How to meter the hot water from a boiler

[Mark Cherv]

I have a 3 multi unit place, but there is only one boiler & oil tank.   Heat is supplied by forced hot water.   Each unit has its on zone, and each zone has an electrical meter that is monitoring how long the circulators run for.   Theoretically I would think I could proportion those numbers to correspond to a % usage, and then I could charge them that % of the oil bill.    Currently I pay the oil, which is not my preference for obvious reasons.   Has anyone had a any experience with trying to monitor water usage like this.   Or do they have a better idea of how I should meter the usage, like maybe an actual flow meter on the hot water side?

Thanks