Just thinking out loud...

Since tankless water heaters make hot water only on demand why would you want to recirculate cold water when turning on the bedroom light?

Wouldn't activating the recirc pump be more sensible by using the kitchen and bathroom light switches?

If you go into the bathroom or kitchen and turn on the light and then the hot water wouldn't there still be a lag time for the hot water to reach the faucet?

What about the times you go into the kitchen or bathroom and don't turn on the light?

Hi justalurker, i believe because our tankless heaters will be put in the garage, there will be some long pipes to the rooms, so the pump is to help push the water through the pipes faster, not necessarily to recirculate cold water. our bedrooms all have attached baths, so we would turn on bedroom lights first. though, i have often wondered myself about the kitchen or if we don't turn on lights, would we not get hot water fast enough, though the kitchen is closer to the water heater, so maybe it won't be an issue? not sure.. would love some advice from others. thanks!

Until you open a hot faucet and the tankless heater fires there is no hot water to move (recirculate) to any faucet any where in your house.

According to your plan... you walk into the bedroom and always flip on the light switch (even in the day?) and the recirc pump starts pumping unheated water. Then you go into the bathroom in the bedroom and open the hot faucet and will get... COLD water because the tankless water heater HAS NOT BEEN HEATING WATER UNTIL just that split second when you opened the hot faucet.

At the moment you flip the light switch (to trigger the recirc pump) the tankless water heater is not making hot water.

Hi justalurker, thanks for your input. i will have to ask my mechanical engineer regarding exactly how it works. just looking at my house blueprints, that is how i gathered it would work. of course, i am not an engineer or plumbing expert by any means, (but have to purchase all our appliances) which is why i am asking for help on the forums, lol..

If the water lines are sized correctly the velocity of flow should already be at the code mandated 8fps so not only is there no need of a pump, installing such a pump to increase velocity of flow would violate code.

Some chain jerking going on here.

Two types of systems available here, once you determine if there will be return line we can understand the system type:

1. A retrofit type system that is activated off a switch that pumps water back via the cold water line to create flow through the heater. The pump(s) are typically located at the fixture locations.

2. A true recir system with a return water line creating a loop and the pump creates flow through the Tankless unit.

You are not really making the hot water flow faster. You are (perhaps) making the appearance of hot water occur sooner.

A 'sensor' is not necessary. You would install the unit under the sink in the bathroom, usually the furthest one from the WH. You then power it from a switch. Same idea for the kitchen. I would keep the switch(es) separate from lighting since you may not want the pump to run when the lights are on.

Having the pump run all the time (lights on) will run up your gas bill.

I think Watts makes one.

weedmeister, zl700, thanks for the info. after looking over our plans again, it says, whenever lights turn on, the recirculating pump will power on for 30 seconds. i believe there is a return line outside of the cold water line (this is a new construction, so no retrofitting involved). is this a typical setup? after searching around on the forums a bit, i ran across a few brands, grundfos, laing, and some taco system for recirculating, would any of these work/are compatible with rinnai tankless water heaters? are any of them better, etc.? thanks very much!

michoumonster, have you picked out a recirculating pump yet? I'm just starting to look into one of these for myself, stumbled upon your post. Your insight is much appreciated. So far in my searching, it seems like the D'MAND system made by Metlund is the best way to go for your setup.

I'm not sure which recirc pump we have, but I know it has a timer on it so you can set it to turn on during certain hours of the day. We primarily went with a recirc pump for the front load washing machine. The front loaders start to fill with a little water then stop, tumble the clothes, sensors detect if more water is needed, water flow starts again and stops, clothes are tumbled, sensors detect if more water is needed, etc. The water flow is never on long enough to trigger the tankless water heater & get the hot water to the washer on the other end of the house without a recirc pump. So our options were cold water wash only or spend the extra $$ for a pump. Having hot water at any faucet in the house in about 4 seconds is a huge added bonus!

hi deprotinator, we haven't picked one yet. i will let you know what we decide on.

The systems that force the cold water back into the cold line are a jury rig solution to eliminate the need for the extra return line.

If you are building you can easily put in the return line (and insulate the hot line).

There are numerous ways to turn on the circulating pump, from using a light switch (often works well in a bathroom) to timers, motion sensors, etc. If you use low voltage equipment and a relay to switch the pump even the wiring is simplified (think thermostat type wiring with lower voltage and current).

If you put eh demand heater in the circulating loop it will be turned on when it detects the flow from the pump and deliver hot water.
If things are sized correctly and set up correctly you should have very close tom instant hot water by the time you turn on the sink faucet.