We've had a Rinnai for probably going on 10 years now. A big caveat off the bat is I’ve never officially done the math on savings. I did comparisons the first year and we were using less gas but haven’t gone in great detail.
Some observations:
We are two in the house therefore a hot water tank would sit idle a lot. That’s a big selling point of the on demand system, you’re not burning gas to heat the water in the tank during the day when you aren’t using it. If you have a larger family with more frequent demands then the savings are less.
You do need to wait for the water to get warm. Basically your flushing the line with the water sitting in the pipe and then the water that runs through the unit as it heats up. We don’t find it a massive issue. When we were looking at the system I read about a lady who hated it because she would give her kids a bath and want to warm up the water part way through and she would have to run the sink first to get hot water. That seemed a fairly specific example but it’s one.
Hot water forever. As the second shower-er with our tank system I’d sometimes run out if I took too long. There were also times where the washing machine got turned on and I’d lose some heat.
Our installer suggested if we didn’t need the space to leave the water tank in place. His reasoning was two fold: 1) the water in the tank would be around room temperature therefore it would take less energy to heat up 2) if there was ever an issue with the tankless system someone could easily hook up the gas line to the tank and we’d have hot water. Both ideas made sense to us.
With the current system we have run showers and dishwasher or washing machine at the same time with no issues. The trick is to let the gas co know you have put in an on demand system and have them install a larger meter. At first we had an issue where we would lose hot water as there wasn’t enough supply to the burners when it really needed to kick in once our tank was empty (see above).
For those who want scorching hot water there is a temp control on the tankless unit which I believe tops out at 140F. I think that is scalding temp.
We have never had any servicing for scaling and it was never discussed or brought up as a need. Our system runs fine.
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