Will a water tank before my hot water heater help keep my hot water hotter longer?


Will a water tank before my hot water heater help keep my hot water hotter longer?

Asked by Michael Centanni

I live in Washington DC where most of the water is surface water (ie. the Potomac River). In the winter the cold water out of the tap gets down well below 40 degrees, sometimes as low as 34 or 35. The result is that when we shower we run out of hot water rather quickly. The cold water coming into the hot water heater cools down the hot water and the cold water mixing at the shower cools down the hot water. The result is to keep turning down the cold which bring more hot water in which bring more cold into the hot water heater which lowers the water temperatur. I was wondering if putting a tank (maybe 50 to 100 gal) before the water heater which would warm up overnight to the ambient temperature in my basement would help this problem since the water going into the water heater would not be as cold and therefore take less time to heat and not drop the temp as much. What kind of tank would I look for and where might I find it?

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David Bergman's picture

At first glance, though I haven't heard of this being done, it sounds like a reasonable approach, similar to the way that a solar hot water system or a drain water heat recovery system preheats the water before sending to the hot water heater. By storing the incoming cold water indoors and letting it warm up to room temperature, the "Delta T" -- the difference between the incoming cold water temperature and the temperature that the hot water heater has to heat to -- is reduced. So the heater has less work to do.

But you asked whether it would keep your water hotter LONGER, by which I'm guessing you mean whether you'll run out of hot water less often. That's a different question from whether it would reduce how much the water has to be heated. Assuming you're talking about using a conventional hot water heater (versus a tankless heater), the limitation on how much hot water you have available really has to do with the capacity of the tank. Once you have used what is heated in the tank (do you have teenage kids?), even if the water is pre-warmed, it still has to refill and heat the water. That cold jolt when the hot water runs out about may be a little less nasty, but it will still happen.

With tankless or on-demand systems, the equation is different. Whether you get that unwanted morning startle has more do with the flow rate of the heater. In this case, pre-warming the water may indeed help some.

If you want to get more analytical about this, what you'd appear to be gaining is energy savings by cutting down that Delta T. But even that may be illusory since that heat has to come from somewhere. In other words, in the winter, you have to heat the house in order to have a warm home for that preheat tank. It's another example of the thermodynamic version of "there's no such thing as a free lunch." It's not as true in the summer, by the way, since you're probably trying to cool the house anyway. In those warm months, you may indeed be able to eat your cake and have it, too.

By the way, a better first step may be simply to insulate the hot water tank. There are kits for this. Unless you have a good new heater, they tend to be pretty inefficient, losing a lot of their heat while just standing around waiting to be used.