I was asked to write this blog about naturally aspirated fossil-fuel water heaters in green retrofits as a response to a rather heated debate at the recent National Green Building Conference. The debate occurred in a class co-hosted by Peter Yost and Michael Chandler; in that debate, I stated unequivocally to “get gas water heaters the hell out of the house — they have no place in a green retrofit!”
Natural aspiration wastes a lot of energy
The efficiency of naturally aspirated water heaters range from 54% to 62%. Believe it or not, the 62% model is 15% more efficient than the 54% model, so until recently it even qualified as Energy Star! That is just one of many reasons why I don’t really pay too much attention to Energy Star, but that is another long story.
Without getting into a lengthy discussion of why naturally aspirated combustion water heaters are so inefficient, let’s just summarize by stating that they cannot be placed inside the conditioned area of the home unless they are in a “cold room” and unless outdoor combustion air is ducted to the room. That means that the residual heat off the water heater is lost — in other words, it cannot help heat the house. This means that at least 38% of the heat you have paid for is gone up the stack, contributing to global warming, air pollution, and whatever else pushes your buttons.
Electric heating is 100% efficient
By contrast, an electric resistance water heater can be placed in the conditioned area of the house — even underneath a stairway or another unusable area — and the residual heat will help heat the house at 100% efficiency.
However, during the cooling season, when you are paying to run the air conditioner, this extra heat is unwelcome. If you are in a climate zone that requires considerable air conditioning, this may not be a good thing. Please consult your NOAA charts on Heating Degree Days and Cooling Degree Days before choosing a water heater. In my Pacific Northwest climate, we have zero cooling degree days, so an electric resistance water heater makes perfect sense.
In a climate with an equal number of heating degree days and cooling degree days, the residual heat would be of zero net-use. If you have more heating degree days than cooling degree days, you would receive a proportional benefit.
Coal is on the way out
OK, now you may argue that an electric resistance water heater may be fine for me, since I live in the land of hydroelectric power — clean energy and all that. But what if you live in a state powered by coal?
Coal-fired power plants are only between 30% and 40% efficient, and they emit all kinds of nasty stuff into the air. Agreed. But the reality is that coal-fired power plants are basically maxed out, running at full capacity just to supply the current loads. Nobody has any plans to build any new coal-fired plants to supply any new loads.
Were you listening to the President (I didn’t ask if you liked him — just if you were listening)? He stated unequivocally that he wants to tax coal-fired plants out of existence. You might not have believed him — after all, he didn’t get the cap-and-tax bill through Congress — but he is already starting to achieve his goals with regulation.
In Washington State, we have one coal-fired plant, located in Chehalis. Our governor has just brokered a deal that will take all coal units off-line by 2025. Those units will be replaced by clean-burning natural gas-fired plants at the same location, presumably using the latest and most efficient technology.
New natural gas plants are around 55% efficient. That might not seem like much next to your condensing 95% efficient furnace, but if you produce the electricity with natural gas, and then use a heat pump to heat your home, the heat pump will be around 300% efficient, depending on the model and your climate zone. Multiply the 300% by .55 (the efficiency of the power plant) and you will be around 165% efficient, or nearly twice as efficient as if you had used the gas directly in the home!
The grid ties us all together
If you live in the Deep South were most of the electrical load is met by coal, and if you are talking about an existing load on the system, you may have a small shred of an argument — until you consider that the entire electrical grid is tied together. As soon as you consider the fact that all electrical generation is tied to the same grid, and that all new loads will be supplied by natural gas, wind, or solar power, you will realize that when you are adding a new home to the system, that represents a new load, and your power is coming from a source other than coal.
The correct calculation to use when determining the efficiency of electricity vs. natural gas is as follows: Gas turned into electricity is about 55% to 60% efficient. Nationwide, line losses average about 6%, and 6% of 55% is about 3%. So reduce the “delivered to the house” efficiency of natural gas-fired electricity by 3%, to 52%. Multiply that by the Coefficient of Performance (COP) of your heat pump, or heat-pump water heater, and that is the net efficiency of your use of natural gas.
That would make a 225% efficient heat-pump water heater about 117% efficient, and your 300% efficient ductless minisplit heat pump about 156% efficient. You just can’t beat that using natural gas (or any other fossil fuel) in the home directly. (Your equipment is probably only listed by HSPF, Heating Season Performance Factor. To get COP, divide the HSPF by 3.412; since 3.0 = 300% efficient, an HSPF of 10.3 would yield a COP of 3.0.)
Focus on efficient equipment, not cheap fuel
Now let’s take a brief look at history: In 1974, the U.S. Department. of Commerce started keeping track of consumer energy prices as a separate component of the Consumer Price Index (CPI). In the last 37 years, the cost of energy at the consumer level has increased an average annual rate of 6.33%. If you are going to calculate the cost of the energy to run your new heating or water heating equipment, you need to factor in that inflation factor in relation to the life expectancy of the equipment — roughly 20 years. Energy that would cost only $30 today (a typical water heating bill) will cost $96 then. Worse, you will have paid over $13,600 for your hot water in that time. What if you saved 23% of that? What if you saved 47% of that?
Remember the numbers up above? The heat-pump water heater is about 117% efficient in its use of natural gas. That represents a savings of about 23% over an ultra-high-efficiency 95% water heater and a savings of about 47% over a water heater that is 62% efficient. If you chose the heat-pump water heater, you would have up to $6,400 to invest in your local utility company stock instead of having blown it up the chimney!
Finally, don’t pay too much attention to the price of natural gas today compared to the price of electricity today. These price relationships can change rapidly. When I built my commercial center ten years ago, heating oil was only 79 cents per gallon, so I put in oil-fired boilers. Boy, was I dumb!
Be smart: go with the units that use the least amount of fuel, not the lowest-price fuel. The price will stabilize over time. All energy is worth the same cost per therm or BTU or KWh, or whatever other unit of measurement you want to use. Save the gas for uses that require portability. Since your house stays in the same place, tie it to the grid.
Much of the discussion at the class mentioned in the first paragraph was related to how indoor air quality issues are affected by combustion appliances and energy retrofit work, especially air sealing measures. By the time the discussion was over, it was clear that no-one present could justify using naturally aspirated fossil-fueled water heaters in those conditions. I hope this article will persuade you that there are no conditions, anywhere, ever, that would justify their use.
Ted L. Clifton is a designer, a builder, and the founder of Zero-Energy Plans in Coupeville, Washington.