Propane tankless direct vent DHW heater as backup heat?

[Nofossil]

So I have 150 gallons of perfectly good diesel fuel in my fuel oil tank, and my oil boiler is 20 years old and kind of sad. About the only thing the oil boiler does anymore is run a few times a year to heat my DHW (in a SuperStor indirect tank) when solar can't keep up.

I'm thinking about ditching the oil boiler and replacing it with a tankless direct vent propane water heater. I'd keep my Superstor and set the tankless heater up with a bronze circulator that would circulate water from the Superstor through the tankless heater on the rare occasions when the SuperStor aquastat calls for heat - basically, when we have a couple of cloudy days in a row.

I'm thinking I could also have it circulate water through a flat plate exchanger to allow me to heat the house and hot tub when I need a backup heat source. Basically, I need backup, and using my oil boiler at a rate of 20 gallons a year just doesn't seem to make sense. I think it's also pretty inefficient when it runs so infrequently.

I've attached a sketch of a possible configuration. I'd run it at 160 degrees - my DHW tank already has a mixing valve. Only one of the two tankless circs would run at a time, and they'd need to be sized for a flow rate of 3-5gpm.

Any reason why this wouldn't be a good idea? Any suggestions for improvements?

Here's a unit that I've looked at:

http://www.pexsupply.com/product_dtl.asp?pID=4921&brand=Takagi&cID=684

 

[Nofossil]

possible minerals in well water can plug up tankless, ive heard

I have a water softener, so that shouldn't be a problem.

The real issue seems to be that the spec sheets don't show what output you can expect at high flow rates, high temperatures, and lower delta T. They're designed to work with temperature gains of 60 degrees and output temps of 120.

The one I refer to above can be set for output temps as high as 180. However, I can't tell from the specs and the manual how much heat it could deliver if it were running at an inlet temperature of 150 and a target output temp of 160, for instance. I have a call in to the engineering folks.

 

[ssupercoolss]

i have heard the takagi is one of the better brands. i have what i believe is the top of the line Rinnai in my vacation home in the mountains. awesome unit, propane, direct vent, i highly recommend looking into it. i think mine is 180,000 btu's, but i think they have a smaller one, which is probably what you are looking for, since you just want to boost up some luke warm water. i believe the rinnai measures the outgoing set temp, not just the rise. which means if you set it at 120, the water leaving the heater is 120. basically, the burner adjusts itself from low to high, its not just on high all the time. i called customer support at one point and found them to be very helpful.. foreverhotwater.com