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Tankless Water Heater/Boiler Supply Piping Question - Noritz Combi Boiler CB199-DV

I had a friend who works for a plumbing company set up my boiler for me. I have yet to fire it up but wanted to ensure that the supply piping was done properly as I have some doubts about how it was laid out.

Currently, the set up for supply piping is:

Shut Off Ball Valve ---> Caleffi Low Lead Pressure Reducing Valve (535951HA) ---> Aqua-Pure Scale Inhibitor Filter ----> Tee ---->

After that, one side goes straight into the DHW cold supply inlet for the water heater.

The other side goes to a Caleffi Dual Check Backflow Preventer (573503A) then the Boiler Auto Feeder Inlet.

I plan on splicing in a Therm-XTrol ST-1 mini Expansion tank on the cold DHW inlet directly following the Pressure Reducing Valve.

My question is, does this piping look correct? Is there anything that should be changed about this supply piping or anything you would have done different? Now is my opportunity to correct any errors on this piping. My plans currently are:

1.) Add the Therm-X Trol Expansion tank right after the PRV
2.) Remove the awkward filter bypass Tee and Valve as that's not necessary
(The scale inhibitor filter Tee (P/N 6838732) does not block so I'll never really need to bypass IMO)
3.) Maybe Add a 3/4" shut off before and after the scale filter to make charging less messy operation.

Also, is the backflow preventer in the correct spot? I've read that he Back-Flow Preventer should always go before the Pressure Reducing Valve. Thoughts?

I also don't like how my friend plumber didn't understand the concept of "closely spaced tees" and instead put a valve between the tees (not something I asked him to do, I asked him to keep the tees as close as possible) but maybe I'll try it like this and see how it goes idk..


  • Jamie Hall
    Jamie Hall Member Posts: 18,953
    There are a number of ways to plumb these things..

    The essentials are first, a double check valve (or better, a reduced pressure backflow preventer) between the inlet to the heating section and the domestic water. An expansion tank on the domestic hot water. An expansion tank on the heating side. A pressure relief valve on the domestic side -- 150 psi. A pressure relief valve on the heating side -- 30 psi.

    Now... to make servicing easier, isolating valves on any filter or other odd gadget. Isolating valves on the feed to the heating side. Isolating valves on the expansion tanks. And there is no such thing as too many unions so you can take it all apart without resorting to a sawzall when you need to.

    NOTE: No valves at all on the pressure relief valves!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • That was a super thorough answer. Hopefully you got what you needed!
    Indianapolis Plumbing Contractors here to answer questions!
  • Amodedude1
    Amodedude1 Member Posts: 20
    edited September 2021
    Thanks. I am only utilizing the boiler auto-feed inlet to fill the boiler heating side of the system. I have a double check backflow preventer on that but apparently not a pressure reducing type (PRZ). I'm guessing the PRZ type you're referring to is something like this ? What is the difference between that PRZ and the double check style that I have?

    I guess the original question I had was do I need an additional backflow preventer before the pressure reducer on the DHW side for the PRV to actually function properly? And now, is the style backflow preventer I have on the auto-feed adequate?

    Skimming through the manual, the DHW hookup seems to be pretty basic, the only thing it really calls for is a backflow preventer on the auto-feed.

    Also, good point about unions. I may add some to the backflow preventer if I can find room to make replacement easy should it ever fail (as I've heard they can and do fail).

    Edit: actually it already has unions so scratch that.
  • Jamie Hall
    Jamie Hall Member Posts: 18,953
    You don't need a pressure reducing valve on the domestic supply, unless you need it for the whole house.

    Whether a double check valve is adequate on the hot water makeup water feed is more a matter of local code than anything else. The principal difference is that there is a pressure reducing valve built into the reduced pressure zone backflow preventer, along with two check valves, which adds a large layer of protection -- and a means of testing. You also still need the regular pressure reducing valve for the heating side, since it runs at a much lower pressure than the domestic hot water side.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Amodedude1
    Amodedude1 Member Posts: 20
    edited November 2021
    Thanks, I ended up adding a pressure reducing valve to the heating autofeeder inlet to reduce down to the required 12-14psi (ish) range required the heating system. I also ended up leaving the existing pressure reducing valve. Seems to be working great. Thanks for the assistance