Breakdown with Mark Irons: Water heaters
By Mark Irons
Heating hot water
accounts for 25 to 45 percent of all the energy consumed in our homes, second only to home heat in winter months. The dollar cost of gas for cooking, hot water and home heating is about half that of electricity. The efficiency of natural gas water heaters and forced air furnaces has improved greatly in the last couple of decades, saving on utility bills and resources. In the past, all forced air furnaces and hot water heaters were about 85 percent efficient, meaning only 85 percent of the heat potential or BTUs were transferred to actual heat, while the rest of the un-combusted gases went up the flue. Today, state of the art furnaces and hot water heaters can be 96 percent efficient. High efficiency forced air furnaces and water heaters are about double the cost of lower efficiency units, but they pay for themselves in gas savings within a couple years. They also have the advantage of not requiring access to an expensive metal flue which is required to vent above the roof line.
Tankless water heaters, which heat water on demand, have become very popular. Without a storage tank, so called demand heaters take up less space and in our mild climate they can even be installed on the exterior of the home. They save on standby losses, or losses to pilots and a standing tank of water radiating heat when there is no demand. Until recently, the average tankless heater averaged only 82 percent thermal efficiency, compared to the much higher-rating
condensation units already mentioned.
Because tankless heaters depend on minimum flow before the heat exchanger kicks in, they do have drawbacks in distribution. They suffer from a phenomenon called a cold water sandwich which occurs between rapid, consecutive uses, when a slug of cold water is left in the unit between each demand for hot water. The newest hybrid condensing tankless unit is one option that eliminates the cold water sandwich.
Radiant hydronic heat entails circulating low temperature hot water through tubing beneath masonry floors for ground floor units and through wall radiators for units above. Concrete or stone floors are ideal for radiant heat because they transfer the heat gradually and uniformly, though they take longer to ramp up to capacity. State of the art radiators can take the chill off spaces in 10-15 minutes. In some cases heat tubing is stapled to the underside of wooden floors, but the efficiency is reduced because wood conducts heat less efficiently than concrete or masonry.
Radiant heat eliminates forced air and therefore circulation of dust and other particulates which aggravate conditions like asthma. Radiant systems utilize a central boiler, which can also heat domestic hot water, and which can can control heat in multiple zones through distribution manifolds and thermostats for each area. These boilers can also do double duty for heating hot water, but the mechanics can be extremely complex and warrant a highly experienced professional for both design and installation.
Excellent, detailed explanations of tankless and condenser water heaters can be found in articles at plumbers Jim Lunt and Leigh Marymor’s website. I also want to offer thanks to master plumber Andrew Carothers-Liske for information on radiant heat systems.
In a future installment I’ll discuss water use, including low flow toilets, which flush much more efficiently than when they were introduced, and also green plumbing systems like solar water heaters and gray water disposal.
Mark Irons is an Alameda-based builder. You can learn more about him by checking out his website.