Tankless with varying incoming pressure from well tank/pump?

[rick.a]

I have a small house with a well and pressure tank set to produce between 30-50 lbs. pressure. This works great. We now want (actually need to for space considerations) to install a tankless WH but wonder if the varying pressure from the well tank is going to screw things up with the tankless which probably likes constant flow rates. Tankless will be Rinnai 53i. Small house with 2.5 gph shower as the longest HW draw. Do you think it will be OK?
Rick.

[jadnashua]

What is your supply water temperature year-round? Does it change much in the winter? Compare your incoming water temp with the specs to see what temperature rise you can get out of it, it may be okay in the summer if the water temps are higher, but not in the winter. And, it's not the duration of the draw, but the maximum draw you might want. You can't prevent someone from turning on the washing machine, dishwasher, or a faucet to wash their hands while you are in the shower. If you have a boiler, an indirect may be much more economical, and the tanks are smaller (typically) than a stand-alone tank.

[rick.a]

Thanks for the reply. Winter incoming water is about 45 deg. Summer is about 55. The unit that we have spec'ed should give us the flow rate that we need with the 60 deg rise in the winter. What I do question is whether the fluctuating pressure will cause problems with the Rinnai unit if the pressure goes down during a HW draw.
Rick.

[Dana]

Thanks for the reply. Winter incoming water is about 45 deg. Summer is about 55. The unit that we have spec'ed should give us the flow rate that we need with the 60 deg rise in the winter. What I do question is whether the fluctuating pressure will cause problems with the Rinnai unit if the pressure goes down during a HW draw.
Rick.

As long at the pressure stays within spec you shouldn't have any pressure. At low/very-low pressures (15psi or less) many tankless units have issues, but not 30-50pis.

[rick.a]

Thanks Dana for the confirmation.

By the way, do you know if the plumbing code in MA requires that the T/P relief valve be plumbed to a drain if using a tankless in a cellar (actually a crawlspace with concrete floor)?

Rick

[valveman]

I get a lot of calls from people with this problem. At 30 PSI the system only has about half the pressure than when at 50 PSI. So the flow through the showerhead and tankless heater vary widely. Many times the flow at low pressure is inadequate to keep the heater working. Then at higher pressure it fires off again. A 40 PSI Cycle Stop Valve will maintain 40 PSI constant for as long as you are in the shower. This keeps the pressure and flow steady and the tankless heater never shuts down. When used with a small pressure tank, the pump is running and the CSV holding a steady 40 PSI before you get the water temp adjusted in the shower. When used with a big tank, you just have to wait for the tank to empty, the pump to start, and the pressure to get to 40 before everything steadies out.