Welcome! Here are the website rules, as well as some tips for using this forum.

# need to fill 90 gallon whirlpool need help sizing indirect

Member Posts: 138
i have a wbv-04-150 peerless oil boiler i think it is 178000 but output. the tub needs close to 90 gallons for them to use it. they have a 40 gallon super stor now no mixing valve. it fills less than half way. i was thinking of using an htp 80 at 150 degrees and mixing it down to 120. i was going to upgrade the feed and return to 11/4 with a larger pump and put it on priority. the tub faucet runs at 7gpm. i don't know how to do the math to see if it would work. i called htp tech support they said they only know how to fix them and can't help with the sizing. any help would be greatly appreciated. i don't want to put it in and have it not work. that would make it hard to get paid and its going to be quite costly to do the install. thanks mike

• Member Posts: 12,145
So you want to draw 7 gpm of 120 degree water 90 gallons of water to fill the tub

90/7=12.9 min to fill tub

If you install an 119 gallon tank 80% of the water is considered hot so 119x.8=95 gallons of water.

stored at 150 you need 5 gpm of 150 degree water mixed with 2 gpm of 50 degree water to get 7gpm of 120 degree water

so 95 (tank capacity)/5gpm (water needed)=19 min of water draw available, you need 12.9 min.

If you install an 80 gallon tank @80%=64 gallons available 5gpm @150 needed=64/5=12.8 min available

So an 80 gallon will give you 12.8 min and you need 12.9 so its about the right size.

But the boiler will make some hot water after you start drawing

But your city water could be colder than the 50 degrees I used

So to be safe you should measure the incoming water temp & then consider increasing the tank size to 119 gallon or increasing the storage temp to say 160

With no storage to heat 5 gpm from 50-150 (100 degree rise) would take 252,000 BTU/hour

maybe someone else will have better math than me

• Member Posts: 621
@EBEBRATT-Ed I think he meant 120 from the tank, probably less at the tub faucet. If it's 107 at the tub, then you only need 4 GPM of 150 mixed with 50. I think @mikedo could win a lot of favor if he first tries a mixing valve and solves the issue without increasing indirect size.
• Member Posts: 12,145
@Hot_water_fan

I took it that he needed 90 gallons of water to fill the tub at 7gpm flow rate. Indirect stored at 150 mixed to 120 would need 5gpm if the city water is 50 deg 7gpm flow

I agree the 120 is hot but your going to lose some temp due to piping loss and water cooling during a 12 min fill into a cold tub and not knowing the city water inlet temp. I think he needs an 80 gallon indirect and up the storage temp if need be
• Member Posts: 16,552
At some point the boiler will fire and that 178,000 will be helping cover the draw
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 19,150
Classic. When filling a tub of that sort (for that matter, even filling an ordinary tub), first, you want hot water. 105 isn't going to do it. Second, you need to hold that temperature. While it is true that the heat from the boiler will be helping, even if all that heat could be transferred to the indirect (it doesn't) you'd still be short. I'm a little conserative -- but not, I think, unduly so -- and I'd want to see the storage tank volume close to the tub volume unless the faucet flow rate is significantly less than the recovery rate.

So -- minimum 80 gallon indirect.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 7,355
That's a pretty big boiler. If you turn the fill flow rate down a bit it will take longer to fill the tub but you won't run out of hot.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
• Member Posts: 16,552
90 gallons to the over flow? 15 or so will go down the drain when someone hops in😗
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 12,145
The tub flow is high at 7 gpm. But that's what the OP provided. 80 gallon indirect should do it. he can always up the storage temp
• Member Posts: 3,334
edited January 29
After reading this discussion, and with the current outdoor temperature, all I can think of is climbing into that tub right now!

We did a 120 gal whirlpool tub with a buderus boiler connected to a Buderus 40 gallon indirect. Used a mixing valve. Set the tank temp to 160° and it took about 20 minutes to fill the tub. The recovery on the indirect was fast enough to keep the tap temperature at 122° for the 20 minutes. The water was cooling the entire time it was filling so by the time it was full, the tub was at about 116°. Just right for me and my wife. and my girlfriend

Edit: My wife and my girlfriend are the same person. What were you thinking?
Edward Young
Retired HVAC Contractor from So. Jersey.
Services first oil burner at age 16
P/T trainer for EH-CC.org
• Member Posts: 138
thanks for the help I'm also considering putting 2 on demand lp heaters in twined together. they are getting a pool boiler so i could tap into that
• Member Posts: 2,460
edited February 20
Add a 2nd SSu 50 in parallel store at 150, should be good to go. Gives you 80 gals hot, with recovery could probably get away with 120 unmixed. Whoops, missed it was a 40, still double up and store hotter with mix valve.
Good luck
Tim