Need help sizing an expansion tank and water heater

[jmed999]

Guys, I have a tankless gas water heater and the main hot water line is 3/4" PEX about 70 ft long. I'm wanting to put in a recirc line. I'm going to use a small electric water heater to keep from having a cold sandwich effect.

So basically, I will have hot water come from the tankless heater and go into a small electric heater. The water will then go through the main line and circle back to a pump (Grundfos UP10). From the pump the water will go back the the hot water line between the tankless and electric water heater.

I got this idea from a Rinnai PDF here Mash

As you can see in the PDF, it has an expansion tank between the electric heater and the fixtures. What size expansion tank do you guys recommend? What size electric water heater do you recommend?

Thanks! Oh and I've already checked with my tankless manufacturer and my tankless WH was designed to run in this type of recirc setup.

 

[Caduceus]

A 2 gallon (approximate depending on manufacturer) is the smallest thermal expansion tank and should be fine for your system. Get a TET for "potable water systems". They are available at most home improvement and hardware stores.
Be sure to check your water pressure, without any water use, and pressurize the TET to match that pressure. It's usually easiest to fill with a compressor, like at a gas station, instead of a bike pump. You'll notice that the thread protection cap on the 3/4" threaded water connection is flattened so that you can set it down to fill with air and not damage the threads. Most come pre-charged with 20 or 40 psi as per the labeling on the tank but must be adjusted to match your house.
Also be sure that the tank is supported in a way not to put stress on the plumbing pipes.

 

[jmed999]

Thanks for the reply!

Now what size electric water heater do you recommend? The purpose of the electric water heater will be to eliminate the cold water sandwich and to maintain the water in ~140ft of recirculated pipe (3/4" PEX) at ~120 degrees. Not sure how much volume I'll need for that in the electric water heater.

I will be able to mount the electric water heater within a few feet of the tankless water heater so I'm thinking a small electric heater would work. I'm just not sure if I should go with a 2.5, 4, 6, or 10 gallon electric heater.

I want it big enough but not oversized and inefficient.

Thoughts?