When the house was built (1982) in New England, no furnace was included (heat pumps).
Later a furnace (1997) was added and a 40 gallon tank was added replacing one included originally. In January 2010, this furnace had to be replaced (leaking). Weil McLean WGO3 tank-less. The installer questioned why there was a 40 gallon tank when one was not needed, but he plumbed it up. I didn't worry too much about it at the time. A small Taco 4 gpm runs from the boiler coil to the indirect tank.
Next, the tank failed (leaking). I was planning to replace it as plug and play.
During a call to Amtrol (tank manufacturer) over a minor unrelated issue, I was told that this configuration is not practical. One-half inch lines and tankless coil, lower water temperature from the source, high-head losses (lines should be 3/4-in), all make this arrangement poor. The solution would be as the Amtrol schematics indicate, running a dedicated zone off the supply to the HEX internal to the Indirect, and back; ditch the tank-less heater altogether.
I agree with this approach for a new installation, but as I say, this was a retro-fit of a system that worked (sort of).
So the question is, could something like this work temporarily or less than optimum or would it be completely out of the question. Suppose there was an additional 10 foot head loss due to pipe friction? The pump runs longer.
According to Weil-McLean, the tank-less produces 3.5 gpm (from 40 to 140 deg with 200 deg boiler water). Amtrol advises this should be 7 gpm and they would like to see the water to the HEX hotter.
I have temporarily plumbed across the tank location and am using the tank-less for the time being.
To complete the project as recommended requires adding a dedicated zone with valves, new 3/4 inch pipe, etc.
I'm looking at having both configurations with the indirect in use during times when DHW only and not heating is required, perhaps on a timer in summer.
Comments are welcome!