I represented Rinnai for over 20 years in my business. I would not bother with a tank. Cold water sandwich is spoken of like some enormous bugaboo, but the fact is, it is not that big an issue. It goes like this. If my wife is in the shower and turns off the water and gets out and I get right in & turn on the hot water, I will get hot water right away as it is residual in the pipe. I know that because the water was off, the unit had to re-start once I turned the shower valve back on. Once the hot water in the pipe has run out, the 4-5 seconds of cooler water will get to the shower. I know it is coming and simply step aside for a few seconds and once it passes through I will have hot water til the cows come home or longer. I would not spend money to avoid this. You learn to live with an appliance. You will like the Rinnai.

More importantly for you is to locate the water heater central to the load. I'm just re-modelling my home and positioned my water heater so my longest hot water pipe run is 18'. Do not put it in the garage and then pipe a million miles to the shower.

Also, these units are very efficient electrically. Before putting a tank in line with the Rinnai, I'd look at getting a battery back-up to operate the machine in the event of a power failure.

What model are you looking at? Where are you?

"Cold water sandwich is spoken of like some enormous bugaboo, but the fact is, it is not that big an issue."

That may be true if your cold water temperature is in the 70's. However, I challenge anyone to withstand a 4 to 5 second blast of Pacific Northwest domestic water, which averages below 50 degrees F year round. And then say again that it is not that big of a deal. Tell us how you like to start your morning with 48 degree water blasting on your bare skin for 4 to 5 seconds, after you just started in with a nice warm shower.

The real hazard with the cold water sandwich is the average person's reaction to correct it. They will instantly grab the temp control and turn it ALL the way to the hot. Then about 4 seconds later, the scalding water blast comes through the pipes, and now they have to go to the burn ward.

Seriously, the cold water sandwich IS a BIG ISSUE with tankless water heaters. In residential systems with small diameter pipes, the effect is amplified significantly.

The smallest little tank is all you need. 20 gallon is way too big. I have a 5 gallon electric tank, plumbed in series, fed directly from my Bosch Aquastar. Before I added this little tank, I was totally dissatisfied with the tankless unit. After, I am 100% satisfied.

Do not let any sales person convince you that you will not notice, or learn to live with the cold water sandwich.

I agree with Aidan_M to a certain extent. We had a Rinnai installed and was not very happy with the results. I would hear about the cold water sandwich when ever my wife would get in the shower after me. So to me it was a Big Deal. You know the saying happy wife happy life.

We did the same thing, had a 5 gallon tank tank installed. In my eyes the cost was minimal, if it keeps the wife happy.

Yes some people with live with the sandwich, but its just not for me.

Here is a link that might be useful: Tankless reviews

The place where the cold water sandwich issue caused us the most frustration, was the kitchen sink. The kitchen sink gets turned off and on more than any other fixture. We like the kitchen faucet water to be warm. Not scalding, not cold, but warm. This was not ever possible until I added the little tank.

Hi all, thanks for responses.

I don't yet know which model we're getting, it's TBD. I'd like two showers to run concurrently and of course we have HE washer, dryer and DW.

In addition to having hot water in case of a power outage, my thought is that having a small tank in front of tankless will allow us to add a cheap but functioning recirc pump. A tankless recirc system was quoted to us at $2700, with separate piping all over.

We're in SC.