ASHRAE Journal - April 2011 - 16

letters
Condensing Water Heaters
I enjoy Emerging Technologies because it makes me think. The column on condensing water heaters in January raised an ongoing concern I have that rated efficiencies for condensing DHW tanks significantly overstate actual or as installed thermal efficiencies. The point at which condensation from flue gasses begins depends on factors such as the amount of excess air, air density and the amount of humidity in the combustion air. I have done calculations that indicate that in most installations condensing won’t happen until flue gasses are cooled below 130°F. If there is a 20°F DT across the flue gas heat exchanger, condensation won’t begin until the flue gasses are in a part of the heat exchanger in which the surrounding water is at or below 110°F. When condensation begins from natural gas-fired flue gasses, about 86% or 87% of the available thermal energy from the flue gasses has been extracted. How do the tank manufacturers get enough thermal stratification to keep the water at the bottom of the DHW tank, or at the cold end of the heat exchanger, cold enough to get the condensation needed to achieve higher levels of thermal efficiency? The test procedures to estimate thermal efficiency that I reviewed a few years ago operated the tank under a steadystate water draw that would ensure that the bottom of the tank was cold, but I don’t think this is how DHW tanks operate in typical installations, especially those with DHW recirculation. I think condensing tanks operate at markedly higher efficiencies than conventional tanks, but until I get a sound technical explanation of how they achieve condensing in typical installations during normal operation, I remain doubtful that they will achieve their rated efficiencies. Bert Phillips, P.Eng., Member ASHRAE, Winnipeg, MB, Canada
AJcover_gatefoldversion.indd 2 12/15/2010 3:49:18 PM

The Authors Respond
We agree completely with several points in Bert’s letter. First, the paragraph summarizing what it takes to get natural gas-fired flue gas to condense is accurate. More importantly, the observation in the first sentence of the last paragraph “I think condensing tanks operate at markedly higher efficiencies than conventional tanks....” highlights an important efficiency characteristic of condensing water heaters, namely that the efficiency of condensing water heaters is significantly better than that of conventional natural draft, center flue gas water heaters, whether operating in full condensing mode or not. Here are our thoughts on the installed and operated efficiency of condensing gas water heaters vs. the rated efficiency. The rated efficiency does depend on (and benefit from) the test procedure and test conditions. On the residential side, the DOE efficiency test is run starting with a tank full of water at the setpoint speci16 ASHRAE Journal

fied for the test (135°F), then there are six 10.7 gallon draws at one hour intervals, then the energy input (to maintain the setpoint temperature) continues to be monitored for the remainder of the 24 hour total test period. This captures standby losses, at least relative to the energy required to heat 6 × 10.7 gallons of water per day. In this procedure, a fresh 10.7 gallons of water at the specified input temperature (58°F) is delivered to the bottom of the tank every hour six times, so for much of the time of the test where fresh water is being heated, the water at the bottom of the tank is cold enough to cause a significant amount of condensation from the flue gas. We think Mr. Phillips is right that commercial gas water heaters are tested on a steady-state basis, so that fresh cold inlet water is always being supplied to the bottom of the tank, maintaining condensation from the flue gas. We agree that it is debatable how good a real-world representation either test procedure is. On the residential side, the finite draws are reasonably representative of large draws such as showers and laundry loads, not representative of the numerous smaller draws for hand washing and other small clean-up tasks. On the commercial side, hot water usage patterns vary drastically, ranging from intermittent hand washing in office buildings (not well represented by a steady draw test procedure) to much steadier usage in health clubs, hospitals and hotels (maybe reasonably well represented by a steady draw test procedure). Recirculation loops in commercial building hot water systems don’t necessarily change this picture much. While the warm water returning from the recirculation loop typically goes back to the inlet of the water heater, the cold vs. hot water at the bottom of the tank situation might not be impacted much. At the extreme of intermittent, small hot water draws, not much condensing is happening anyway, so mixing warm return water with already warm water in the bottom of the tank won’t change anything. At the other extreme, if large amounts of hot water are being drawn, the return water from the recirculation loop will mix with the much higher flow rate of inlet supply water, with the mixed temperature probably still being low enough to cause condensing. In between these extremes, the warm water returning from the recirculation loop could be raising the average temperature in the bottom of the tank enough to reduce the amount of condensing that is occurring. Another important real-world variable is the seasonal and geographic variation of cold water supply temperatures and differences in user setting of the hot water supply temperature. When those temperatures are toward the higher end of the range, less condensing will occur; when they are lower, more will. So, I think that the bottom line is that condensing water heaters will not always attain their rated efficiency. In some applications on an average annual basis they will; in others they won’t. When selecting a water heater, the anticipated usage patterns should be considered in light of the qualitative points in this discussion. Again, even without any condensing, the condensing gas water heater efficiency is significantly better than conventional natural draft, center flue gas water heaters, because the thermal efficiency is incrementally higher and standby losses are drastically reduced. John Dieckmann, Member ASHRAE; Alissa Cooperman; James Brodrick, Ph.D. Member ASHRAE
ashrae.org April 2011

ASHRAE Journal - April 2011

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