combustion test question

Generally

yes. No change would be acceptable. However if the blower is depressurizing the room you might see a change.

All

The best

Robert

Some \"intersting\" returns

Here are a few pics of some returns we have seen.

Some done by the HO and some by "pros".

Enjoy!

Mark H

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What brand of furnace ??

Does it have a clam shell HX ?

David

> Thank you that was the info I was looking for.

The age is about right.

On Serpentine HX from ICP I am finding the rings that hold the two halves together are falling out. These rings are located in the center of the sections. When they fail they allow air to infiltrate into the HX rather then out. If enough rings fail heat will roll out the front of the burners. The presssure switch will not shut off the furnace in this type of failure.

I do not know if you have an ICP HX in your furnace. But it is worth looking at.

David

> Concord 90% approx 6years old. Not sure if the

> primary is clamshell.

No Doubt about it.
When I first moved from NJ to Upstate NY I was like a fish out of water. 95% of all heating in this area was scorched air. To see someone heat their home with nothing but wood was an even bigger shock.

David


> this would never happen with hot-water or steam

> heat.

>

> _A

David

If what you say about the rings falling apart between HX's I should be able to turn on the indoor fan, drill a 1/4 hole in the pvc vent pipe, insert my Mag gage probe and find pressure?. I may have to block the flue and fresh air pipe in order to have a reading if its a small leak but I should find pressure. Then by turning the fan off I should lose pressure correct. Thanks J.Lockard

O2 testing heat exchangers

If a furnace passes the ventilation air test as outlined in the International Fuel and Gas Code (Appendix D) the O2 test will produce valid results and depressurization due to duct leakage or other venting problems will not effect the results.
The O2 test has several advantages over the trace gas test including testing during normal operation under normal temperatures. Leaks below the burner will be evident. Multiple tests can be performed at once including combustion air testing, heat exchanger testing, CO testing, and efficiency testing. A combustion analyzer can be used to determine unacceptable leakage, atmospheric draft appliances and draft-induced appliances using O2 as the trace gas. The O2 will be below 21% and stable during normal operation. When performing this test, it should be noted that as of this writing, there has been no formal field study to document the correlation of minimum leakage rates and O2 changes. However, during lab testing, we were able to determine leakage through a single 1/8” test hole drilled in several locations in the heat exchanger. After each test the hole was plugged with a screw and an additional location was tested. Multiples of holes were also tested to simulate pin holing in a heat exchanger. Locations were specifically selected that avoided air blowing directly in the heat exchanger, and rather depend upon system static pressure to force additional air from the supply side into the heat exchanger. If the O2 changes when the blower starts, there is leakage into the heat exchanger, and further investigation is warranted. Closing main dampers will increase supply side static and further amplify the leakage rate. If cracks or holes cannot be found, the customer should still sign off in writing that they have been notified there is a potential problem with the operation of their furnace.

PERFORMING the O2 TEST
A flue gas combustion analyzer can be used effectively for finding leaking cracks or holes in a furnace heat exchanger. Not all cracks or holes will leak. Smaller cracks and holes found only by a thorough visual inspection may not be leaking during the time of testing. They still pose an potential danger to your customer, as cracks will continue to open over time due to the tremendous forces on the metal at the ends of the crack.

On all furnaces the static pressure achieved by the system blower can overcome any positive pressure in an atmospheric draft appliance. On draft-induced appliances, the pressure within the heat exchanger is always negative, causing them to leak in rather than out. Any leakage in a heat exchanger in or out poses a danger to your customer. Leaks out can allow flue gasses that may contain CO into the living space, and pressurization of a draft induces appliance heat exchanger can result in a rollout and possible fire. With all combustion, the readings on the analyzer should be stable after several minutes. When the stack temperature stabilizes, all other gas readings on the analyzer should also remain stable. Readings that change during operation after stabilization has taken place are indicative of a combustion air, venting, or mechanical problem such as a cracked heat exchanger.

Important notes:

1. Oil furnaces and older gas appliances can have cleanouts leaking that will test positive for leakage. This is not a heat exchanger failure. Inspection gaskets should be replaced and properly sealed following the manufacturer’s recommendations. If the manufacturer’s recommendations are not available, an industry-approved method should be used to seal the cleanout.

2. No inspection method is “fool proof.” The three-part method should always be performed to maximize the safety of the appliance.

PROCEDURE:

1. Follow the manufacturer’s instructions to properly zero the combustion analyzer.

2. Insert the combustion analyzer in the appropriate test position in the furnace.
a. For atmospheric draft appliances this would be directly in the heat exchanger cell.
b. For 80+/90+ furnaces this would be in the stack.

3. Start the furnace and observe the oxygen reading for stability. (1-3 minutes)

4. When the blower starts, watch for a change in the O2 reading. If the blower starts prior to stabilization of the O2 reading, a piece of cardboard can be inserted and removed during operation to observe if any changes in the combustion readings take place.

Corrective action:
Attempt to visually find the crack or hole.
A. If you can find the defect, show it to the customer.
B. On the service invoice, write that your testing indicates a leak in the heat exchanger. (Do this even if you cannot find the leak.)
C. Inform the customer, in writing, that the heat exchanger has a defect and poses a potential danger. (Do this even if you cannot find the leak.)

Hope this helps
Jim Bergmann
HVAC/R technical Specilist