This is very interesting. You mentioned it was a vacant house. Is it possible that what you encountered was Hydrogen Sulfide buildup in the water heater and that it interfered with your carbon monoxide detector?
You describe hypoxia as your symptoms and I am wondering if this was due to high Hydrogen Sulfide levels.
Below is an article on sensor technology and anomalous CO readings due to Hydrogen Sulfide.
I would contact the manufacturer of you CO detector and find out if their sensor is subject to this error.
** Toxic Sensors
- Electrochemical (Wet Chem) Toxic Sensors
** These sensors react to a specific chemical (substance). Chemically specific sensors are available for chlorine, ammonia, carbon monoxide, carbon dioxide, nitrogen dioxide, nitric oxide, hydrogen cyanide, hydrogen sulfide, sulfur dioxide and hydrochloric acid. The manufacturer’s technical information will indicate what sensors are available for their unit. ( Figure 3).
**The electrochemical sensor housing contains two (and sometimes three) electrodes sitting in a liquid solution (either a base or alkali, depending on what the sensor is looking for). The housing is covered by a Teflon membrane that keeps the fluid in the housing yet allows air in. As air molecules enter through the thin Teflon membrane, the fluid will react with a specific substance if found. When the detector is working a small current passes between the two electrodes. Any change in the fluid’s density caused by a reaction to the substance in the air will affect the density of the fluid and change the amount of current passing between the two electrodes. The current then passes through a temperature compensating circuit. The electron flow is then read as a specific amount of the substance. The manufacturer creates a Wet Chem Sensor’s ability to detect specific types of gases based on the choice of membrane, the number of electrodes, the alloy of the electrodes, the alloy of the electrode (gold, lead, etc.) and type of electrolyte fluid.
These sensors have very good linearity which makes them very accurate for the substance they will react to. They can measure either large or small quantities and these sensors have a typical life span of approximately 1 year.
As with all sensors, Wet Chem sensors have their limitations. The fluid can freeze when left in environments having temperatures lower than 0 degrees C. They are also adversely affected by altitude. Air pressure at sea level (14.73 psi absolute) is the force required to induce the air into the sensor. As one rises in altitude, the less force is available to push the air into the sensor, thus reducing the accuracy of the reading. Some substances, (e.g. moisture) affect the sensor by changing the make up of the fluid, thus reducing the amount of electrical resistance which impacts the reading. Check the manufacturer’s instructions to see which substances will affect the sensor.
Abnormal readings are another issue with regards to Wet Chem sensors. Abnormal readings are generally readings that don’t make sense. For instance you are working in a sanitary sewer and your instrument is showing a CO reading of 300 PPM (current TWA in Ontario is 35 PPM) and a low reading (below the TWA of 10 PPM) of hydrogen sulfide. What you likely have is an interference from the hydrogen sulfide. Some electrochemical carbon monoxide sensors are subject to interference from low levels of hydrogen sulfide. The knowledge that carbon monoxide is not a common occurrence in sanitary sewer applications (whereas hydrogen sulfide is) would lead you to consider that you are probably having an interference problem.
Awareness of the hazards in your workplace, some basic understanding of chemistry, knowing what interferent gases adversely affect your unit and strict testing protocols will minimize this problem