satellite water heater or recirculating pump?

Question

I want to eliminate the long wait for hot water in my kitchen. I've ruled out a tankless unit there, so it's between a small storage-tank unit (6-12 gallons), or a recirculating pump (between the main 50 gallon conventional electric unit and the 20ft-distant kitchen).

Energy usage is my main concern, and that energy goes to one of three places: hot water used, hot water stranded in pipes after the faucet is turned off, and heat lost into the environment. Seems the first two are the similar for both the tank-style and the circulation pump. Heat loss is related to the surface area through which heat is lost (the tank surface area, or the surface area of the recirculation loop piping, since the pump essentially extends the surface area through which heat is lost), the temperature differential (hot water versus ambient), and the R-value (of the tank jacket, or of the piping). I wrote a little spreadsheet. For (both cases) a 60-degree temperature differential between the hot water and the ambient crawlspace, and R-8 insulation, a typical small tank has losses equal to about 22 watts, and a 20ft round-trip 1/2" pipe has a loss of about 12 watts. That's not much difference: less than $10/year at 10cents per kwh, or 100-200 pounds of carbon for typical electricity. Also, the pump uses some electricity - though of course it can be turned off a lot of the time with a timer, whereas even a turned-off tank heater is losing energy as the water cools off when the timer is off.

Also, the pump seems like more complexity, noise, and proneness to failure, and fairly similar in price.

Is my energy-efficiency analysis correct ? Are there other factors to consider ?

Answer 1

I moved to a house in rural area with propane heat and hot water. Another propane water tank is expensive. And because of higher electricity rates an electric hot water tank would also be a poor choice. The place came with a Grundfos recirc unit (25 watts) which is piped to pump outlet water into hot water tank inlet. It draws water from a run of tubing attached to a far bathroom hot water feed under a vanity.

The pump is actually very quiet, and they can be adjusted to shut off during low usage periods to reduce energy cost. They can also be set to run based on the recirc temperature at the pump inlet, useful on weekends when you have visitors. It sits on side of tank, about the size of a 3 inch pipe about 6 inches long. The one in my place has an old style mechanical timer and is hard to figure out, and needs re-adjustment after a power failure.

It is rated at 25 watts, and can supply 3 gpm max, but only under ideal low height between the pump connection at main hot water tank and the height of the recirc connection. Maximum is stated at only 4 feet. It would not be my choice to buy. It costs around $300 Can. and I could choose my own other parts required for less.

The pony hot water tank would likely be the same initial cost as the recirc option. You would need a 20 amp 220 volt circuit wired to it, and it could be trouble free for years. It would take up some space, but would always be quickly available hot water. It will use more energy than a recirc set-up, since the 140 degree pony tank has a high temp difference to radiate lost heat. But it offers the most convenient method of a short time delay.Your hot water would arrive in seconds, depending on how close your tank is to the taps. If hydro rates were cheaper here, I would prefer it.

Have you put insulation on the long run of hot water supply piping?