The flow of a recirc pump may not be enough to trigger the tankless heater to turn on.

I have seen this discussion before and IIRC it is an either recirc or tankless decision with current technology.

Your right the navian has a circ pump build into the machine, I am a takagi rep you need a 26-99 grundfus or a 00-9 if you didnt have the pump already in the sytem. What you really want and it makes sence as a long term goal is have a takagi unit. I will assume your going to have at least 2 units to keep up with the use of water. having he demand system from taco. Look atthe taco-hvac website. going tankles and with the deman d system is a big purchase but you will be happy in the long run....
the demand system is a button or motion detecter on a wireless remote. You only push the button as you need the hot water circulated. so it only usedas you need it. which is an estimate of 15 to 25 minutes of recirluation at a time.
or get a high eff water heater by a. o. smith with the demand system which willalso be great.

Did you check out "Eternal Water Heater". It is what we installed, and it works with the recirculating pump.

Hey the master chimed in

Tankless natural gas and tankless electric are generally expensive ways to heat water. Crunch your numbers, initial cost, fuel cost and efficiency go into lifetime cost.

We were recently going to install a Rannai unit in our home but decided against it. While a great idea, the cost and practicality didn't seem to make sense. Some of the reasons we decided against it included dedicated 1" gas line run up the outside wall of our house (two stories), needed the recirculation to provide hot water to HE washer (uses small bursts of water), discussions of intermittant bursts of cold water, more stringent annual flushing requirement, need soft water and the total cost. Talked to a few installers and Rannai seemed to be the preferred choice. Considered Navian for the included recirculation but they are still fairly new on the market. Just seemed better to replace our two tank water heaters. I think tankless water heaters will eventually become the standard but right now they are still an expensive option.

I have a little Takagi-Jr tankless and I put it on a recirculating loop so I could have "instant" hot water at all the faucets. Like WiseHVAC, I thought that a motion sensor would be a smart way to trigger the recirc pump. But years later, and I haven't got around to doing that upgrade. I'm sure I've wasted some gas pumping 24/7.

Here is a link that might be useful: another post with pic

My daughter just opened a café. She had 2 commercial tankless on demand water heaters installed. A café needs 110 -120 at all times. This takes a long time for the water too heat up. You use a lot of water because you have NO recirculation pump because the heater only works when water is needed .

A cafe needs water to a certain temperature in its dishwashers for proper sanitation which mechanical dishwashers will provide all by themselves. It may cost less, however, to provide hot water feed to them as hot as practical.

IMHO recirculating water with a tankless heater is a misapplication of an expensive technology where a simpler high efficiency (well-insulated tank) water heater would make more sense. I suspect that some number crunching using lifetime costs of the equipment, maintenance and water costs would reveal that it is a very expensive way to heat water.

Random thought, people who want a recirculating system better have all their pipes insulated well, otherwise in the summer you basically have a radiant heat system active while you are trying to cool the house...

What many homeowners don't realize is that tankless water heaters, though very efficient, require a few seconds for the burners to reach temperature enough to heat the water passing through the heater. Cold water sandwiching effect is illustrated on youtube. To eliminate the delay water must always be in motion through the heater....or....us a external small utility heater to absorb the cold tempered water before it heads down the pipe. I'm trying to find out if incorporating an intelli-circ and external pump is better than just getting a tankless water heater with a pump.

The recirculation system could be looked at as a small storage tank, albeit a very inefficient one. It radiates a lot of energy even though it insulated. The piping is very poorly insulated compared to a standard indirect tank. Then there is the electrical energy that goes into pumping the recirculation loop. There are mod con combi boilers that have very small storage tanks built in. I don't know if any dedicated water heaters are designed this way.