Definition of Overfired

[AbbyNormal]

I see a lot of heated debates on the various forums involving the correct firing rate of natural gas appliances.


The residential and light commercial furnaces will have a label stating an input and a rated output. The heat content of natural gas can fluctuate from one region to another, as well as fluctuate in a region over a period of time.

Some organizations train combustion testing and analysis to heating technicians and make great strides monitoring CO at light off, trending the products of combustion while a furnace operates, but seem to tune furnaces based on adjusting gas pressures to force an output they can measure in the supply air, as well as to control the amount of 02 measured in the flue gas.

They seem to advocate that as long as what they do is with in the air temperature rise range of a furnace and CO levels are acceptable in their opinion then a furnace is not overfired and therefore clocking a meter, when it is right there, is a waste of time.

Some residential/light commercial appliances on the market today seem to be designed around more excess air than a power burner with air adjustment would work around, and higher airflows with lower temperature rises. Seems that they are having gas pressures adjusted to meet parameters they are not meant for and the air temperature rises getting pushed up to 70F or better, typically near the top end of a residential appliance range.

Measuring airflow always has an error and so does measuring temperature, so errors measurement errors can multiply out and compound the error. When there is a meter available I think it is still worth checking the volumetric fuel input.

The residential manifold pressures are a lot of times listed on the label as 3.5", and with that pressure set, the gas meter may spin too fast or too slow.

Some of the combustion training however seems to advocate setting pressures as high as 6" to get a measured output, without perhaps first checking the function of the gas valve or even the size of the orifices.

So is clocking a gas meter a waste of time when it is readily availble to do so?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[SeattlePioneer]

As a furnace repairman, my aim is to insure that equipment I repair does not exceed the BTU input rating specified by the manufacturer.

Clocking a gas meter often gives a reasonably good estimate of the BTU input if you get the BTU value of the gas per cubic foot from the gas utility.

But it's NATURAL gas. The heating value varies significantly from week to week. Around here, that goes from about 950 to 1100 BTUs per cubic foot of gas.

The utility suggests using a figure of 1040 BTUs/cubic foot as a reasonable average value.


A more accurate way to measure input is to use the burner manifold pressure and the burner orifice size to calculate the input. However, all burner orifice sizes aren't equal, and different orifice designs have different aerodynamic characteristic and allow different amounts of gas to pass depending on the design, even give the same nominal burner orifice size.

Often a gas appliance manufacturer has a technical services or engineering department that will give you the BTU input if you give them the model and serial number of their equipment, the orifice size, specific gravity of the gas and BTU content of the gas and the burner manifold pressure.

Given that information, they can often give you the burner manifold pressure that will give the rated input.

Of course, since the BTU content of natural gas varies quite a bit as described above, getting the rated input remains a crapshoot.



Seattle Pioneer