Indirect Water Heater running out of hot water

Indirect Water Heater Running Out of Hot Water

There is nothing worse than jumping in the shower or bath and you run out of hot water. We are not going to address a technical problem like a control or circulator failure. We are going to address everything on the tank is working as it should be but, the tank is running out of hot water.

Let’s understand first the sizing of the tank. It should be sized for the domestic hot water usage in the home. We have discussed the indirect hot water tank sizing on a different web page. I am more concerned here with the draw of hot water is too great for the recovery time of the tank.

4 things are important for good operation and not running out of hot water
  • Tank sizing
  • Boiler Size
  • Pipe Size
  • Circulator size

Looking at a spec on the Utica H2O indirect water heater tank. We need to understand the information the manufacture is telling us. Here is the ratings chart

Utica-H2O IWH Chart

The Ratings chart is telling us the important information we need like the model number, first hour rating and boiler size to meet the available DHW spec. We’ll look at the Model H2O140UB and a tank temperature of 140f. The rating is 221 gallons per hour. First hour rating means storage and recovery of water.

Usable Water volume

The Model H2O140UB is a 40-gallon water tank. The usable water is how much water you can draw until it drops below the tank control setting in this example is 140f. We can calculate the usable water with the following formula.

Tank size in gallons * 0.75 = Usable Capacity

40 (tank gallons) * 0.75 = 30 gallons usable hot water

After you draw about 30-gallons and the tank is not operating you will be running out of hot water.

Smaller Boiler Recovery Rate

Let’s assume the boiler is an 80,000 btu boiler instead of 139,000

Boiler Output/(8.33*60*90) = Recovery

8.33 = Weight of 1 gallon of water
60 = 1 hour
90 = Temperature rise between incoming and out going water temperature

80,000/45,000 = Recovery 1.7 gpm.

Usable Water + (Recovery *60) = New First hour rating

30 +( 1.7 gpm*60) = 132 gallons

Difference of 89 gallons less in the first hour rating  (221 gallons – 132 gallons)

First Hour Rating/60 = GPM Draw

Faucet draw to not run out of hot water 132 First Hour/60 Minutes = 2.2 gpm


Faucet Draw

If you exceed this you will run out of hot water. The boiler may be smaller to meet the heat loss of the home which is common to do and is the proper way to size the boiler.

We can see the smaller boiler will produce less water to the faucet. If we increase the boiler size we will cause a problem with the boiler short cycling during the heat demand and increasing the fuel bill.

Running Out of Hot Water

If there is a problem with running out of hot water from an indirect tank

Possible causes (new install);

  • Mixing valve fault
  • House pressure too high – increases flow through the faucet
  • Exceeding GPM flow through tank
  • Piping between the boiler and tank is undersized
  • Circulator undersized
  • Incorrect pump setting
  • Under sized boiler
  • Undersized tank

Before throwing away the tank or boiler try this

Is there a way to increase the amount of DHW draw without changing the tank or the boiler? Yes, this may be an easy fix by just adding a mixing valve. Although this is a way to add dhw draw it still may not give what you need. It always comes down to the GPM draw required.

We first need to determine the tempered capacity. The formula to determine the storage capacity is;

(Tmix-Tinc) / (Tstored-Tinc) = SF


SF = Storage Factor
Tmix = The selected mix water temperature
Tinc = The incoming water temperature
Tstored = Tank Storage temperature
We will use a DHW supply temperature of 120°f, a tank temperature of 160F and a determined 50°f incoming water temperature.
70/110 = 0.63 Storage Factor


Usable Capacity/Storage Factor = Tempered Capacity

30/0.63 = 47.6 Tempered Capacity

This is an increase of 17.6 -gallon capacity

New First Hour Rating 132 (above) + 17.6 Tempered capacity = 149.6 

You could run 2.5 gpm all day and not run out of hot water with this smaller boiler.

If you raise the temperature on the tank up to 170f and apply the formula abobe you new draw rate would be 3.05 gpm.

I am not addressing the tank pipe sizing or the circulator sizing as I have cover them on other pages.

Disclaimer: The information found on this web site is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information or application, damage to property or bodily injury from applied information found on this web site.