Furnace cycles on and off

[firozhirji]

I have a YORK furnace - Ultra Diamond 95 installed in 1996. It is a variable speed two stage furnace rate 112, 000

Since day one we get it comes on runs 20 secs and turns off. We have had a number of service persons come out to check and no success.

Finaaly, I spoke to York and they sent out a tech who checked the following:

1) intake and vent pipes for blockage

2) control board

3) pressure switch

4) ventor motor

5) condensate drain

He said all was fine -- left the furnace running and of course it stopped as soon as he left.

But he did recommend changing both the intake and vent pipes to 3inches. They are 2 inchees now.

Should this be done? The quote was $300.

Thanks

[Terence McCann]

Do you have the original manual that came with the furnace? In it they will clearly spell out what size pipe is needed for combustion and exhaust air. Fwiw I have a York 95% furnace that has 2" intake and exhaust piping and operates fine.

When you say it comes on for 20 seconds and shuts off - what exactly shuts off? Does the whole unit shut down and not come back on? Does the burner come on for 20 seconds then shuts down but the blower motor starts and runs? Just recently my burner started to short cycle and then lock-out. Turned out to be a dirty flame rod - after cleaning it's running fine again.

[Scottpat]

Sounds like a high heat limit control or the anticipator control. Both can cause the unit to short cycle and I would think would be the first things a technician would look for.

[firozhirji]

Blower comes on for 20 secs.

[Terence McCann]

Blower comes on for 20 secs.

The main blower that blows warm air out of the duct work or the little fan inside the furnace that is hooked up to the 2" pipe?

There is also a little red LED (typically it will flash green if everything is normal) that will flash X amount of times when the furnace locks out on something (you can view it through one of the little plastic view plates in the main panels). If you look in the manual it will describe what each flashing pattern means. Flashing rapidly 6 times is a pressure switch problem, flashing rapidly 8 times is a faulty gas vale or flame rod etc. Have you checked this as well? One other point, the circuit board will store these fault codes in its memory (unless someone erases it). Next time a tech is out ask him/her what the past fault codes were.

[Rocon]

I had a unit that was acting up similarly, and the problem was the thermostat. The thermostat had a run cycle setting that was causing the problem. Try disconnecting the thermostat and connecting the wires directly together.

[firozhirji]

I will try and answer all the replies in here:

Tech checked the thermostat.

The fault code was 3 but was not produced while the tech was there (like a car problem that disappears as soon as you get to the garage and comes back as soon as you leave the garage!!).

The tech checked the pressure switch and checked the blower and ventnor motor.

The tech checked all the pipes --including the runoffs for the condensation in the vent pipe.

He could only think of the pipes being 2" instead of 3".

I have attached the pages from the manual that says 3" pipes are needed for 112 MBT units

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[Terence McCann]

A fault 3 code is "pressure switch stuck open"

Armed with the information in your manual take a look at your installation and see what needs to be done with regards to supply and exhaust piping. Then, if you're skilled, tackle the job. If not the last tech who said the piping needs to be changed sounds like he's on the ball.

Start with the obvious first.

Good luck!

[Robert E Lee]

It is my understanding that high efficiency furnaces over 100,000 btu require 3" intake and exhaust, as noted seems like that may be the problem.

[Marc]

A 3" pipe has more than twice the cross sectional area of a 2". If the spec call for a 3", a 2" would be only 45% of that spec. That's significant.

Marc

[plummen]

basic furnace 101 for those not familiar with them:

1-stat calls for heat sends signal to board(newer furnaces)

2-inducer motor should come on for a set amount of time creating a vacuam to close pressure switch

3-board senses switch is closed sends power to spark or hot surface igniter ,igniter will either start to glow or spark depending on type.

4-gas valve will open allowing burners to light if all else is working .

5-as long as the flame sensor (takes place of old style thermo couple)senses a flame gas valve stays open.

now check the simple things

1- jump red and white wires together off the board in bottem of furnace,it it stays running check your stat as previously mentioned.

2-if burners go out and try to relight themselves soon after check the flame sensor,plastic scuff pads work good for cleaning them.dont use sand paper on them

3-if a limit or roll out switch is causing the issue it normally will not come back on and the blower motor will kick on and try to cool furnace down till you manually reset the switch.

those are just a couple simple things to check,if it was a pipe sizing issue id think it would of been a problem since it was installed normally anyway []

[AROB]

I have a Ruud 95% gas furnace, single speed. Its only about 3 years old. It has always cycled on and off before coming up to target temp. It will run about 10 mins, the gas shuts off and it continues to run to cool itself (or whatever). The instant the main blower shuts off the pre-blower starts up again and it relights and turns on and repeats...I tried replacing the thermostat batteries and made sure the filter is clean. I even tried running without the filter just in cast and it still happens. Any ideas on what to do in this case?

[Rob Amaral]

Is is possible the furnace(s) are too big for the space being heated?

[Tom Raymond]

Is is possible the furnace(s) are too big for the space being heated?

I think you meant "probable".

The vast majority of furnaces and boilers I see are the same size, regardless of what size the house is, or how well insulated.

[rkenney]

I have a Ruud 95% gas furnace, single speed. Its only about 3 years old. It has always cycled on and off before coming up to target temp. It will run about 10 mins, the gas shuts off and it continues to run to cool itself (or whatever). The instant the main blower shuts off the pre-blower starts up again and it relights and turns on and repeats...I tried replacing the thermostat batteries and made sure the filter is clean. I even tried running without the filter just in cast and it still happens. Any ideas on what to do in this case?

Sounds like the high limit switch turns off the gas (180 deg.) on the first cycle.

Blower continues to run to pull the high limit switch low (90-95 deg.). This is normal operation.

Furnace cycles on again because the target temperature (T-stat) was not reached first time around.

Your on the right track. Most likely cause is an air flow restriction. i.e. dirty restricted filters (you already checked that), blocked intake or outlet registers, dirty A coil (pet hair is typical).

Of course since you say that this situation existed since installation it is possible that the installation (installers) did not accomodate the duct size correctly (small ducts - low flow).

[Garet]

As others have said, it's almost certainly insufficient air flow through the system. Is this new construction or a replacement? Either something is clogged (AC coil since you've already removed the filter), ductwork is blocked or the furnace is over-sized for the ductwork.

It's done this for 3 years? Too bad furnaces don't come equipped w/ a "Check Engine" light. The metal parts inside have gone through a lot of thermal cycling. Last week I saw an 8 year old furnace that's now shot (failed heat exchanger) because of similar issues.

[Brandon Whitmore]

It's done this for 3 years? Too bad furnaces don't come equipped w/ a "Check Engine" light

Oh, but these newer ones do, you just have to read the fault codes.......