Variable speed pumps

sixoilguy

I'm a 6 oil mechanic (yeah, go ahead and laugh) in NYC. On all my new construction accounts I'm seeing variable speed DC pumps on the hydronic systems. Someone told me they speed up on temperature drop to compensate for pressure loss. Why would there be a pressure loss if the expansion tanks are set and working properly. I'm sure it's a complicated question so I don't expect a detailed answer. Maybe a good web site or book explaining theory of operation. Thanks, Neil

Gordan

Seeking knowledge is far from laughable

Variable speed pumps don't compensate for static fill pressure, but for head loss created by the friction of water circulating through the piping. The added head loss happens when zone valves (or proportional valves such as TRVs) shut off flow to a given zone  (or partially shut it off, in the case of TRVs.) The pump detects that increase in head loss, and takes it as a signal that it should dial down the amount of flow that it tries to generate. So it compensates for an increase in head loss by turning itself down, not up. It speeds up on differential pressure drop (zones "calling" for more heat by opening their valves) and slows down on differential pressure rise.



Most (residential) ECM circulators on the North American market are controlled by some kind of differential pressure based control, not on delta-T. The exception (to the extent of my knowledge) is the Taco Bumblebee, which is a new product and its control tries to maintain a set system delta-T, like the 00-VDT (which is not an ECM pump - it is variable speed, though.)