The head of ING Direct Canada and several economists are warning Ottawa not to make abrupt changes to mortgage rules, lest that trigger the very thing everyone wants to avoid - a swooning housing market. While Ottawa should increase scrutiny of the Canadian real estate market, acting too quickly to impose new blanket rules on mortgages could have the unintended consequences of toppling the market, they say. Most expect the market will cool off by itself later this year even as some, including the Bank of Nova Scotia, acknowledge house prices are now in bubble territory.