Bill Doering, whose "green" Santa Barbara remodel was featured in the Real Estate section of the L.A. Times, has gotten quite a few questions regarding his tankless, on-demand water heater. So, as Bill is an engineer who runs an energy-oriented engineering firm, and as engineer types tend to be pretty thorough, he has more clarification on the issue. Here's what he said:
I wanted to address a few concerns from people interested in on-demand heaters regarding "the time waiting for hot water."
A number of people have asked me about this and I was quick to explain that indeed you do have to wait for hot water with an on-demand hot water heater. So, optimizing its location and the plumbing runs is the correct and efficient thing to do.
However, what I failed to mention is that the additional time waiting for hot water is not significant when compared with the time waiting for hot water with a traditional tank-based system. The point is that in both cases the water in the pipes between the point of use and the water heater (on-demand or tank-based) has to be expelled before the hot water can get to the faucet — in both cases you have to initially wait for hot water.
In the case of an on-demand hot water heater you do have to wait longer because when the burners fire up it takes a few seconds for the heated water to get to full temperature. How much longer?
Well, it depends on the specific environment. But I did run a very crude test with my on-demand unit that was about 45 feet away from the kitchen sink and compared that to a faucet that was about the same distance away from a traditional tank based heater in another house.
A 45′ run from my on-demand heater took 75 seconds to get hot water.
A 40′ run from a traditional tank based heater took 45 seconds to get hot water (in another house)
This is a very crude comparison, but it highlights that this issue is not unique to on-demand water heaters.
What if you don’t want to wait for hot water? Or you don’t want to waste water?
There are a few things you can do:
1) optimize the on-demand heater location and pipe runs. Use two heaters, if necessary.
2) install a recirculating system at the points of use that are furthest from the heater (these recirculating systems can be turned “on” when you want to use the hot water and don’t have to on all the time)
3) install an electric on-demand water heater at the points of use that you are concerned about.
4) pre-heaters are available in dishwashers and clothes washers
5) wash your hands in cold water. I do.
On that last point, unless hot water is near boiling it won’t kill any gems. Hand washing is about soap and scrubbing not hot, hot water.
We put our on-demand unit near the bathrooms so that the water would be hot for showers almost immediately. And it is.
More questions? Please post below. Or you can reach Bill at bill@DoeringDesignandEngineering.com.