While it is commonplace for property inspectors to note the electrical service size and often the size of the main water supply, the size of the gas meter is frequently overlooked. If a gas meter is undersized, the attached gas appliances could be starved for gas – especially when the major appliances are running at the same time.
When a gas meter is installed by your gas utility provider (PG&E in our area), they size the meter based upon the total capacity of the gas appliances installed in the building when the meter is installed. The meter label (see below) will indicate the rated capacity of the meter in cubic feet per hour (cf/h), and out in the field, we typically see meter sizes ranging from 175 cf/h to 275 cf/h.
Meter Sizing – Rated Capacity vs. Maximum Continous Capacity
Unfortunately, it’s not quite that easy. After consulting with Armstead Ward of PG&E’s gas division, it turns out that a meter can provide more gas than its rated capacity. For example, a meter that is rated for 275 cf/h, can actually provide 385 cf/h of “maximum continuous capacity.” This is based on laboratory testing of each gas meter, and unfortunately there is no standard formula to determine maximum continuous capacity vs. rated capacity. Mr. Ward recommended adding 15% to the rated capacity as a conservative estimate for the maximum continuous capacity.
Determine your Demand
To determine if your gas meter is undersized, you first have to know the total gas demand for your building. The easiest way to do this is to add up the “British thermal units/hour” (Btu/h) ratings of all the gas appliances. Each appliance will have a date plate with this information, though some can be hard to find. Example building: furnace (80,000 Btu/h) + water heater (40,000 Btu/h) + kitchen range (50,000 Btu/h) + dryer (30,000 Btu/h) = 200,000 Btu/h total demand.
Size vs. Demand
The Btu/h per cubic foot of gas can vary, but PG&E uses an average of 1,000 Btu/h for every cf/h of gas. Thankfully, this makes them math pretty easy! If you have a meter with a maximum continuous capacity of 250 cf/h, it can provide a maximum of 250,000 Btu/h. As long as the continuous meter capacity is larger than the demand, everything is fine. For the example building above, the total demand of 200,000 Btu/h is smaller than the meter capacity 250,000 Btu/h and the system should perform as designed.
What can go wrong?
Adding a new gas appliances or upgrading gas appliances (think a kitchen remodel with a new commercial style range) can upset this balance. Just the act of switching to a modern tankless water heater can increase demand by 160,000 Btu/h or more. When gas appliances are starved for gas, they will not function at maximum capacity, and will be prone to soot buildup.
What can be done?
Basically, it’s time to call your gas utility provider. They can come out and switch your gas meter with a larger size. After consulting with PG&E, they confirmed that they would upgrade the meter size at no cost to the customer. Time to calculate your total gas demand and then see what size gas meter you have. Let us know what you find and leave a comment below.
Note: this post was updated on 8-1-16. The original post did not clarify between the rated capacity and the maximum continuous capacity.