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APPENDIX A: WHIRLPOOL SIZING
1) If not already done, use one of the procedures in Part III to size a Maxi-Therm/boiler combination for all
fixtures EXCEPT the whirlpool tub.
2) Find the nominal volume of the whirlpool tub. This should be listed on the tub manufacturer’s literature.
3) If the whirlpool tub has a volume greater than the Maxi-Therm(s) selected in (1), increase the size of the
Maxi-Therm selected to one with a volume greater than or equal to the volume of the tub. The size of the boiler
does not have to be increased from that selected in (1).
4) Determine the recovery time for the Maxi-Therm. This is done as follows:
Recovery Time = 44982 x V divided by Q
Where:
V is the volume of the Maxi-Therm in gallons.
Q is the output of the boiler to be used in BTU/hr.
Recovery time is in minutes.
5) If adequate hot water is to be assured for filling the whirlpool tub, no significant amount of hot water may be
used for other purposes while the whirlpool is filling or for a period equal to the recovery time before the
whirlpool is filled. Also, hot water may not be available until the recovery time has passed after the tub is
filled. If this is not acceptable, the whirlpool should be supplied with its own source of hot water.
EXAMPLE:
A 70-gallon whirlpool tub is to be installed in a residence. Using one of the procedures in Part III, an
MT040G has been selected with a boiler having a 120,000 BTU/hr output to handle space heating and all
domestic hot water loads except for the whirlpool tub.
SOLUTION:
Since the whirlpool tub has a volume (70 gal) greater than that of the MT040G (38.0 gal.), select an MT079G
(79.3 gal) instead of the MT040G. The boiler output stays at 120,000 BTU/hr. The recovery time is:
44982 x 79.3 = 29.7 minutes
120000
This means that hot water may not be available, for purposes other than filling the whirlpool, during the fill,
and for approximately 30 minutes before and after filling the tub.
