Many contractors simply do not know how to properly size a furnace; or they do, but just don't want to spend the time and diligence in doing so. One of the key elements in sizing a furnace is accounting for the chimney and its condition. We find that when we are bidding on jobs the majority of our competitors neglect the chimney, thus leaving any chimney liner out of the equation and out of the price.
We at Energy Services believe in doing the job right. Part of that job is making sure you and your family are safe and protected. We are also looking out for any future repairs that may incur if the job is done incorrectly. Not accounting for the chimney in the sizing process can lead to very expensive repairs like having to hire a brick layer to completely replace your brick chimney, having to replace your hot water heater, or having to prematurely replace your furnace.
If you have a masonry chimney and need to replace your furnace, you need to know this information.
Most masonry chimneys are obsolete for venting today's more efficient furnaces. This is because most masonry chimneys are too large in diameter to properly vent the cooler flue gasses that today's 80% efficient furnaces produce.
1. Most masonry chimneys are either not lined with a metal chimney liner or not completely lined with clay tile.
2. If a 90%+ efficient furnace is purchased, it can not be vented into a masonry chimney. In this situation, the only appliance left to vent into the masonry chimney is the hot water heater, which typically has a three to four inch vent connector. Therefore, the hot water heater can not be reconnected back into the existing, drastically oversized common masonry chimney. See the discussion below that describes the need for a flexible chimney liner.
The typical gas furnace and hot water heater uses household air to mix natural gas in the burners to create the heat we need. The byproducts of the burning natural gas include water vapor mixed with the household air. This water vapor mixture includes the off-gassing of the household chemicals we use on a daily basis. This, in turn, causes the water vapor to become acidic. There is about a gallon of water vapor in a Therm of natural gas.
During the time the gas furnace and hot water heater are operating, the products of combustion, which include this acidic water vapor, enter the vent connector metal pipe and travel horizontally to the common masonry chimney and up into the atmosphere above the home. As long as the "water" is in a gaseous vapor state there is little to no harm to brick and mortar. However, as the flue gasses travel from the furnace and water heater vent connector and into the masonry chimney, the flue gas temperature drops from 575 to 375 degrees to below 200 degrees. This is the dew point or point where the flue gasses begin to condense into droplets of water.
Over time, the acid water seeps onto the mortar joints and into the brickwork. This acid water then freezes in the winter and thaws in the spring which causes the mortar to disintegrate and cement in the brick to start leaching out of the chimney chase. The leaching looks like streaks of white or off color powder either at the eight foot level or near the top three feet of the chimney. Eventually, more and more mortar breaks loose and gaps appear. To prevent the chimney from completely crumbling to the ground a mason contractor is called in to temporarily fix the chimney with tuck-pointing.
If a chimney liner is not installed to fix the problem, tuck-pointing will continue at an accelerated pace and/or the entire chimney will have to be replaced. Tuck-pointing and/or replacing the chimney could cost from hundreds to thousands of dollars.
We have seen masonry chimneys so deteriorated that carbon monoxide seeped into attic areas, crossed the insulation barrier and spread into the living space of homes where people resided. In a few ranch homes we discovered stained inside walls where the acid water leached through the mortar joints onto drywall in living rooms.
If the furnace to be installed is an 80% efficient model, a five or six diameter metal chimney liner is installed in the chimney. If a 90%+ efficient furnace is installed a three to four inch diameter chimney liner is installed. The 90%+ furnaces are vented with PVC out the side of homes. 90+ efficient furnaces should never be vented into masonry chimneys.
*Note: many furnaces in the Chicagoland area are vented into a masonry chimney along with a fireplace. There is almost always an offset for the furnace common chimney that is not clay tile lined in the offset section. This offset is usually made out of brick and mortar. Sometimes we see about two feet of brick and mortar just below the top of the chimney instead of clay tile. Both the furnace manufacturer and the GAMA Venting Tables say this type of chimney is not clay tile lined and therefore requires the addition of a new metal liner. Unfortunately, many heating contractors don’t know this because they don’t read these manuals or are not trained by the manufacturer. If you have both a masonry fireplace and furnace flue in the same brick chase, make sure your contractor inspects your furnace chimney from the top down to verify the construction. If they don’t and claim that venting is not required, seek out a different contractor.
Lastly, it is against furnace manufacturers and GAMA tables codes if the existing furnace and water heater vent connector runs horizontally over twenty feet toward the masonry chimney. The solution is to install a 90%+ efficient furnace and a side-vented, power-vent hot water heater and cap off the furnace portion of the obsolete masonry chimney.