IIRC, the general rule is a tank water heater provides approx 2/3 of its gallon size in usable heated water. So a 66 gal tank gives you about 44 gallons of "usable" 125ðF water ... although of course you get more effective capacity in a shower when the hot flow is tempered by adding cold.

Don't guess at the gpm rate of that rainfall shower. Measure it by timing the fill of a 1-gal bucket at whatever is the flow rate you normally run. Also check the manufacturer specs for comparison against your timing test.

All that being said, maybe your water heater should be checked for a bad element.

dadoes, your general rule (which I think is more like 80/20) is just for electric water heaters, not gas. With an electric one, the area below the lower element is typically unheated (cold). With a gas unit, the burner is at the bottom so that the whole tank heats up.

Additionally, the recovery (the time it takes to reheat a tank of cold water) is lethargic with an electric unit. While a gas one can often reheat a cold tank in an hour or less, an electric one can take 2-3 hours. So that when the hot water in an electric's tank is used, that's it for a long while.

jb3710, once you've filled a bathtub or two and then started the dishwasher, there's not much hot water left to use from a 65 gallon electric tank. If you want to do all that usage within a period that's too short for an electric heater to recover (presuming gas is not available), you'll need either another heater plumbed in parallel or a larger sized tank.

You'll get a bit more capacity by turning up the temperature setting. If your dishwasher has an internal heater for water, it'll work fine with cold water, start it after your shower and bathe the kids earlier.

This post was edited by snidely on Sat, May 25, 13 at 17:32

The kids will only get bigger and those quick tub baths will turn into long showers. You need to look at a long term solution. Like putting in an 80-100 gallon heater. Preferably gas. Or, doing a gas tankless for your master and still upgrading the tank size for the kids/utility end of the house.

Need more information. Children bathe at 7:30-- we get that. Do the children get only a few minutes of hot water or do they get all that they want? The next use at 9:45 should have plenty of hot water even if the heater was started from cold at 7:30. So is the few minutes of hot water at all times or only after 9:45?

Have you maybe lost your lower element?

I recently lost the function in my lower element. It meant that all of the sudden our hot water time in the shower was cut extremely short (5-10 minutes). We only have a 50 gallon tank, all electric.

I noticed that if I left it long enough, we'd have hot water again, so knew my entire hot water tank hadn't failed.

Turns out, the lower element does most of the work in a hot water heater and ours failed. Replaced it (not an expensive part, but the plumber to hire was expensive. Had I known how easy it was to do, I'd have done it myself. The next time though, we'll likely be due for a new tank).

Ours happened to fail about the time of a remodel. Coincidental. Perhaps yours is the same. Maybe the rainhead has less to do with a drastically short shower time than you think.

Really good one question. And i really appreciate your thought.