Larry, I'm far from expert, but I've been working with a company to spec out a new system for my house. Here's what we're doing: A single high-efficiency modulating tankless boiler (Trinity) feeds two closed-loops. One goes to an indirect water heater (hot water coil inside a conventional glass-lined tank), and the other feeds the coil in the A/H.
My original intention was to use a high-eff tank heater, like the Polaris, to make domestic HW and feed the coil. The DHW would have to be tempered of course. Three problems with that approach are cost, my water chemistry, and available fuel. The Polaris and other 90+ percent eff tank heaters are bloody expensive. My ultra-soft high-pH water will eat up a stainless steel tank, and the only glass-lined one I could find won't burn LP gas. So I had to drop that approach.
The Trinity boiler is 92% efficient, and modulates its flame to match demand. Its coil is stainless, but will be in a closed loop of treated water. The A/H has a variable-speed blower.
The efficiency is in having only one burner venting warm exhaust gas. Now, if they're adding on all kinds of other contraptions (storage tank etc), there will be more losses, I think. Why not just use a larger indirect heater? Why extra controls? The A/H has all the controls you need, and the boiler simply goes on when water starts flowing through it.
Know what it sounds like to me? You've been raked over the coals! I think the installers will always specify every cool gadget on the market to maximize profits. I got two companies to bid on my system, bids from $18,000 to $25,000. But the outfit I'm dealing with now will be about $9,000 for parts, and I do the installation.
Sounds like it's too late, but if you're interested, they are
www.descoenergy.com, in York PA. They are GREAT to work with!
