Tankless & Shower w/ Body Sprays

[Aaroninnh]

Parents have an existing 180,000BTU Rinnai RL75 with a single bathtub/shower combo. House is located in New England with estimated incoming water temp of about 45 degrees. Heater has been doing the job just fine. At Delta T of 60 degrees (to get to 105 degree shower temp), estimating their existing heater can put out about 5.5GPM looking at charts.

Second bathroom is being added with custom shower. Custom shower has a single shower heard + 4 body sprays. Plumber has not provided estimated GPM even after being asked, but based on ~2.5GPM for a shower head and~1.8GPM per body spray I told my parents their current WH probably wont fly.

So they ask him and he agrees the existing water heater is probably not sufficient and suggests an upgrade. His suggestion is to REPLACE existing heater with a 199,000BTU Rinnai RU199. I ask, are you sure you don't mean add a second heater in parallel, and the answer comes back confirming the proposal is to rip and replace.

I don't know what the plumber knows that I don't, but the charts on that only seem to put our about another .4GPM at a Delta T of 60 degrees, for a total of 5.9GPM.

Thoughts? Plumber is right and his idea is great? Ask for a parallel tankless proposal? Ask for a tank proposal?

 

[Fitter30]

Gas meter size, pipe size to run both and would want to check with Rinnai to see if the two heaters will talk with each other.

 

[Sponsor]

 

[Jadnashua]

Where I live in NH, I've measured the incoming cold water after a cold spell at 33-degrees, so 45 may be optimistic! Depends on where in NE they live.

Body sprays can be all over the place from maybe 0.7gpm and up to the max. That combination could tax not only the WH, but the supply lines and valve, too. The copper institute recommends a pipe water velocity of NGT 5fps for hot water, and that's only 4gpm with a 1/2" copper line...so, depending on how hot your hot is, and how cold your cold is, in the winter, you'd be likely to exceed that with 1/2" valve and supply lines.

Some tankless units will throttle the volume when they can't achieve the temperature rise, and that would affect the sprays, too. Keep in mind that the sprays only accelerate the water when there's a restriction, and between the WH and the valve, you may not achieve excess volume so there's a restriction...that can make it more like an open hose with no nozzle on it...the water just comes out without acceleration.

 

[Dana]

For gusher showers with side-sprays and for less money than a second tankless you can get a lot of capacity boost out of a drainwater heat exchanger, assuming you have more than 4' of vertical drain downstream of the shower. It also cuts down on fuel use too, taking heat that was literally going down the drain and putting it into the incoming cold water stream (to both the shower and to the water heater).