Start up gas pressure check

[mike3]

Our standards call for clocking input on every new furnace and leaving them at rated input. Taking pressure without clocking doesn't gurantee input unless orifice is also sized. Am I missing something?

[Black Adder]

We always check gas pressure when servicing a furnace, about 50/50 on whether it needs adjustment or not. The limit switches will protect the system if the temp is too high but this is not how your unit should operate, if the the unit is cutting out on high limit you will have a very hard time heating your home as you will have the fan running and no heat from the burners. I have seen oversized furnaces that will not heat a house while a smaller furnace will with no problem because of cycling on the high limit. Temperature rise check is done to confirm that the furnace is running within manufacturer specs. As far as checking the gas pressure, it must be checked at the gas valve with the burner on. Takes less than 5 minutes to check and adjust.

[Jim Davis]

Originally posted by mike3
Our standards call for clocking input on every new furnace and leaving them at rated input. Taking pressure without clocking doesn't gurantee input unless orifice is also sized. Am I missing something?

Input is a theoretical value that is unatainable in the field. Under normal residential equipment operation the maximum usable BTU's produced from a fuel is about 90% of what it rated. 100 cubic foot of natural gas rated at 1000BTU per cubic foot can only produce 90,000BTU available to transfer. Of this 90,000BTU we may transfer 40%,50%,60%,70%,80%or 90%. This has been verified by actually measuring the BTU output.
How do you clock a meter on Propane or Oil? If clocking meters provided useful information wouldn't meters be on all fuels? I have found clocking a meter on gas provides about as much information as clocking an electric meter on air conditioning.

[mike3]

Jim-- I guess my question would ask if all the techs that proclaim they take pressure are also verifying orifice size?

[Black Adder]

Orifice size is one thing I take as a given that the manufacturer has done correctly. Sure you could pull the main gas bar off and check the numbers stamped on the orifice or check to see what size drill fits the hole, but where do you stop when checking a furnace. When I buy a car with a carburator I don't expect the mechanic to check the jet sizes in the carb.

[Jultzya]

I do check orifice sizes when installing a conversion kit. I have a couple of times found an orifice to be of the wrong size in the kits!

[Black Adder]

Granted you may find a wrong size orifice in a conversion kit but I don't think checking orifice sizing in a new furnace is part of the recommended manufacturer start up procedure. We use mainly American Standard furnaces and I believe they all use the same size orifice in them. The burners are rated in 20,000 btu increments and the bigger the furnace the more burners. Don't imagine they would have a lot of different size orifices around when they are assembling them at the factory.