The Evolution of High-Efficiency Water Heaters
By Bob "Hot Rod" Rohr
Radiant Expert
Ready for a crash course in high-efficiency water heaters? Here’s the scoop.
The basic tank styled water heater has seen a number of high-tech changes over the past decade. Manufacturers are now offering a wider range of high efficiency tank type heaters. Stainless steel is the choice for some, yet others offer a glazed steel tank.
Along with the condensing high-efficiency burners comes the ability to vent the byproducts of combustion with schedule 40 PVC, CPVC or ABS materials. Vent lengths vary based on pipe size and the number of fittings. Some water heaters will push 100 ft. of developed vent length. Some models vent with 2-in. pipe. In my shop, I have installed a 130,000 BTU input 80-gallon model with the internal coil, vented with 2 in. schedule 40 PVC.
Yet another feature is the sealed combustion design. Just as the flue products are vented outside, combustion air is piped directly to the unit. With sealed combustion there are no more concerns about sucking airborne chemicals into the combustion process. It’s not unusual to see cleaning chemicals stored along side a water heater in a commercial application. The warm space seems to attract this sort of clutter.
Sidewall venting is allowed on all the models with which I am familiar. Attention to the vent termination location is critical. Concentric kits are available for 2 and 3 in. sized flues to provide intake and exhaust in one pipe penetration. The side wall venting option opens a lot of doors for the retrofit market. I’ve also seen PVC inserted inside the old abandon B vent pipe for a condensing appliance update.
Some of the current versions of condensing heaters have much quieter fan assemblies than the models I installed 10 or more years ago. A quieter blower allows a unit to be installed in a mechanical closet space alongside hotel rooms, for example.
At least one manufacturers offer models with an additional heat exchanger coil inside. This allows for some space heating without the worries or liability issues involved with open loop combined DHW/heating applications.
One manufacturer has upped the ante yet again by offering a modulating burner version. It also is available with or without an additional heat exchanger coil in the bottom. This clever design would allow for a thermal solar interface to supplement the DHW production all in one neat small footprint package. By providing the coil in the bottom, the heat exchanger coil sees the coldest incoming water temperature to maximize the solar delta-T.
The burner in the solar version is located midway up to allow the solar more “working room.”
I know engineers and owners are faced with a lot of choices for DHW production these days. Instantaneous seems to be the rage. Time will tell how well these perform in the widely diverse water conditions this country presents. I notice some of the valve companies offer special shut off/purge valves to facilitate heat exchanger flushing. With instantaneous type heaters I worry about longevity issues. Consider the effect of a large burner flame under a very small tube bundle with who-knows-what in the combustion air if it is drawn from the room.
With well-insulated, stainless steel tanked, condensing technology, I’d put my money on these high-efficiency heaters for lowest lifecycle and operating costs in the long run.
Sometimes there is no substitute for a dump load appliance. I’ve seen condensing water heaters offered all the way to 130-gallon capacity and input ranges up to 499,900 btu/hour. ASME listed units are offered by some manufacturers.
A digital read out for temperature and diagnostic is common. Multi-colored lights indicate the sequence of operation on some models. Take some time to shop all the brands currently on the market for the size and features that best fit your application.
Hope this helps! Reach me at www.showmeradiant.com with questions, concerns and ideas.
Bob "Hot Rod" Rohr is particularly interested in hybrid radiant systems -- systems that utilize solar and wood energy in addition to, or in place of, fossil fuel. Also, systems that incorporate different methods of heat delivery. He owns and operates Show Me Radiant Heat and Solar, Inc. in southwest Missouri. Reach hot rod at hotrodradiant@aol.com or by phone at (417) 753-3998.