Selecting a water heater for a 100 gal soaker tub

[RobRing]

Hello, all. I've been reading through the archives for the tank and tankless water heater forums trying to decide how to spec a new water heater. We’re in Rochester, NY where the winter groundwater temp hovers between 40 and 45 degrees. It’s currently just me and my wife, but kids are a definite possibility in the near-term. We’re in the process of a master bath remodel and are installing a 100-gallon soaking tub, which replaces an existing 85-gallon jetted whirlpool tub. Our current heater is a 13 yr old 50-gallon natural gas tank (State, I think). It still works but is probably nearing the end of its useful life. The current water heater just barely filled the old tub, so I don’t expect it to do the job on the new tub. The new tub would be likely used a few times per month on average. I’ve long been drawn to the tankless concept, but everything I’ve read that’s relevant to our location and usage raises a lot of red flags. But for what it’s worth (and as a point of price comparison), our installed cost for the Rinnai R75LSi tankless (after NY rebate) comes to roughly $2,200.

My question for this forum is what type of tank setup would be advisable for us? Without the new tub in the equation our 50-gallon tank has been sufficient for our needs, even with lengthy showers. I’m reluctant to incur a huge additional cost for a new system just to accommodate the 100 gallon tub given its infrequent usage, but that may be what we signed on for with the bigger tub. The Bradford White sizing guide recommends a 75 gallon (M-I-75S6BN), 100 gallon (M-I-100T6BN), or 2 40 gallons (M-I-40T6FBN), satisfying 100%, 112% and 113% (respectively) of our estimated demand. I don’t have a quote, but I’m guessing the 75 gallon tank runs from $1,200 to $1,500 installed.

Would another option be a new high recovery 50-gallon tank, set to a water temp of 140-degrees, with a 120-degree water tempering valve installed on the tank outlet? If so, do new tanks typically come with a tempering valve installed or is it a costly add-on?

If my math is right a 100-gallon tub with one small adult (120 lbs) needs about 85 gallons of water to be full. Add one medium adult (160 lbs) and the water needed goes down another 20 gallons to 65 gallons. With a tub fill rate of 5.5 gpm it would take around 17 minutes to fill to 85 gallons. Using a formula I found in another thread on this site I calculated that I’d need about 60 gallons of 140-degree hot water mixed with 25 gallons of 40-degree cold water to give me 85 gallons of 110-degree water. Surely a 50-gallon tank could accomplish this over a 17-minute period, right?

I’ll stop with my rambling and speculating now and let those who know better than I weigh in. I appreciate any suggestions you can offer.

Thanks!

Rob

[Dana]

If you heat your home with a hydronic system (forced hot water baseboards/radiators/radiant floor) the hands-down best solution for you is an indirect-fired HW tank run as a "priority zone" off the boiler.

Beyond that, a 75gallon tank with a 75KBTU/hr or larger burner would fill the bill, but you may have to crank up the temp a bit, but not much. For more money (but also with a rebate) the high efficiency big-burner 50 gallon Vertex 100 would make it as well.

Tempering valves aren't too expensive, would be a sub-$100 cost adder at the time of installation. IIRC they're now required by code on all HW tank installations in MA- might be in NY too, in which case the point is moot.

A typical 50 gallon tank has about a 35K burner operating at 80% efficiency, delivering ~28KBTU/hr to the water. You're mid-winter incoming water temp is probably ~45F and you're needing to raise it another 65F up to 110F. In the 1/4 hour during the tub fill it's only delivering 7000BTUs to the water, so it's only heating up ~108lbs or ~13 gallons (2 minutes-worth at 5gpm) to 110F, which makes it extremely marginal at best. With a 75KBTU 80% burner behind it you have a tiny bit of margin ,but may have to slow the flow a bit. With a 75gallon tank & 75KBTU/hr 80% burner or the 50gallon Vertex 100 with a 95% efficient burner the margins are much better.