Help with an Undersink Instant Hot Water Circulating System

Does anyone have an Undersink Instant Hot Water Circulating System? Are these safe? It seems like forcing the any water from the hot side to the cold side would create increased pressure in the cold side, and run the risk of rupturing seals in the pipes. Is this anything to be concerned about?

No, there is no cause for concern, as long as everything is plumbed correctly. Just make sure you arent using the on-demand heater for anything more than a sink - I’ve found the sink models to be almost useless. Just go with a whole-house unit. You can get a nice Bosch for $700. I assume you are talking about an on-demand heater. The link you gave is mainly for standby generator sets.

CynicalGabe, he’s talking about a system that cross connects the hot and cold supplys with a pump and sensor. When activated, water is drawn from the hot supply, but it’s not hot, and pumped into the cold supply. When the water being drawn from the hot supply gets hot, the sensor shuts off the pump. Then when you open the faucet you have hot water without running any water down the drain.

I have the Metlund system ( in my house. It works great with no problems. Had it about 19 months. The cross connection is in the master bath, but on it’s way there it also serves the two guest baths.

I stumble out of bed in the morning and press the button at the sink. By the time my eyes are really open I have hot water available. It takes about 20-30 seconds to get the hot water to the sink.

If you’re really lazy they make a motion activated switch that will activate the unit when you enter the room.

Strange that my link takes you to a generator page. I am looking at the AutoCirc Undersink Instant Hot Water Circulating System, model no. 6754. I have not even been to home depot’s generator page…

I found the unit on the HD site by searching for the model number 6754. It looks like that one cycles continuously unless shut off by the timer. Don’t know if I like that as well as mine which is activated as required by the user. Guess it depends on how often hot water is used. Of couse two years ago when my construction started there weren’t to many choices in this type of thing. And of couse you need an outlet under the sink where the unit’s to be installed.

And their statement that “Instant hot water will be available at all fixtures between the water heater and the Autocirc”, depends somewhat on how your plumbing is routed. It could be all your fixtures, or just the one where the system is hooked in. Mine services the master bath and two guest baths, but not my kitchen sink nor mudroom sink. But that’s OK with me.

No. When the pump pushes water back into the cold side, there’d be increased pressure only if there was no place for that water to go. Remember that the inlet of the hot water tank is connected to the cold water supply.

So all you’re doing is pushing water in a circle from HWH tank through the hot lines to the pump, then back through the cold lines to the HWH tank. Meanwhile, just almost like every minute of every day, the water in the rest of the system is stationary under the static pressure from your external supply.

The pump does NOT have to overcome the supply pressure; that same pressure is on both sides of the pump before it turns on. It only has to supply enough power to move the water in a cricle through the pipes, which is a negligble pressure rise.

Excelent, thanks for giving me a clue guys