How to Reverse the Usage of a Thermostat Controller

I dare say noone can tell without:

1. Thermostat data sheet, or at least the make and model.

2. A schematic of your existing circuit

3. Data on the fans you propose to use - V, A, how many, etc.

If the controller has a relay output with both NO and NC contacts, then the controller can do what you want. If not, a simple relay can be added.

By adding this relay, would that be OK?

That might be suitable as a fan motor contactor (if it can handle the amps), but it does not solve the opposite logic issue between heating and cooling.

This relay is used to reverse the rotation of a three phase motor...

Just add a summer/winter aux switch that disconnects the heat control circuit and connects the fan control circuit..

Beg to disagree. The functioning of the thermostat needs to be changed as well, which is not possible unless it has a changeover contact and many thermostats don't.

The original poster needs to give a lead on the make and model of the thermostat in use.

The typical thermostat has heat and cool settings, if that's not the case and it only has heat, then the aux switch will send the power through NC contacts of the control relay to the cooling fan/s, and the heating circuit will be disconnected in summer setting...So the result being when the thermostat makes on temp fan shuts off and when breaks on temp, control relay then makes on NC contacts sending power to cooling fan/s...

Indeed. It's all about circuit design.

You are going to have to reset the thermostat from 19, to 28 degrees, but with no upper cut out limit. Might be better off to use second thermostat to trigger the fans, and just use a single power supply for both the heaters and the cooling fans as they will never both be on simultaneously. Good luck.

You need a DPDT heat/cool switch and two SPDT relays.

When in the "heat" position- relay one is de-energized by power circuit in pole one of the switch and thermostat output goes to heaters as currently programmed through pole two of the switch.

When in the "cool" position- the first relay (which controls power circuit to fans through its NO contacts) is energized and the second relay (which controls the fan starter through its NC contacts) is controlled by the output from the thermostat through pole two of the switch.

Heat position- heaters work as planned and fans are de-energized.

Cool position- heaters are out of control circuit, fans are ALLOWED to work through fan relay one and when the temperature is high enough to open the thermostat circuit, the fans are "told" to work by the fan relay two being de-energized. When space temperature falls below "high" thermostat setting, thermostat circuit closes and fan relay two is energized, opening the fan starter circuit and de-activating the fans.

The fan power control through pole one prevents accidental fan operation in the event of a relay mal-function.

If you intend on using the fans of the heaters I would suggest that you don't since they will end up being severely undersized for the application. It most often takes a much bigger fan to cool a greenhouse than to heat it. Also, the extra wear on the heater fans could potentially be costly in the future.

If you install separate fans, larger ones than contained on the heaters, located at a strategic location such as at the end of the greenhouse they will outperform the heater fans. An appropriate thermostat, on with rise/off with lower temp, and a manual on/off switch providing power is then utilized to control the system. This type of arrangement exhausts the hot air out of the house and allows cooler ambient air in to replace it. Also the installation of vents on the roof will help.

This is how the commercial greenhouses do it and they have been successful for a long time. I spent several years in a commercial greenhouse, many acres under glass and plastic, and this is the most efficient, economical and maintainable method of cooling houses (green).

Good Luck, Old Salt

Amending my previous reply-

No disrespect intended but many of the replies to your question are making this relatively simple problem much more complicated than it needs to be. There is very little scientific or engineering required to solve your dilemma. No complicated schematics, extensive data, integrations or derivations are required. Remember- KISS = Keep It Simple, Stupid! Two separate but both simple systems working independently of each other with you determining which is going to be turned on/off for the particular period of the year is the easiest to operate, maintain, cost effective and to operate.

I strongly suggest that your next step is something that all of us are familiar with, find out how the pro's do it. Isn't that how most of us got our extensive knowledge? Ask the pro's with years of experience and knowledge how to do it and for further information! I know that's how I got mine, formal education and listening to those who were wiser on the subject than me. It wasn't by asking a bunch of strangers whose qualifications I had no idea what they were. To expedite and obtain excellent information GOOGLE "greenhouse ventilation" and you will come up with at least a dozen excellent sites, both commercial and academic, which can show you how to make your system cheaply, effectively and better than you ever thought. (Sorry, my "copy and paste" for this forum isn't working or else I would list the sites).

I just did and there is more than I ever imagined. I spent several years as an engineer in an extremely large commercial greenhouse and there was some information on the search that I very little knowledge. There is always room to learn!

Good Luck, Old Salt

For a more accurate response it is advisable if you can draw and provide the complete circuit diagram of your present set up including all operating voltages. As mentioned by others, using relays can reverse the logics but thermostats are normally low voltage devices 24 - 48 volts maybe?), while the intended loads are line voltages (110 or 220 volts?).

Example of problems you may encounter:

i.e., the relay coil as shown maybe 220V and therefore cannot be driven directly by the 24volts output of a thermostat! You may need a 24v coil relay with contact ratings of 220volts with current capacity to match your intended loads of several amperes??

Since you failed to provide us with your exact technical abilities I would recommend that you go to Lowe's or Home depot and purchase an electronic (Hunter's is a good one) thermostat and simply add a contactor relay to handle the current of the heaters and fans.

Hi Youngken,

I tend to agree with the majority of the replies to your enquiry where they point out that you have asked a question but have given not enough information to allow anybody to give you an answer.

Normally when controlling any load with a thermostat it is good practice to switch the load (heaters or fans) with contactors which are operated by the thermostat.

In order to perform the control function you require you would need a two stage thermostat with one stage set for cooling (contacts to close on temperature rise) and the second stage set for heating (contacts to close on temperature drop). In between the two stages you would have a 'dead band' where no output would be enabled (i.e. temperature requirement is satisfied/at set-point).

With the parameters that you envisage I would suggest that you look at setting up the controller like this:-

Set-point = 22°C

Heating = On @ 19°C Off @ 21,5°C

Cooling = On @ 28°C Off @ 22,5°C

Once setup has been completed the greenhouse temperature would be able to be reset by just adjusting the set-point up or down.

The above settings would satisfy your temperature requirements plus ensure that, by having a dead-band setting, you do not end up with a hysteresis problem where you would be cycling between a demand for cooling or heating caused by overshoot.

You mention that you need this change from heating to cooling to cope with the seasonal change in temperatures, however with the system operating on automatic this would handle the control requirements during the changeover seasons when heating or cooling may be required on the same day.

I hope the above helps to answer your rather vague question.

Good luck!

Keith

I have to agree somewhat with your comment about over analyzing problems - we do get carried away sometimes.

I also find humorous the assumptions we sometimes make because the OP doesn't respond to requests for more information, and we proceed on assumptions based on our own past experience and observations.

In the case of this thread, we have at least 17 comments, while the OP hasn't provided basic info requested in the first comment, i.e. the make and model of the thermostat he has.

I think most posters genuinely want solutions, but sometimes I have to wonder if they keep things vague just to see how far off-track the discussion will go .

"... see how far off-track the discussion will go."

Well when we start analyzing the answers to the question, we have usually hit the wall...(sic)

The back side of my thermostat is like that

Finally, I discover the logic can be changed by changing the pin location for this type of thermostat

My problem has been solved. Thanks everybody!

Thanks for letting us know the resolution!