I have a RV thermostat that I replaced with battery powered Honeywell Home and the new thermostat turns the fan off right away then the air-conditioner turns off. I would like to extend the time the fan runs after the relay opens for 1 minute at the thermostat side.

The thermostat just connects the RC that's hooked to the original thermostat ground wire and the fan to operate. I measured the voltage between the fan and the ground when not running and it's 12vDC at the fan wire and the ground is ground.

I have 12vDC power available at the thermostat.

Attached picture was what I was thinking. Using the 12vdc behind the wall to power it, the thermostat fan hooked you trigger, then the fan and ground from thermostat wires hooked to the NO side

Any thoughts or suggestions here?

 

Welcome to AAC!

Honeywell, with all the chinese characters? I am surprised. See the R7 potentiometer at the bottom of the board? It just might do what you ask for. Check the operating pamphlet that came with the thermostat. A simple 555 timer and fan relay could get you a 1-minute delay. I'd be thinking more time as in 5-10 minutes or so which would take more than a 555 timer. My own personal HVAC keeps the fan on low-speed full time due to moisture issues down here on the coast of Georgia.

 

For longer times with an analog timer consider the MC14541

 

I have a question or two for you:
First: the Thermostat (TS) turns the Air Conditioner (AC) ON. The compressor runs to compress the freon and make cold (not actually make cold but to remove heat). When the TS shuts off, the compressor shuts off, and so does the fan. THIS is the question: Can the TS control the fan independent of compressor?

Second: IF the TS turns everything off at the same time and you desire the fan to continue running for X period of time; to do so you would have to supply an alternative power source for the fan. Will powering the fan beyond the TS shutdown cause back-feeding power into the TS? This is important - will back-feeding power into the TS damage it?

Those questions are posed for consideration. I don't need the answers, but you might.

 

I see that the thermostat does not include a separate fan control connection. DID THE ORIGINAL THERMOSTAT have a separate fan control contact?? The most effective scheme will be a thermostat in parallel with the contacts of the fan control relay inside the AC evaporator housing. Consider that most home-residential air conditioners do not run the fan after the compressor switches off, why are you trying to achieve?? OR is this RV actually a MOTOR HOME, rather than a travel trailer?? In that case you need a relay to control the fan when the thermostat is set to cool.

 

Thank you everyone for your response so far. Sorry I wasn't able to edit the post to make it read better.

The part on bottom of OP is what I was going to buy. It's described as a "DC12V Adjustable Timer Relay, Relay Timing Module, Adjustable Time Switch Module, Digital LCD Display Countdown Timer" on the jungle site

This is the diagram below for the thermostat it originally had below. With the Honeywell Home thermostat, the GND from the air-conditioner is connected to RC/RH on the Honeywell, The compressor is on Y, and the fan high and fan low are hooked together to operate fan high on G.

This shows the original wiring for the DuoTherm thermostat I had on top, below is how the new one is wired in, it shows a switch but that's only if you want to keep low fan

 

I see NO SIMILARITY between what we see in post #1 and any of the connections shown in post #6 . But now I understand that the honeywell PCB in post #1 is a timer device, of which we have no hint as to how it functions.
So now I am guessing that the HUNTER 42999B is the new thermostat, and that it is intended to control ONLY the cooling function.
Further, I am GUESSING that the R on the Hunter thermostat is the power connection, although it might possibly be the W wire IF it is a cooling only thermostat.
So WHAT I KNOW FOR CERTAIN is that without more information it is not possible to provide any reliable advice without additional information.

IF the present connected digital thermostat, with the yellow wire actually controlling the compressor, and the orange wire actually controlling the blower, is actually functioning as a cooling only thermostat, in a cooling only system, then what will serve you best is an on/off switch action thermostat inside the cooling evaporator section, that will keep the fan running until the evaporator coils warm up to some temperature.
The actual connections would be based on information that we have not seen yet.

 

I see NO SIMILARITY between what we see in post #1 and any of the connections shown in post #6 . But now I understand that the honeywell PCB in post #1 is a timer device, of which we have no hint as to how it functions.

Yeah sorry, I would have been more careful if I knew I couldn't go back and edit the original post.

The part on bottom of OP is what I was going to buy. It's described as a "DC12V Adjustable Timer Relay, Relay Timing Module, Adjustable Time Switch Module, Digital LCD Display Countdown Timer" on the jungle site

So now I am guessing that the HUNTER 42999B is the new thermostat, and that it is intended to control ONLY the cooling function.

That was just a picture I found on Google images, instead of the Hunter, I have the Honeywell Home. If controls the heat and the furnace. The furnace is a separate propane furnace, but it runs through the air conditioner controller in the ceiling, it uses

Further, I am GUESSING that the R on the Hunter thermostat is the power connection, although it might possibly be the W wire IF it is a cooling only thermostat.

the GND from the air-conditioner is connected to RC/RH on the Honeywell

It shows black on the picture but mine is actually green. The high fan wire is brown. It's just powered by 2 AAA batteries and closing the contacts between each function and RC (ground)

Here's what the original thermostat terminals looked like...

Here's the diagram from the original thermostat

 

OK, One puzzle is how does that temperature sensor know if it is controlling heating or cooling.
Consider that for heating it must activate the heat system when the temperature is BELOW the set-point, while for cooling it must activate the compressor and fan when the temperature is above the set-point. So there is a bit of information missing.

 

OK, One puzzle is how does that temperature sensor know if it is controlling heating or cooling.
Consider that for heating it must activate the heat system when the temperature is BELOW the set-point, while for cooling it must activate the compressor and fan when the temperature is above the set-point. So there is a bit of information missing.

It's manually switched between heat, off, or cool.

 

OK, but still, the scheme needs to be different than just like the drawing shows. AND still, a thermostat that is closed when the cooling coils are cold would be a simple way to do it. That is a similar function to the "bonnet' switch that controls the fan when heating.