I have a two stage furnace with a heat pump, and a thermostat that isn't capable of controlling the second stage of the furnace when in heat pump mode.

However, I can configure it in such a way that I get two 24CVAC outputs when I want it to engage the second stage of the gas furnace, and either one of them should engage the first stage. I need to design something like an AND gate and and OR for two 24VAC outputs:

Current Outputs:

  • Heat Stage 1 (labeled Y1): Heat Pump Only (No Gas Furnace)
  • Heat Stage 2 (labeled Y2): Heat Pump + Gas Furnace on Low
  • Heat Stage 3 / Aux Heat (labeled W1): Gas Furnace on Low

I need to keep Aux Heat to be Low, as in the coldest winter months I have to select Aux Heat only as it's too cold for the heat pump. However, when the thermostat calls for Heat Stage 2 AND Heat Stage 3 (which it never would do in Aux Heat mode), I want to provide the 24VAC signal to the Gas Furnace on High.

Desired Outputs:

  • Heat Pump (Y1) - No change
  • Gas Furnace on Low - Y2 OR W1 (24VAC)
  • Gas Furnace on High - Y2 AND W1 (24VAC)

Such that when Y2 & W1 are carrying 24VAC, I then have a new 24VAC output that is effectively an AND gate between them for HIGH, but either one triggers LOW.

Were this DC, I could just easily use a relay. I could probably also rectify the AC to DC, and then use an AND Gate IC or Relay. However, I'm not that familiar with working with AC like this - is there a better way to do this?


Someone commented briefly that I could use a 24VAC relay - and this seems like it could easily work to suit my needs here. Is there a solid state / non relay way to do this with AC?

Table 1. List of inputs and outputs.

Input 1: Thermostat Stage 2 (24VAC)
Input 2: Thermostat Stage 3 / Aux Heat (24VAC)
Output 1: Input 1 AND Input 2 (24VAC for Gas Furnace HIGH)
Output 2: Input 1 OR Input 2 (24VAC for Gas Furnace LOW)

Table 2. Logic table.

Input 1 | Input 2 | Output 1 | Output 2
 Off    | Off     | Off      | Off
 Off    | On      | Off      | On
 On     | Off     | Off      | On
 On     | On      | On       | On
  • \$\begingroup\$ I can't make any sense of your question. I've added some tables for you to edit, if possible. (Delete them if they don't help.) \$\endgroup\$ – Transistor Feb 1 '19 at 18:08
  • \$\begingroup\$ I filled them in. Does that clarify it for you? \$\endgroup\$ – Ehryk Feb 1 '19 at 18:47
  • \$\begingroup\$ For relay logic, there's nothing quite so much fun and education as: relay computer, relay logic info, and relay logic circuit diagrams. You can't escape being able to find a way to produce your logic there. \$\endgroup\$ – jonk Feb 1 '19 at 19:24
  • \$\begingroup\$ I believe there are solid-state relays that are AC-in and AC-out. While if there aren't I'd be astonished enough to eat my hat, I'm not sure sure, so I'm leaving this as a comment. 24VAC is common enough that 24VAC in/out SSRs should be a thing, and I can't imagine not being able to connect two in parallel. \$\endgroup\$ – TimWescott Feb 1 '19 at 19:27
  • \$\begingroup\$ @TimWescott I'm looking into PCB AC SSRs right now, and I'm confident I can piece together what I'm after with enough of them. I was mostly asking this question (as I realize now) if there is a relay-less way to do logic gates with AC signals. \$\endgroup\$ – Ehryk Feb 1 '19 at 19:30


simulate this circuit – Schematic created using CircuitLab

Figure 1. A suggested scheme.

  • Furnace Low will turn on if either input is on.
  • Furnace High will turn on if both inputs are on.

A pair of 24 V AC coil, single-pole relays - one with changeover contact - should be all that is required.

Just to close the loop then, there isn't really a thing as AC logic gate ICs the way there are DC logic gates, or similar shortcuts like how two diodes can make an OR gate with DC?

There isn't an easy way to do it in the same way as there is with DC logic. I'm sure there were some sort of electro-magnetic devices used in times past to achieve some sort of AC logic without using moving contacts.


simulate this circuit

Figure 2. An SSR implementation of the scheme. D1 to D4 represent the input side of the SSR which will be more complex than a simple LED.

This arrangement may not switch reliably due to the AC control of the opto drivers running on low voltage. The LEDs may turn on the opto-triacs rather late in each half-cycle.

  • \$\begingroup\$ Great, thank you! Just to close the loop then, there isn't really a thing as AC logic gate ICs the way there are DC logic gates, or similar shortcuts like how two diodes can make an OR gate with DC? \$\endgroup\$ – Ehryk Feb 1 '19 at 19:52
  • \$\begingroup\$ See the update. Thanks for accepting my answer. For future logic problem solving try the truth table approach. It will clarify your thinking greatly and is a simple way of reducing a complex description. \$\endgroup\$ – Transistor Feb 1 '19 at 20:39
  • \$\begingroup\$ Thanks for the tip, I'll use the truth tables next time. You helped me formulate this into a question that made sense, so thanks. I went with two TE RT114524 24VAC relays, as I'm sure they will work for this. I am curious about AC Optocouplers, so how would I do this with only SPST connections? \$\endgroup\$ – Ehryk Feb 4 '19 at 3:44
  • \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Feb 4 '19 at 7:23
  • \$\begingroup\$ Thanks again! As I said I'm going to stick with the relays for now, I was just curious how it would work with SPST / optocouplers. \$\endgroup\$ – Ehryk Feb 4 '19 at 8:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.