CIty Water Pressure and System Demand for Water. Higher Pressure the Answer?

[acbruno]

I have two Navien NR-240 tankless on-demand water heaters that lead to a holding tank feeding 7 self service bays. The incoming water pressure is regulated to 60PSI and passes through a water softener first before going to the water heaters. Problem is, during moderate use, one of the water heaters has issues keeping up with at least 30PSI water flow to maintain operation because of other demands of the car wash. This eventually leads to emptying of the holding tank and starving pumps.

I contacted the city about this and they said I could get a higher pressure PRV to allow up to 100 PSI into the system. I first tried cranking down the pressure set screw on my existing 75PSI max regulator and got it to 95PSI not under load. Of course under load it dips back to 60 -70. I then removed the set nut on the threads so I could crank the set screw down even more and got it to 130 PSI but the PRV started leaking (very old and rusty) so I had to back it off to 80PSI no load.

I have a plummer coming in next Tuesday to install the larger high pressure regulator but until then he said I could just bypass the regulator if I needed to if it is leaking too bad. I guess that would be the ultimate test to see if the higher pressure will fix my capacity problem. I’m not sure if 130PSI will cause other issues but for the short time I had it up there nothing happened.

Any thoughts?

 

[MEP001]

No, higher pressure is not the answer. By increasing the pressure with no flow, you've only increased the static pressure.

The proper way to solve this problem is to find where the restriction lies. Put gauges before and after the PRV to find if it's restricting flow, and if it's not start testing other things like bypassing the water softener.

 

[JGinther]

I'm making a lot of assumtions, but likely you need another water heater. You only have 400K btu. If you live in a cold area in wintertime, the delta rise will likely be 80 degrees or so. 7 bays at 3.5 gallons per minute would be 24.5 gallons per minute. Look at the heater charts on 80 degrees change in temp. Do you use cold water rinse? That might "fix" it cheap.

 

[acbruno]

Yes, I do have cold rinse. The only functions that are heated is the soap and wax. I spoke with Navien about the error code 39 flow sensor shutting off water flow. They told me what set of wires to pull from the circuit board to prevent the water shutoff. After I did that, I haven't had another error trigger and the last few days have been the busiest in years with high water demands. I kept an eye on the pressure right after the PRV and it was around 60 most of the time. Hopefully with the higher pressure PRV, I can set it on 70 or 80 PSI and get that consistently. That should help or solve this problem I hope. The only way to get more water to the equipment is to increase pressure or increase the incomming pipe diameter right?

I know there is a pressure drop from the incomming water to the water heaters and the hot water exit point. I have the heaters set on 120 degrees. I can't say that they reach that temperature or not when demand is at it's highest but it is an acceptable temp when it gets to the holding tank.