Water Heater


For years, I wanted to have the efficiency of a tankless water heater, but I never knew how to get a heater installed in our house since it contained a recirculating pump connected to the existing tank heater. I didn’t find any information on the Internet on how to deal with both a tankless water heater and a recirculating pump. The challenge with this configuration is that in order for the tankless water heater to turn on the burner it requires flowing water. The recirculating pump doesn’t draw enough water to ensure that the burners turn on so therefore another solution needs to be created to have a hot water configuration that includes a tankless water heater and a recirculating pump. This article will describe the hot water topology to use for creating a hot water system in a domestic house that contains both a tankless heater and a recirculating pump.


Designing a water heater system for a domestic house that contains both a tankless water heater and a recirculating pump requires thought and consideration. This article will describe how to design a tankless hot water installation, what you need to purchase, how to prepare for the installation, installing the heater itself and expectations with a tankless water heater.

Design a Tankless Hot Water Installation

When designing a tankless water heater system, you need to consider: water hardness, gas flow, exhaust and venting, drains, and the dreaded recirculating pump.

Water Hardness

Hard water will destroy a tankless water heater. The calcium build-up destroys the efficiency of the heater and will eventually prevent water from moving. Testing your water before you install the heater is critical, and you want water that is less than 50 ppm. If your water isn’t soft, you must install a water softener.

Gas Flow

Tankless water heaters need a lot of gas to the heater when it is running. If the heater cannot get the necessary flow of gas, the onboard computer will generate a fault and the heater will not function correctly. Most heater manufacturers specify that you need a 3/4″ gas line to the heater. Some manufacturers suggest that you test that you have enough pressure from the gas meter and to install a larger gas meter from the gas company. For my installation, I didn’t perform any modifications to the gas line since it was already 3/4″ from the meter. I was definitely nervous the first time the heater fired up awaiting a fault from the computer, but gas pressure has never been an issue with my installation.

Exhaust and Venting

Exhaust and venting is the biggest source of issues with tankless hot water systems. Many installers connect the heater directly to existing duct work which will most certainly lead to system failures. You have to make sure that you use the venting that is specified by the heater manufacturer. If you do not use the correct venting the heater will absolutely fail. The venting manufacturers have worked directly with the heater manufacturer to make sure that the vent pipe is sized correctly and that the heater operates at peak efficiency. Also, make sure that you follow all of the recommendations for connecting the vent to the heater and venting it to the outside. Expect to pay 20% of the heater cost in vent pipe. Vent pipe for a tankless water heater is not galvanized and not something that you can pick up at the big box home improvement stores.

The heater not only uses the vent pipe for exhaust, it also uses it for fresh air intake. Using the correct vent pipe is critical to the success of your installation. There is so much caustic molecules on the exhaust of the heater that you will destroy the heater if you don’t use the correct venting.


Many homeowners choose to install a condensing heater. A condensing heater is more efficient, but it does have two drains on it. The first drain is a pop-off valve in case of a over-pressure situation like a regular tank water heater and the second drain is for the condensing liquid. My heater came with a plug installed on the condensing drain. If you forget to remove this plug, or you don’t correctly install a drain to remove the condensing fluids your water heater will rust out.

The combustion byproducts of natural gas are water and other petroleum byproducts. The water that is produced from the combustion needs to be drained. I chose to drain the liquids into an outside area that is full of vegetation. You should consult your city and or county code on the proper disposal of the condensing liquid. Water heater manufactures provide mechanisms for treating the condensing liquid.

Recirculating Pump

Challenges with a tankless water heater is that they need a flow of water to turn the burner on. I honestly wasn’t sure that the recirculating pump that was installed on the existing tank heater in the house could generate enough flow to trigger the burner to ignite. Not wanting to take a chance with this installation, I chose to install a buffer tank that would keep a small amount of hot water always hot. Installing a buffer tank also eliminates the phenomena known as a “cold-water sandwich” when the heater takes time to respond to new demands on the hot water flow as faucets are turned on and off throughout the system.

I calculated that my house had over 6 gallons of water stored between the location of the water heater and the furthest faucet. You can calculate how much water is stored in your pipes with this formula:

Gallons = [1/2 * pi * [radius of pipe (in)]^2 * length (in)]/231

1 Gallon = 231 cubic inches

pi = 3.14159

Since I had to move 6 gallons of water from the furthest faucet to the heater, I felt that a 7 gallon water heater would be big enough for the demand. I purchased a 7 gallons point of use water heater from Bosch.

I connected the recirculating pump to draw water directly from the tank instead of the tankless water heater. The cold water from the point of use water heater is fed directly from the tankless water heater.

System Expectations

With this hot water system, you will never run out of hot water and you will gain the efficiency of a tankless heater. There is one design flaw with this system, and that is if you run the recirculating pump without drawing any hot water from any of the faucets the hot water coming out of the faucet will be luke warm. This is due to the fact that the water is constantly heating the pipes and slab if the pipes are in the slab. This heat exchange is a lot for the small point of use water heater to keep up with. In my house, I manually control when the recirculating pump is turned on and wait about 30 seconds for the hot water to reach the furthest faucet. I can then turn on the hot water and not notice any cooling of the water because the tankless water heater is mixing water into the recirculating loop.