Home & Garden Columns

About the House: Time to Consider an On-Demand Water Heater

By Matt Cantor
Friday August 31, 2007

The Europeans have had it all over us for some decades when it comes to energy efficiency. This might have something to do with a political attitude toward wasting energy or sheer economy. In any event, our European brothers and sisters are more inclined to pinch a BTU (that a British Thermal Unit for those of you new to the energy game). 

One of the things that they’ve long embraced has been small, “on-demand,” “flash” or “tankless” water heaters. Before I get into a description of what an on-demand water heater is, let’s take a brief look at “tanked” conventional water heaters. 

A conventional gas water heater (electric water heaters are worthy of discussion but are relatively rare in our area) is made up of a vertically mounted water tank (usually 30-50 gallons) with a gas burner mounted at the bottom. Water is heated in the tank until it reaches the set temperature (adjustable at the front with a non-specific dial, i.e. warm, hot, hotter) and the water is held at that temperature by the intermittent operation of the burner. In other words, the burner comes on many times each day to keep the water at the desired temperature. This is true whether you are home, sleeping or at work. All so that you’ll be able to enjoy a nice hot shower whenever you’re in the mood. 

This requires a lot more energy and cost than if you heated the water as you needed it (which we’ll get back to that in a little while). 

These water heaters take up several square feet of space and need to be mounted on a floor or stand (in a garage, they should be placed on a stand so that the flame is at least 18” above the floor to prevent ignition of gas fumes unless a special FVIR model is used). 

Water heaters are also very heavy and can become a bull in a china shop during an earthquake. If they do move more than a tiny bit during an earthquake they can break their gas lines and cause a gas build-up and explosion. This is one of the really serious seismic issues everyone should be looking at and every “tanked” water heater should be heavily strapped, top and bottom (and a third if they’re over 50 gallons). 

Lastly, a conventional water heater runs out of hot water after a while and must “recover” which can take anywhere from 15-45 minutes, depending on the model. In short, your shower is over at some point. 

An on-demand water heater is different from this conventional gas model in several respects. First, there’s no tank (I’m not actually this stupid. Yes, of course you knew that). Instead, there is a very long coil of tubing seated above a large burner. When you turn on your hot tap, water begins to flow through the coil and a sensor picks up the movement. The burner comes on and heats the water as it flows through the coil. The whole heating process takes a few seconds and hot water emerges at the other end of the coil of tubing. You have hot water in a few seconds at your point of use (shower, sink, laundry).  

This means that you get to have hot water for as long as you want. It never run out. You can stay in the shower all afternoon! What a concept. 

Because you are only heating water when you need it, you end up saving serious money on your gas bill. You also save the earth a little bit by decreasing green house gas emission (CO2) from your home.  

You may save two hundred or more dollars per year with a unit like this. Because these units are not filled with many gallons of water, they are much lighter and much less likely to move during an earthquake. This means that the likelihood of a gas explosion after an earthquake is much less. The units are typically bolted to a wall as a part of their normal installation and many models are designed to be hung on an exterior wall. These features decrease gas danger even further.  

Lastly, I frequently get asked by my clients how we can get the water heater out of laundry room or kitchen to make more room and the installation of an on-demand on an outside wall is often my response. Although these “new wave” of water heaters are costlier, they can give you back a room that’s been taken over by that looming old figure by the stove. With the increased efficiency, lower operating cost, added interior space and increased seismic safety, it’s hard to argue against these newcomers, even at their higher cost.  

A number of models exist (I’m only looking at gas units at this point, but electrics do exist) and new ones are coming along every year. If you decide to go this route, make sure you get a large enough model so that you don’t run out of water when you’re running shower, laundry and sink. Although these units keep producing hot water, they do have limits on how much they produce per minute. It is best to talk to an expert about the model that’s right for you. A downside you should expect with these units is that you will have to turn the water to a good flow to get them to kick on. They can generally be throttled down somewhat afterwards.  

Life is good. You can finally achieve your life’s goal of staying in the shower all afternoon. 

The above article was published in the Berkeley Daily Planet in June 2005 and, since you’ve all gotten so much smarter since then, there are some finer points I think you’re now ready to handle so here’s a little update: 

On demand water heaters do a lot of heating in a very short time and so need a huge burner (about three times as powerful as your central furnace). This means that they need a big gas line.  

When installing one, you should expect to have to provide at least a three-fourth of an inch gas line to the unit. If you’re more than about 30 feet away, it may have to be 1 inch. If you get a more powerful unit, the gas line may be bigger. This calculation is a critical part of the plumber’s job. Putting the device closer to the gas main is a good idea since it can not only decrease the length (and cost) of the gas piping, it can also decrease the size (and more cost) of the gas piping. Try to put the unit on the outside of the house and on the same side as your gas main (unless your main is on the front since this will look lousy). 

It now looks as though these devices have a very bad relationship with galvanized water pipe and can be killed in as little as two years if the piping leading to them is galvanized steel (the old stuff). For now, there doesn’t seem to be a service repair for the part that gets damaged by steel so it’s very important that you have your unit installed with copper piping coming to the unit from the street (3/4” is best for most houses). 

Lastly, placement of the unit on the outside can save hundreds of dollars on the purchase of very expensive stainless steel flue piping as well as creating more inside space. Remember how these burn a whole lotta gas. Well, the corrosion caused by this is too much for the common double-wall metal flue that we usually love so, if you have your unit inside (including inside your basement, garage or crawlspace) you have to spend mucho bucks on Class 3 stainless flue pipe. This can easily add hundreds of dollars SO, save the money and put it out with the cat. Make sure that the unit is at least a couple of feet away from (and not below) an openable window so that the exhaust doesn’t blow back inside. Be proud if you make this bold move. It’s a not-all-that-small part of saving the planet. 


Got a question about home repairs and inspections? Send them to Matt Cantor at mgcantor@pacbell.net.