Best Of

Are you sure its wired for either or both operation. A lot of the systems like this were set up to run together to meet the demand of the building as there usually is not a single residential furnace that had the output to meet sanctuary's etc. We would set them up with back draft dampers if it was an either or system. If one was off and other on they would recirculate warm air from the running one backwards from the one off and back into return of one running, Not good. So both furnaces would have backdraft damper. Carrier also made a twinning kit with If I remember right, dpst relay and limits for blower compartments so if one blower failed and started to recirculate it would shut them both off. This was for the all on twinning systems. Did them for churches, memorial halls etc. Was decent cost effective solution when pulling a large old low boy furnace that may have been 400000 btu and 3 or 4000 cfm.

I thought this looked familiar:

I just answered this on a PM

This is what I said:

If I recall correctly, the Carrier Weathermaker with a 9200 designation was a furnace that used a hot surface ignitor to light the burners each time there’s a call for heat. So, your statement about a separate pilot is somewhat confusing. That particular furnace series has at least three different models that I recall. The 58MVP was a variable input furnace and the 58MCA and 58MXA were fixed input furnaces. I believe that all three models used a secondary heat exchanger to reduce the exhaust temperature below the condensing point and therefore have some sort of water from the exhaust that will drain out of the furnace during operation. Also, that lower exhaust temperature allows the venting system to be made of PVC plastic piping.

Many of the furnaces models in that range had a maximum input of 150,000 BTUh making them a great choice for most homes, however, older equipment in some larger homes had capacities of 200,000 to 300,000 BTU input. In order to use these new smaller units in those older buildings, a twinning kit was developed by many manufacturers, like Carrier, to allow the use of two smaller furnaces to do the job of one larger furnace. This is no doubt what has happened here.

In order to further assist you in how this system should operate and how it does operate, I would need the actual model number of each furnace. The model numbers of all Carrier furnaces start with the number 58 then usually 2 or 3 letters like MCA the several more numbers to designate the size of the burner like 090 or 140. After that are some letters and number combinations that indicate the size of the blower fan and cabinet with and other things. If you can provide me with the model number of both furnaces, I may be able to answer your query.