Yes, you can use a Bitcoin mining rig to heat your home.
Whether or not it's worth doing depends on a number of things, such as:
- Whether you have a heat pump, or would otherwise install a heat pump (they generally give you 3kW of heat for around 1kW of electricity)
- Whether of not you have other heating that is more cost effective than electric heating (eg a wood or gas fire)
- Whether it's convenient to have a mining rig in the location you want heated (you lose efficiency if your heat is produce away from where you want it)
- The current exchange rate of Bitcoins (and the difficulty of mining them), and how those values change over time for at least the payback period of the mining hardware
- The price of electricity
- The noise produced by a mining rig. They tend to be quite noisy, so it may be a disturbance to leave it running overnight
- Your technical ability to maintain the equipment, and your time available to do so. If it has a problem and you don't have time to fix it, you stop earning mining income.
There's no guarantee that mining earnings will pay for the electricity to run them, let alone pay for the cost of the equipment and the labour incurred maintaining it - this is dependent on the demand for Bitcoins, and your costs. There's always a risk that the exchange rate will drop to the point where it's not worth mining at all, or that new technology (eg ASICs) will make your hardware obsolete before it's paid for itself.
Any web-based profitability calculators that exist won't necessarily take into account all the factors above, and none can predict the Bitcoin exchange rate, so their usefulness will be minimal. I recommend just creating a spreadsheet to do the calculations. To include the value of the heat output in your calculations, consider what you would need to otherwise spend to obtain the same heat output, and include that figure as profit on top of the mining profit (but also of course account for the cost of electricity to run the rig).
To predict the electricity usage of a particular setup, see this hardware comparison - a carefully built rig can run 4 graphics cards which might output 600-800W (or more) of heat. Using a desk fan (or similar) to keep strong air flow on the rig should reduce the potential of a convective flow of air taking the rig's heat straight up to the ceiling, where it won't be felt in cold weather.