Heating two buildings

Holycack

I have my boiler plumbed to heat two seperate buildings. One being my house a 1652 SQFT very well insulated house (R50 attic, R35 walls, and R50 basement (styrofoam blocks))
domestic hot water via indirect tank and two being my barn at 1296 SQFT. The house runs at 18*C and the barn runs at 6*C all radiant. The system uses a water boiler model burnham V84 (135000 BTU) with a teckmar 369. I have run thermopex tubing between the buildings 110 FT buried at 6-8 FT deep (thermopex is a 4" insulated line with a 1" supply and return line, loss is 1*F for 100') The question I have is the barn only calls for heat every 24 Hrs roughly but it takes the boiler running for 40 minutes continuous to bring the heat up to demand. The return line is very cold and the boiler has trouble reaching setpoint temperature. What I am wondering is if it would be more efficient to:
1:Run this way and dont change a thing (40 min. firing time)
2:Install a bypass with a flow valve in the barn in order to mix the supply and return lines before they enter the slab (hoping to return more heat allowing the boiler to catch up) longer circulating time.
3:Installing a bypass line with a flow valve and a NC zone valve before the slab header and continously circulate the sluppy and return lines in order to keep them at boiler temperature and not have such a cold supply and return line.

Which takes more energy? Maintaning an underground line temperature or trying to bring the lines up to temperature when the barn calls for heat? Ground temp is roughly 5*C. This current case is at an outdoor of -10*C unseasonable warm for this time of year coldest day is usually -40*C. The barn is insulated R20 walls and R40 Ceiling with a garage overhead door well insualed and sealed as apposed to sliding barn doors. The horse actually will raise the barn up to 7-8*C when he is brought in at night.

Any input is appreciated as I want to run as effecient as posible. Last years bill was averaged to $70 a month this is without the barn connected yet. Have not calculated with barn due to the fact I just got the system up and running last month.