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We just moved from our old house which had great shower pressure to our new house which is pretty pathetic. I'm trying to figure out what/where the problem is.

The old house was built about 40 years ago and has plumbing fixtures from about 15 years ago. The showerhead is about 25 years old and is awesome (named Country Club). This house has 2 - 55 gallon hot water heaters (I have 3 daughters).

The new house is brand new so all plumbing & fixtures are new. This house has a Rinnai tankless hot water heater (model RL75).

At both houses I set the pressure regulator at the max. At both houses using a RainBird pressure gauge I get 80psi on the ground floor faucet at the old house and 81psi at the new house. On the second floor shower I get 76psi at the old house and 75psi at the new house. So that appears equivalent.

Please note all measurements below are off the pipe out the wall, not the shower head.

The problem appears to be two things.

First, with the shower head off, the old house shower delivers 5gal/min (hot) while the new house it's 2.86gal/min (warm) & 2.67gal/min (hot). The warm/hot is setting the Rinnai to 106/140 and setting the valve to the middle/mostly hot.

And at the new house the ground floor outside faucet only delivers 3.43gal/min.

Why such a difference in water delivered? The pressure is the same. In both cases nothing else was using water at the time. And the fixtures all use 1/2" lines so even if it's 3/4" up to the fixture, it's 1/2" from there. So shouldn't both be delivering the same volume?

Second problem. At the old house I would put the valve pretty much even between hot & cold to shower. At the new house, with the Rinnai set to 140, I still have it about 90% of the way to full hot.

So what is my main problem? I think the first problem is getting 5gal/min. As the outdoor faucet doesn't hit that I don't think the hot water heater is a part of that problem. But what/why/how do we not get that when the pressure is the same?

And then do I need a hot water heater that can deliver 5 gal/min at 140 to have the shower be half hot/half cold to get the full pressure?

Edit: As one of the comments pointed out, my question is really how do I increase the volume. And one thing I don't understand, if 2 1/2" pipes both have the same pressure, shouldn't they deliver the same volume?

Update: With the shower head on I get exactly 2 gal/min while the shower head is spec-ed at 2.5 gal/min. And what I want is a shower that pushes me against the wall. One of my greatest joys is a super hot shower that blasts me.

  • Tankless heaters are limited in the amount of water they can bring up to tempature. If you put the tankless at its max setting you may be able to add more cold because at a lower setting the unit quits heating when at the set point. This limitation is the one negative with tankless units , the nice part is you never run out of hot water. – Ed Beal Jan 12 '18 at 21:35
  • Your new valves may have flow restrictors to save water. At your old house you may have been using more water than the regulatory authorities really wanted you to use, and what you really "should" use. You probably are now using the amount of water you should. We have a bottom of the line Bosch NG fired tankess water heater (117 BTU/h) and under the slab copper water lines that does not deliver hot water like the tank we used to have. I installed hand held shower heads which allows proper showering with maybe 1/3 of the water we used to use. – Jim Stewart Jan 12 '18 at 22:30
  • Any shower head sold now in the US must deliver no more than 2.5 gal/min and lower flow heads are designed for 2.0, 1.5, and 1.0 gal/min. Ours are 1.5 GPM. Your new shower heads should have the delivery rate printed on the face, but you may need a magnifying glass to read it. Try to get used to this new showering experience. What is the BTU/h rating of your Rinnai tankless heater? Do you have PEX piping? – Jim Stewart Jan 12 '18 at 22:37
  • @JimStewart that is about flow. OP is complaining about pressure. My guess is the new house was built to accommodate the flow of new style shower heads, and he has brought his old hi-flow shower heads, and the piping just can't keep up. Nor should it. 5GPM is shockingly wasteful unless you're in a captive watershed like the Great Lakes. – Harper Jan 12 '18 at 23:47
  • @Harper, he said in the title it was about pressure, but then said he had the same static pressure in the new house as the old, and reported quantitative measurements of flow. I think the OP should revise his title. – Jim Stewart Jan 13 '18 at 13:33
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I would suspect you have a two fold problem. 1: Your tankless water heaters ability to heat and deliver water - GP/m. 2: Your city pressure (indicated by your outdoor faucet) - not sure if you tested that with the faucet removed or still attached; Just because you have a certain amount of pressure from the city does not mean you have the same flow rate. Your flow rate might be different as in GP/m's supplied at X pressure.

Item 2 is what I would examine - check with your municipality to see if there is a flow rate restriction of some kind (what is their spec for flow rate at what pressure), or perhaps your house water supply valve is not open all the way.

  • I upvoted this answer because I thought it brought up an additional mechanism for flow restriction to what I had suggested, namely, that the municipality has implemented designs for new construction which restrict flow rate compared to older construction. Presumably this would be in the water meter or would be the size of the water meter. Maybe the OP could have the city install the next larger size water meter and this would give a higher flow rate in his showers and the outside faucets. dallascityhall.com/departments/waterutilities/DCH%20Documents/… – Jim Stewart Jan 17 '18 at 12:28
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    However, the builder of the OP's house may have sized the water supply lines to fit with the current best conservation practices for supplying a house of that type (size and amenities) with the size of meter the city specifies as the default for that house. In other words going to the next size larger meter might or might not give significantly higher flow rates at the delivery points. Find out what size meter you have and if the city will consent to installing a larger one. (We have a 5/8" meter for a 2000 sq ft 2-bath house on a 10,000 sq ft lot. last month we used 3300 gal.) – Jim Stewart Jan 17 '18 at 13:18
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Most probably the plumbing in your new house is performing exactly as it was designed. Do not assume there is any fault until you have gotten used to how the new plumbing works. There is a small possibility that the Rinnai is not adjusted properly, but I very much doubt it.

Given the size and composition of your family I would say to set the tankless heater to 140 F and tell your other family members that the plumbing automatically restricts the flow rates at the delivery points and the shower valves limit the temperature of the water to prevent scalding and to save energy. Tell them that although very hot showers feel great, they cause damage to the skin that will show up later in life.

Question 1. With the Rinnai tankless heater set to 140 F and one or two shower valves on full hot, can you get very hot water (significantly greater than 120 F) out of the kitchen faucet?

Question 2. What is the design of your plumbing system? Is is PEX with a central manifold and valves controlling dedicated output lines to the various delivery points?

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