I have an HRV (heat recovery ventilator) which has a bathroom booster switch input. The HRV supplier specified a SPDT momentary switch for the bathroom booster switch. 18/2 shielded wiring is specified for the switch to HRV.
The SPDT switch has three screws plus a ground. The supplier told me that I should jumper the screws on the same side together and then use 1 wire on the left side of the switch and one on the right side of the switch. Does the jumpering really just turn this SPDT switch into a SPST switch? I asked why they didn't specify a SPST momentary switch and their answer was it was out of stock. My other thought was the dual throw could be used to later install a CO2 sensor in the bathroom and that could be another input to the switch to cause it to activate the boost mode – not sure if that is accurate.
SPDT wiring diagram in the following link. The jumper goes from A1 to B1.
https://www.leviton.com/en/docs/PK-93107-10-02-0D-W.pdf
I have the leviton 56081 2w switch. The way I understand this working is that when the momentary switch is activated a timer on the HRV puts the unit into boost mode for 5 minutes. I imagine that the momentary rocker switch automatically turns itself off so really it makes a contact and then breaks the contact – is that correct?
The manufacturer quick guide says don't use a standard single pole single throw switch.
The manufacturer documents says the switch is 1mA (lmax).
https://www.zehnderamerica.com/wp-content/uploads/2020/02/Option-Box-1.pdf
Best Answer
This is something of an opinion question - a reverse-engineering "why'd they suggest this?" 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 "do it wrong" by pressing the wrong end of the SPST-type switch, likely resulting in a customer support call or worse.