Broke,
When trying to justify a new unit like this by citing the payback as a reason, you have to somehow factor in the reliability too. We can never do that, so it just becomes a best case bet. If the payback is 8 years, you're even money at 8 years and haven't saved a thing. Now, after that is when the savings begin, and your starting out with an 8 year old appliance.
If that appliance is not 100% reliable the payback moves farther and farther into the future.
If your existing water heater failed, it would make more sense to replace it with something you believed was better and weigh the costs at that time. I like the utility rebate idea too. Maybe yours does that. Of course you'll have to fill out the forms, get a permit and inspection, etc. So factor that in too.
It has become abundantly clear to me, with a hydronic heating business for almost 40 years, that simplicity and proven components are the key to success. I strive to make systems simple. That doesn't mean they don't have capability, just the opposite, they do it all with the minimum of parts and therefore increased reliability and low build cost.
I've mentioned it already, but if you only want BTUs at the lowest cost with the best reliability for the long run, you probably can't do better than with thermal solar.
Another trap that people fall into is to make the leap that since they have an efficient system, or a perceived efficient system, that they can be more careless with use. Of course they can, but if the original intention was to save money, they don't. That is part of the fallacy of tankless water heaters. "If it's more efficient, we can take longer showers, and we never run out of hot water".
People always have only two answers to my question after a year or so of having one of my systems. The question is: did your bill go down? If the answer is no, it's only because they are now heating to a higher temp or heating more rooms than before. In other words, they answer that they are heating more and like the added comfort, for the same cost, or they noticed a significant reduction in the bill.
In your case you've said you are willing to work on it to save repair costs. Cool. You'll have fun hunting down parts while your family takes cold showers. Hmmmm, is that negative enough?
In my case, when I have given a guarantee, I have to perform or take the hit.
I really hope heat pump water heating is reliable. It's a great idea. If the Rheem unit is as reliable as a household refrigerator they will have a great product.
