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:
- 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.
- 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