Heating Help

Freezing Tankless Water Heater

My neighbor has a BOSCH tankless water heater in his rental house..  What's happening is cold air coming down the flue is freezing the HX and bursting the copper pipe in the HX.  This unit uses electronic ignition, so no pilot to keep it warm.  Is there something he could do short of a motorized vent damper.  This thing is totally non-electric it has a small turbine is the gas line to generate current for a spark, so would have to have a temp senor wired into a relay to activate the damper unless you guys know of a non-electric one.  Any suggestions welcome.

he probably needs a back flow preventing vent

something like

http://www.houseneeds.com/shop/heatingproducts/waterheating/aquastar1/aquastar_ventingkit.asp

The right thing to do

The proper thing to do is discover and solve the negative pressure issue in the house that is drawing the air down the chimney. If the chimney spills there is a safety issue. Is the unit allowed to be vented into a chimney?

Vent issues

 Here is what the Bosch installation manual covers on the topic: ( Page 9 in the manual)
[url=http://www.boschhotwater.com/Portals/7/TechManuals/GWH2700ES_English_high_altitude_10.2008.pdf]http://www.boschhotwater.com/Portals/7/TechManuals/GWH2700ES_English_high_altitude_10.2008.pdf
WARNING:
In areas where outside temperatures routinely come close to freezing, sealed combustion operation is required. Use separate terminations for combustion and vent, which must be installed on the same wall or roof surface, however never facing the direction of prevailing winds. Faliure to do so may result in heat exchanger freezing up and bursting. This failure is not covered under manufacturer's warranty.
Is it possible this may be the cause?