches as a back-up for a solar water heating system.