Q: I have a century-old home with copper and cast-iron drain pipes. I recently installed a high-efficiency furnace but learned the condensate may be corrosive to the drain pipes.
I am wondering what pH level can harm the drain pipes. Also should I filter the water though a material such as limestone to reduce the pH. Would this an effective solution for treating the water before going down the drain,. I currently have a condensate pump that pumps it to the laundry sink. Thanks for your time. — Adam
A: Water with a pH below 7 is generally considered acidic and below 6.5, water is corrosive to most materials. The EPA recommends that household water be between 6.5 and 8.5 for health reasons, not because of damage to pipes.
Condensate from a high-efficiency furnace typically has a pH of about 5.0, but can range from 3.5 to 6.5. It’s important to note that pH is a logarithmic scale, so pH 5 water is 10 times more acidic than pH6.
If the water sits in a trap and evaporates, the acidity and corrosive effect will increase. In older homes a whole-house trap might be buried in the floor and the main sewer line to the street is most likely cast iron, even if modern PVC drains have been installed in the house.
In a 100-year-old home, it’s likely that the drain pipes already have considerable corrosion, so you don’t want to add to the problem. Cast-iron pipes tend to corrode from the inside out, so what appears a small rust spot or rusted seam on the exterior can be a sign of imminent failure. In some cases you can push a screwdriver right through the pipe at the visible rust spot.
Cast iron is about 10 times more vulnerable to corrosion than copper, so corrosion from condensate can rapidly destroy cast-iron drain pipes. While corrosion of copper is slower, it still occurs over time. PVC is immune to corrosion from acidity, but some plastics such as ABS are vulnerable.
For example, a house I owned for several years had well water with a pH of 6.3. This caused blue stains on all plumbing fixtures from copper leached from the inside of the water-supply pipes. Eventually, acidic water will lead to pinhole leaks in the copper pipes. How many years this will take depends on a lot of factors and is difficult to predict.
Adding a condensate neutralizer to the drain line from the furnace is a good idea, and is required by some plumbing codes (but not always enforced). There are a number of commercial neutralizing kits on the market. Most are pretty simple inline filters with a neutralizing agent such as crushed limestone that needs to be replenished periodically. Just search the web for “condensate neutralizer kit” to see what’s available.
Manufacturers recommend that you test the pH of the filtered water every month for the first year to determine how often you need to add new filter media. The larger the filter, the less often it will need to be replenished.
For safety, it is important to plumb a bypass line in case the filter gets blocked. This can happen with some types of media when left unattended for too long. It can harden into a block. Also, all high-efficiency appliances have internal traps in the condensate line to prevent flue gas from escaping out the pipe. This should be checked during servicing.
In warm climates where high-efficiency gas appliances are less common, the condensate drain is often routed directly to the outdoors. Other than stains on the brick or siding, this usually works well unless a cold snap hits. Then the condensate line can freeze and shut down the furnace. People have tried a number of remedies – increasing the diameter of the drain, insulating the drain, or – if all else fails – adding an electric heat tape to the pipe which kicks on when the weather falls below freezing.
One other thing to consider in an older home is the effect on the chimney flue of switching to a side-vented furnace or boiler. In cold weather, the older, inefficient furnace or boiler kept the flue warm. This minimized condensation of corrosive exhaust gases on the inside walls of the flue.
If you have gas-fired water heater sharing the same flue, the cooler flue will be now subject to increased damage from corrosion. This is mainly a problem with older clay-tile or unlined brick flues. A bigger concern with oversized flues is backdrafting, particularly if you’ve weatherized the house. If you have a gas-fired water heater, check with an HVAC contractor to make sure the flue size is matched to the appliance.
— Steve Bliss, Editor & Publisher, BuidingAdvisor.com