Get a Co alarm anyway one with digital read out, ones I have seen hold
any peak reading in memory. Co is bad and any reading relating to the
furnace heat exchanger means replace it as they only get worse. Chimney
issues can cause high Co readings also, but you don`t indicate any
issue. Get qualified bids and a written load calc , I will bet yours is
probably 80% efficient and 94%+ is the way to go,not 80% your heating
guy just wants an install. Ive had qualified bids of 29- 3200 for 70000
btu 94.5% VS DC units with good thermostat, plus 10 yr warranty. 2800
for an 80% sounds a bit off, but my bids were in the off season, not now
when all are busy.