Bruce Cahoon

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.

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.

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!

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.

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'?

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.