All other values being equal, i did a model with a gas tank @ 0.83UEF and a gas tankless at 0.92UEF. I expected the tankless to consume less gas, but the output showed slightly more gas used by the tankless. Is there another category I have to alter when i switch from modeling a tank to a tankless gas water heater?
Thanks
One thing to note about the gas tankless water heaters is that there is an assumed derate on the efficiency consistent with what's in ANSI-RESNET 301 of 0.92 for EF and 0.94 for UEF: https://www.resnet.us/wp-content/uplo... This factor is intended to account for the impact of cycling: as the tankless comes up to temperature when a draw starts and cools down after it ends, a little bit of energy is wasted. So the actual efficiency will be closer to 0.92 * 0.94 = 0.8742.
That's not enough to explain the full difference here, since that's still higher than the 0.83 UEF of the tanked unit. So one possibility here is that the gas tank is undersized and not actually fully meeting the load. You might want to check the hourly outputs here to see if the gas tank is able to fully meet the load, or the outlet temperature is dropping during times of high hot water use (the tankless will ALWAYS perfectly meet the load). If that is the case, you might need to upsize the tank/burner for the amount of hot water use you have in your model. Water use scales mostly with the number of occupants, although other factors (like if you have a dishwasher or not, or low flow fixtures) matter too.
Another possibility my colleague brought up: if you have the water heaters in the conditioned space and a furnace, the tank losses will reduce space heating loads, but the tankless will not.
If you can share your BEopt project file I can take a look at this specific scenario and probably give you a more exact answer on what's going on in this case.
Asked: 2025-03-05 13:31:33 -0500
Seen: 94 times
Last updated: Mar 06
