Flue Gas Temperatures Raise Concerns About PVC Venting

Combustion Test Results

TJ's Plumbing and Heating has recently spent some time servicing newly installed, high-efficiency boilers. As part of
the service, they always complete a full combustion test. At one particular service, they had a 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 due to the fact that Schedule 40 PVC pipe and fittings are only rated for 140 degrees Fahrenheit. In New England, it is possible that we may see PVC failures in the near future, directly related to the high flue gas temperatures of the new high efficiency boilers.

PVC Alternatives

TJ's Plumbing & Heating uses stainless steel, CPVC, or concentric venting kits, for flues, on the high efficiency Viessmann, Buderus, and Lochinvar boilers they 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. We would expect the boiler to downfire; but we don?t see this happening quickly enough. During the above scenario, we 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

There is a link on the TJ's Plumbing & Heating website blog to a PVC pressure and temperature relationship chart. It
clearly shows the maximum operating temperatures for the various piping systems.

Carbon Monoxide Safety

Always remember... for the protection of you and your loved ones, always have a carbon monoxide detector, hard-wired, installed in your boiler room.

TJ's Plumbing & Heating
(508) 226-5514
www.tjradiantheat.com