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Bruce Cahoon

Airco Turbo 8300 model TH-150H cycles on and off

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This furnace was installed in 1985 as part of construction of a new R 2000 home.  It still heats the house fine.  However at startup the burners are on for only about 20 seconds and after about 3 seconds the burners are on again for only about 6 seconds and for the rest of the heating cycle the burners are on for about 2 seconds the off for about 3 seconds. 

If I squeeze tightly the white plastic tube with the rubber band around it (see image), the burners stay on as one would expect.  We have cleaned the flame detector.

100_4764[1].JPG

Edited by Bruce Cahoon
re wprding

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I'm not sure which tube you're referring to but I suspect those two white tubes are involved in proving that the inducer fan is working before allowing the burners to ignite by checking for a partial vacuum at the inducer fan suction.  Normal atmospheric pressure at the suction is taken as meaning that the fan isn't working.

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What Marc said. The pressure switch is trying to prove negative pressure in the flue. 

I'd start by looking for an obstruction in the flue and checking to be sure that the furnace room has an ample supply of combustion air. If those are both fine, then I'd test the pressure switch (or just replace it - they're cheap enough). 

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2 hours ago, Jim Katen said:

What Marc said. The pressure switch is trying to prove negative pressure in the flue. 

I'd start by looking for an obstruction in the flue and checking to be sure that the furnace room has an ample supply of combustion air. If those are both fine, then I'd test the pressure switch (or just replace it - they're cheap enough). 

Another thing is to check for an air leak on the suction side.

Edited by Marc

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What Marc and Jim said and check both tubes for moisture.   Pull them off on the low side while unit is idle. 

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18 hours ago, Marc said:

I'm not sure which tube you're referring to but I suspect those two white tubes are involved in proving that the inducer fan is working before allowing the burners to ignite by checking for a partial vacuum at the inducer fan suction.  Normal atmospheric pressure at the suction is taken as meaning that the fan isn't working.

Thank you for your response!!  Of the 2 white/grey tubes connecting the flow points near the fan motor with  what I believe is the pressure switch, one of them has a rubber band around it.  When I 'finger close' the tube with the rubber band, the burners function properly.  When I finger close the other tube, all burner activity ceases.  The fan and the ducts to it seem to work properly.  I am unable to find a pressure switch designed for this furnace. I am told that parts are not available to this furnace.  How do I find the right pressure switch to use?  If I should want to replace the fan motor ( preventative maintenance) would it be necessary to replace the fan motor and pressure switch as a 'matched pair'?

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42 minutes ago, Bruce Cahoon said:

Thank you for your response!!  Of the 2 white/grey tubes connecting the flow points near the fan motor with  what I believe is the pressure switch, one of them has a rubber band around it.  When I 'finger close' the tube with the rubber band, the burners function properly.  When I finger close the other tube, all burner activity ceases.  The fan and the ducts to it seem to work properly.  I am unable to find a pressure switch designed for this furnace. I am told that parts are not available to this furnace.  How do I find the right pressure switch to use?  If I should want to replace the fan motor ( preventative maintenance) would it be necessary to replace the fan motor and pressure switch as a 'matched pair'?

Why not just replace the rubber band and try it again. Try a hose connector in lieu of a rubber band.

Edited by Marc

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1 hour ago, Marc said:

Why not just replace the rubber band and try it again. Try a hose connector in lieu of a rubber band.

 

1 hour ago, Marc said:

Why not just replace the rubber band and try it again. Try a hose connector in lieu of a rubber band.

 

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The rubber band is only used to distinguish between the 2 hoses.  I will replace the two hoses and try again.  Can you give me any guidance as to the replacement pressure Gage?  The one displayed in the picture is the first try at a replacement and is already suspect (after about 2 years) as part of the problem.

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Is this a condensing furnace? 

If you remove the non-rubber-band hose and suck on it, does the furnace work properly? (It should - for as long as you can hold the suction.)

If this is a replacement pressure switch, how did you choose it? Do its markings match those on the original switch? 

Who wrote 0.5" wc near the switch? This stands for 1/2" water column and might be a guide to choosing a new switch. 

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Thank you Marc, Jim and Les for your responses!

This is not a condensing furnace.  When it was installed the builder called it a high efficiency furnace.  The conbustion chamber is not sealed, but it has an intake 'flue' from an outside wall for combustion.  A furnace service specialist installed the replacement pressure switch, but the furnace still had an on/off cycle  (burners stayed on for about 20 to 30 seconds which was not much of an improvement).   

I did get a rough measurement of the CFM of the combustion  fan motor - between 13 and 18 CFM.  So I do not believe the problem is this motor.

Would a WC number higher or lower cause the burners to stay on longer??

Thanks again!

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1 hour ago, Bruce Cahoon said:

This is not a condensing furnace.  When it was installed the builder called it a high efficiency furnace.  The conbustion chamber is not sealed, but it has an intake 'flue' from an outside wall for combustion. 

I only ask because the pressure switch has two tubes, which seems unusual if it's a regular furnace. The 2nd tube leads to what looks like a collector box that you'd find on a condensing furnace. I know that in the '80s manufacturers experimented with some odd configurations, so this might be one of those. Do you have a picture of the data plate, which would show the "category" of the furnace and several other technical details? 

2 hours ago, Bruce Cahoon said:

I did get a rough measurement of the CFM of the combustion  fan motor - between 13 and 18 CFM.  So I do not believe the problem is this motor.

Who suggested that the problem would be with the motor? (It's called a draft inducer, by the way.) The purpose of it is to draw a regulated amount of air over the burners and into the heat exchanger. After the flames ignite, the buoyancy of their hot air draws them up and out of the flue. The draft inducer has nothing to do with pushing the gases up and out of the flue. When the furnace is operating properly, the pressure inside the flue is less than the pressure outside the flue or in that collector box (or whatever it is) behind the draft inducer. If there's no negative pressure in the flue, the pressure switch senses this and shuts down the flame. Since that pressure switch has been changed and is still cutting out the flame, its reasonable to assume that it's just doing its job and that there might be a problem with the flue: an obstruction perhaps or some other condition that's preventing the buoyant gases from rushing up the flue. 

2 hours ago, Bruce Cahoon said:

Would a WC number higher or lower cause the burners to stay on longer??

No. I was just wondering who wrote it and whether it's a spec for the switch or if it was a measurement of the actual pressure in the flue. 

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I took another look at the photo and realize I have never seen a configuration like that.  I, of course, have not seen everything, but this looks like some sort of field modification.  Is it original? 

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Thank you for your response.  This is the original furnace as it was installed in the winter or spring of 1985 in a custom house certified as "R 2000'.  The furnace was one of the highest efficiency available at the time.  My first and only service call (until now) on this furnace (except for cleaning) was in about 2005 where a service representative installed the pressure switch shown in the above picture.  The 2005 service call was because I felt the burners were going off and on too frequently (burners going off then on about every 20 seconds).  This fix did not resolve the issue but the burners stayed on a bit longer.   

I would not be surprised to learn that the combustion blower which blows the burner gases thru a duct out thru the basement wall of the house was field installed, at the time of the original installation of the furnace.

NOW the burners are going off and on about every 7 seconds - on for 3 and off for about 4 seconds.  The service rep wanted to replace the combustion blower fan and pressure switch shown in the picture but parts are not available.   He would like to install a new furnace..  My next step is to replace the hoses between the blower and the pressure switch.

I have attached 2 more pictures.  

100_4773[1].JPG

100_4774.JPG

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the bracket that holds the pressure switch sure looks field fabricated to me.  it has different fasteners than the remainder of cabinet shown in photo;  raw edges, fasteners, discoloration at throat of inducer, etc. the burner cycle you describe is of concern.  I really can't imagine aftermarket parts not being available. 

Maybe things are different in Canada.

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Frankly, as an AC guy myself. I'd have made the same recommendation: change the inducer blower assembly, if that doesn't fix it, and its an '85, change the entire indoor section.  AC guy is seeing things from a different place than the home inspector.

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Thank you for your response.  This is the original furnace as it was installed in the winter or spring of 1985 in a custom house certified as "R 2000'.  The furnace was one of the highest efficiency available at the time.  My first and only service call (until now) on this furnace (except for cleaning) was in about 2005 where a service representative installed the pressure switch shown in the above picture.  The 2005 service call was because I felt the burners were going off and on too frequently (burners going off then on about every 20 seconds).  This fix did not resolve the issue but the burners stayed on a bit longer.   

I would not be surprised to learn that the combustion blower which blows the burner gases thru a duct out thru the basement wall of the house was field installed, at the time of the original installation of the furnace.

NOW the burners are going off and on about every 7 seconds - on for 3 and off for about 4 seconds.  The service rep wanted to replace the combustion blower fan and pressure switch shown in the picture but parts are not available.   He would like to install a new furnace..  My next step is to replace the hoses between the blower and the pressure switch.

I have attached 2 more pictures.  

100_4768[1].JPG

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Try to contact:

Howard Pike Location: Coquitlam, BC Canada Bio: Chimo Furnace owner since 1988 Airco /Olsen Product Engineer 1979 to 1988 BCIT Grad (Mech Tech) in 1979 CSB Ministries Rep since 2001 Company Information: Chimo Fur...

You can search here for posts by Howard, but it has been several years now since he was active on this forum.

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3 hours ago, Bruce Cahoon said:

The furnace was one of the highest efficiency available at the time.

Not really. It's input is 150,000 btu/hr and its output is 124,500 btu/hr, which amounts to 83% efficiency (and is probably why it's called an 8300 turbo). The great majority of furnaces in the 1980s were 80% (or thereabouts) efficient and lots of them used draft inducers, as yours does. Several manufacturers were experimenting with much more efficient designs, which pretty much all sucked, by the way. Be thankful that you didn't have one of those. These old Aircos last a long time. 

4 hours ago, Bruce Cahoon said:

My first and only service call (until now) on this furnace (except for cleaning) was in about 2005

Perhaps part of the problem . . . 

4 hours ago, Bruce Cahoon said:

I would not be surprised to learn that the combustion blower which blows the burner gases thru a duct out thru the basement wall of the house was field installed, at the time of the original installation of the furnace.

OK. Hold on here because this is really important for you to understand. The draft inducer fan should not, does not, *must* not "blow" the burner gases out through the flue. In an 83% furnace, those gasses are going to be in the 130 to 170 degree range and the furnace is expecting them to rise out via the flue as a result of their buoyancy. That means that after the flames are ignited, the pressure in the flue has to be less than the pressure in the heat exchanger. If it's not, the furnace won't vent properly and exhaust gases will enter the house. 

The whole point of the pressure switch is to monitor the pressure in the flue and shut down the flames if the pressure ever gets too high. (I suspect that it's looking for a difference of 0.5" wc.) 

The short cycling could be caused by the pressure switch, but if so, it's likely that the pressure switch is just doing what it's supposed to be doing and there's a problem with the natural draft of your flue. Describe it. How far does it run horizontally before it runs up vertically. What's the overall vertical height. Does it have a proper cap at the top. (These sometimes rust and fall down, obstructing the flue.) 

The short cycling could also be caused by the high temperature limit switch, (a failed heat exchanger could cause this) but when that happens, the cycles aren't usually that short - more on the order of minutes than seconds. 

Check your flue. 

Check your flue.

Check your flue. 

 

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