Colman Evcon DGAT070BDD Blows Cold

[chasvez]

Hi Guys!
Here is a little background. At the start of the winter season, the furnace had blown intermittantly warm and then would short cycle blowing cold until shut off at the switch. When it would blow warm it would go through a normal cycle of induction, preheat, then blow, gas off then blow till cooled down. then a few minutes later it would short cycle till i got tired of hearing it kick on and off. I had also noticed that for a time it had a no call for heat error. Figuring it was the board after some testing I resigned to tinker some more before pulling it and sending it to you for repair. After thirteen years iIm kinda getting good at it, I have to put a new one in every other year or so. Yesterday i thought I would try cleaning the board with electrical componant cleaner because I noticed some areas where dust had accumulated around some traces underneath from static and might be causing a short. low and behold it worked. Normal operation and no error flash. While I was tinkering though I had removed the A/C sending wires too. If I reconnect them, it doesn't flash a code but starts short cycling again. Here is the clincher. I had replaced the thermostat. Changing the wires to the new matching the colors as best I could. As long as I have the A/C sending wires disconected, the furnace now works fine. So I believe that the wiring at this point is messed up. So I need some help with the wiring to make sure it is correct. Here is the wiring pattern. From thermostat to controller the wires are blue green white red yellow. from ac condensor red and white.

from therm--> B G W R Y so BW GG WC RR YY
at controller--> W G C R Y these are the connections at controller
from cond --> W R so WC YR

at Therm B O Y G W RC-RH RCRH are jumpered
Y G B R so YY GG WB RCRHandR
With the heat working fine I guess that the wiring for the A/C is the culprit. Right now the white wires are disconnected at both therm and controller but red is still connected to R on controller. I hope that is enough info to help me im getting frustrated with this whole mess

[chasvez]

I have an update on the function of the furnace... just a little while ago I caught the furnace doing what it did before fan running with no heat. I did notice that if I turn up the thermostat it will begin the heating cycle and warm the house while the fan runs. only out of order

[gordo48111]

chasvez,

Normal color coding for t-stat wiring is:

R - red wire - 24V feed from furnace transformer.
W - white wire - on a call for heat, 24V is switched from R to W, and to W on furnace control board.
Y - yellow wire - on a call for cooling, 24V is switched from R to Y, and to furnace control board to outdoor condensing unit.
G- green wire - on a call for fan, 24V is switched from R to G, and to G on furnace control board.
B - blue wire generally not used, would be for a second stage of heat or cool.

generally a white or black wire from furnace control board (C) is 24V common to condensing unit common. Also to C (common) on higher end t-stats, usually multi-stage or commercial.

[chasvez]

Thanks for the reply gordo
so if I understand correctly t the condensor...r to r and w to common black on controller? By the way the fan still kicks on occasionally running for no reason. I'm still Confused by this.

[gordo48111]

No. R to R will give you a constant 24V to the condensing unit. Since you mentioned changing out the t-stat, make sure you did not install one designed for a heat pump. What make/model t-stat is it? Also read the instructions to see if there are any switches that need to be reset for gas heat cycling and forced air fan (FG). Wire the t-stat as I described. The red wire can connect to either RC or RH which are jumpered together. Then find the Y wire coming from the t-stat back to the furnace. It usually will join a red wire at Y on the control board then goes to the condensing unit. The other wire leading to the condensing unit should come from the C or common terminal on the control board. It is usually white and will also have another black wire connected to the same terminal. The colors of the wires leading to the condensing unit don't matter as long as they are connected to the proper terminals on the control board (Y and C). Make sure the t-stat wiring is correct, as I described. I have the same furnace in my home but less BTU output, a DGAT056BDD. The only thing that I see controlling the blower on my model is the control board relay. The on and off delay times of the blower can be changed on the board but if it has been operating fine in the past you shouldn't need to adjust them. And I don't recommend changing those settings without a temperature meter to monitor the air temperature across the heat exchanger. If you do replace (or repair) the board, look for two black plastic jumpers that plug into the board. Make sure the jumpers on the new (or repaired) board are in the same position as they are now.

[chasvez]

The blue wire is actually in place of the white wire from the thermo to the controller. So I think im good there other than a difference in colors, but i can change that no prob. I couldn't honestly tell you if the jumpers are in the right place now or not. ive never compared it to others ive replaced. If I had to guess the problem it is residual heat still comming off the heat exchanger. Last year we had it replaced because of the recall on the furnace. and since the exchanger feels warmer after the cycle because it is working like it should. My question is, is there a way to have the fan blow longer after the gas valve switches off to cool the exchanger more of residual heat? If there is an adjustment I can make I would like to try. since things have worked better after the above, it only cycles a few times after heat and then stays off till the next call for heat. Thanks for the help.

[gordo48111]

First you said the blower was short cycling and blowing cold air. Now you say there is residual heat that you want to circulate. I'm not sure what the condition is that you want corrected. Maybe Robert can better understand what is going on.

[chasvez]

the furnace is short cycleing and blowing cold. Basically the blower would blow mostly on and off after a heating. sometimes it would run at random times too with no heat before I cleaned the board. Now since I cleaned the board it seems to only blow intermittant two or three times after it has finished heating then it will stay off till the next call for heat. thats why i thought it might be residual heat making the fan run more... I'm no HVAC so I'm not super good at explaining myself, sorry

[gordo48111]

Ok, I think I'm a little clearer on what's happening - thanks. As I mentioned earlier, the only thing I see on my same model furnace (only smaller) that controls the blower is the control board. There is no fan-limit that I see on the furnace or the schematic diagram that would cycle the blower motor. If the blower is cycling off and on after the heating cycle is over, I suspect either a faulty board or the blower motor is overheating and shutting off on internal overload, then restarting after it cools down. You will need to determine if power is being interrupted to the blower motor or not. If it is, the board is most likely the problem. If there is 120V to the motor when it is off, the blower motor needs to be replaced. You need to have the means to check for 120V to the motor after it begins to short cycle (when it is off). If you are not comfortable with doing this then have a technician come out and investigate.

[Robert]

As long as you have 24vac across W and C on the board, you should also have 120vac across the HEAT terminal and one of the NEUTRAL terminals.


Test when all is well and when all is not.


IF at anytime when you have the 24vac across W and C and no 120vac across Heat and neutral the board is bad.


That said, it may just be the board fan relay. We sell the boards, but Mark also repairs them in his store if it's just a bad relay.


You can call him at 605-229-2627 to discuss that option if it tests bad.


Thanks,
Robert

[gordo48111]

As usual, Robert is spot on. Just be aware that there won't be 120V at the Heat and Neutral terminals during a call for heat until the "blower on" delay time has elapsed. It could be 90, 120, or 150 seconds depending on how the jumper on the board is configured.

[chasvez]

thanks alot guys ill look into it and post my findings!!!!

[Robert]

You're very welcome, happy to help. The delay time for blower is usually around 90 secs give or take.


Best to test once blower starts, then when it stops to see if it dropped power from board.


Thanks,
Robert

[chasvez]

Sorry Fellas been busy... ill check it out in a couple of days... will post back...

[chasvez]

I dont know guys.. I checked everything and so far it checks out. been running fine since last post... if I have any changes ill report