Question about Bosch AquaStar 2400ES-NG

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Frozen tubing in heat exchanger

After a cold snap overnight, there was no hot water in the morning. There was no flow of water from the hot side of any of the faucets. I opened the pressure relief valve to determine if water was flowing thru the heater (there was none.)

I opened up the exterior and interior covers and saw that the way the exhaust was exposed to the outside air directly. I believe that the cold air was able to make it down into the combustion chamber and cause the tubing surrounding and thru the heat exchanger to develop ice blocks. I was able to use a hair dryer to thaw the tubing out in about 20 minutes.

My questions: Is there a way to minimize or prevent the exterior air from entering the unit thru the exhaust tubing?
Is there a suitable high temp damper that would at least reduce the amount of air flow?
Is it possible to program the Secondary fan to run all the time at a low speed to provide positve pressure in the combustion chamber to force warmer interior room temp air into the exhaust duct?
Can the front and combustion covers be left off to allow interior air to keep the tubing at room temperature?
Any assistance that you can provide will be greatly appreciated.

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  • gmestrada85 Jan 14, 2009

    No. This unit has the 3" single wall stainless steel high temp tubing from Bosch (Kit AQ3ES). The exhaust runs 18" vertically, 90 degree elbow to horizontal, then 12" horizontal thru the thimble to the exterior of the home. Terminates with the exhaust vent hood.

  • Anonymous Jan 17, 2009

    I have the same problem. The heat exchanger froze, burst and flooded my basement. Having a hard time trying to locate a replacement heat exchanger.

  • gmestrada85 Jan 19, 2009

    If both the exhaust and the intake pipes are routed outside (per your recommendation and the installation instructions), then the heat exchanger is now totally exposed to the outside air. Not sure if there will be any actual air MOVEMENT thru the pipes when the unit is not operating (like I observed initially). With the covers installed, then the heat exchanger is now isolated from any warmer interior air in my basement. Will the freeze protection heating elements be able to keep up?

  • Allen Neal
    Allen Neal May 11, 2010




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The 2400ES in cold climates needs to be a twin piped unit. 3" SS venting with a combination air port made of pvc also 3 feet apart I believe on the same wall and at the same plane. Keep the cover on at all times and this will seal of the heater from the rest of the house and from freezing.

Posted on Jan 18, 2009

  • Tim Hamlin Jan 19, 2009

    Heat exchangers can be purchased online at

  • Tim Hamlin Jan 20, 2009

    Thats the hard part with any tankless unit. That particular design does not use a flapper in the exhaust vent. So to stop the air from coming in on its own no, their is no way to stop it. Your heater was designed to be a twin pipe set up. It doesnt seel it off from inside heat. It seals it off from another heating system used in the house.

    Most of the time your tankless is off unless being used. The freeze protection in the heater will keep it warm enough, plus heat from your house.

    If you twin pipe the heater, then another heat source cannot draw cold air down the vent for combustion purposes. with the cover on and the twin pipe set up it seals it off from another heat source drawing cold air down its venting.If you keep the cover off of the tankless then you will build a lot of condensation and cause the unit to short out on the electronics. High temp dampers are not allowed on tankless units per NGC codes. Also there is no way to actually set the secondary fan assembly to run when the unit gets cold. The freeze prevention will activate at 32F and is suppose to be good to 0F. But with warm basement air it would actually be better than that. But then again, we havent seen -20 to -40 below temps in a long time. The only other possible thing I can think of to use would be a small wall mounted fan like they use for pellet stoves and wood stoves. They are about 6" round and you can mount them anyweres with the bracket they have so you could force more warm air directly at the heater.



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1 Answer

Low water pressure

Hi Bob,

You can determine if the tank is the problem by connecting a garden hose to the drain that is provided on most water heaters. Before doing this, be sure to shut of the electrical supply to the heater - otherwise you can damage heating elements if an electric type or waste fuel by heating an empty tank if a gas / propane fired type.

If you get a slow flow out of this drain, it means that somewhere in the cold water supply side to / in the tank - there is a problem. Inside the tank, the cold water is delivered by a "dip tube" that distributes the cold water to the bottom in the tank. Dip tubes are used in both electric and gas / propane fueled heaters. Here's a cut away view of a typical tank:


If the tube is clogged, or there is build up, scale or debris in the supply (cold water) side of the heater, flow rates will suffer. Likewise, if there is a problem with the output of the tank, it will directly affect flow as well.

If you get a good flow rate from the drain, then the issue is going to be on the hot water output side of the heater. Again, the same build up, scale, debris issues on the hot water out put pipe will cause problems with flow. Typcial tanks do not have dip tubes for the hot water side of the tank - as taking hot water from the top of the tank is most desireable and is furthest away from the cold water that rushes in to make up the hot hot water going out.

There's nothing really in the tank itself that can cause issues with flow outside of the dip tube, and piping to and from the tank.

Good luck!

Jun 29, 2012 | Water Heaters


Find your only getting Luke warm water @ your faucets??


This is a common fault with new to used installed waterheater.

It is call a diptube ,made of pvc in part description. It is located on cold inlet,top of waterheater.

Malfunction of dip tube is usually a fracture caused by age mostly,or faulty install when new and tube was exposed to high heat from welding .

This tube allows the cold water supply to flow into W/heater and exit at bottom where it is heated while pushing already heated water to hot side and to your faucet. If this tube is defective ei...cracked the cold will [leak] mix with the hot before reaching bottom to be heat..thus lukewarm water syndrome. Solution $8 part H/depot>remove coldside copper fittings and pull white pvc out,about 4feet long,less rotted area, install new 3/4 inch pvc dip tube with flange up.

Thanks jj505 The HomeRepairman

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I am going to guess you have crawl space and the you have some frozen water pipes.

Turn the water faucets on to the hot side and one of them to the cold at a trickle and add heat to the water piping area or go down there and use a heat gun/hot air not flame and apply to the pipes. If you have a part of the piping close or in an outside wall I would start there.
Good luck and hope for the best.

Most people don't know that the hot water as it cools will freeze first because the water has more air in it then cold water dose.

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I have had the same problem. I took the cover off, and put an electric heater (1400 watts, with built in fan), on a stand 1 1/2 feet away, blowing at the upper half of the Bosch. When you take the cover off, you will see a rectangular copper shaped thing - that is the heat exchanger. The copper water lines run through it and freeze from the cold air coming back down through the flue.
Whenever the temperature is going to be 15 degrees or less overnight, I turn the electric heater on. In fact, I leave it set up, on a stand, with the Bosch cover off, all the time... just in case.
This is really a poor design. With the money we've invested.....what a joke. Oh, and I do have the anti-freeze heat trace kit installed as well. Hahahaha, what a joke.

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