You are claiming operating costs rather than capital costs.  Claiming 20% of your heating bill doesn't impact the capital status of your home anymore than expensing a tank of gas impacts the capital status of your car.

As Droisa says stay away from depreciation related items or capital related expenditures (e.g. a major renovation that benefits the business rather than the residence).

Like most things tax related the more you push the envelope the more room you give the CRA to challenge what you are doing.  Stay within what is fair and reasonable.   There appears to be some ambiguity as it relates to mortgage interest with following link explicitly saying you can't - however that might be for those who are not self employed.  I have worked as both a self employed and incorporated business owner and have never had a problem deducting home office expenses.

http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns206-236/229/cmmssn/hm-eng.html