Sizing boiler for DHW production only

Bob_9

I want to put a boiler and two 92 gallon indirect hot water tanks in to replace two existing A.O.Smith 100 gallon water heaters. How do you calculate the btu requirements of the boiler to heat the 184 gallons of water?
Lets say 40-45 degree city water raised to 135-140 degrees.
This is a building that houses a model shop, research lab and corporate HQ for a large firm. Extra quick recovery isn't ultra important but a reasonable recovery rate would be good.
Thanks, Bob

kf_2

DHW Sizing

Formula

BTU Requirement = GPH needed X 8.33 X Temp Rise

In other words, lets say the requirement was 500 gallons per hour.

BTU = 500 X 8.33 X (145-45) or 500 X 8.33 X 100 = 416,500 BTU's per hour.

8.33 = the number of pounds per gallon of water.
BTU = the amount of energy required to raise one pound of water one degree Farenheit.


kf

sootmonkey

cold h2o

multiply your ans. of 416500 by 60. that will give you the ans. you want. 24990000 btus needed to maintain this load. do you need 500 gals. per hour every hour?

Bob_9

gph requirement

I don't know what the gph requirement for the building really is. They have your typical mens and womens bathrooms, a lunchroom with a sink and coffee makers, and a lab. The lab is the largest demand on the tanks.
The 500 gph is an example for the formula listed.

[Deleted User]

2 - 92 gallon

indirects sounds like overkill. You should know what the requirements are before starting the sizing calculations.

You should also be aware that sizing indirects can be as much art as science. Some manufacturers rate recovery based on the gross rating of the attached boiler. Some use net. All rate on first hour. Finding the actual per hour recovery is seldom straightforward. The ratings also depend on the gpm @ specified head for the boiler water circulator.

What temperature do you want? Be careful. Most rate on a 65 or 90 degree temperature rise. Does the system have a recirc. line? If so, that can effect the ratings and btus required. And. How you pipe the heater(s).

My suggestions. Find out how much h/w is needed. Then go to the indirect rep. w/ ALL of your questions. Then, if you decide to do the job, get the installation instructions BEFORE working up a price.

Good luck.

Jack_23

On-demand

Check out Rinnai's sizing guidlines at www.rinnai.us in the service tech section. If you can tell me the number of fixtures and type I can assist. They do very well in this type application.