High GPM requirement tankless help

[GeoffS]

Hi guys, I've learned a ton from following on here and want to first thank everyone for advice. I'm installing a high volume shower upstairs in a high end small airbnb. Outside of the shower the rest of the hot water demand should be minimal, kitchen sink, bathroom sink, dishwasher, and washer dryer, but it's a nightly rental so the concurrent use of devices should be pretty small. In the shower is a rainfall head (2.5 gpm) and 4 body sprayers (1.5 gpm a piece), I was considering only installing 3 of them to reduce demand. I live in Northern Michigan, on a well, water temp should be around 40. I had planned on installing a tankless, the best I can get for a 70 degree rise is about 5.5 GPM. I was going to install two in parallel, but according to my research even after repiping the place to 3/4in gas lines (was previously 1/2in) I won't be able to support the BTU demand. Repiping to 1in isn't possible without trenching all the way to the propane tank. A buddy owns a very busy and reputable plumbing business who suggested I just put a hot water heater before the tankless. 50 gallon up to about 105, then run the tankless to heat higher, then put a mixing valve after to make the water go further. But that doesn't make since to me. Even if I heat the water higher, I get like 30% extra water, so I've got maybe 70 gallons of water, after which my tankless will be taking in cold water right off the well and I'll throttle my GPM back down to the max capacity of the tankless at 5.5. Is that the correct thinking? So I'll get 10 minutes of running the sprayers and the rainfall before I have to turn the sprayers off to accomodate the pressure loss? Is there any other way to configure the system to handle the gpm without adding another tankless? I asked him this and we was very confident there is noway that I'd run out of water or lose pressure, but I really don't want to drop several thousand into the plumbing and be wrong. Thoughts? Thanks for the help

 

[Valveman]

A 50 gallon water heater by itself will supply the 5.5 gpm shower for 10-12 minutes. Two of those would supply the shower for 20-24 minutes. Tankless heaters can be a pain. They need a big gas line and a steady water flow to work properly. Many times just the 10 GPM well pump cycling on and off while using a 5.5 GPM shower will cause enough pressure fluctuation to make a tankless heater cut on and off. Not to mention how bad the cycling on/off is for the well pump. But if you want to use a tankless heater adding a Cycle Stop Valve to hold a constant 50 PSI instead of the well pump cycling on and off between 40 and 60 over and over makes instant water heaters stay on as they should. 50 PSI constant will also be much stronger shower pressure than when the pump is cycling 40 to 60 over and over. Having strong constant pressure and all the hot water you want in the shower is a real luxury, so I hope you get it figured out.

 

[Sponsor]

 

[1dk2la]

Hello gents, can you guide me to a website i can buy a shower head with 5 GPM output. I live in CA and you know the limitations, but my 2.5 gpm just isnt cutting it. how to circumvent the limitations ?

 

[Reach4]

Ebay. I suggest this search term there:
high flow shower head gpm