Water Heater Physics and Economics 101

[fingpilot]

OK, so here's the setup.

Private well, one gallon per second unrestricted flow. 1/2 HP submerged 'torpedo' stlye well pump. No flow restrictions in house, all 3/4 supply lines, no pressure drop anywhere when toilet(s) flush, etc. 50 gallon propane Energy-Star water heater, wrapped in insul, and in an interior utility room.

Temp on water heater is currently set to just warmer than necessary for hottest shower. Dishwasher has aux heat if necessary to get sanitary wash temp.

Two hypothetical questions for the PP Brain Trust.

1) Keep water heater setting as is. Most shower water then comes from heater, neither of us ever runs out. Minimum mixing of cold water to get desired temp at showerhead. Seems most efficient to me. So long as shower temp selector is not on max hot, but close to it, is theorhetical 'best'.

2) Conventional wisdom. Set water heater hotter, thus using 'less' hot water in each shower due to mixing with cold to get same desired temp. My thinking is that all the variables being equal, will require more propane to get and keep water hotter when not being used for shower.

Wish there was a programable water heater thermostat for this kind of situation.

What's the consensus?

[Clay Perrine]

Replace water heater with a tankless version.

No propane used to keep the contents of the tank hot.

[fingpilot]

Not really an option.

Brand new house, and would be throwing a perfectly good water heater (and the money it cost) down the drain. One of my previous places had a tankless, and it never got hot enough for an endless shower. Took it out and put in a conventional WH again. Maybe things have gotten better with those, but my experience was that they are a total failure.

[Ned, NYNA11]

I believe that the lowest temperature setting reduces the heat loss.

Radiant losses, a function of temp difference between the water and the outside air temperature is less with lower water temp.

the smaller temperature difference between the water temperature in the pipes and the air temperature will reduce the conductive loss.

The primary heat transfer in the tank is convective as the cold water comes in the bottom and heats up as it mixes with the already heated water. Convective heat transfer in the pipes is neglible,

[Rot 911]

What Ned said. You want the lowest temperature at the hot water heater that you find comfortable. Less heat loss at the tank.

[cmccuist]

There is no way you're getting 1 gallon per second through a 3/4" line. Not with that pump.

That's 60 gallons per minute!!

That equates to a velocity of 43 feet per second in the piping (6 fps is a good number).

Anyway, that wasn't the question.

I believe option 1 is correct as you are not having to cool the hot water in order to take a shower and you aren't keeping the water in the tank at an unnecessarily high temp.

[fingpilot]

Sorry, I should have been clearer about the well. The gallon a sec was from the well head 4" pipe. Is 1 and 1/2 inch pipe down a 200' drop in elevation to the house from the well. Well pump sits 250' below well head. I sit on extended slope of Mt. Lassen. Incredibly good water here year round. My point was that water pressure and flow were not issues in the house.

I think my question has been answered.... I'll keep the water heater temp just above the full hot setting of our showers.

During the summer months here (ambient 100F daytimes), shower was half hot and cold setting. Now getting 40F nights and only 65F daytimes, just last week, had to bump water heater temp up, and started thinking about the question. Have burned less than 75 gallons of propane (cooktop and water heater) since May. All heating is with woodstove/blower. Neighbors up here have a heat exchanger on their woodstove flue for aux water heating as well.