Is a tank water heater or gas tankless water heater more environmentally friendly?

In My Humble Opinion
1

Basically, what the subject line says. And by environmentally friendly, I mean the emission of greenhouse gases.

There’s been talk recently about preferring electric ranges over gas ranges. How does this apply to gas tankless water heaters? Does the fact that tankless heaters only come on when you need them greatly reduce the amount of greenhouse gases emitted? Versus electric tank water heaters which keep a 50 gallon tank hot constantly?

Power in my state is largely generated by natural gas, with nuclear power, coal, hydroelectric, and renewables in these proportions (I’m eliminating all of the 0’s and “mega” to simplify, but the proportions are right):
Natural Gas: 44
Nuclear: 22
Coal: 9
Hydro: 3
Renewables: 7

There are 2 people in my house and I’d characterize our hot water use as low to moderate.

I realize every situation will be different and there’s no 1 right answer, but given the above info, is electric tank or gas tankless clearly better?

Thanks,
J.

2

You may want to consider throwing into the mix, electric tankless water heaters.

3

Our old electric tank heater, which came with our condo, crapped out last month, so I just had a tankless gas heater installed instead. It’s LPG rather than natural gas (which isn’t available here), but based on my research it’s still supposed to be much cheaper than electricity. It’s certainly more fun - as much hot water as we want, whenever we want, no wait. It also takes much less room, although that’ll only really matter when I get around to removing the old decommissioned tank (which will probably be never).

Incidentally, I considered installing a solar/electric water heater, which is even more cost effective in my climate, but seeing as I’m on the 3rd floor of 9 in my building, the amount of insulation needed to make sure hot water actually reaches my apartment is prohibitively expensive, not to mention that even if I installed it, I’d still have to run the faucet for five minutes for the water to get here.

4

Which are severely inadequate for whole house use.

5

Given that, my sense is that you might be better off with an electric tank water heater. The newer ones have a lot of insulation to minimize heat loss in the stored water and if your showers aren’t insanely long (as are mine), you might need just a thirty-gallon tank.

6

The most environmentally friendly is probably going to be a heat-pump based electric water heater. They are more efficient, because it takes less energy to move heat from in your house into the water than it does to create heat in the water. The problem is they are more complex and more expensive than either gas or electric water heaters.

Most things I hear in the last decade or so do not like tankless water heaters from an energy use perspective. I’m not sure exactly why, except that keeping the water hot in a modern hot water heater does not take very much energy at all.

The dirty air problem with gas stoves is because they vent into the house. A gas water heater will send exhaust gases out a chimney. In my case, one shared with the furnace.

If you go electric and pay time of use rates, then you can do like in the video below. He only lets his water heater turn on when electricity is cheap, and just uses from the tank the rest of the time. That only makes sense if it’s automated, and your daytime hot water use is low enough.

Whatever you do, a lesson I learned when replacing mine a couple years ago is know what rebates and stuff are available. I few different plumbers wanted to install regular gas water heaters that didn’t qualify for any rebates. Upgrading to a more efficient heater that did qualify was too expensive to make sense when going through a plumbing company, because they only stocked low end and very high end models.

Then I shopped for water heaters at Home Depot and Lowes. There the price difference between low end and high efficiency was about the amount of the rebate, so I could upgrade to a high efficiency heater for free, and pay less to use it going forward. The store just contracted installation with one of the local plumbing companies. In the end I paid about the same amount as I would have for a basic heater from a plumber, but instead got one that costs me less to run.

The only downside was the particular heater I picked needed to be shipped to the store, so it took an extra week. Not a problem as my old worked, it just was 20 years old and leaking.

One last thing, if you do shop at the chain hardware stores, the price of the exact same model water heater was different at different locations in the same chain. That’s why I went with ship to store, because the store a few miles away that had it in stock wanted $150 more.

7

Heat pump water heaters would actually be my choice, too, but the location of the heater is severely space-limited, and I can’t find any that would fit.

J.

8

BTW, I just watched this video, and I recommend it to everyone. It’s a masterful explanation of how an electric water heater works, and the surprising physics that allows a 40 gallon water heater to put out 53 gallons of hot water before running out.

J.

9

How noisy is your tankless water heater? My present water heater is in a closet near the center of the house and I don’t want to introduce a noise source if it is too loud.

Thanks,
J.

10

Completely silent. But the gas heater has an exhaust pipe, so you have to make arrangements for that.

11

You could also throw solar water heaters into the mix. My university has a solar house (or at least they did when I attended) that they built to demonstrate how solar energy could be used in a typical home. They actually built it way back in the 1980s; it was kind of funny to tour it and see the dated looking brown 80s kitchen appliances.

But anyway, one thing I remember they did was to put a solar water heater on the roof, and plumbed the hot water it produced into the “cold” water input of a regular tank water heater. As long as the solar water heater was producing sufficient hot water, the other water heater never needed to turn on. It was pretty much there as a backup for when the solar water couldn’t keep up, like on a cloudy day. Of course for a setup like that you’d have to buy two water heaters, and make some modifications to your plumbing, so you’d be paying a lot more upfront.

12

My Rinnai, a fairly large whole-house heater is not silent, but neither is it loud. I doubt one could hear it operating in a closed closet.

13

That would depend upon which one you select and individual household needs. Personally, I do not want one because of potential power outages. I have a 12k whole house generator which is insufficient to power one of these devices.

14

Bear in mind that gas tankless heaters also need to be plugged in to work, making them useless during power outages.

15

If the concern is CO2 emissions then gas comes out ahead of electric resistance for the same type of appliance. Beyond that it gets complicated. Even gas tank water heaters have the issue that they’re less well insulated than electric tank heaters, because their heat exchanger runs right through the middle and when it’s not running the heat from the tank is bleeding out the flue. More sophisticated ones with power draft fans and dampers help that a lot, but they’re still more difficult to keep well wrapped in insulation.

For tankless, it’s a no-brainer that gas is better. Most homes don’t have a beefy enough electrical service to power an electric tankless heater anyway. When comparing tank to tankless, yeah that gets more complicated because it’s also based on usage. I agree with @echoreply that a heat pump water heater would be the best. Perhaps even more so if it was used as a pre-heat tank for a smaller gas or electric tankless unit (like @WildaBeast’s solar setup which is awesome) or even point-of-use heaters, but that’s a lot of reworking of existing plumbing and electric. Anyway, the issue with heat pumps its that they can produce tons (hah) of heat, just not very hot heat. I.E. they’ll make 100º water or air forever, but boosting it further to 120º hurts the efficiency quite a bit. So relying on them to do all the work may not always be ideal.

16

An electric tankless? Sure.

The power required for a gas tankless heater is minimal - 80-100 watts max, making it ideal for even the smallest generators. I imagine even a UPS would power the largest gas heater for a good long time. Standby power required is only 2 watts.

17

The OP’s question is about net greenhouse gas emissions from each of the water heater options available. I haven’t got any training to even begin an analysis, but ISTM that what must be considered is:

  1. Emissions from the electric coal/gas generating plant to provide the electricity for each type of water heater.

  2. Emissions from the natural/propane gas generating plant to provide the electricity (or the actual gas) for each type of water heater.

  3. Emissions from the use of each type of water heater (applies mostly to natural/propane heaters), and relative efficiencies of each type of heater per annual average BTU of heat provided by each type of heater.

  4. Emissions from the transmission or transportation of each type of energy supplied.

All of the above focusses on emissions only and ignores the average annual cost of running each type of heater.

18

I believe the power damper and electric igniter (instead of a pilot light) are the only difference between my Energy Star high efficiency gas water heater, and the cheaper model. The power supply maxes out at 12 watts, so it is easily powered from a battery during a power outage.

Similar to an EV, an electric water heater that runs off a coal plant today might be using solar or wind next year, so it is worth thinking ahead about carbon emissions. A gas hot water heater will also need to take into account methane leaks as part of the carbon footprint, not just it’s carbon emissions.

Over the summer I use about 10 therms of gas for the hot water heater and dryer. That means I pay about as much for the privilege of being hooked to gas, as for the amount of gas I use. Only counting the consumed gas, the maximum my hot water heater is costing me is less than $0.50/day.

That also works out to 120 pounds of CO2 per month. Xcel used to provide CO2 pounds for electricity used, but they don’t anymore. The old number was about 1.33 pounds per kWh. That would mean I’d have 90kWh per month of power to use for clothes drying and water heating to break even on CO2 emissions. I do not know if that is a reasonable number.