April 28th, 2008

The Low Down on Tankless Hot Water Heaters

Posted by Jason Pelletier, Low Impact Living

hugg itdigg it add to del.icio.ustechnorati

Tankless hot water heaters are becoming a must-have appliance in homes these days. They purport to offer many benefits - instant and endless hot water, less wasted space, and a smaller carbon footprint as well. They can even be used to power radiant underfloor heating that is a luxury found in many green homes.

But are they truly the best option? What can you expect from one, and what do you have to worry about with installation? Finally, are there other options available that offer the same benefits without some of the challenges? We’ll try to cover most of that in this piece.

First off, some basics about how tankless hot water heaters work. Standard non-electric storage hot water heaters typically are about 60% efficient, meaning that 60% of the energy in whatever fuel they burn is converted into heat for your water. The other 40% of energy is wasted, some due to inefficient burners and some due to the gradual cooling of water stored in the tank through time. Tankless or on-demand hot water heaters address both of these issues: they eliminate the storage-related losses, and they employ sophisticated burners and controls to improve on the combustion efficiency. Top-of-the-line models can be 80-85% efficient, which results in a 30+% savings on your hot water energy bills and your carbon footprint compared to your tank in the corner. (electric unit savings will be lower, since storage electric HW heaters already have efficient heating mechanisms).

There are some important caveats associated with tankless hot water heaters, though. If you’re considering one, make sure you take into account the following issues when you’re weighing the pros and cons:

1. Instantaneous hot water means it will be hot at the outlet of the water heater right away, not necessarily at your tap. If you have a home where your shower or sink is far away from the hot water heater, replacing a storage with a tankless hot water heater might not solve your cold start problems. All of that water sitting in your hot water pipes will lose heat quickly, and you’ll have to clear it out when you turn on the shower before you get to hot. Many modern green homes employ new plumbing techniques where the hot water heater is located in the center of the house and each fixture is individually plumbed with smaller and less expensive plastic tubing to solve this problem and reduce hot water pipe losses.
2. Tankless hot water heaters consume more fuel per unit of time than do standard hot water heaters, since they can’t rely on the stored hot water in the tank. This means that you might have to replace your gas line with a larger diameter one, all the way from the gas meter to your hot water heater. This can be expensive depending on your layout. Most tankless HW heaters need a 3/4″ line, while most older homes have 1/2″ lines by default.
3. Proper sizing is critical. The required size of a tankless HW unit depends on two main factors: the temperature of the water coming in, and the maximum demand you expect at any one time. When people are disappointed by their new heater’s performance, it is often because they under-sized the unit, either because they under-estimated how frequently they need multiple hot water uses or they underestimated how cold their water is coming into their house. Most manufacturers provide charts, tables or calculators to help with proper sizing - make sure you are honest about your uses, and make sure you follow the manufacturer recommendations. And, be sure to plan based on your winter water temperature - it would be a real bummer to wake up on the coldest day of the winter to find out that your new water heater underperforms!
4. You might have to improve your venting. Tankless HW heaters burn alot of fuel, so your existing venting might not be sufficient. You may have to install larger, straighter ducting, or perhaps install the unit outside. Both can add to the cost, so be prepared.

The tankless hot water heater segment is increasingly crowded. All major manufacturers make models that will suffice for an entire single family home and that meet the upcoming Energy Star standards for tankless hot water heaters. Top manufacturers such as GE, Bosch, Noritz, Rinnai, and Takagi all make models that comply with the performance characteristics of Energy Star, but we found it interesting that the warranties for current whole-house models from only three of the five companies met the 10 year heat exchanger / five year parts levels set by Energy Star: the new GE 9.4 GPM model (10/5), the Rinnai R75LSi (12/5) and Takagi Mobius T-M199 (10/5). The Noritz 0751 and the Bosch 2700ES met the performance characteristics, but the warranties fell short of what will be required for Energy Star labeling in the future. We suspect that all will soon upgrade their warranty programs to meet Energy Star standards.

What about other ways to get the same benefits? Today, you have many options.

• First off, solar hot water heaters are the most environmentally friendly units you can buy, and they’re much more affordable than a solar electric system. You still get attractive rebates and tax incentives in many areas to offset the cost. They’re still 2x as expensive as a tankless hot water heater, but if you’re really trying to cut your footprint this is the way to go.
• There are several new ultra-efficient storage hot water heaters that are even more efficient than the best tankess versions - up to 95% efficient! How do they do that? They employ specialized burners, heat exchangers and insulation to get the absolute most heat out of your fuel. AO Smith’s Vertex hot water heater is good example. You’ll probably pay about the same or perhaps a bit more as compared to a tankless version, and they are large, but they are even more efficient.
• Consider either a point-of-use tankless hot water system or a recirculating pump if you have a decent hot water heater but are tired of waiting a minute or two for hot water each time you take a shower. These units fit right under your sink and will provide almost instant hot water. The point-of-use hot water heaters (such as the Bosch PowerStar) are just smaller, less complicated versions of electric tankless heaters, so they have the same benefits. A recirculating pump will redirect the cold water that has accumulated in your hot water pipe back to your heater, thereby saving water and some energy in the process. You can connect them to switches, timers or motion sensors so that there’s hot water waiting before you even get to the bathroom. These pumps can be HUGE wasters of energy, though, if they are used for a long time each day. If you are eco-minded, make sure you buy controls that only run the pump when you truly need hot water. Versions that allow this kind of customization include the RedyTemp and the Metlund D’MAND system.

If you are wondering how much these various options will save in your home, create a profile in Low Impact Living’s Environmental Impact Calculator and go to the Projects page. There you can review how much you can save by installing a tankless or high-efficiency storage (such as the Vertex above) hot water heater versus just insulating your current version.

Efficient hot water heating is a true green no-brainer. Not only are they light on the planet, but they will also increase the comfort of your home and the effectiveness of your appliances. Fortunately, you now have many great options to choose from!