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Jeremy Hillary Boob, PhD 01-30-2012 09:34 AM

electric versus propane water heater
 
I'm not sure whether this is the correct forum for this question, but here goes...

I've got an electric 40 gallon water heater and was considering replacing it with a 40 gallon propane water heater. It seems that most people in this area believe propane is a much better way to heat water, but it seems to me that propane will be more expensive.

Here's my calculation.

BTUs
------
per gallon of propane: 91500
per kwh of electricity: 3412

Efficiency ("energy factor")
-------------------------------
40 gallon electric water heater: ranges from 0.9 to 0.95
40 gallon propane water heater: ranges from 0.59 to 0.67

Let's suppose an electric water heater has an efficiency of 0.9 and a comparable propane unit has an efficiency of 0.67 (note that I'm taking the low number for the electric and the high number for propane, which should skew the numbers slightly in favor of propane). Then

"Effective" btus when heating water
------------------------------------------
per gallon of propane: 91500 * 0.67 = 61305
per kwh: 3412 * 0.9 = 3071

The ratio here is 61305 / 3071 = 20 (approximately). That is, we effectively get 20 times as much water heating from 1 gallon of propane as from 1 kwh of electricity.

The cost of my electricity is $0.12 per kwh up to some limit, and then it goes to $0.15 per kwh. So, lets take the higher number, $0.15 per kwh. That means propane would have to cost less than

$0.15 * 20 = $3.00 per gallon

to be cheaper than electric for water heating (and at the $0.12 per kwh, it would have to be less than $2.40 per gallon). I haven't checked propane rates in a while, but it seems that it's at or above $3/gallon right now. In addition, propane water heaters are more expensive than electric. So, electric water heating wins.

Am I missing something here?

TarheelTerp 01-30-2012 10:55 AM

Quote:

Originally Posted by Jeremy Hillary Boob, PhD (Post 839130)
It seems that most people in this area (and where might that be?) believe propane is a much better way to heat water, but it seems to me that propane will be more expensive.

Fuel cost is only one of several factors in the "much better way" evaluation.
Natural gas is almost always the best bargain /Btu.

Another key ingredient and very high on my list is 24-7/365 reliability.
When storms or other power outages happen (as they always will)...
my plain vanilla WH will produce hot water so long as the gas is flowing;
and my plain vanilla gas stove will always be cooking us food.

AandPDan 01-30-2012 11:11 AM

Another thing you should consider is recovery times. Electric water heaters are much slower in recovery than a gas fired unit.

If you have a large demand for hot water you won't have to wait as long for recovery.

hummer4x4guy 01-30-2012 11:49 AM

Have you looked into the geospring water heater from GE? I am going to be posting about it asking for input from anyone who has one also. I am 50/50 on this water heater but because of other work I want to do on my house I may need to switch from my gas fired heater to electic.

Daniel Holzman 01-30-2012 12:55 PM

I have a Geospring GE hot water heater. I purchased it about 1-1/2 years ago to replace an indirect oil fired hot water system. The Geospring unit is an air to air heat pump, meaning it extracts heat from my basement air, and uses it to heat hot water. It is claimed to be about twice the efficiency of a conventional electric water heater.

This is difficult to evaluate, because the efficiency of a heat pump is measured differently than the efficiency of a fuel based system. In a fuel based system, the overall efficiency is simply the ratio of the actual amount of heat extracted from the fuel to the theoretical amount of heat in the fuel. For gas hot water systems, this can be as high as 95 percent, more typically in the mid 80 percent range. For electric hot water heaters, efficiency is 95 percent or so. For fuel oil hot water heaters, efficiency may be as low as 60 percent.

With a heat exchanger, efficiency is defined as the ratio of the amount of energy you get out of the exchanger to the amount of energy you put in. The ratio can be as high as 4:1 with the GE system, which sounds like free energy, but the missing link is that you are extracting energy from the air surrounding the heat exchanger, and that energy had to come from somewhere. In the summer, my basement is warm and damp, and the heat exchanger is very "efficient", with a ratio probably approaching 3. In the winter, when the air is dry and cool, the ratio is much lower, and in fact the heating coils have to operate to heat the water much of the time. Over the course of a year, I really don't know how much less costly the system is than a straight electric heater, but for sure in the summer it costs about 1/2 to 1/3 what it would cost to run an electric system.

The kicker of course is the price of the system. It cost me $1400 for the unit, with the energy efficiency rebate from taxes the effective cost was $1000, for a 50 gallon system, or at least $500 more than a straight electric heater. I pay close to 19 cents per kilowatt hour, so I thought it would be worth it, and it has operated flawlessly for the time I have had it. If I get ten years out of it, it will probably pay for itself.

Jeremy Hillary Boob, PhD 01-30-2012 03:54 PM

Quote:

Originally Posted by AandPDan (Post 839188)
Another thing you should consider is recovery times. Electric water heaters are much slower in recovery than a gas fired unit.

If you have a large demand for hot water you won't have to wait as long for recovery.

This is definitely an issue. With our current electric 40 gallon unit, by the end of the 2nd shower, it's getting cold. Still, with 4 of us here, and a little care, we never run short of hot water.

Jeremy Hillary Boob, PhD 01-30-2012 04:04 PM

Quote:

Originally Posted by TarheelTerp (Post 839170)
Fuel cost is only one of several factors in the "much better way" evaluation.
Natural gas is almost always the best bargain /Btu.

Another key ingredient and very high on my list is 24-7/365 reliability.
When storms or other power outages happen (as they always will)...
my plain vanilla WH will produce hot water so long as the gas is flowing;
and my plain vanilla gas stove will always be cooking us food.

Another good point. We have shoddy electric service here in the Santa Cruz mountains of California. Our electricity goes out at least a couple times each winter---sometimes for days on end. The good news is that I've got a standby generator on an automatic transfer switch (I installed all of that myself, and it was an adventure). The generator is necessary to keep our pellet stove going in the winter, so we don't freeze. The bad news is that the generator doesn't supply power to the water heater, so we've got no hot water when the electricity is out.

One thing I've considered is getting a propane water heater and keeping the electric water heater. Then, I could switch back and forth based on the current price of propane/electricity. A side benefit would be that we would have hot water during power outages. Is something like this reasonable?

AandPDan 01-30-2012 05:23 PM

Quote:

Originally Posted by Jeremy Hillary Boob, PhD (Post 839419)
One thing I've considered is getting a propane water heater and keeping the electric water heater. Then, I could switch back and forth based on the current price of propane/electricity. A side benefit would be that we would have hot water during power outages. Is something like this reasonable?

It's doable but you don't want to keep water stagnant. You couldn't just say shut off the electric unit and use the gas one with the water just sitting in the electric. Bacteria can grow.

Legionella is just one such bacteria.


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