How much does a mining rig heat up the room it is in?

Essentially all the electricity a mining PC consumes is released as heat. A 400W rig outputs roughly 1,365 BTU per hour - enough to noticeably warm a small office or bedroom over a few hours. In a 10x12 foot room with the door closed, expect the temperature to rise 5-10°F (3-6°C) above the rest of the house. In winter this is welcome free heat that offsets your furnace. In summer it makes the room uncomfortable and can force your air conditioning to work harder, doubling the effective cost of those mining hours. Most home miners scale back during the warmest months for this reason.