Indirect with jetted tub sizing question

I currently have a 50 gal direct vent water heater, it's 20 years old. It's stll runing strong but it's time is up, want to replace it before it leaks.

The question is since I have a fairly large jetted tub and the 50 gal just about does it if you are carefull not to use any other household hot water and the heater has completed it's cycle before you fill the tub, by the time the tub is full the water has gone cold. What size indirect should I use.

Most seem to be sold with first hour ratings of around 100 gal with 30 in storage, Since the tub has high flow filler and can be filled in about 6 minutes (about 60 to 70 gal) I feel that I need to disreguard the first hour rating and have all the water I need already stored. The reason for my thinking is that if the cast iron boiler starts cold it takes at least 5 min to get the boiler water up to 150 so for that first 5 min I would be drawing down a 30 gal indirect and it would run cold before the tub is full.

I think I need 60 gal in storage to be happy with the system. I am also finding some brands call their tanks 60 gal but only have 50 gal storage because the coil takes up room and other wishfull thinking factors. So I need a real 60 gal storage.

Is my thinking correct, last thing I want to do is replace a working gas heater with an indirect that cannot meet the high demand. The bolker has 110,000 BTU output and I could make the indirect a priority zone but would rather not so that could slow the recovery even more.

The general rule of thumb is you can only get about 75% or so of the content of a WH before you've diluted it enough to dissapoint you. Some of this depends on what temperature you maintain the WH...a tank with 140-degree water will fill more of a tub than one held at 120-degrees. Most indirects are maintained at 140-degrees, and most codes require a tempering valve on the outlet to restrict the output. Some boilers can let you up the storage temp even further, thus making a smaller tank 'look' like a bigger one. The higher the storage temp, the bigger the storage losses, but on a good indirect, it's an order of magnitude less than a gas fired standalone tank. In the scheme of things, making it a priority zone is still probably your best bet. I have a 60-gallon SuperStor Ultra on a 60K boiler, fill a 6' air tub, and have never run out of hot water. Mine is plumbed as a priority zone, and is maintained at 140 with a tempering valve to limit the outlet to 120 or so. It does take about 10-minutes to fill the tub, as the supply is 1/2" pex, but that's another issue.

Right now I run the 50gal at around 130, I fill only with hot water it does go cold near the end of the fill but the result is a tub that is at the temprature we want. We almost never use the cold faucet, just open the hot all the way till the tub is full. This works alright as long as no one in the house uses any hot water and the heater has cycled off from any previous use.

I am thinking 60 gal at 130 degrees would do it just fine, the recovery of the indirect is faster so the rest of the house will have hot water shorty after the tub is full.

What does everyone think about Amtrol Boiler Mate, I know they had leaking problems years ago but they say that has been corrected. The price is less than other 60 gal units and my wife likes the blue color.

First hour gallons is a useless number when looking at tub-fills. You are correct in assuming that it's the first 6-minute performance is that counts. In 6 minutes at 110KBTU/hr (=1833 BTU/min) and a fill rate of 8gpm, the effective [6x8=]42 gallons of incoming water would only raised [(1833x6)/(8.34 x 42)=31.5F, which is fine- you could fill tubs all day at that rate if your incoming water temp is ~85F, but represents a 6 minute performance shortfall otherwise.

If you're barely cutting it with a 50 gallon standalone, the performance of a 30 gallon indirect won't be satisfactory, even with 110K (or even 200K) of boiler input. Bumping the storage temp helps, a bit, (surprisingly little, if you do the math) and if you have a condensing boiler going over 130F on storage temp pretty much kills condensing performance. A 60 gallon indirect at 120F would do you just fine, and you'd likely see a ~90% average combustion efficiency (depending on flow and heat exchanger size.)

Thanks Dana

That math made my head hurt, thanks for doing it for me. In addtion to the lack of recovery I also have to factor in that the boiler may be starting from cold so my recovery will be even longer. The boiler is cast iron so it takes at least 5 min to produce any meaningfull output.

I think the 60 would do it just fine, just have to make sure it is a real 60 in storage and not some manfacturers hyped up rating.

Only question remaining is the Super Stor any good, remember my wife likes the blue color but I do not want to buy a lemon LOL. I got 20 years out of my direct vent gas and it's still going.

The SuperStor Ultra has a SS tank and cupronickel heat exchanger - 10-year warranty. Should last much longer.