Lots of pieces of good advice here. Maybe I missed this, but how much storage do you have? I saw 1200 sq ft shop, well insulated. What height for the sidewalls?
Here is my experience which may help you determine what will be best for you. You are in NE WI which may have somewhat similar weather to me in north central MN. Typical mid-winter temps -10F to +5F; really cold spells might be a couple of days at a time, down to -20 to -40F. My shop is 1500 sq fit and I consider it well insulated: R19 walls, R40+ ceiling, 2" foam under entire slab and perimeter, also 2" foam down 30" around the perimeter. I have 1000 gallons of storage, horizontal tank. My Tarm is rated at 140,000 btuh. I use a 5 x 12 x 30 flat plate hx, as the floor is antifreeze, while the boiler and tank are water.
Taco 009 supplying 6 x 300' loops of pex in the slab. Flow is 2.75 gpm to floor. I set the slab mixing valve at 100F. Floor sensor for heat to the slab is set at 61F, with 1F differential. Interior air temp ranges between 50-58F at outside temps of -20 t0 + 20F. Actual heat load ranged between 11,000 - 18,000 btuh. Delta-T on slab supply/return is 30F +/- (100F supply, 70F return), which at 2.75 gpm = 41,250 btuh. Typical 1000 gallon tank draw-down on a slab heating cycle is about 12-15F, which is 100,000 - 125,000 btu's.
Comments:
1. Valid point that your near bottom of tank temp may be too low to provide the heat you need for the slab. You will need to determine how much low temp water you have, therefor how many low temp btu's you have available to heat the floor.
2. I did not find it practical to play around with the slab temp. It takes a long time and lots of btu's to heat the slab up if I let it drop below the 61F set point. In your case it may be practical to have a lower set point, but I would be cautious in letting the slab drop much below the setpoint.
3. Your low-flow circ on the sidearm is resulting in tank mixing, as return water from the sidearm likely is quite a bit higher than near bottom of tank water. When my slab is being heated, my tank mixes almost completely, as again hot supply water mixed down to 100F results in a little less hot return water to the tank. This is not a problem for slab heating, but it would be a problem if the need is for high temp water for space heating.
4. A btu is a btu, regardless of where you draw it from, and it will need to be replaced from your boiler. This is a guess, but I would expect that unless you have a lot more than 1000 galllons of storage, your stored btu's will not get you very long between boiler burn cycles. For me, tank 185 - 100F is 709,000 stored btu's, and at average 15,000 btuh heat load for my shop, that gets me two days between boiler firings. All of this past winter my firing rate was every other day or longer, depending on outside temp. You need to know your total heat load, determine your stored btu's, and then decide whether trying to use low temp water makes any sense. How low can the hot water supply for the house get before you need to re-fire your boiler? Again, unless you have lots more than 1000 gal of storage, if you can't go below 160F, that's only 167,000 btu's for a 1000 gal tank (adjust for your storage gallons), divide that by your space heating load, and you will have your answer. If you have to re-fire your boiler frequently anyway, you might want to re-think your approach.