No space for an eco drain so......

[inetdog]

I used a 12VDC pump using only about 4W, which only runs when the temp at the end of the line gets too much lower than at the heater.
The hysteresis is adjustable, and the sensed temps or differential can be selected for display.
Bruce Roe

Just keep in mind that the largest factor in the energy cost is generally not running the pump but reheating the water that stands in the line from heater to point of use.

[J.P.M.]

Just keep in mind that the largest factor in the energy cost is generally not running the pump but reheating the water that stands in the line from heater to point of use.

Actually, without a recirc. system, or when a recirc. system is off, the water is not being reheated. It's usually being dumped at the point of use and replaced by heated water from the heat source. But it is almost always correct to say that the relative amounts of energy used t pump water is usually very small compared to the energy required to heat water.

Assuming a non recirc. system, and assuming the water in a line loses 60 deg. F. in cooling to ambient temp, 100 ft. of 3/4" line will lose the equivalent of ~ .25 kWh (~ 868 BTU + a few BTU for the line material) of energy. With a system such as Bruce's, depending on the what the thermal hysteresis is, to a loose first approximation, the thermal losses for his recirc. system will be a bit less to a lot less than a continuously running system in something like the ratio of the of the running times of the pump for the two scenarios.

[bcroe]

Energy to run the pump is probably on the order of 1.5% of the energy for the water heating. The pump comes on with
around an 8.5 deg F differential from water heater to the end of the line. It turns off around 5.5 deg. I suppose a study
of the duty cycle could give more info on the actual heat loss. It certainly changed a lot when insulated. Bruce Roe

[J.P.M.]

Energy to run the pump is probably on the order of 1.5% of the energy for the water heating. The pump comes on with
around an 8.5 deg F differential from water heater to the end of the line. It turns off around 5.5 deg. I suppose a study
of the duty cycle could give more info on the actual heat loss. It certainly changed a lot when insulated. Bruce Roe

With attention to detail, and no gaps, 1/2 thk. insulation can cut the line thermal loss by ~ 2/3.

Roaming the halls around here @ ~ 3 A.M. today, the thought crossed my mind that a constantly recirculating hot water distribution system is somewhat analogous to, and has some of the same logic behind it as if I lived in a cold climate and kept my car running constantly with the heater running so that the vehicle interior will be warm when I get in it for a foray into the elements for a trip to the grocery store or to work.

[inetdog]

With attention to detail, and no gaps, 1/2 thk. insulation can cut the line thermal loss by ~ 2/3.

Roaming the halls around here @ ~ 3 A.M. today, the thought crossed my mind that a constantly recirculating hot water distribution system is somewhat analogous to, and has some of the same logic behind it as if I lived in a cold climate and kept my car running constantly with the heater running so that the vehicle interior will be warm when I get in it for a foray into the elements for a trip to the grocery store or to work.

And "on demand" recirculating systems would be analogous to remote starters to warm your car up a few minutes before you need to use it?

[J.P.M.]

And "on demand" recirculating systems would be analogous to remote starters to warm your car up a few minutes before you need to use it?

Probably - or at least not far off - provided the pump started only long enough prior to the beginning of water draw to ensure hot water was at the faucet when the demand began (with that time = line vol./pump flowrate), with recirc. pump shut off once water draw began.

Not my money/house/life/business, but this whole thing about residential recirc. systems seems very wasteful and a bit ostentatious to me.

I suspect if folks took a few minutes to learn how much such systems waste in terms of energy and how little water that actually save, instead of blindly and ignorantly swallowing what comes out of peddlers who sell these energy hogs, the more thoughtful users would think twice about their continued use.