That sounds extremely high to me. We had a heater replaced here at my house about 3 years ago and it seems like it was around $4500. My house is only a little bigger than at 1500 SF.
I know a lady that got taken by a company that "LEASED" her a new heater, AC, and Hot Water heater on a 10 year payment plan. Man, did she get taken. Be careful out there. Some unscrupulous people.
Did you get a quote from anyone on what it would take to just FIX it? This isn't the time of year you find good deals on heaters. Maybe get it replaced next summer when the prices are down. Just a thought.
