Not likely. The heat loss from the 85 gal glass cylinder wrapped in fibergalss will not keep the water hot for long.

Thank you for your response! I don't know if this changes your thought, I just want to clarify that the 85 gallon tanked hot water heater is still connected and would still be running.

Yes, it’s a silly idea. That’s not how any of that works.

The Rinnai M160CN is a 160,000 BTU/hr combination boiler/hot water heater. Are you using it or do you plan to use it for hydronic heating and heating of domestic hot water?

I have an 85 gal. hot water tank and we never run out of hot water even when 4 of us are showering one after the other .

I am curious as to why you spent the money on a tankless system when you had a 2 year old 85 gallon hot water heater?

Homes yes, I use it for the radiant heat in my house I have two zones. And since I already have it, I thought I'd be able to increase my total hot water capacity.

Consulting thank you for your response, I have 4 of us who like to take long showers, and I was assuming that at 6.1 gpm it would go quickly.

@mike_home we bought it to replace our 20yr old old boiler.

Wonderdog thank you for your answer, from the responses (yours specifically), it looks like its not as straightforward as I had thought. I'll have to go back and talk to the plumber when I start these bathroom updates.

Yeah, I'm not going to participate in this one.

Four people taking long showers at 6.1 gpm each is obscene.

each?

@suraj989 - I know it’s not what you asked, but make sure you even like a rain showerhead. I disliked the one I had in a previous home, and would have to use our second bathroom’s regular shower instead. The rain showerhead never had enough water pressure. Also, because the water falls as a straight vertical, you have to stand directly under it, and then you get water in your eyes and mouth. I never had realized before why regular shower heads spray water in a more cone shape at an angle, but if you think about it, that allows, for example, to rinse your hair thoroughly (standing with your back to it) without the water getting in your eyes and mouth. The rainhead shower did not allow that - at least in my experience.

@Shannon_WI Thank you for the opinion, this is something ill need to look into, have you used a shower with the body sprayers? does this make much of a difference?

@Jake The Wonderdog, I thought I was asking an innocuous question, didnt mean to offend. my apologies.

If you live in California here is the plumbing code:

According to the California Plumbing Code, effective July 1, 2018, all residential construction projects must comply with the following regulations:

• Kitchen faucets: may not exceed 1.8 gallons per minute (gpm)
• Bathroom faucets: 1.2 gpm
• Showerheads (handheld and fixed): 1.8 gpm; in showers with multiple heads, the combined rate of all showerheads controlled by a single valve may not exceed 1.8 gpm in total
• Toilets: flush volume may not exceed 1.28 gallons per flush (gpf)

(Public and commercial fixtures may be subject to different regulations.)

January 1, 2019 will bring another change: the law requires homeowners to replace noncompliant plumbing fixtures in homes – single-family as well as multi-family and commercial buildings – built prior to 1994. Those selling their homes will be required to inform prospective buyers of noncompliant plumbing fixtures prior to the sale. Additionally, homeowners will no longer be “grandfathered in” and will technically be required to update their fixtures even if they are not remodeling or selling their homes.

"have you used a shower with the body sprayers? does this make much of a difference?"

My mother installed the side body sprayers in her condo. For her, after a couple of uses the excitement wore off, and she never used them again. I tried her shower with the body sprayers, and for me at least, felt it was not worth the expense of installation. And of course, the more plumbing pipes, fixtures, handles, and spray heads that you have, the more there is to maintain, and the more there is to break.

Body sprays also use up hot water like nobody's business. If you are using that tankless water heater this won't be a problem re running out of hot water, but there will be the high energy bill of heating up that amount of water instantly.

First off let me say that is an insane amount of water, for a residential setting. Really insane for California where there is a shortage of the resource to begin with. It is an interesting idea though. I know nothing about this kind of setup. There are only two people in my home and we get along fine with 40 Gallons of water in a tank with plenty to spare. When the kids were here though we could see issues if everyone decided they wanted to shower at the same time as we do have 3 bathrooms. If I were going to attack this idea, I would put the tankless after tank. In this way (at least in theory) the tankless would only operate at full capacity once the water in the tank was exhausted? Or as the water began to cool the tankless would ramp up. That theory based on how hot the water is flowing into the tankless and whether or not the tankless would operate at a lower setting or not at all if hot water was already flowing through it. I don't know the answer to either one of those questions. As far as those rain shower heads, not a fan. I have a family member who has one, not the best shower experience I have had. To each his/her own though.

Tankless water heaters moderate their heating capacity (and energy usage) to meet the output setpoint. Water flow rate and supply/incoming temperature are both factors.