Ciculator pumps that are good for gravity flow? (power outages)

Hi, first-time poster here at 'The Wall'. But I've been lurking for a bit and learning a lot, what a great resource!



My single-zone ancient gas-fired boiler needs a new circulator pump. I really like the fact that I still have heat when the power is out : There's enough gravity-flow through the pump to heat the house (to a point).



I don't want to lose that when I replace the circulator with something new, so I'm wondering if anyone can recommend a pump that allows for a decent amount of flow when it isn't running?



I'm pretty sure I don't want one with an integrated check valve.... But beyond that, any advice for something to look at?



This is for a typical single-story + basement residential home, old Burnham boiler, 120,000 BTU.



Any advice much appreciated!



JoeJ