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It could be set to run full hash while you are away from the house quite easily and 10% or less capacity while at home. Bitmain is already considering low power options for home miners!
Still, it doesn't make sense from an economic point of view if you want to save money on heating bills.
Let's assume 5 cents per kwh and a run time of 12 hours with 4 months of heating and that revenue won't change in 5 years.
The s19 will earn you 3.5$ a day, and consume 2$ of electricity, so a total gain of $1.5 a day, $180 in profit a year of heating, $900 over 5 years, minus $2000 the miner, that's loss of $-1100.
The normal Walmart heater will obviously just burn electricity, making it $240 a month, $1200 over the 5 years period, plus the cost of the heater 100$ , which would make it more costly by $200 in 5 years versus the miner.
Assume now that the income will not change, the miner will not fail, you do need 12 hours of 3kw every day for 4 months at least.