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Watts, BTU's Volts and all that stuff for water
bpost
Member Posts: 32,664 ✭✭✭✭
I know some basic electronics but am rusty on power and BTU's so if you can help I'd appreciate it.
I installed a new electric water heater. I put a timer on it. We are trying to determine when the standby loss is so great that the timer becomes a losing proposition. The timer is now set to shut off at 8:00 PM and turn back on at 7:00 AM. It is a high efficiency heater with lots of insulation around the tank.
The heater has two 4500 watt elements, supply voltage is measured at 224 VAC, under load.
If the heater is OFF for X time and the water temp drops from 120 to 80 degrees during that time how many watts are used to heat the water back up to 120 degrees. The tank holds 50 gallons.
I guess it would take knowing how many watts it takes to heat a gallon of water forty degrees, I don't remember that class, it was 20 years ago. [:0][:I]
I installed a new electric water heater. I put a timer on it. We are trying to determine when the standby loss is so great that the timer becomes a losing proposition. The timer is now set to shut off at 8:00 PM and turn back on at 7:00 AM. It is a high efficiency heater with lots of insulation around the tank.
The heater has two 4500 watt elements, supply voltage is measured at 224 VAC, under load.
If the heater is OFF for X time and the water temp drops from 120 to 80 degrees during that time how many watts are used to heat the water back up to 120 degrees. The tank holds 50 gallons.
I guess it would take knowing how many watts it takes to heat a gallon of water forty degrees, I don't remember that class, it was 20 years ago. [:0][:I]
Comments
I will be interested in the answer, if you can get an accurate answer.
I know some basic electronics but am rusty on power and BTU's so if you can help I'd appreciate it.
I installed a new electric water heater. I put a timer on it. We are trying to determine when the standby loss is so great that the timer becomes a losing proposition. The timer is now set to shut off at 8:00 PM and turn back on at 7:00 AM. It is a high efficiency heater with lots of insulation around the tank.
The heater has two 4500 watt elements, supply voltage is measured at 224 VAC, under load.
If the heater is OFF for X time and the water temp drops from 120 to 80 degrees during that time how many watts are used to heat the water back up to 120 degrees. The tank holds 50 gallons.
I guess it would take knowing how many watts it takes to heat a gallon of water forty degrees, I don't remember that class, it was 20 years ago. [:0][:I]
Shutting down during periods of non-use will never be a bottom line loss. Here's how that works: at full operating temp, your heater loses X BTU per hour, all of which is replaced by X BTU of electric input just to maintain temp. As the temp drops, the BTU loss decreases, so the total BTU loss is less, and the elec needed to replace the lost BTU and restore temp is less.
So, it will take 417.5BTU to heat that tank 1 degree in 1hr.
If you need 40degrees of heat, you're looking at 16,700BTU/hr.
If probably takes 1/2hr to heat, so double that BTU to 33,400.
33,400btu/hr = roughly 9,780W = 10 KWH but for only 1/2hr = 5KWH
All of this would rely on a 100% effecient heating system, which we all know isn't feasible.
I could be entirely wrong however.
Temp rise needed @ 40 degrees
BTU formula already posted so: 417x40=16680BTU's needed.
4500 watt element (only one runs at a time) makes 15355 BTU/hour.
So it is taking over an hour of strait running to make up the heat. If you have demand billing where some hours cost more than others it may cost more to shut it off than let it keep temp.
50 Gallons of water = 8.35lb/gal * 50 = 417.5lbs
So, it will take 417.5BTU to heat that tank 1 degree in 1hr.
If you need 40degrees of heat, you're looking at 16,700BTU/hr.
If probably takes 1/2hr to heat, so double that BTU to 33,400.
33,400btu/hr = roughly 9,780W = 10 KWH but for only 1/2hr = 5KWH
All of this would rely on a 100% effecient heating system, which we all know isn't feasible.
I could be entirely wrong however.
Your very close, but only one element runs at a time so it will take over an hour to rebuild the heat.
Post, the Voltage really doesn't matter unless you want to know the Amp Draw which would be roughly 20A.
You will have to know how much a kilowatt/hour costs you.
Here in Idaho we have the lowest electrical rate at 7.5 cents per kilowatt hour.
Margaret Thatcher
"There are three kinds of lies: lies, damned lies and statistics."
Mark Twain