After ours was installed I stumbled across a company that makes an all in one unit called Eternal Hybrid (link below). You may want to contact them and see about getting the energy efficiency specs to go that route. I think Navien also makes an all in one system now too. I would think either company would be able to give you more accurate energy specs and better answer your question. The other thing to keep in mind is that without the loop (& pump), you are in effect wasting gallons of water waiting for the hot to kick in--think showers, sinks, etc. How much does the water wastage cost in terms of both water & sewer bills + the unnecessary waste of a resource? In addition to washing machines not pulling enough water through at a time to trigger the heater and then remain on long enough to actually get the water down the line to the machine, some dishwashers are having problems too. Here is a link that might be useful: Eternal Hybrid Hot Water Heater Site

Thank you. When I re-read my question later, I realized it was impossible to answer without more details. I do hear the unit go on when I run hot water in the sink. And if I remember, it's an indirect feed since there really isn't anywhere to store it. I bought a 100 gallons. I can't imagine that won't last a few months for hot water only. Hoping I can get this complete before heating season.

I can;t argue with the fact that replacing an old boiler with a more efficient one can cuts costs considerably. If natural gas is available, there is nothing else to consider. If not, then relative fuel costs of electricity, propane and fuel, as well as climate conditions (for heat pumps) need to be considered. When faced with high fuel prices in an old home, the biggest bang for the buck can be in improvements in the home envelope and that , as Jim pointed out, should be done first if possible. If you get heat loss low enough, fuel cost can fade as an isssue. Jim, you have a condensing boiler? Can you be specific? Did you have trouble with new, lower temperature, flue location? How are your return water temps running? Do you have a lot of heating zones? What type of heat emitters are you using? If your improvements caused short-cycling, you might consider a buffer tank. I recently saw an interesting commentary on two-pip vs. four -pipe plumbing for buffer tanks. If set up right, they can make larger improvements in condensing boiler efficiency than if set up without condensing in mind.

The point of a tankless water heater is to have a continuous supply of hot water that, unlike a standard water heater, will not run out. It can only provide instant hot water if the tank is at the point of use. There is nothing magic about it that will repeal the laws of thermodynamics and fluid flow. If there are many feet of piping that the heated water needs to run through, particularly if some of the run is in unheated space, there will be the type of lag you are describing. This is because the heated water has to displace whatever cool/cold water has been sitting in the pipe, and because that heated water will also will lose a lot of heat to the cool piping until that piping gets to be about the same temperature as the hot water. Only when the sitting water has cleared and the piping is heated up will you get water coming out at the temperature you want. This is not unique to tankless units...it's true of any type of water heating system where the heated water has to travel a bit before it gets to the point of use. If you want hot water faster, you'll need to move the tankless unit much closer to where you are using the water.