sizing water/air exchanger for wide temp swing

Trevor_3

Hello all--

I'm an advanced DIY-er about to embark on installing a wood gasifier boiler (Econoburn 150) (arriving next week) along with a really big (1350 gallon) (under construction) stainless steel water tank so that I can run the boiler hot and fast (and efficient), heat up the big tank, and then the house can draw the heat back out of the tank while I'm away at the day job, etc.

Someday I may go with radiant, but the house (an 1830 VT farmhouse, now mostly insulated, at least for an old house) has an existing forced warm air system (125kBtu ThermoPride) with all the ducts in place.

The wood boiler/ tank, etc., is doing to be sufficiently pricey that for the short haul, that my plan is to put a water-to-air heat exchanger fin-coil in the main trunk of the ducts right over the oil furnace, and use the oil furnace's air blower to move the air.

From what I've read so far, I know that these hybrid "hydro/air" systems are sometimes used, although I also know that people's opinion's of them vary- in any event, it's what I need to do for now.

So here comes the question-- with "normal" hydronics, I know that you design and select components around a "delta T" and an assumed, and probably pretty predictable water temp, to size things like radiators and heat exchangers.

In my situation, the variables start to get a lot fuzzier and wider ranging. The max that the "big tank" will probably hit when fully charged, will be about 175-180 F. The lower I can draw the temp in the big tank and still throw BTUS at the house, the better (I know that someday things like radiant floors or Euro-panel radiators open up a lot in that direction, but I just can't affort those in the foreseeable future)

If I can size/find/buy a water-air HX that will fit in my main duct above the oil furnace (which happens to be 22x22 inches) that will still add some useful BTUs into the airflow down to the _lowest_ possible temperature, then the effectiveness and capacity of my heat storage will be much better than if I use a "regular" HX that just assumes something more normal like 180F in/ 160 F out.

And for purposes of my question, forget about worrying about boiler return temp protection- I am dealing with that separately.

For now, please just help me pick, or figure out how to pick, a water-air HX that'll fit in a 22x22 duct and that will put out as much heat as possible as the water temp sags as low as possible (it would be great if I could still be getting heat into the ducts when the big tank is down around 120).

Thanks very much
Trevor