Custom sized inlet and outlet and very small internal diameter. Culligan has another much more RV friendly HF-150A. The inlet and outlet are 3/4 inch in diameter, which is what any RVer would want. Save a lot of time and money when looking for accessories. I like the 90 degree plastic elbow with the plastic quick couplers. Get some spare elbows. Less stress and the filter doesn't tip over as easily. Camco makes a good Short Pressure Protector out of lightweight plastic that you can use even when your elbow is down. Not found here, found on eBay. A tube repair kit may be required to replace the end of the drinking tube. I have an RV so I need it to be as compact and light as possible. I use a Culligan D40 filter which is difficult to set up. If it's night time and you can't see good fast links, it's much easier. There is also one on the underside of the camper for the same reason. Or use it as-is with your hoses.
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