Repair or Replace: Heil DC90

[informedconsumer]

hello there, me and my wife recently moved in a home with the HAVC system as pictured below. The furnace is a Hiel High Efficiency DC90 80000 BTU from around 1995 so beyond end of life. It is connected to an ecobee thermostat. The system had been working fine for most of the last few months, except the previous 24 hours and once around two months ago.

If useful also including the ecobee data. First the ecobee was completely off as it was not getting power from the furnace, and the furnace refuses to start. Based on a short discussion with the ecobee technical support, they mentioned that the furnace is not providing the required voltage to the ecobee and the furnace is getting rapidly switched off.

We had this issue once briefly three months ago. At the same time the furnace started by itself after the break switch was reset. It returned out that a probable cause was an air filter that had not been replaced for a while which was restricting Any thoughts on what possibly could be the issue and whether it is best to replace the furnace.  The questions I have are:
1) should this or a new furnace require a fresh air intake vent, since it seems that currently all the air intake is the air being circulated in the house?
2) how to determine what is the right size of the furnace as there is conflict viewpoints that the furnace is over-sized.
3) any other things you suggest to troubleshoot the current furnace and is there any value in trying to have it be repaired by a professional given its age?

[Admin]

Once you reach 12 - 15 years it's usually best to replace if the repair is more than $500.

The humidistat should be at least 6 - 12" above the humidifier.  You likely need to upgrade the plastic vent to ULC 636 venting.

That model is known to have heat exchanger problems so I would have it checked.  It looks like it has a smart valve which can also be problematic.  I just replaced my mom's 19 year old furnace.  Same brand but mid efficiency and it never failed once.

Usually if one of the safeties open it interrupts power to the thermostat.

[NoDIY]

I have seen hundreds of failed heat exchangers on this model..
Popped rings and cracks.

It owes you nothing. Have it replace or if u must keep it, have it properly inspected. I can usually see defects by looking through the high limit opening.