There’s no doubt that housing is recovering. Existing home sales—which account for the bulk of the market—have topped year-ago levels for 20 months in a row and existing home prices have bested year-ago levels for 12 consecutive months. In addition, inventories of those homes have dropped to a 4.7 month supply — far below the more normal 6 months.

But unlike past housing recoveries, this one is heavily supported by investors — big and small. They account for about a third of home purchases in the existing housing market, according to the National Association of Realtors.

Among those big investors are the Blackstone Group (BX) which has been buying $100 million worth of single family homes a week since early last year, spending a total $3.5 billion to date, according to the Wall Street Journal.

Related: Toll Brothers CEO: 'Housing Recovery is the Real Deal This Time'

Institutional investors like Blackstone as well as individuals are buying homes to rent as landlords, taking advantage of another growing market. About 12% of U.S. households rented single family homes in 2011, versus 9% in 2004, according to the most recent data from the U.S. Census Bureau. Many of those renters are folks who lost their homes to foreclosure.

While these investor purchases are boosting the housing market they are also creating more risks because investors are not necessarily in the market for the long-run as the typical individual home owner usually is.

What happens when these investment firms leave the market?

“That’s a huge risk," says The Daily Ticker’s Aaron Task. “If they decide…they don’t really want to be in this business all of a sudden you could have a ton of new homes coming back into the market and then that supply situation will get flipped very badly against the market itself.”

Related: This is Housing Bubble 2.0: David Stockman

Former Budget Director and current deficit hawk David Stockman calls this Housing Bubble part two.

Investors in the housing market also face their own challenges.

“It’s not that great of a business to be a landlord…it’s a little trickier than just a typical investment,” says The Daily Ticker’s Lauren Lyster, noting that landlords have to make repairs to maintain their investments.

Owning real estate as a landlord is also “not a huge return business,” says The Daily Ticker’s Aaron Task. “What if the tenants don’t pay the money or if something goes wrong and you have to fix it.... It’s not as simple a business as it appears on paper.”

Indeed, Och-Ziff, a private equity fund that was one of the first to buy into the housing recovery a couple of years ago, announced late last year it was exiting the business because returns weren’t as big as they has expected. (And they probably also wanted to cash in their profits.)