Why a SPDT momentary instead of a SPST momentary?
This is something of an opinion question - a reverse-engineering &quot;why'd they suggest this?&quot; question, but I'll take a swing.
In Leviton's low voltage catalog there are SPST and SPDT momentary switches in the Decora style. Both have center-off. It's easy to guess how the SPDT style works. Think about the implication for SPST with center-off: probably pushing one end of the switch causes a connection while pressing the other end does nothing.
A schematic diagram of the switch was not easily located in Leviton's literature, but based on outward differences between SPST and SPDT models, I expect the one-screw side is the common terminal and the two-screw side has both of the NO terminals. Tying those two NO terminals together does simplify the SPDT switch to an SPST function -- it's different from the SPST switch, though, because pressing either end of the switch activates the output.
The HRV vendor likely recommended the center-off SPDT switch to avoid the possibility that the user might &quot;do it wrong&quot; by pressing the wrong end of the SPST-type switch, likely resulting in a customer support call or worse.