Relay question

[imagineer]

Ok electrical gurus, here’s a conundrum…

The greenhouse I built for Mrs. Imagineer gets too warm on sunny days. For example, over the weekend, it was 33F outside and because it was sunny out, the greenhouse was 107F inside. I do have 2 automatic roof vents (that open at ~78F) and am planning on installing two more. Also, once the threat of snow is gone, we’ll be installing a 70% shade over the south roof face.

To combat the excess heat gain, this weekend, I’ll be installing a 110v thermostat controlled 14” gable fan that will pull air out of the greenhouse. There are already two passive vents located near the bottom of the east/west walls to allow outside make up air to enter.

For your reference, there is a 220v heater, hardwired in the green house with the thermostat set to keep the greenhouse above 55F in the overnight winter hours. If, while the heater is running, the temp up near the roof gets warm enough, it can actually activate the automatic vents (so I have the one closest to the heater pinned closed).

Depending on how accurate the independent thermostats are for the gable fan and heater, for the next month or so, there is a possibility that both could be running at the same time (further inflating my current electric bill).

I want to install a relay or something that will prevent the gable fan from running when the greenhouse heater is running, or vice-versa. What would be easier to pull off; having a 110v load trip a relay to cut a 220V circuit, or have the 220v load trip a relay to cut the 110v circuit? What type of relay should I look for?

 

[rlitman]

Having one thermostat with a relay that locks out another is a road to disaster. You may get relays clicking up and back fighting each other until something fails catastrophically. All it takes is one little power glitch while they're both trying to be on. If you had a control circuit with de-bounce logic, you can do it safely, but just a relay, no way.

I'd suggest a timer on each that prevents the heat from running during the day, and the fan from running at night.

Alternatively, you can use a single thermostat that is capable of controlling both heating and cooling, that has an auto-changeover capability (most require you to flip the heat/cool switch manually, but auto-changeover is available if you search it out).

How do you pin the windows? You may be able to lock them down while the heater is running so you can let them work freely during the day.

 

[imagineer]

Moderator, please move this over to the Electrical forum. I goofed and posted it in the wrong forum.

Rlitman, you mis-interpreted what I'm trying to do. When the 220v heater kicks on, I want a relay to cut the electrical power to the 110v gable fan.

I'm not interested in trying to sync up the thermostats, or try and reduce down to one master thermostat to control both.

 

[cherokee]

My wife is wanting a green house, I would love to see photos if you have any to share. She has been looking at fakebook marketplace for "free" windows for a while now. I am not sure how much longer I can dodge this bullet.

 

[nh_yota]

If you use a lockout contactor you still might run into issues.

For instance, greenhouse cools down at night and heater kicks on. Heater warms greenhouse and turns off but it's warm enough to trigger the fan, which turns on and blows all the heat out, causing the heater to kick back on. RInse and repeat all night long.

I think timers are a better solution.

 

[LXCam]

Go with an off delay timer. That way the vents don't open instantly once the heater turns off.

https://www.grainger.com/category/e...EJ3u8h0YNjrUVZ7FADhoCsLoQAvD_BwE&gclsrc=aw.ds

 

[imagineer]

If you use a lockout contactor you still might run into issues.

For instance, greenhouse cools down at night and heater kicks on. Heater warms greenhouse and turns off but it's warm enough to trigger the fan, which turns on and blows all the heat out, causing the heater to kick back on. RInse and repeat all night long.

I think timers are a better solution.

I didn't think of timers, but that might be an option.

When the heater runs, it brings the greenhouse to about 60F ambient, but at the roof peak, the temp can be high enough to activate the roof vents. FYI, the roof vents are passive and are activated by small gas cylinders that extend when heated above mid 70F.

https://www.amazon.com/dp/B08CKK7WD6/?tag=atomicindus08-20

The 110v gable exhaust fan I purchased is thermostat controlled. We'll set it at 85F - 90F so there should not be a situation where the heater and gable exhaust fan would run at the same time, I just want to make sure.

 

[MongoTA]

Would a small muffin fan and a floor-to-ceiling length of 4" or 6" duct work? Have the fan run while the heater is running? The muffin fan would be low electrical consumption. Circulate cooler floor air up towards the ceiling, exhausting where the vent actuator(s) is (are), helping to reduce stratification? Or something along those lines. Not sure of the size and scale of the greenhouse.

 

[inphx]

Arduino board and some sensors?

Arduino Greenhouse Control - Humidity and Temperature

Arduino Greenhouse Control - Humidity and Temperature: This project will regulate the humidity and temperature of any greenhouse through the use of heaters and fans. Additional tutorials will be added over time for automated watering and lighting, among other things. Everything is done through...

www.instructables.com

 

[Denwood]

How do you feel about some automation? A Hubitat hub and a few sensors and you'd be able to control everything quite reliably as you'd be able to monitor all temps, detect if windows are open and control devices appropriately. You'd also be able to monitor and/or control remotely from your phone if desired. I use one to control our pool solar system, HRV, lighting etc. You can do just about anything with automation these days using off the shelf bits.

 

[nh_yota]

More reliable than a timer is a photocell wired to a contactor so you don't need to reset the timer when the power goes out. Sun's up - fan can turn itself on - sun's down - fan can't turn itself on.

 

[Innovate1]

Automation would allow you to do more and see what your actual temps are. Gives you lots of flexibility. But if you want to keep it simple then disabling the fan when the heater is on is probably the easiest due to the fan being lower current than the heater. Without the current draw I can't make specific recommendations. As others have said a delay feature would be good to prevent the vent fan from operating as soon as the heater stops.