gas furnace blower exhaust problem

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  #1  
Old 12-16-04, 04:54 PM
shasson24
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gas furnace blower exhaust problem

Hi,

Here is my general information:

Live in Ottawa Canada in a townhouse. Current temperatures are about -3 to -10 celcius.

Gas Furnace was installed in 1997 and is a HEIL Ultimate III DC90 Ultra High Efficiency. The front of the manual says "NUGM Series"

Thermostat is a Honeywell T8132A,B,C Programmable with air conditioning attached.

So, for the most part the system seems to work as expected. Turn the temp. up and it comes on (with system on indicator on thermostat) and turns off when the wanted temperature is reached. Turn the temp. down and it turns off. The problem is when the temperature of the thermostat is turned down and the temperature of the house was higher, but then declines to reach the thermostat temperature. Once this occurs the blower exhaust will turn on for 10 seconds and then switch off for one second and then repeat this cycle constantly without stopping. I have not left it like this long enough to see if the whole system will turn on once the temperature of the house declines below the themostat temperature. The system on indicator on the themostat does not turn on when this occurs. The only way to stop this from occuring without turning off the heat on the thermostat is to turn up the heat, then the entire system will come on and turn off when expected.

Why would the blower exhaust turn on by itself like that?

Thanks in advance for any help.

Scott
 

Last edited by shasson24; 12-16-04 at 04:56 PM. Reason: more detail
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  #2  
Old 12-17-04, 05:31 PM
shasson24
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thermostat isolation relay or resistor

Hi,

I did some more looking around the net and saw that some people have experienced the same problem as me. They seem to be blaming it on their digital thermostat, but no one really gives a solution. Some people suggest installing an isolation relay or a resistor.

I am unsure how to do this or where they go? I have a pretty good background in electronics, so I would feel comfortable doing this if I had it explained to me. Does anyone know anything about this?

Thanks.

Scott
 
  #3  
Old 12-18-04, 07:15 AM
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Sounds to me like the heat in the xchanger is turning the fan back on and off .

Have you had this thing cleaned in the last few years?? A dirty flue ar xchanger can cause this problem.
 
  #4  
Old 12-18-04, 08:13 AM
shasson24
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Furnace servicing and resistor info

Hi,

I did have the furnace serviced at the end of October of this year.

I belive I have found a good diagram of what people are talking about to do with a resistor addition.

http://www.xpedio.carrier.com/idc/gr...tstat-0-28.pdf
- on page 6 of 8 Fig. 2—Typical Air Conditioner.

However, since my furnace may be different I may need a different resistor value or wattage? Any comments on how I can be sure the resistor they use is the one I need?

Thanks in advance.

Scott
 
  #5  
Old 12-18-04, 04:44 PM
shasson24
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Could this be the problem?

Hi,

I noticed something that may be peculiar, but I wanted to get a second opinion. I have 5 wires running to my thermostat for W, G, Y, R and Rc. On my control board W come from W, G comes from G, Y comes from Y and R comes from R. Rc comes from R as well though. However I have a C point on my control board with no wire from it running to my thermostat. Should Rc be running from C on my control board? Or is it in the right location on R presently?

Thanks.

Scott
 
  #6  
Old 12-18-04, 07:03 PM
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For your set up, RC and RH should have a jumper wire on the t-stat itself. (R on furnace should tie together on RC/RH on stat)

RH alone is used for System with it's own transfromer for heating.

RC alone is used for A/C system that has it's own transformer.

How's the battery? Fresh?

Anyway, Sounds like it's something on the furnace itself that causing the problem..
 
  #7  
Old 12-18-04, 08:44 PM
shasson24
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Additional information

Hi,

Yes, I agree with you about the R and Rc on my thermostat. I had two lines running from R on my furnace control board to both of these places on my thermostat. I decided to just have R on my control board run to R on my thermostat and then jumper R to Rc on my thermostat. That reduces one line going to my thermostat (leaving 4 lines in total now from my board to the thermostat), but the same problem remained.

I have also tried one other thing. When the system was in the bad state of the exhaust blower cycling, I took the W line off of my thermostat, which made the system stop. If I touched the W line back to the W pin on my thermostat, the system would continue to be in the bad state. However if I touched the W line to R, the system would start up as expected. This shows that it is the thermostat that is causing the problem right? For some reason it is not allowing the entire connection of W to R when it wants the furnace to start. It gets in some half way state that the furnace control board cannot interpret correctly and starts and stops the furnace frequently.

Again I would like to hear more about this resistor from W to C on the control board. All it looks like is a pull-up resistor so that when the thermostat isn't driving the W line low, it will automatically get pulled high to 24V. Is that correct?

I have also noticed that my thermostat does not appear to be keeping the current temperature very well. It thinks the room is 21C and my other digital thermometer thinks the room is 19C. I may try a new thermostat to see if it solves the problem. Not sure why this one would be bad though.

Oh and yes I tried really good new batteries, which didn't change anything.
 
  #8  
Old 12-18-04, 08:49 PM
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I not done anything like that with the resistors in line.. So i couldn't help you there.

I would suggest trying a new stat.
 
  #9  
Old 12-19-04, 02:04 AM
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Just read this thread and got confused by the first post which confused me as to whether the fan being referred to was the vent fan or the air handler. It seems like it is the house fan that is being referred to.

Had a look at the circuit diagram posted. To me it seems like the 270 ohm resistor is simply a dummy load. The fact is it rated at 10W suggests it is designed to dissipate a lot of power for a resistor. A typical resistor of the kind that is used in electronic circuits (such as biasing transistors or ICs) are usually rated around 0.25 watts. My guess based on the different circuits shown is that if the stat is swiitched to a state which is not used by the control board (eg no air-conditioning while switched to cooling) the stat still wants to see some current drawn. No idea why without seeing the internal circuit diagram of the stat.

If you are good with electronics why don't you hook up a multimeter at the stat terminals and look at the voltage on the G terminal. If it keeps pulsing then the stat may be faulty. If the W terminal is being activated normally, the fault may actually be on the control board rather than the stat. A few minutes with a meter should help you figure out what is going on.
 
  #10  
Old 12-19-04, 05:22 AM
shasson24
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a little more explanation

Hi,

Sorry about any confusion. What is coming on is the blower exhaust. The blower exhaust is what first comes on when the furnace is starting up. So I think it's the W line that is fluctuating. The furnace fan never comes on in this bad state.

But you are right. I should use a multi-meter to check the voltages on the W line when it's in that bad state. It should be 24V when it's trying to start right? I think I have pretty much proven that it's the thermostat though since when I connect the W control line directly to the R control line the system starts up correctly, but when I connect the R control line to R on the thermostat and the W control line to W on the thermostat I return to a that bad state with the blower exhaust cycling. It would incur that the thermostat is putting some weird voltage out on W between 0ish and 24V that the furnace control board is thinking is on then off and then on and then off.

I'm just worried that if I get a new digital thermostat it may happen still. Some people have said that thermostats with no C line are power stealing and need this resistor.

Thanks for your help.

Scott
 
  #11  
Old 12-19-04, 11:47 AM
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OK - I understand now - it is the vent fan that is being referred to. In that case, yes, a fluctuating voltage on the W line will most likely cause that and I agree that it sounds like it is the stat but before changing anything you should duouble check with a meter - yes, the voltage should be 24V when the heat is active. If the voltage is simply fluctuating between 0 and 24V once every 10 secs or so you can easily get a reading with a meter. The other possibility is as you say the voltage is somewhere in-between which causes the furnace to misread the W like and pulse in this way. If the voltage is pulsing erratically and very fast the only good way to see this would be with an oscilloscope but a meter would at least tell you there is a problem without a profile of the fluctuation.

If this is a probelm with this particular brand of stat why don't you get another one like Honeywell. I believe all of them are 24V compatible which is a standard control voltage for HVAC systems.
 
  #12  
Old 12-19-04, 11:54 AM
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try using the extra wire for common. that sounds like a "power stealing" stat, bleeding voltage to power the display from other unused low voltage circuits. if that does not do it, change thee stat
 
  #13  
Old 12-20-04, 04:01 PM
shasson24
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thermostat problem

Hi,

I wanted to let everyone know that I ended up changing my thermostat and things seem to be working fine now. The problem has not happened since I replaced the thermostat.

The problem did sound like a "power stealing" stat, but my thermostat has no common terminal to connect the common wire to. I have seen some thermostat manuals suggest connecting a resistor from C to W as well to correct this "power stealing" problem.

I decided to just change the stat, because thermostats that don't have a common terminal and don't suggest to use this resistor, should not be "power stealing" (and my thermostat's manual does not mention these things). And I definitely determined that the problem was the stat when I moved the W wire between the W and R terminal on the thermostat when the problem was occuring. (See other post above).

So far so good, I'll let you know if things change. Thanks for all your help.

Scott
 
  #14  
Old 01-04-05, 02:28 PM
tkimberling
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Stat

Which thermostat did you replace with. Brand Model? I have same problem.

Thanks
Tim
 
  #15  
Old 01-20-05, 04:01 PM
shasson24
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Hi,

I ended up using this:

Honeywell
7-Day Programmable Thermostat w/LCD
Model CT8602C 2009

You can look it up at home depot. Still works fine since I replaced it.

Good Luck.
 
  #16  
Old 02-07-05, 12:21 PM
rkeens
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Question Thermostat Problem?

I'm having the exact same problem with the cycling flue exhaust fan and just found this thread. I have tried a Honeywell CT3200A and a Noma 7 day programmable thermostat (don't know the model but same 4 wire configuration) and the same cycling of the exhaust fan happens with both. In both cases no mention that installation of a capacitor is required. No provision on either thermo. for a C connection; the Honeywell has a jumper from R to Rc and the Noma just has a R terminal so it must be jumpered internally. It cyles both before the heating cycle starts and after the cycle when the main blower has shut down. Ideas?
 
  #17  
Old 02-08-05, 03:45 PM
shasson24
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Hi,

Your problem sounds slightly different than the problem I had. Mine only cycled when starting up and when it did cycle, it never actually started. It never cycled when shutting down. And it would cycle properly and startup fine if the heat was turned way up.

The way you could tell if it's your thermostat or furnace is to take the thermostat off the wall. Then take the W line and touch it to R (be careful to keep all the lines separated once your themostat is disconnected). If your furnace starts up with no problem, then it's the thermostat that's causing the problem. If the problem still exists than it's your furnace or furnace board that is causing the problem.

Either way I don't think I can help more than what I've posted before.

Good luck.

Scott
 
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