November 16, 2011 Flue Gas Temperatures and PVC Venting – Cause for Concern?

Combustion Test Results

Over the last few weeks, we have spent some time servicing newly installed, high-efficiency boilers. In one particular instance, I was personally servicing a 96% condensing gas boiler. As part of my service, I always complete a full combustion test. I had my Testo high-end combustion analyzer in the venting system. The boiler was running at 100% firing rate and the flue gas temperatures were 167 degrees Fahrenheit. This particular boiler used PVC pipe and fittings to vent the combustion gas outside.

PVC Failure on the Horizon?

The high temperature of flue gases is a major concern to me due to the fact that Schedule 40 PVC pipe and fittings are only rated for 140 degrees Fahrenheit. I think we, in New England, are going to see PVC failures in the near future, directly related to the high flue gas temperatures of the new high-efficiency boilers.

PVC Alternatives

Our company uses stainless steel, CPVC, or concentric venting kits, for flues, on the high-efficiency Viessmann, Buderus, and Lochinvar boilers we install. Viessmann specifies stainless or CPVC only – no other material – for flues.

Why do High-Efficiency Boilers Cause High-Temperature Flue Gases?

Let’s use the following example…
A boiler is calling for it’s hottest water, such as a domestic hot water need. The boiler ramps to high-fire and, in seconds, is producing 180 degree water. The domestic hot water tank is sitting at, let’s say, 100 degrees. The boiler water is being pushed through the fairly small coil at a flow rate high enough to accommodate the boiler flow rates and the tank loss. The tank can’t absorb all of the BTUs the boiler is pushing out. The remaining BTUs exit through the flue, causing the rise in temperature. The boiler is designed to run at 100% capacity when you get a DHW call. I would expect the boiler to downfire; but I don’t see this happening quickly enough. During the above scenario, I can quickly see a 20 degree, or less, temperature difference between the supply and return piping. This is bad news for the flue gas temperatures. The closer the supply temperature to the return temperature, the higher the flue gas temperatures.

PVC Pressure and Temperature Relationship

Please reference the attached PVC pressure and temperature relationship chart. It clearly shows the maximum operating temperatures for the various piping systems.