Negative Air & Freeze Prevention for Bosch Tankless Water Heaters

Negative Air in the older Bosch naturally drafting units and the new Bosch tankless water heaters such as the Bosch Therm 520PN, 520HN and 330PN is something that needs to be considered when installing the tankless.

Negative air pressure in a structure occurs when air consumed by fuel burning appliances or exhaust systems in that structure exceeds the amount of air available.  Here is a list of items that consume air within a typical household:

• Stove exhaust fans
• Furnaces
• Water heaters
• Fireplaces/ Woodstoves are the worst as they consume a lot of combustion air
• Bath fans
• Dryers

All of these items must be taken into account when making provisions for a proper amount of fresh air from the outside.  If the building is new construction, air infiltration from the outside will be limited and provisions for extra air supply should be made. If the building is older construction, air may infiltrate into the house through unplanned sources such as window frames, door jams, sill plates or idle chimney flues. These sources still may not be providing the adequate amount of air and additional sources of fresh air should be considered.

Why Negative Air Is Bad

Cold air from the outside is drawn through the water heater’s flue pipe because other fuel burning appliances or exhaust systems in the structure require more air than the dwelling can supply. When the required amount of air for the fuel burning appliances or exhaust systems in the structure is not being supplied to them, they find the flue pipe on the water heater an acceptable means of supplying themselves. Thus, freezing cold air is pulled down the water heater’s flue, drawn over the copper heat exchanger, potentially freezing it and the water valve below. Because, when idle, the water heater holds only cold water, freezing can happen quickly possibly causing leaks or ruptures in the heat exchanger and/ or the water valve. The water heater does not pull air through its flue pipe – if you were to remove the heater, the air would continue to come down your flue pipe. You must have the air balanced in the structure to correct this problem. Consultation with a heating, ventilating and air conditioning (HVAC) specialist is recommended.]

There is also another alternative to combat negative air for the Bosch 1600P, 1600H, 520PN and 520HN naturally drafting heaters and that is to use the AQ4 powervent kit.  The AQ4 powervent kit is for venting the appliance out the sidewall of the building and comes with a termination hood with built in flapper to keep the cold air from coming in when the heater is not being used.  The AQ4 powervent kit requires power to operate the blower motor which blows the exhaust gases to the outside.  The AQ4 powervent kit is not approved for the older Bosch 1000P and the new Bosch Therm 330PN as the unit is not large enough to use the powervent motor.

All larger Bosch tankless water heaters that are direct vent units meaning they have a built in fan, are sealed combustion and have the ability to take the intake air from the outside should always use this method and take the combustion air from the outside in all cold climate areas.