Questioning Savings from the GFX

Your feature on water heating in the October 2002 EBN (

Vol. 11, No. 10) included discussion of the GFX wastewater heat recovery unit. The article suggested that this device can reduce water heating energy by 12–15% and that installing one for $500 to $800 is cost-effective. Given small savings and high costs, I wonder if the GFX makes sense for most households.

I estimate that each month my family of four uses about $10 worth of natural gas for hot water (@ $.75 per 100,000 Btus), about half of which might be for showers when there is simultaneous use of hot water and hot water going down the drain (when the GFX would work). The GFX would produce savings of $2.50 per month if it could recover 50% of the heat in the $5 worth of shower water we use each month—with a payback of 17 to 27 years given installed costs of $500 to $800. And that may be generous when one considers that the temperature of drain water from showers can be quite cool because showerheads tend to aerate, releasing shower water heat into the bathroom. This would lengthen payback further.

Walter Simpson

Buffalo, New York

Editor’s reply: With such low gas costs, you are correct that the payback for a GFX installation will be quite long. Tacked onto a 15-year home improvement loan at 6% interest, that $800 cost would increase your loan or mortgage payment by $6.75 per month—more than the monthly savings of about $2.50. However, if you install the GFX yourself for $300 and pay for it through the same home improvement loan, the savings would just about pay for the additional monthly loan payment ($2.54). Note that if your gas prices were to double in a few years, the savings from the GFX would double, and the economics would be much more favorable. A family heating water with electricity or using a lot more hot water per month would find much better economic justification for installing a GFX. Charlie Stephens, of the Oregon Office of Energy, says that your comments about aeration effects aren’t really relevant; the savings estimates of the GFX quoted in the EBN article were measured savings at the water heater, so heat dissipation is already accounted for. – AW