Heat exchanger is cracked. Still under warranty accept for install costs.

in Heat exchanger what cause the flu gases to separte from the regular combustion air that goes out to the rooms for circulation?

I have heard that there is a minimum temperature at which you should have your house in the winter to protect the heat exchanger on a high-efficiency furnace. I’ve heard stories of people going on vacation and leaving their thermostat temperature set to 45 degrees or something, and it does damage to the heat exchanger. Is this true, and if so, what is the recommended minimum temperature that I should set my thermostat to ?

I can not replace the whole furnace due to limited budget and mostly attic access problems. So I need to find a heat exchanger that I can replace the cracked one with. York furnace model #P1HGD15N10001a (1980ish). Or can it be retrofitted with a newer heat exchanger. Is there a creativeway to do that?
Thank you,
Freezing near Detroit.

Checked online, but the first entries were for gall bladder, liver and kidney tests related to a HIDA test.
Supposedly it helps find microcracks in the heat exchanger.

I have a high efficency gas furnace that runs good. But after it shuts off a little water leaks from underneath it. The pcv vent, drain and pump are fine. Does this mean my heat exchanger is broken or could there be a loose hose connection inside the furnace?

My furnace guy said the heat exchanger of a unit has cracks. I also saw it; but it looked like a crack on the a little bit rusted surface, but i did not see a hole or anything like that. The guy put CO detector at the flue and the CO level shot up at 200 ppm and came back down at 25-30 ppm which I think is normal because that is what the flue is for. He said that is too high, and he says he sees usually 5ppm or low level. Potentionally, it can leak into the main airstream. But there is no CO in the air stream into our house. And then, he said I need to replace the whole unit or the heat exchanger. Is what he said reliable?

I have a Payne HE Gas Furnace (plus 90, I know about the lawsuit and settlement) and simply wonder what would happen if the secondary heat exchanger would fail? We are having a water leak from it, and I am wondering if it links to a failing secondary heat exchanger failure. Any answers?

My gas company red-tagged my furnace due to carbon monoxide leaking, because of a failed heat exchanger.
Should I get the heat exchanger replaced for $1000 (parts and labor) or just get a new furnace for about $3500?
The furnace is a 1996 Tempstar 90% efficiency model.