Heat ON then OFF then straight to COOL??

[Fett]

Hey everyone. I've got a pretty common problem, but its beyond me. I have 50 AC Units on the building that I work at. 23 split and 27 rooftop over under. The splits are 3 and 5 ton carries and the rooftops are 5 and 6 ton carrier weathermasters. The splits are operating good. Bassically we have a two story building and the splits service the first floor and the rooftops service the second floor. Well on the second floor theres a conference room served by two of the 6 tons... its not that big of a room and I think 12 tons of cooling is way overkill... both thermostats are less than 3 feet apart from eachother and the units just cycle heat and cooling all day. I have the temperature set to heat when it gets to 68 degrees and cool when it gets to 75 degrees. As soon as it reaches 67 degrees the heat kicks in and as soon as the unit should cut out from heating it drops into cooling. The actual room temperature never reaches 75 degrees it stays between 68 and 71. Now we have a computer program called VNC and all the thermostats in the building are wired to a server so I can change temperatures and occupied/unoccupied settings from my computer. If I change a temperature setting in the computer it reflects on the thermostats settings so I'm fairly sure that its not a problem betwen the T-stats and this program and more with the unit. Any advice?

[Fett]

Ok, I ran a test. The room temperature was 70 degrees. I set the temperature for heating to 75 and for cooling to 80. I went up on the roof and watched the units. Both should be in "HEAT" (blowing cold air outside) one was in heat and one was in cooling and about 3 minutes went by and the one in heat switched to cool. Any ideas?

[Brian_UK]

Always a problem when you have two units fighting each other.

One starts heating and blows hot air over the second unit which then starts cooling to counteract it - and then the fight started ;)

You need just one thermostat/sensor controlling the space temperature.

[Fett]

Always a problem when you have two units fighting each other.

One starts heating and blows hot air over the second unit which then starts cooling to counteract it - and then the fight started ;)

You need just one thermostat/sensor controlling the space temperature.


So it would seem although I also have a set of offices (six to be precise) that are run off of one 6 ton and this unit does the same thing. The thermostat is in one of the offices.

[nike123]

So it would seem although I also have a set of offices (six to be precise) that are run off of one 6 ton and this unit does the same thing. The thermostat is in one of the offices.
Maybe you have some interference from power source cables passing alongside low voltage cables from indoor units. Check that all low voltage cables are screened.

[Fett]

Maybe you have some interference from power source cables passing alongside low voltage cables from indoor units. Check that all low voltage cables are screened.

I will look, they did run all the cables for everything in the cieling of all places...

[wombat]

There should be a dead zone to prevent the A/C going directly from heat to cool. Check the setting on the computer program and if possible monitor the sensor temperature to see what temperature heat and cool start and stop.
Maybe there is a wiring fault.

[frank]

Can you put the thermostat into the return air path (Just before the return air enters the unit) rather than the room? Even if it is a temporary move, it should make the control smoother.

With one unit supplying 6 offices and a meeting room, your control will always be difficult, but return air sensing would be a better option.

[Fett]

There should be a dead zone to prevent the A/C going directly from heat to cool. Check the setting on the computer program and if possible monitor the sensor temperature to see what temperature heat and cool start and stop.
Maybe there is a wiring fault.


The computer program that I have just monitors the temperature and allows me to change it so I don't have to go to the T-stat. Our security system and some other systems are run off of this program exclusively like our lighting. I've not been very impressed with the company that installed all these units. I have had to have them come out to fix three splits due to a loss of refrigerant so a wiring fault might be it. They will be out by the end of the week to combine the two 6 tons onto one thermostat and also run power to a thermostat downstairs(which they never wired up from the beginning).

[Fett]

Can you put the thermostat into the return air path (Just before the return air enters the unit) rather than the room? Even if it is a temporary move, it should make the control smoother.

With one unit supplying 6 offices and a meeting room, your control will always be difficult, but return air sensing would be a better option.

Well the problem with this building is they chose to go with "plenum return" so there is no return ductwork and the air just goes up through grates in the cieling and theres about 4 feet of air space above the cieling tiles. Theres one unit supplying 6 offices that acts irratically and the conference room is supplied with two units and two thermostats. Hopefully combining into one thermostat will solve that but I am still unsure of what I will have to do to keep the one unit from reversing before it should.

[back2space]

You need to discuss that problem with the installers who will in turn discuss with the manufacturer, there should be a buffer zone where the units "hold out" before switching over into cooling/heating

Its a control problem it seems

[Fett]

You need to discuss that problem with the installers who will in turn discuss with the manufacturer, there should be a buffer zone where the units "hold out" before switching over into cooling/heating

Its a control problem it seems


Ok, I talked to the installers. They say it is impossible to put two seperate units on the same thermostat... lies ofcourse... anyways they said the supply air temperature sensor which they forgot to put as one of the displayed fields on my computer program should be set the same (on each unit) and if the temperature is coming out different from eachother (from the supply diffuser). Then there is a problem with the unit and they will come and look at it (they didn't say they would come fix my computer program because all these temperatures should be displayed, outdoor temp, indoor temp, return air, supply air, humidity... ect...). So I ran my check (stuck a thermostat in the supply air stream) and the temperatures spread was 7 degrees apart. So I sent out my 3rd email and will see what results I get wether or not they are going to come and look or not...

[Fett]

I now know what the problem is. Since the units are conventional... they have them set up as heat pumps in the computer program and its making them switch over on 2 minute intervals. It has to do with how the unit is wired... compressor run with y and w connected and the computer program thinking its a heat pump.