I guess I should have mentioned that I'm a "function over form" kind of guy. No I'm not married - I guess a typical wife wouldn't like the looks of the collector. That's why the internal loops are probably popular. You'd still have the exposed piping to some extent in my situation due to the concrete base below the slate hearth floor.
As for performance, the tank was at 121 degrees this morning but the house was at 62 degrees
The larger-than-I-remember nut coal got jammed in the hopper and the fire was nearly dead at 5:00 this morning. I may have to start mixing nut and peal coal so the smaller pieces help the bigger ones drop down onto the grates. I don't remamber having this problem last year, but apparently I've got it now. And me with 2 tons of nut and one ton of pea on pallets...
When I finally got the stove going again after unjamming the coal, I went downstairs to monitor the system. The pump was off most of the time and when it did cycle on for 30-60 seconds, the temperature at the tank feed line from the collector jumped to almost 150 degrees before the pump shut off again due to low collector temperature. I may start fiddling with the Delta-T on the controller to allow more of this cycling. It can slow the recovery rate but should raise the maximum temperature.
I really wish Stiebel Eltron had imported the original Resol DeltaSol controllers with their solid-state relays instead of the mechanical relays. The Resol DeltaSol BSpro is nearly identical to the SOM-7si but can control the pump speed to maximize temperatures. It duty-cycle modulates the AC to the pumps from 30% to 100% based on some extra parameters in its programming. I wonder why Stiebel Eltron doesn't buy that model and relabel it as their own? Must be some reason (like maybe Germany uses 240 volts which allows lower current switching circuits). But then again, there's a Canadian Resol distributor that has 120 volt versions. Must be an OEM product built that way for a reason (not that I'd agree with the reason).
Still shiverring in Massachusetts.