How can I connect a portable propane bottle to a tankless hot water heater?

One major problem is that most propane regulators are intended for small appliances, mostly less than 75,000 BTUs/hr typical of an outdoor grill. The Rinnai V65EP can use up to 150,000, so a regulator designed for a little more than that would be the desired fit.

Shopping around, I notice many don't say what their maximum flow rate is, and of those which do, it is hard to find a "low pressure" (corresponding to less than 13.5" water column) in that range.

Here are a few I could find after several minutes searching:

• http://www.ebay.com/itm/Regulator-Stainless-Steel-Hose-Type-1-LP-Gas-Grills-Parts-21-BBQ-Low-Pressure-/261136984141#vi-content
• http://www.ebay.com/itm/Propane-Regulator-POL-LP-Gas-Low-Pressure-grill-BBQ-smoker-stove-Parts-/251219429378#vi-content

Lowes has a two stage regulator capable of up to 200,000 btus. It is inexpensive at $40.

I am hooking up a Bosch 520 on demand to a 20# propane tank. I found this site and this product to make the connection. It satisfies the requirements for low pressure (under 14 WC), high flow (117,000 btuh).

http://www.tejassmokers.com/lowpressureregulators.htm part #108053P-60

Using a small portable tank will most likely create problems when using a tankless water heater. Many things come in to play with your scenario. Distance, pipe size, total BTU of ALL appliances, and tank size. Most propane tankless W/H are between 120,000-199,000 BTU's so using a small container as mentioned, your going to have a volume and flow issue in a short period of time. I've been in propane service for 25 years and almost always I install a dedicated 2nd stage regulated at the W/H. As you consume the vapor from the small tank, there isn't enough wetter space(liquid) to make up for what the W/H is consuming, so you'll get an ice build up at the bottom of the small tank

We camp at a cabin that has a Paloma so I used the following to hookup the Paloma Tankless Water Heater to the 20lb propane tank. Camco screws right into the tank and connected the Universal Kit to connect to the Paloma. Use PTFE Thread Seal Tape on all connections except the hand thread Tank Connector. Universal Kit comes with tape, but it's not labeled as rated for gas and it's white. Heard the yellow is the true gas rated tape, so I bought it separately. Test connections with soapy water. Make sure Camco vent is pointed down. Just for shower use, so did not see ice up on the tank.

Camco 2 Stage Regulator https://www.amazon.com/gp/product/B0024E6TX2/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

Universal Gas Appliance Hookup Kit https://www.amazon.com/gp/product/B01LY9SZ82/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

Gas PTFE Thread Seal Tape https://www.amazon.com/gp/product/B000FKBXCE/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1

I actually have this setup, except I have teed two 40lb tanks together. A comment above mentioned wetter space, and that is indeed the issue. I have a high BTU regulator, but the tanks simply cannot evaporate enough propane to power the water heater at full blast (157,000 BTU). It's working "fine", not the end of the world, but occasionally when the washer is filling and we are using hot water in other places it does stop working. After a few minutes the tanks thaw and we return to normal. I decided on 2x 40lb tanks because I could fill them myself and carry them inside a vehicle and would not have any kind of contract or anything.