# Bit of Hot Water Tank Science

• 10-11-2007, 11:39 AM
molo
Bit of Hot Water Tank Science
How would you figure out if it is cost effective to turn off your water tank for a certain number of days. eg. We have an 82 gallon electric hot water tank. How many days away from home could justify turning it off, and turning it back on to reheat upon returning to the home; 4, 10, 20 days? How would you determine this?

TIA,
Molo
• 10-11-2007, 11:55 AM
markts30
It is hard to say...
Where are you located?
What time of year is it going to be? (outside temps etc...)
Do you have other forms of heating that will be left on?
Is any piping in un-heated/poorly insulated areas?
If you are going for a while and not planning on heating, are you going to have the house winterized?

Unless you really need to, I would just turn the temp down to the lowest setting (vacation mode etc) and leave it as is... You should not use too much power doing that....
• 10-11-2007, 11:59 AM
molo
The space will be kept 45-50 degrees. The pipes are uninsulated. Cold New York.
• 10-11-2007, 12:16 PM
You need to do an experiment. YOu'll need an accurate thermometer. Right after the burner finishes heating things up, take a small sample of water to determine the starting temp. Turn the burner off so it can't run. Either overnight, or preferably more often, take another small sample to develop a curve of how fast the temp drops (the quality of the insulation). Knowing how many hours it takes to cool off the tank's worth of water, you can figure out how many BTU's it would take to just keep it hot. If you are really lucky, the specs are available on the manufacturer's website, but they often have the tank at a nominal 70-degrees. The bigger the delta between the room temp and the water temp, the faster the thing loses heat.

You also might get an idea by monitoring how often the burner turns on when there is no useage. This would require knowing and believing the accuracy of the thermostat and its repeatability. Say it allows a 10-degree drop before it turns back on, and it takes 4-hours, with 40-gallons, you can figure out how many BTU's it is losing. 40 gallons * ~8 pounds, * 10-degrees = BTU. Extropolate that for the time it would take to get to the room temp.
• 10-11-2007, 12:23 PM
master plumber mark
depends on how long and how old
It really depends on how long you are going to be gone and how old the wate heater is...

All you have to do is shut it down at the breaker
and it only takes 30 minutes to heat back up anyway...
when you arrive home.....

if the heater is old , sometimes just turning it off could
cause the elements or thermostats not to come back on
or burn out when it is switched back on.....

I doubt that the cost of the electricity would be more than one meal out
while you are on vacation

• 10-11-2007, 04:43 PM
molo
Quote:

The bigger the delta between the room temp and the water temp, the faster the thing loses heat.

.

This raises an interesing question. The tank is in an area that stays around 70 during the summer, but in winter gets into the 30-40 range. I've wondered about how to determine how much heat the heater is losing during these cold months. Does anybody have an idea how I can determine this?

TIA,
Molo
• 10-11-2007, 04:59 PM
theelviscerator
Just turn it to low heat most have a vacation setting...
• 10-11-2007, 06:16 PM
GrumpyPlumber
Molo, in an area where the temps might get to freezing...it just might not be a good idea to leave it off.
"Vacation" is a good idea.
• 10-11-2007, 07:02 PM
Same thing...see how long it takes in between running when there is no use. You can relate the time to the temperature change the thermostat allows to the size of the tank.
• 10-11-2007, 07:55 PM
molo
Along the lines of MPM's "tankless experiment", Is there some way I could instal a usage meter on this electric water heater to isolate the usage of the heater?

Molo
• 10-11-2007, 08:53 PM
hj
heater
It is immaterial. If the heater is going to be idle for any period of time, you can turn it off. If still has some hot water in it when you return, it will just use the same electicity as if it was maintaining the temperature, and if it is cold, then it only uses the energy to fill the tank, which will be less than if it were maintaining the heat.
• 10-11-2007, 09:28 PM
molo
I'm not going to turn it off, It is allready at it's lowest setting. I'll just leave it there. I am interested in the isolated meter for monitoring month to month usage and determining how the temperature of the room effects the operation
• 10-11-2007, 10:04 PM
Dunbar Plumbing
Just put one of these in.....set on low and it will heat the entire house in the winter.

I just had to take a picture at the supply house this morning.

No two-wheeler for this one. Maybe a fork lift.

http://img.photobucket.com/albums/v70/DUNBAR/BAG.jpg

That heater is going into a Senior Citizen Care center.

Gotta keep those bedpan washers piping hot!!!
• 10-11-2007, 11:38 PM
Mike Swearingen
I read somewhere once that an electric water heater can represent up to 14% of your electric bill.
I turn the water heater breaker and my water OFF even if I just leave for a weekend. I had an inrteresting flood experience over a weekend once with a burst washing machine hose.
Mike
• 10-12-2007, 03:24 AM
Marlin336
Quote:

Originally Posted by Mike Swearingen
I read somewhere once that an electric water heater can represent up to 14% of your electric bill.
I turn the water heater breaker and my water OFF even if I just leave for a weekend. I had an inrteresting flood experience over a weekend once with a burst washing machine hose.
Mike

It doesn't hurt to shut the main when you go away. The only thing it could possibly damage is your boiler if it springs a leak and you don't have a low water cutoff.