Atmospheric Draft water heaters installation and maintenance are very important for “Back Draft” issue. What does “Back Draft” mean?
Lets take a look at the how Combustion occurs. This short info will help you to visualize the back draft importance.
The main principle of all fuel burning appliances is Combustion and there are by-products of combustion, namely;
For combustion we need FUEL + AIR + Igniter
When “ Ignition ” occur we have HEAT +
. + Water vapor ( H2O ) , (inside flu gases)
. + Carbon dioxide ( C02 ) , ( inside flu gases)
. + Carbon monoxide ( CO ) , ( inside flue gases)
. + Nitrogen oxides ( Nox ) , (inside flue gases)
. + and other particulates, (soot, ash, etc…)
Combustion is an exothermic reaction, so it releases HEAT. We want to use heat but we cant use others. We need to make sure the gas burning by-products such as CO, Nox, C02 and moisture have a safe and easy path out of your home . But some conditions of flue gases can not deliver it out of your home and potentially letting hazardous exhaust gases come back into the home, which we called BACK DRAFT.
Silent killer Carbon Monoxide and other life threatening harmful gases leakage can change our future and for our families. ( Please check “Carbon Monoxide-Silent Killer” page for more )
I know no one would want to be in this situation and I hope no one would have to face it.
There are many potential causes for atmospheric combustion appliances to back draft.
Pressure difference between outside and inside (Negative pressure), Chimney and vent condition or size, Vent Connector design problems, Improper vent connector slope, Long horizontal vent connector, Neglect!
Sufficient indoor air supply is very important for home comfort if you have atmospheric combustion appliances. These atmospheric combustion appliances sucks the room air into the combustion chamber and burn the air-gas mixture.
As you know hot air rises. Hot exhaust gases leave the house through the chimney with stack effect. Stack effect is the movement of air into and out of buildings.
The hot flue gases creates a negative pressure, inside the chimney. This negative pressure pulling the flue gases out of the building which is called “Draft”. When atmospheric appliances exhaust the flue gases from chimney use dilution air.This extra air enter the vent from draft hood and help the stack effect and flue gases to move very easy.
A basic rule of building science is that for every cubic foot of air that leaves the house, another cubic foot of air comes in.
When the pressure inside the house is lower than the pressure outside, the house is under negative pressure and air leaks into the house.
Especially cold winter days or windy days we can have this condition. And the pressure difference is the force that pushes air through the air pathways. Under these conditions, if the atmospheric combustion appliances (furnace or water heater or both) start to work, flue gases has a hard time too transfer it outdoor. And outdoors high pressure pushes flue gases back into the house. If this situation occurs under one minute it is called “Spillage”, if it occurs more than one minute its called “Back Draft”. Also kitchen exhaust aspirator fan or bathroom exhaust fan or clothes dryer can make negative pressures inside the home too. When they work at the same time along with natural draft water heater or furnace, these fans will make negative pressure inside the house and flue gases will be back draft.
* This is a general problem of tight structure houses. If they do not have fresh air intake or good ventilation for air balance within indoor and outdoor, pressure difference negative effects will occur every time.
* Another reason of back draft is the condition of the chimney or the vent. Some times animals can make nest in the chimney opening, or some of the chimney building materials can collapse and this obstruction can effect the chimney stack effect and flue gasses back draft into the house. Chimneys need to be checked periodically for obstructions.
* Also another common mistake is an installation, of a long horizontal vent connector. Atmospheric combustion appliances vent connectors should be short as much as possible. Too much horizontal pipe will create restriction. And connector slope should be minimum rise 1/4” per foot. Otherwise back draft is inevitable.
Neglect is the most common mistake and one of the most important issues. Please do not neglect and keep clean top of the water heaters.
Dust, pet hairs, loundry dryer paper sheets , etc. These materials blockage dilution air intake and effect chimney stack effect and draft. If these materials enter the vent pipe and in time blockage the vent pipe or chimney liner.
If you have power vent water heater , dust or dryer sheets or other materials can blockage vent motors “squirrel cage” and minimize or stop the draft. Some reason pressure switch does not stop the combustion and back draft is inevitable.
( It is happened and CO detector alarm save residents life. Next day home owner make excuse ” I didn’t know that, no one told me keep clean of the water heater ! ” )
Here are some common signs that your water heater is back drafting:
- Melted plastic at the draft hood,
- Corrosion at the top of the tank,
- Moisture at the top of the tank,
- Heavy condensation on the cold water pipes,
- Using smoke test or a mirror at the water heater draft hood to see if there is spillage.
Signs of Back Draft
If you see any sign please do not neglect and call licensed technician for service.
When heating equipment and vents are well-maintained and household air pressure is balanced, the risk of back drafting is minimized.