When you are having a gas or oil fired heating system serviced or installed, the very least you expect is a safe, efficient and reliable operation. The only way these three areas can be checked is by extracting a sample of the flue gasses and having the sample analyzed for Carbon Monoxide (CO) production and what is referred to as Steady State Efficiency (SSE).
SSE simply represents the proportion of heat lost up the stack as opposed to the amount being introduced into the heated area.
The SSE is calculated based on two variables, the amount of Oxygen (O2) remaining in the flue gasses after the combustion process and the temperature of the flue gas sample.
All gas and oil fired equipment is engineered and designed to operate within very specific ranges of flue gas oxygen and temperature.
The air we breath is about 21% O2. Depending on the design of the furnace, manufacturers' have very specific recommended O2 ranges.
The stack temperature is a reflection of how much heat is being removed from the flue gasses.
As most people are aware, Carbon Monoxide is a deadly gas. In addition, it represents unburned fuel being exhausted with the flue gasses which increases your fuel bill.
MAKE ABSOLUTELY CERTAIN that the heating company you use has the equipment and knowledge to take combustion and CO tests. Simply ask if they do combustion testing with equipment providing a continuous digital readout of Oxygen, Carbon Monoxide and Stack Temperature.
Obviously, when selecting a HVAC company, there are a lot of factors to consider. But at least a company which has invested in the test equipment is demonstrating its commitment to a safe and efficient heating system for customers. That's a pretty good start. It probably also means the company has invested in training personnel to keep up with the changes in the industry - and this industry has changed tremendously in the past several years.
By the way, if you find a contractor with the equipment, have him check your gas fired kitchen oven and range tops for carbon monoxide production. There's a good chance that you will find your furnace flue gasses (which are vented to the outside) are producing lower levels of CO than your oven (which is venting into your kitchen)!