My main problem is that when I am taking a shower, I lose all hot water when someone turns on cold elsewhere. I have been led to believe this is because of an insufficient water supply to the water heater, leading to a loss of pressure on the input and thereby decreasing the pressure on the output. I believe I could fix this by bringing in a 3/4" or 1" line first to the water heater via PEX manifold, and then do PEX home runs to all the fixtures in the house.

I have attached pictures below of my setup, and I welcome any input.

(Originally I had pretty much the entire flow backwards. I have updated my images accordingly.)

I have a 1/2" line D that feeds both the boiler and the water heater E. F comes out of the water heater and reduces to a 1/2" line to feed the house fixtures. plumbing diagram I used the installation instructions for the 9DM3 backflow preventer pictured, and the following is an installation diagram. I have both the 9DM3 and the 1156F pictured between D and C, and I am confident this part of my home matches the installation instructions.
correct install instructions

1 Answer 1


With the new diagram, we can ignore A&B as they are part of the heating loop.

As for upgrading the 1/2" pipe feeding the water heater, yes, that should be possible, though it may not make much difference if all the hot water for the house is still being sent via a single 1/2" line after the water heater. Now, if you increase the pipe size to the water heater and provide individual PEX from the water heater to the fixtures, you should see some improvement, but it may also be limited improvement if the cold water is still being supplied from the 1/2 pipe in (or coming out of) the wall here. If you can increase the pipe size all the way to the water meter (or actual cold-water entry from the meter), that will help more, but a shorter section of 1/2 pipe is better than a longer section of 1/2 pipe if you cannot get all the way back to a full-sized feed line.

  • Thank you. I will try closing A and B individually as suggested and update with results. Mar 1, 2014 at 17:47
  • I have updated the question because everything was backwards. Also, A won't shut off because B is in the way. :-/ Mar 1, 2014 at 18:30
  • @NathanDeWitt You can detach the handle of A and put it back in the position that right=open and down=closed, since it's profiled, it should go just above B when open instead of clashing.
    – yo'
    Mar 2, 2014 at 0:47

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