Coalbrookdale Darby Boiler Question

First let me interduce myself. My name is Tom and a newbee but a real oldy ! I HATE buying oil !! I have a collision shop that has a spray booth that burns 5 1/2 gallons a hr. A heating unit for the shop that burns 2 gals. a hr. And a house the that burns 1.25 a hr. which I rent out ( my old house) on the shop location. I built a new log home and installed radiant floor heating in the whole house . Took me Ten Yrs. After the first heating season Last Yr. I bought 5 275 tanks of oil . YIKS !! So I went looking to Alt. energy. I had in the past a Harman Stoker I loved it but SHE did not ,in the old house . My back is shot 3 blown discs in the lower and one in the neck so I went with soler. I am very happy with it ! Very expensive but works well I don't think I will ever see a payback at oil prices now but what the hell. My problem is cloudy days and nights when the storage tank gets low say under 90 deg. I am wondering if a Darby will keep the tank up to over say 120 deg. It is 415 gal. of storage . the guy wants 500 buckeroos for it I have not seen it yet. Any input would be appriciated . And please don't pick on my grammer.
Thanks Tom

I forgot to mention I put a century wood insert in works ok for the lower part of the house for heat. But not the upper. I put a hilkoil coil on the side got to 110 deg. out of it that was it. So I got a top coil 5 loops for the top and all that did was put the fire out !! He has a good warrenty, Yea Right ! Un used for fit not function. You use You Lose!!
Tom

The Darby is a radient stove and moving it will fix your back for good.

There is a small cast iron boiler sold on Ebay for $500 called Bucket a Day which should work.

Richard