replace gas with electric water heater

[young707]

according my PG&E bill:
Gas 12.13333 Therms @ $1.06335 = $12.90
Electric
Baseline Usage 286 Kwh @ $0.12845
101 - 130% Baseline 63.2667 Kwh @ $0.14602
total $45.98
How do I compare the electric and gas between Therms and Kwh?

[jadnashua]

for 100,000BTU in electric, it would cost you around $4.27; for 100,000BTU in gas (one therm) $1.06. Now, with an electric WH, you get almost all (not quite) of the energy applied as heat, but with NG, some of it goes out the flue and doesn't heat. A low-end WH is about 60% efficient, some are over 90%. It would have to be about 25% efficient to cost the same as electric, and none are that bad.

Plus, a NG WH will recover quicker than an electric one, at least most of them.

[Gary Swart]

My gas water heater is a power vent and uses pipe. What this means is, there is very little heat loss through the vent. Anyway you slice this pie, natural gas is far cheaper than electricity.

[Dana]

for 100,000BTU in electric, it would cost you around $4.27; for 100,000BTU in gas (one therm) $1.06. Now, with an electric WH, you get almost all (not quite) of the energy applied as heat, but with NG, some of it goes out the flue and doesn't heat. A low-end WH is about 60% efficient, some are over 90%. It would have to be about 25% efficient to cost the same as electric, and none are that bad.

Plus, a NG WH will recover quicker than an electric one, at least most of them.

Using source-fuel BTUs isn't a very good way to estimate when efficiency factors of the equipment differ so widely. You can't just use the source-fuel BTUs to come up with a good cost comparison.

The EF of an electric tank is ~0.90, the EF of a typical atmospheric-drafted tank is about 0.55.

So for that $1.06 therm of gas you're actually only getting (0.55 EF x 100,000 BTU=) 55,000 BTUs in hot water...

...at a cost of ($1.06/55=) 1.9 cents/1000BTU

With the 0.90EF electric tank that 14.6 cent kwh of electricity you get only (0.90 EF x 3412 BTU/kwh =) 3071 BTU in hot water...

... at a cost of (14.6/3.071=) 4.75 cents/1000BTU

So heating hot water with electricity would be about (4.75/1.9=) 2.6x as expensive as using gas.

While still dramatically more expensive, it's much less egregious than the $4.27/$1.06= ~ 4x cost multiplier estimate based on source-fuel BTU only, ignoring efficiency factors.

Very rare is the local US market where heating hot water is cheaper with electricity is cheaper than heating with natural gas. If you have 5-cent/kwh electricity it's still possible though, and those markets DO exist (but not in CA.)

[Runs with bison]

Locally the cost of service for gas runs in the ~$30/month range, tax included. The gas itself is dirt cheap, running less than $0.60/ccF with tax at present. However, if I didn't have gas heating I would consider electric water heating, even though it is far less thermodynamically efficient overall (when the power generation loss is figured in.) Electric water heating has greater health concerns, slow recovery, etc., but eliminating a fixed monthly charge has an appeal when you spend less than $10/month for fuel for actual water heating for a family of four (our winter time usage, summer is 1/3 less.)

[young707]

Thank you all your comments.
Now you convinced me and we are going back to gas water heater.

Its just would be nice if we can easily turn the heater off and on, for long or short trip we regularly take.

[jadnashua]

Almost all gas WH have a vacation position on the gas valve...it's pretty simple! The pilot light, if it has one, doesn't use much.

[Runs with bison]

Its just would be nice if we can easily turn the heater off and on, for long or short trip we regularly take.

From what I've found, gas usage while away for 10 days is rather low, about 1 ccF when I measured it. I haven't tried shutting off the gas or going to lower temps (vac'n mode). I also haven't tried to measure a gas rate of pilot only...but consider that the pilot running helps maintain tank temp, so the pilot gas usage rate sets the lower bound. The primary reason to leave setpoint where it normally runs is that temps below 120 F provide a good breeding ground for Legionella bacteria. This is also why I don't recommend messing with timers (that and the lack of payout for them unless one is on electric and has peak rate charges.) Gas water heating is not as susceptible to legionella, while electric is.

If you want to reduce water heating costs, insulate exposed hot water lines (but not around the flue area unless you use a non-flammable, non melting insulation), put some insulation on the T&P line and its valve, and if your tank has only about 1" of insulation, add a water heater blanket to roughly double its insulation value. Better still, consider 1.5/1.6 gpm shower heads and HE washing machines.

Insulating lines makes a lot of difference in how soon you get hot water on repeated draws and even on first draws. I wouldn't build a house today without requiring all hot water lines be insulated all the way to the fixture.

[Dana]

Thank you all your comments.
Now you convinced me and we are going back to gas water heater.

Its just would be nice if we can easily turn the heater off and on, for long or short trip we regularly take.

Turn the thermostat all the way down, then back up to your desired setpoint when you return. With an electronic ignition unit that would cut the standby loss by well over half.

That works for standing-pilots too, but it doesn't turn off the pilot, so it's standby use will have a lower-limit determined by the BTU-rating of the pilot, which is substantial (about 5-7 therms per month), but at a buck a therm that's 17-24 cents/day. While not big money, it's not breaking the bank even for a 3 week vacation.

What Runs with bison said, insulate all of the hot water distribution plumbing, and even the cold-feed within 10' of the tank , and the temperature & pressure outflow piping with 5/8" wall closed cell pipe insulation. It'll save an order of magnitude more money in the course of a year than turning off the HW for a few weeks of vacation time, and save water too, since you'll have higher temps available between successive draws. See:

http://www.leaningpinesoftware.com/h...er_pipes.shtml



According to in-situ measurements by CA utilities in the past decade, in a typical CA home something like 15-20% of all hot water heating energy gets abandoned in the distribution plumbing. Under IECC 2012 code, hot water distribution plumbing must now be insulated to at least R3 (half-inch wall closed cell foam), but it's still cost effective long term to take it up to R4 (5/8"-wall foam.)

[young707]

I did not know I should flash the water heater tank? How often?