GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Understanding water heater specs

user-5946022 | Posted in Energy Efficiency and Durability on

I have some questions about tank type water heaters:

How do you compare the sizing needed for a gas tank vs. electric heat pump type tank. I read (perhaps on this site?) that heat pump type tank water heaters have much slower recovery and thus, for the same hot water demand, a house would need a larger tank with the heat pump type.
1. Is there a chart to compare needed tank sizes between gas & heat pump type tanks, including how the recovery rates of a specific tank impact this?

Regarding gas tanks, none of the gas tank direct vent (ie NON power vented) water heaters seem to be energy star rated.
2. Are there any Energy Star rated Direct Vent water heaters available in the US? (ie a direct vent gas tank that does not need electricity to work)?
3. Does anyone sell a gas vented water heater (not direct vent, but with a fan) whose fan works off the gas instead of needing electricity?

4. Can anyone explain why these two direct vent AO Smith units have different first hour ratings – ie what other factors influence this besides tank size, btu’s and energy factor?
Model G6-DVS4038NV, 40 gal, 38k btu, 0.6 energy factor, 64 gallon first hour
Model GDV-40, 40 gal, 38k btu, 0.6 energy factor, 72 gallon first hour

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Here are some resources:

    1. Check out this useful page from the Department of Energy: Sizing a New Water Heater. Scroll down to the section that begins, "Sizing Storage and Heat Pump (with Tank) Water Heaters."

    The page notes, "To properly size a storage water heater for your home -- including a heat pump water heater with a tank -- use the water heater's first hour rating. ... The EnergyGuide label lists the first hour rating in the top left corner as 'Capacity (first hour rating).' The Federal Trade Commission requires an EnergyGuide label on all new conventional storage water heaters but not on heat pump water heaters. Product literature from a manufacturer may also provide the first hour rating. Look for water heater models with a first hour rating that matches within 1 or 2 gallons of your peak hour demand -- the daily peak 1-hour hot water demand for your home."

    I am also reproducing a worksheet from the DOE page (see below). For more complete information, visit the web page.

    2. You should also check out this previous Q&A thread on GBA: Heat pump hot water heater tank size.

    3. Remember two rules of thumb: with a heat-pump water heater, a bigger tank is better than a smaller tank; and installing a drainwater heat recovery device can make a heater with a small tank behave like a heater with a big tank. For more information on drainwater heat recovery, see Drainwater Heat Recovery Can Lower Your HERS Score.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |