rooftop zone control hunting between cooling and heating

[comfortable air]

The rooftop is used in a commercial office building. It was controled by a zone electronic board. It has three zone. each zone has a thermostat. The thermostat has "G", "Y","W", "R", "X". Does anybody know how does the control board control the rooftop. I set all of the thermostat at cooling mode and tem 70. but one room is hot. and another two room is very cool. The thermostat indicate 68. The unit fighting between heating and cooling. I just disconect the "W" to the power for a temperary solution.


[Edited by comfortable air on 06-15-2005 at 10:05 PM]

[comfortable air]

just refresh to absorb the attention

[joey791]

What brand unit?

[chill factor]

does each zone have a bypass damper? is it working?
Need to know what kind of vvt system it is and the devices attached to it?

[maxster]

you already have your heat source within the space so lock out the heat side of that rooftop with and O/A thermostat say set at 50F.set the rooms for 68F-70F and the unit control for 55F-60F,should be discharge air...if not it is on the return so go for 72F.the stats might vote the swing on the unit thru that board,if you have no zone dampers in the unit into the space that has to be the sequence.the trick is to keep the stats all within range of each other if 2 spaces are cold but the other is good tweek up the cold ones then use the warmer of the spaces as your mark to satisfy

[comfortable air]

It is a very old keeprite rooftop. The control is add on by later. I don't know whether each zoom has damper or not. When the control board told me the damper is off, but the grill still has strone cold air come out. I just wonder there is no zoom damper, because the celling is drywall. there is no excess to the ductwork. Mr. Maxster, how can we lock out the heat side of the rooftop? what is a O/A thermostat? I did not find any unit temperature control.only three zone thermostat go to the zone electric control board. and 5 thermostat wires coming down from the rooftop connects the control board instead of the thermostat.

[maxster]

to lock out the heat take a Honeywell 0F-100F temperature control with the copper bulb and mount it in the condensing section.then pull your W off the unit and wire it to the common of the control,then come off the terminal "closes on drop" and put that on the W on the unit.what "closes on drop" means is at your setting of that control if the O/A goes below it,the heat will be OK to run again still depending on the unit call for heat.the mystery still is should that unit have zone dampers within it or mounted away and what are the stats controlling.if your saying that the board within the unit has a "damper closed indication" then you are controlling the air delievery to that zone with the stat in the space.the unit has to have some sensing of the air being discharged if you have no dampers then put the sensor going to that board from within the unit(no connection to the space)and mount it in the return air...is there any type of setpoint controller on that board in the unit possibly 50F-85F range?

[comfortable air]

I did not see any setpoint controller in the unit. you mean the zone damper and thermostat just control the distrubiton of the space air volume? it has nothing with the unit calling heating or cooling? do you think it must have a discharge air sensor or reture air sensor sense the temperature of the air and sent the singal to the unit for heating or cooling?

[maxster]

i would say return air sensing,it is more consistant on quick changes from compressors coming on/off...it is possible that those stats vote the swing of the unit as set and they are like averaging the entire space as one back to the board as one...idea!!!!disconnect 2 of them and run the unit with the other.with dampers being controlled by the stats as they satisfy the air,then would back up into the unit and the system temp would rise or fall and a sensor(within) would cycle the heat or cool down until you start moving air again?

[comfortable air]

good ideal. but I could not come back for this fun.

by the way, mr. maxster, how long it take you to chang a york compact rooftop blower moter. I spent my whole day for that stuff.

First I could not take the fan out the moter. so I use the flame to cut the moter shaft. then I took it back to shop, use the drill to get the old shaft out of the blown wheel. The blow is so big, and the space is so compact. it took me lots of time to put it back. when all this done, I turn on the power, no start. I mesured the voltage. one phase is zero. at first I think one fuse is browed. after I took one hour to located the power switch, The fuse is ok. finnally I traced the problem from the 3- phase disconnected switch. then I went to buy one. but the new one is not fit the space. I have to use the old box. the bad thing , when I install the switch, one screw is broken(what a bad product), so I had to go back to get one screw.

You see the whole day just for one service. the guy diagnose the blower moter need replace. I think the problem is the one phase lost. anyway, when I left, the rooftop works fine. what a hard day. I did not have time to have lunch.




[Edited by comfortable air on 06-22-2005 at 08:42 PM]

[maxster]

any time you do a follow up on motors,compressors that somebody condemmed,check it to ground with a meter and between the legs and give it a quick spin(could of been a weak fuse and the motor was ok).on compressors do the same ringing out to ground and between the legs,if you see all is good the compressor could be locked rotor....do a visual on the contactor if it is cooked and arched their is your locked rotor hit.

[comfortable air]

Mr.maxster, Do you think a three phase fan moter or compressor would be damaged if lost one phase? I check the ground with a quick way. I usally diconected the power, and put the multimeter to the leads of the load side of the contactor.the reading is 27M . do you think it is ok? what is a locked roter. what could we do for it? could you explained the last sentence to me(do a visual on the contactor if it is cooked and arched their is your locked rotor hit.) sorry, I did not followed it




[Edited by comfortable air on 06-26-2005 at 11:23 PM]

[maxster]

when you lok at a contactor info plate and it says 40 amps at 240 volts that is the rating of the contact points.then say your compressor is rated on the tag for 29.0 amps at RLA at 240 volts....so when that contactor pulls in and the compressor starts and for that milli second you are drawing LOCKED ROTOR and the amp range drops down then into your RLA range and all is goooood?if you come up on a compressor and it rings out good to ground and between the windings has continuity..WHY is it blowing the fuses?pulling the cover on the contactor and seeing the contact points melted together or large carbon deposits all around the points is that LRA hit and the fuses blew and that is a visual check.that is why your always change your contactor on a burn out or locked rotor compressor,and you can have a locked up compressor or motor that if you put your amprobe on the leg and push it in you won't "blow the fuse" and for that split second can see 10X the tagged amps.any time you check a belted motor for this always take the belt off just for that quick test,and when you have a unit with multiple condenser fans on say an inspection bump the fans just to check if their solid

[maxster]

a note on 3 phase fusing......if only one is blown that is a weak fuse that opened during the running of the unit(no cooling),if you come across 2 blown fuses and obviously the call again is no cooling,one of the loads shorted or a contactor shorted causing the fuses to blow.if you have say a condenser fan that sat all winter and the bearings are tight from the weather...that could knock out 2 fuses on start but the motor rings out ok!

[comfortable air]

thank you very much. Is it that one phase fuse blow could be caused only by a weak fuse? It is not caused by the ground, overload, or shorted? just replace the fuse and everything will be ok? only two or three phase blown could be a problem?

[Edited by comfortable air on 06-27-2005 at 04:55 PM]

[maxster]

i have never had that 1 fuse blown and a load grounded or open on the windings,the starting of the comp/fan is the critical time when fuses are used.same with circuit breakers that lock rotor start draw is the key factor.i had a supply fan motor on a new rooftop and with the belt off if ran and rang out ok to groung and the windings were solid on tose readings....then with the belt on i would trip the breaker on start UNDERSIZED breaker!some jobs they go to the mimimum on the fuses to save money.example....was a pump that drew on the tag 38 amps and they put in a 50 amp fuse,and in a hot machine room "out it went" 2 legs after running for 30 minutes!i am on the chiller and it goes out on flow,and that leads me to the pump/starter and the fuses.....undersized on the install$$$$$$$ and that is why fuses come in different sizes(diameters and lenghts) when your fuse'g loads

[comfortable air]

If the unit draw 38 amp, how big the fuse is ok? Is it larger 175% and less than 2 tiems?

[maxster]

keep in mind that the fuse sizes are for the entire unit amperages but the main concern is the wire sizing to that unit if something shorts or burns out.Yes,the fuse will open up but better that then the insulation overheating on say a 100'-- 500' 3 phase wire pull from the switch gear room in the basement.if you read the number off the unit 3 phase going in the code will tell you of max amps allowed for that size..adding up the unit tag amps and the wire max amps will give you an ideaa of the fuse sizing.