Changing Old zone valves to Circulators

[kwieg1]

I have a Smith Cast Iron Boiler that supplies 6 zones of hydronic baseboard. As iot is piped now, it is a 2 pipe direct return system with boiler pump and 6 white rogers zone valves. The zone valves are starting to go bad and getting annoying to replace and want to replace the with Circulators + switching relay. Do you see any problems with keeping boiler pump and adding 15-58 grundfos brute pumps instead of the Zone valves?

[Dana]

That's a pile o' pumps!

If you do that it's more than a simple swap out with the zone valves. Plumb it primary/secondary, hydraulically separated and move the expansion tank to the input side of the old pump, using the old pump on the boiler loop, and use check valves on the zones (one per pump) to keep the pressure difference of the active zones from creating a reverse flow on those not calling for heat. (If every zone has quite a bit of head the reverse flow may be slow enough to not much matter in practical terms.)



(Ignore the fact that this diagram is pumping toward the boiler as opposed to away, and move the expansion tank to the other side of the separator.)

The hydraulic separator need not be a purchased component- it can be fabricated with short length of pipe with closely spaced Tees if you do it right. Alternatively you could use a buffer tank as a hydraulic separator to add mass and inhibit short-cycling on single-zone calls for heat, etc. In some instances it makes sense to use a "reverse indirect" type buffer tank & hydraulic separator to make domestic hot water for the building, combining thermal loads to one boiler to enhance duty cycle & improved net efficiency, but those options are a bigger redesign than just swapping out zone valves for circulator pumps with a fabbed-up hydraulic separator.

[hj]

A residential system such as this probably does NOT need a "boiler circulator", hydraulic separator across both lines, or temperature sensor in the piping. The expansion tank and make up feed should be connected at the point of "zero pressure change" which is the boiler itself. Since the pumps are pushing INTO the zones, the check valves should be on the returns, because that is where the zones interconnect after the pressure source, the pump, and where a backflow could occur. It also needs an air removal device in, or close to the boiler on the feed line. You would be money ahead in parts and labor, and complexity, to just replace ALL the zone valves and keep the existing format.