The "heating" efficiency out of any ASIC-based cryptocoin miner is actually very high - directly competative with any "space heater".

 The PRICE efficiency though might be a different story, depending on the cost for the old miner - and temperature control might be an issue unless it's only a SMALL part of the total heat input to the room or you set the room with a termostatically-controlled damper/fan setup (or the room is on some sort of zone-controlled heat setup already).