Geo-types,

My reading indicates two somewhat common issues in a normal gas tankless set up, that is, cold water directly to the tankless (keeping in mind the benefits of higher efficiency and no cold showers).  First, some don't turn on with very low flow, for instance washing hands at a low-flow fixture.  Second, if they do turn on under a low-flow scenario in which the faucet is turned on-off-on-off there will be plugs of cold water because the tankless doesn't fire up until some water has gone through.  For instance, when manually spraying dishes to rinse them the sprayer turns on-off-on-off etc.

Navien, and maybe others, make rather expensive models that have a small built-in buffer tank that addresses one, or maybe both these problems.

I need to buy a water heater now, but will put in a renewable water heater in later (solar thermal or geoexchange).  So I'd like what I buy now to back up, or provide the last bit of heat, for the renewable system down the line.  If the gas-fired tankless is used as a back-up to, or to top off the heat on a renewable system don't the two problems mentioned above go away?  The water in the tank heated by solar or geoexchange should always be between warm and the set-point (right?), so...  If a low flow faucet does not turn on the tankless, at least warm water will be passed through the tankless and delivered to the sink for hand-washing, right?  And if a sink is being used intermittently, the "plugs" of cold water that come through would actually be plugs of warm water, right?

The idea here is to save money by not buying the top-of-the-line tankless with a buffer tank in it, and live with those two "problems" for a couple years, knowing that when the renewable system comes along the tankless won't have those two problems anymore because it will be fed warm or hot water.  Am I thinking correctly here or missing an important point?

(I do realize that some tankless heaters won't kick on if the incoming water is too warm and if I go tankless I'll look for a model that is flexible in this respect.)

Thanks,

Gordon

Space and wiring permitting, go with a conventional electric storage water heater for now. They are cheap durable, and reliable.

Down the road you can depower the tank heater and let it become the buffer / storage for either solar or geo desuperheater and let it feed a tankless heater.

That said, I dislike tankless heaters for their cost and complexity but will concede they have a role in tight install spaces and / or far flung points of use.

In thread titled "straw poll" most of the heating pros indicated they do not have or plan to install tankless systems.
Joe

If I were to set up a solar water heater I would use an electric tank 80 gal and make sure it is well insolated. Solar can heat water to over 180 degF use a mixing aka tempering valve to reduce this to 120 degF. Now, how many sunny days do you have? Here in Prescott AZ we have 300 plus days a year. And this system would just about never use electricity here. And I would have no need for a thankless water heater. If I found my self needing more hot water I would add more storage. You need to factor in your local weather.

Another factor is freezing if you are subject to freezing you need to account for it. One way is to use a dump valve that empties the external components of the system. Another is to use antifreeze and use a heat exchanger.

Again local conditions must be a factor in the design.