In my 30-plus years as a plumbing contractor, I’ve replaced countless water heaters. Our company, which works in the San Francisco Bay area, installs three basic kinds: conventional gas models, tankless models, and — increasingly — condensing storage heaters. Customers who want to save energy nearly always ask for a tankless water heater. That’s no surprise — tankless heaters have been getting a lot of media coverage and are the only efficient heaters most people have heard of.
In most cases, however, we recommend condensing storage heaters to our replacement customers, because they are arguably more efficient than tankless models and — when used to replace an existing heater — frequently less expensive to install. For homeowners, switching from a conventional heater to a condensing model is not a big change. If they consume the same amount of hot water as before, they’ll have lower gas bills and run out of hot water less often. Switching to a tankless heater, by contrast, requires some lifestyle adjustments: The homeowners will have to wait for the heater to produce hot water and they won’t be able to get it at very low flow rates (see “Is a Tankless Heater Right for the Job?”).
In this article, I’ll discuss condensing storage heaters and how they’re installed. Since natural gas is the primary fuel in our region, I’ll be describing gas-fired models, many of which can be field-converted for propane.
How They Work
In several respects, a condensing storage heater is like a conventional model. Both burn gas, have exhaust flues, and store hot water in an insulated tank. But a condensing heater is much more efficient because of how heat is transferred to the water.
In a conventional heater, the fuel is burned in an open chamber, and hot combustion gas rises through a flue in the center of the tank. A lot of this heat is transferred to the water in the storage tank, but a good portion exits through the vent pipe and is wasted.
In a condensing heater, a draft-inducing fan pushes air and fuel into a sealed combustion chamber inside the tank. As the fuel burns, combustion gas is exhausted through a secondary heat exchanger — a coiled steel tube submerged inside the tank. The combustion chamber and heat exchanger have large surface areas to maximize heat transfer to the water. So much heat is transferred that the combustion gases cool to the point where the water vapor in the exhaust stream condenses, releasing its latent heat, which is also transferred to the stored water. By the time the exhaust gas leaves the heater, it’s cool enough to be safely vented through inexpensive plastic plumbing pipe. (Each manufacturer accepts different kinds of pipe; options include specified types of PVC, CPVC and ABS. All are far less expensive than stainless steel.)
A cutaway drawing of a Vertex heater (above) shows the combustion chamber and secondary heat exchanger coil inside the tank. In the Phoenix model at right, the combustion chamber and secondary heat exchanger are located in the upper part of the tank, with a second heat exchanger for a solar thermal collector below.
The thermal efficiency (TE) of a condensing storage heater is quite high, typically between 90 and 96 percent. (For an explanation of thermal efficiency standards, see “Making Sense of Gas Water-Heater Ratings.”) Standby losses are low because the storage tanks are covered with thick foam insulation — plus these units all have electronic ignition, so there is no standing pilot.