Indirect Water Heater Running Out of Hot Water
There is nothing worse than jumping in the shower or bath and running out of hot water. We are not going to address a technical problem like a control or circulator failure. We will address the issue where the tank is working as it should, but it 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 that the draw of hot water is too great for the tank's recovery time.
- 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 manufacturer is telling us. Here is the ratings 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 140°F. The rating is 221 gallons per hour. The first hour rating refers to the storage and recovery of water temperature.
Usable Water volume
The Model H2O140UB is a 40-gallon water tank. The usable water is the amount of water you can draw until it drops below the tank control setting; in this example, it is 140°F. We can calculate the usable water with the following formula.
Tank size in gallons * 0.75 = Usable Capacity
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
8.33 = Weight of 1 gallon of water
60 = 1 hour
90 = Temperature rise between incoming and out going water temperature
Difference of 89 gallons less in the first hour rating (221 gallons – 132 gallons)
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 home's heat loss, which is a common practice and 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 increase 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 drawn 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, 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 120F, a tank temperature of 160F, and a determined 50F incoming water temperature.
(120°f-50°f)/(160°f-50°f)
70/110 = 0.63 Storage Factor
Usable Capacity/Storage Factor = 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 in the tank to 170F and apply the formula above, your 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.
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