Fine so you want to assume that heat loss is 0, and delta T is 14, here is the math. air density at 3c = 1.223kg/m 1.223*900*1005 = 1,106,203 * delta t of 15,486,849 joules > the heat required to raise the temps inside 900m3  by 14 degrees Celsius. T = Q/W   Example 1: for 1*antminer S21 = T = 15,486,849J/3500W/60s = 73 mins Example 2: for 2*antminers T = 15,486,849/3500*2/60 = 146 mins If you really want to assume no heat loss at all and up to 1 day to reach that delta t of 14, then you just use Q and T and solve for W Since  T = Q/W then W = Q / T Q = 15,486,849J > We got it from the above equation  T = 24*60*60 = 86400 > Converting 1 day to seconds W = 15,486,849/86400 = 179W That means even 179W is good enough to heat a 900m3 area in 24 hours with 0% heat loss and perfect conditions, obviously, since there is no way that heat won't escape from whatever walls you are building, this never will never do it. But if you really are okay with waiting a whole day to bring that temp up, then I think with good insulation, you can probably do well with just 5-10KW, your main issue now would be to discover how much heat loss you are going to encounter, if the place was already built, you could make a cheap test like Phil suggested, you get a 3KW heater put it there for 24hours while monitoring the temps, whatever that gets you is what 1*S19 would do, doesn't matter the source of heat, you just focus on the KW ratings and get an idea of what 3kw, 6kw, 9kw would do to your greenhouse if you get 9KW heater and it fits just fine, then 9KW worth of miners would do the exact same thing, air doesn't care how you generate that heat, heat is energy after all. If the average person outputs 100w of heat, you put 90 lazy people inside the greenhouse and they would raise the room temp exactly the same as 3kw miners or 3kw heaters would.
Fine so you want to assume that heat loss is 0, and delta T is 14, here is the math. air density at 3c = 1.223kg/m 1.223*900*1005 = 1,106,203 * delta t of 15,486,849 joules > the heat required to raise the temps inside 900m3  by 14 degrees Celsius. T = Q/W   Example 1: for 1*antminer S21 = T = 15,486,849J/3500W/60s = 73 mins Example 2: for 2*antminers T = 15,486,849/3500*2/60 = 146 mins If you really want to assume no heat loss at all and up to 1 day to reach that delta t of 14, then you just use Q and T and solve for W Since  T = Q/W then W = Q / T Q = 15,486,849J > We got it from the above equation  T = 24*60*60 = 86400 > Converting 1 day to seconds W = 15,486,849/86400 = 179W That means even 179W is good enough to heat a 900m3 area in 24 hours with 0% heat loss and perfect conditions, obviously, since there is no way that heat won't escape from whatever walls you are building, this never will never do it. But if you really are okay with waiting a whole day to bring that temp up, then I think with good insulation, you can probably do well with just 5-10KW, your main issue now would be to discover how much heat loss you are going to encounter, if the place was already built, you could make a cheap test like Phil suggested, you get a 3KW heater put it there for 24hours while monitoring the temps, whatever that gets you is what 1*S19 would do, doesn't matter the source of heat, you just focus on the KW ratings and get an idea of what 3kw, 6kw, 9kw would do to your greenhouse if you get 9KW heater and it fits just fine, then 9KW worth of miners would do the exact same thing, air doesn't care how you generate that heat, heat is energy after all. If the average person outputs 100w of heat, you put 90 lazy people inside the greenhouse and they would raise the room temp exactly the same as 3kw miners or 3kw heaters would.

Fine so you want to assume that heat loss is 0, and delta T is 14, here is the math. air density at 3c = 1.223kg/m 1.223*900*1005 = 1,106,203 * delta t of 15,486,849 joules > the heat required to raise the temps inside 900m3  by 14 degrees Celsius. T = Q/W   Example 1: for 1*antminer S21 = T = 15,486,849J/3500W/60s = 73 mins Example 2: for 2*antminers T = 15,486,849/3500*2/60 = 146 mins If you really want to assume no heat loss at all and up to 1 day to reach that delta t of 14, then you just use Q and T and solve for W Since  T = Q/W then W = Q / T Q = 15,486,849J > We got it from the above equation  T = 24*60*60 = 86400 > Converting 1 day to seconds W = 15,486,849/86400 = 179W That means even 179W is good enough to heat a 900m3 area in 24 hours with 0% heat loss and perfect conditions, obviously, since there is no way that heat won't escape from whatever walls you are building, this never will never do it. But if you really are okay with waiting a whole day to bring that temp up, then I think with good insulation, you can probably do well with just 5-10KW, your main issue now would be to discover how much heat loss you are going to encounter, if the place was already built, you could make a cheap test like Phil suggested, you get a 3KW heater put it there for 24hours while monitoring the temps, whatever that gets you is what 1*S19 would do, doesn't matter the source of heat, you just focus on the KW ratings and get an idea of what 3kw, 6kw, 9kw would do to your greenhouse if you get 9KW heater and it fits just fine, then 9KW worth of miners would do the exact same thing, air doesn't care how you generate that heat, heat is energy after all. If the average person outputs 100w of heat, you put 90 lazy people inside the greenhouse and they would raise the room temp exactly the same as 3kw miners or 3kw heaters would.

Fine so you want to assume that heat loss is 0, and delta T is 14, here is the math. air density at 3c = 1.223kg/m 1.223*900*1005 = 1,106,203 * delta t of 15,486,849 joules > the heat required to raise the temps inside 900m3  by 14 degrees Celsius. T = Q/W   Example 1: for 1*antminer S21 = T = 15,486,849J/3500W/60s = 73 mins Example 2: for 2*antminers T = 15,486,849/3500*2/60 = 146 mins If you really want to assume no heat loss at all and up to 1 day to reach that delta t of 14, then you just use Q and T and solve for W Since  T = Q/W then W = Q / T Q = 15,486,849J > We got it from the above equation  T = 24*60*60 = 86400 > Converting 1 day to seconds W = 15,486,849/86400 = 179W That means even 179W is good enough to heat a 900m3 area in 24 hours with 0% heat loss and perfect conditions, obviously, since there is no way that heat won't escape from whatever walls you are building, this never will never do it. But if you really are okay with waiting a whole day to bring that temp up, then I think with good insulation, you can probably do well with just 5-10KW, your main issue now would be to discover how much heat loss you are going to encounter, if the place was already built, you could make a cheap test like Phil suggested, you get a 3KW heater put it there for 24hours while monitoring the temps, whatever that gets you is what 1*S19 would do, doesn't matter the source of heat, you just focus on the KW ratings and get an idea of what 3kw, 6kw, 9kw would do to your greenhouse if you get 9KW heater and it fits just fine, then 9KW worth of miners would do the exact same thing, air doesn't care how you generate that heat, heat is energy after all. If the average person outputs 100w of heat, you put 90 lazy people inside the greenhouse and they would raise the room temp exactly the same as 3kw miners or 3kw heaters would.