Tuesday, August 6, 2013

Tankless Water Heater Musings

One way of saving money and energy that is touted greatly is a tankless water heater. As I understand it there is still some controversy over just how much money this will save you, and they tend to be rather expensive so the payback can be long. A year ago or so I strapped a temp sensor to the inlet and outlet pipes of my traditional tank water heater so I could measure it's usage and see just how much it's running when nobody is using water to give myself some idea of the use of a tankless water heater here.  Specifically they are dallas 1-wire sensors connected to a Barix, Barionet 50 and finally reading their data in to XTension.

Here is a few recent days worth of data. Yes, I know I have the temperature turned up too high, but this is the kitchen, laundry and master bedroom heater and not the one in the kids rooms. My kids are big enough now that it's not that much of a danger to them. The second water heater that serves their rooms  is still turned down to 120 though so don't worry about us.

The blue line is the cold inlet temp and the red line is the hot output line. The first thing I see is that the cold inlet temp rapidly reaches the same temp as the outlet as soon as you stop using water. That makes me suspect there isn't a heat trap or whatever that fixture is that is supposed to reduce heat loss through the pipe on that line. I'll have to investigate that. But the other thing you see is that in these 3 days there isn't a single point where the cold water inlet temp spikes without a corresponding spike in the outlet temp. Meaning that the water heater hasn't run once not in response to demand in the last 3 days. As it turns out I have to go back to July 24th to find a graph that shows a "maintenance" cycle just to keep the tank warm.

There, at 2:30 in the morning the cold temp spikes with no hot water use measured. I have to go back several weeks again from this to find another one. If I switched to a tankless heater those would go away, and any energy wasted by them. But how much would that be? 2 or 3 waste cycles a month isn't very much to pay back the high cost of a tankless water heater.

The other factor to consider is efficiency or how well does it put the energy that it uses into the water. It was my understanding years ago that electric tank water heaters are nearly 100% efficient, meaning that almost none of the energy is lost, it all goes into the water. Gas water heaters like mine do lose quite a bit of energy up the smokestack, but so do gas tankless. This is an area for further research, but I do not believe that it's a huge difference at this point. At least not enough to make me replace a currently functional hot water heater. When it's time to replace it though I will look seriously again at these figures and it may make sense in the future.

There is an additional consideration as well in cost. Gas tankless water heaters require a lot of gas, they have a minimum pipe size in order to get enough gas. At the placement in my attic where it would replace the existing water heater I would have to have new gas piping run to support a tankless of enough capacity for my home. At considerable expense. The same holds true for electric, you have to have a high amp service run to where it is, they can't just be plugged in to a handy outlet. They use a huge amount of power. If you're on a peak metering system that changes you more if you use large amounts of power your electric tankless is very likely to put you close to or over the limit so the power you use for it may be much more expensive. Do the math on this or you may be very unpleasantly surprised.

Just thinking out loud here, everyones situation is different and it may make perfect sense for you to get yourself one of these. If you use a LOT of hot water you'll never run out, and if you use very little hot water you dont waste energy keeping the tank warm. Though if the tank is like mine the number of those phantom cycles might be very few.


  1. The heat traps, as I understand it, are little expansion tanks. Like a little 2.5 gallons tank hanging off your main tank. So I think you'd know if you had or didn't have one of them. My last hot water heater didn't and when I had a new one installed, they didn't put one on either. Seems like one of those things that's a good idea in theory, but maybe the marginal loss isn't enough to justify adding another thing to leak onto the plumbing.

    Speaking of that, I notice your inlet temperature is already 80-100°. I'll confess that I don't know how cold Georgia winters are, but in Utah the water in winter was almost ice cold. I'd think colder weather would increase your maintaince cycles since / if the hot water heater is in a unheated or poorly heated space (usually its a space that's vented to outside anyway, in the case of a gas hot water heater)...also the cooler inlet water would cause more of a parasitic loss if that expansion tank is not present.

  2. The little tanks are expansion tanks, and I'm not entirely sure the purpose they serve. Plumbing is generally a task I hire out to professionals due to lack of interest and the very expensive cost of DIY system failure, not to mention it's effect on my spousal project approval rating should water bring the ceiling down... But they keep your pipes from breaking as the water expands and contracts in a closed or high pressure system perhaps? The heat trap is just a couple of inches of pipe that has some plastic inside and does some magic to keep the heat in. I originally though that the convection was through the metal of the pipe but looking at pictures of it perhaps they are meant to keep the water itself from moving backwards up the pipe, cooling and falling back down into the tank to be replaced by more hot water? So i've now read the wikipedia article on expansion tanks and so should you as the picture they use in an example is from a 1987 Saab 90.

    My water heater is in the attic, so in the absence of water flow the pipe there eventually returns to ambient temp which can be quite high during the day. All that solar energy in the attic and my water heater is actually insulated to keep it OUT ;) Have to build some kind of a heat exchanger loop for it someday. There is a neat system that puts a pex loop just all along the ridge vent and lets you capture some heat from the attic as it vents. But we are saving up to completely foam in the attic which would put an end to that. I'm seriously considering some DIY loops just inside some of the rafter space close to the underside of the roof deck, but someone would almost certainly drive a roofing nail into one if I needed roof repair or shingle replacement so I haven't done more than dream about it. We shall see...

  3. This is only prove that Tankless Water Heater really works great. Compare to conventional water heating, tankless is a lot better.

    However, considering the dependency of fuel, it is not reasonable enough to use. Again, tankless works great. However for me, I'll prefer to use solar water heating as it is natural, renewable energy source, and safe to the environment.

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