Clocking Boilers/Furnaces Vs. Combustion Analysis

You must have listened to the HVAC School podcast on furnace commissioning with Jim Bergmann. HVAC School is an excellent podcast and there was even an episode featuring Dan Holohan himself a while back!

Thanks for asking this question regarding clocking meters in commercial applications, it is something I was wondering about myself.

I work in a commercial setting as well. I always clock meters when setting up power burners or when the manufacturers literature instructs me to do so (I’ve seen some furnaces, power burners, some atmospheric boilers). This can be a real pain in a commercial setting. I have never clocked meters when dealing with RTUs, but this podcast for me thinking that it would be best practice to always clock the meter when starting up new equipment.

To summarize, the process recommended by Mr. Bergmann;
1. clocking the meter to measure calculate input BTU/h to the appliance
2. make fine adjustments to gas pressure to achieve proper input if necessary (3.2”-3.8”)
3. if this is not possible, change the orifices as necessary to achieve proper input
4. perform combustion analysis

This process is being recommended for residential furnaces. Jim points out that under-fired equipment will not perform at the advertised efficiency level.

Does anyone have or use their own portable meter? I suppose you could install on temporarily in-line if it were impractical to shut down every other gas-fired appliance in a building. I’ve heard this may also be more of a thing for propane guys since there usually aren’t meters on propane tanks.

On most mod-con boilers, the startup instructions give an acceptable range for inlet gas pressure and give acceptable O2 and CO ranges (CO2 as well, even though most analyzers are calculating this number based on O2 measurement). Does anyone here regularly clock meters when dealing with mod-cons?

I do commercial and industrial. One day I might be working on a tiny power burner or atmospheric, the next day I might be working on a 1,500 hp steam boiler for a plant, and the day after that I might be working on a heat recovery steam generator in a cogeneration plant.

I like to clock as much as I can, but it isn’t always practical to do. If it’s a larger burner, I will do whatever necessary to clock it. If the building gas and boiler gas all come off the same meter, I’ll either try to shut gas off to the rest of the building if possible, or clock what the building is using with the boilers off, then subtract that from the boiler clocking. I’d say where I’m at, 90% of the boilers I work on have their own meter, although it’s usually hidden away somewhere in the building and may require a second person to get it done.

The other thing that can make it complicated is in most commercial/industrial settings you have different incoming gas pressures and temperatures, which both need to be accounted for. Luckily some gas meters already do this for you, but not all of them. Some have a spot you can wave a magnet to see what the pressure and temp is. For example, most of Washington DC has 20psi supply gas to buildings. If you were to clock the meter without accounting for that supply pressure, your clocking will be way off.

For smaller burners (mostly on/off) I generally go off of manifold pressure. Once furnace pressure is correct, and draft is correct, you will be pretty spot on with input if you set the manifold pressure properly as the nameplate says.

When going by manifold pressure you must know if the manufacturer is providing that number as gross or net. Gross refers to the total pressure you measure. If they want a gross manifold pressure of 2.5 then you will be looking for 2.5. Net means pressure in addition to. If you have 0.5” of backpressure on the manifold, either from the blower or the furnace pressure, you would add that 2.5” manifold pressure on top of the back pressure. Can you see why clocking is so important? If furnace pressure and draft change, so could your input.

I would try to clock as much as you can. Once you do it more often it becomes less of a chore.

In 11 years I’ve never worked on a commercial burner/boiler that did not list O2/CO/CO numbers. You can’t tell input off of O2 readings. Could be running at half input but still have correct O2 reading. But yes, if specified manifold pressure is pretty close along with stack temp (at operating temp) is in range you’re probably in the ballpark for input.

At some risk of pointing out the obvious... you're talking apples and oranges here, and it isn't the one vs. the other, but two completely different things.

Clocking the burner -- or obtaining the fuel flow rate in some other way -- will tell you one thing, and one thing only: your gross BTU input. This may be a nice number to know, but...

Doing a combustion analysis, together with stack temperature, however, will tell you how close you are to having a proper air/fuel ratio, and thus how well the burner is adjusted and how clean and to a large extent how efficiently it is running. Not the same thing at all.