Great question @Nick Symon, and thanks for bringing this up as I'm in a similar (but not quite as awesome) situation. Kudos to you on the position you're in! Our primary residence has a decent amount of equity in it currently, and we are moving soon, but considering keeping the current house as a rental.
I'm running into the roadblock of a low theoretical "Cash on Cash" return, even though the property cash flows taking into repairs/capex reserves, 1 month a year vacancy, property management, etc. While there would be ~$80k in equity left in the property once we draw on the HELOC to pull the down payment for the next house, after taking into account the above vacancy/reserve items the projected cash flow is ~$150/month. I consider the "cash" for this portion at roughly $60k, since if we sold the house there would be $15-20k in commissions and closing costs deducted from that equity ($80k, as mentioned above).
@Joe Villeneuve - You mentioned that as long as the property is positively cash flowing, it sounds like it's a go. Do you take into account the amount of equity in the house required to hold on here, or from your perspective is that irrelevant?
@Greg Weik - Good point on the property manager. On the building equity note and future expansion, would you sell a single family residence now and continue to save for a 3-4 unit? Thinking long term, if we were to grow a rental portfolio this seems to be a more efficient method.