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.


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).


  • I don't know anything about "sides" on a switch. Switch screw functions aren't designed by position, but rather, by screw color or other marking. Feb 25, 2021 at 20:52

2 Answers 2


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.

  • Thanks. I had this thought too but I am not clear on the SPST momentary as my assumption is that you hit either up or down and it creates the contact and automatically breaks it later? Feb 25, 2021 at 19:46
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    @FreshCodemonger Installation instructions.. of course! I looked at the product data sheet, the brochure.. Anyway, I expect SPST has the same guts as SPDT except that one terminal is omitted. If so then pressing one end of the SPST would be no-op while pressing the other end would close a circuit. Only one of up or down (depending on orientation) would have effect. Because it is momentary center-off the contact is broken immediately when the rocker is released.
    – Greg Hill
    Feb 25, 2021 at 20:31

Yes. That does turn it from SPDT to SPST. Who is 'they'? I am sorry I don't believe there is sufficient info to answer your final question.

You should research the specs on the switch and HRV to ensure that 'they' haven't told you something that is incomplete, or conditional, or just plain wrong.

For example, some things to consider: is the switch break-before-make, or make-before break? What voltage/current is the switch rated for? What voltage/current is the HRV rated?

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