Hot water recirc line w/ instant water heater?

I have an instant water heater (as backup for solar heated water) and was wondering if anyone has any experience using a hot water recirculation pump with a demand heater.
I understand that I will be circulating hot water back through the demand heater. Mine is designed to be used with preheated water, and has a modulating gas valve to turn off the flame when not needed.
I just want to hear about any other issues folks might have run into with this setup.

I do not have a return line, and have been looking into the valves that return back hot water into the cold line. Any thoughts?

Thanks and Happy Labor Day!
Matt

If you read the instructions, those that I've looked at specifically say do NOT use with a recirculator. Yours may be different.

I believe this would overwork the on-demand heater causing early failure. There are recirculating systems that do not require a return line, but I would suggest that if at all possible you install a return line and use a recirculating pump designed to be use with a return. I have such a system in my home, and it works very well.