# Furnace double cycles



## RandyB1986 (Jan 2, 2009)

I have a Trane X80 gas furnace. Model #TDD100C945BO Ser. # J29548348 Mfg.Date 0994

It got really cold here the last couple of nights, single digits and high wind, and I noticed something I didn't like. My furnace goes to kick on and the draft motor starts, then burners ignite and everything seems normal, once house is warm the burners kick off and blower motor shuts down and draft fan continues for a few minutes then instead of shutting down it clicks and the burners will refire and the cycle will run again then shut down.

I am wondering if perhaps my furnace is running and just cant warm the house to the temp requested on thermostat, because it is so cold outside. Then I was thinking maybe the furnace senses too much heat and shuts down until unit cools. Am I even close or do you guys have any advice?

I do think I am going to go get a CO detector after reading the other post. Should I have a CO detector if I only have a gas furnace?

One other question.....what would be a good replacement furnace for this old unit, with easiest install?

Thanks,
Randy


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## Doc Holliday (Jan 2, 2012)

The furnace with the closest measurements, length and openings to match your old Trane, would be best.

A few ways to determine if your stat is simply telling the furnace to come on or not.

1.) Turn the stat OFF.
2.) Jump R to W at the control board in the furnace with the door switch pushed in so the control board has power to it. Jumping terminals R to W will bring on the heat. 

R is your 24 volts. Once connected to it do not let anything touch metal or you'll blow a fuse. Hook to W first and then R.

First your exhaust inducer motor will start (if you have one, I'll find the manual for your unit), then the pressure switch closes, then the ignitor glows and then the gas valve opens up. Then flames on until the unit gets hot and lastly the blower kicks in, providing your living space with heat.

When you remove the jumper (with the door switch still pushed in so the system has power) the call for heat should stop. The gas immediately (no flames) and then approximately 30 seconds after the blower motor should stop. 

Wait for the blower to stop completely before you bring heat back on at the control board. This this mulitple times to ensure proper start and stop.

If the unit fires and stops without doing this double cycle than your stat is the issue, temperature related.


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## Doc Holliday (Jan 2, 2012)

Here you go. Tells you everything you need to know about your furnace. And to answer your other question yes, any time natural gas furnace is used it is an excellent idea to have CO detectors as well. 

http://www.trane.com/webcache/un/furnaces (furn)/product/22-2311-16_1q_fu_01012009.pdf


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## Doc Holliday (Jan 2, 2012)

Btw, it's not the furnace that senses temperature but the thermostat. By bypassing the stat and controlling at the furnace you remove that sensing variable. 

Good luck.


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## user84377 (Jan 7, 2012)

RandyB1986 said:


> I have a Trane X80 gas furnace. Model #TDD100C945BO Ser. # J29548348 Mfg.Date 0994
> 
> It got really cold here the last couple of nights, single digits and high wind, and I noticed something I didn't like. My furnace goes to kick on and the draft motor starts, then burners ignite and everything seems normal, once house is warm the burners kick off and blower motor shuts down and draft fan continues for a few minutes then instead of shutting down it clicks and the burners will refire and the cycle will run again then shut down.
> 
> ...


Couple of questions Randy.

1) Has this behavior always been noticed in the heating mode?
2) You stated that after the house warmed up is when you happen to notice the furnace cycling the burners....how long approximately between the burners cycling? And is it the same amount of time?
3) Mercury/Mechanical thermostat or digital?


Normally the sequence of operation following a termination of heating call by the thermostat should shut down the inducer motor shortly after the end of the heating cycle.

A real good starting point is to check the on board diagnostic LED. 
1) Flashing slow...normal, no call for heat
2) Flashing fast...normal, call for heat
3) 2 flashes...external lockout
4) 3 flashes...pressure switch error
5) 4 flashes...open limit device
6) 5 flashes...flame sensed when no flame should be present
7) 6 flashes...polarity reversed
8) 7 flashes...gas valve circuit error
9) 8 flashes...low flames sense signal
10) 9 flashes...Check igniter

Some other issues that could be causing your problem are:

If flue does not terminate properly after breach of building/home structure you could be having issue with draft pressure during excessive or windy situations. If the flue is really bad off the problem could exist even during calm days.

Failed or incorrectly set heat anticipator on a mercury or mechanical thermostat. If you in fact have a mercury style stat then there is a heat anticipator located on the thermostat behind the mercury switch or contact. These should be set for the total amount of current being consumed in the heating circuit. This is done by either finding that setting in the literature...sometimes the actual gas valve will have it stamped. Otherwise they may be set by wrapping 18 gauge wire approx 10 times around an amp clamp meter ( the thermostat is removed from the sub base during this process and the sensing wire is connected between R & W). Whatever the reading...you divide by the number of wraps, which is ten). However if the heat anticipator has failed there is nothing you can do, except replace the stat.....Improperly adjusted or damaged heat anticipators can either cause too many cycles or too few.

Limits or safeties opening can also be your problem but the onboard diagnostic LED will flash that code for you to see.


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## RandyB1986 (Jan 2, 2009)

Just wanted to say thanks to you all....I think it is my old honeywell mercury stat. I think it may be something to do with the heat anticipator.

I need to just get a new furnace or heat pump this summer. If I switch to heat pump do I have to replace furnace anyway? I would like to get rid of gas furnace and have heat pump with electric furnace....my gas furnace is the only gas I use now. Would total electric heat pump be cheaper to operate then natural gas furnace?

Thanks!


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## Doc Holliday (Jan 2, 2012)

On average electric costs three times as much as gas.

If you are wanting to go electric you can, it's very common with heat pump applications. Electric would be second stage or emergency/auxilary heat.

Buy a heat pump compatible thermostat now which you can use on your current set up so you don't have to purchase one later.


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## user84377 (Jan 7, 2012)

RandyB1986 said:


> Just wanted to say thanks to you all....I think it is my old honeywell mercury stat. I think it may be something to do with the heat anticipator.
> 
> I need to just get a new furnace or heat pump this summer. If I switch to heat pump do I have to replace furnace anyway? I would like to get rid of gas furnace and have heat pump with electric furnace....my gas furnace is the only gas I use now. Would total electric heat pump be cheaper to operate then natural gas furnace?
> 
> Thanks!


In order to know whether or not the heat pump w/electric aux heat would be cheaper than utilizing your current gas furnace you need the following:

Keep in mind that the following will continually fluxuate and therefore there are no guarantees which will constantly be cheaper.
Although if you are willing to pay for the install geo thermal is always cheaper than gas.

Cost per KW HR of electricity from your utility
Cost per CF of gas
Amount estimated to used at design winter temperature for your area

Either way, good luck and let us know if your problem gets resolved.


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## Dr Heat (Dec 25, 2008)

you can simply add a heat pump to your existing furnace and set it up for dual fuel. Have the best of both worlds.


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