When you want to achieve the highest energy efficiency possible, HVAC equipment must be tuned properly, gas pressure and input most likely need to be increased. It’s also important to make sure the necessary airflow is available before any adjustments are made. But the question is, how can you easily calculate or estimate the potential energy savings for your customer?
It is hard to calculate actual dollars a person can save with new equipment or a proper tune-up because prices of fuel and weather conditions are unpredictable. However, we can calculate a percentage of savings or reduction in fuel usage.
There are several methods that can accomplish this. In this article, we’ll address two of them:
- Modified HeatMaxx
- O2 and Flue Temperature Comparison.
The energy savings formulas that follow are based on the assumption that it takes one cubic foot of gas to create 1,000 Btus of heat.
Modified HeatMaxx for Calculating Energy Savings
Before adjustments: Use temperatures at the furnace (two supply runs and return temperature) versus taking temperatures at the registers, then use fan airflow plotted using the measured total external static pressure and fan performance tables.
Then use this formula:
After adjustments, us this formula:
Using this method we can calculate either a reduction of usage or a percent of change.
For example, before adjustments:
Quick math shows this is at least a 41% increase in efficiency. If a customer’s annual heating bill is $1,500 then you can approximate a $615 energy savings (1,500 × .41).
Remember, this is a quick estimate for technicians, not a sophisticated computer program like “ComfortMaxx™” which involves additional data and calculations.
It is also effective to convert this to usage and explain it in a different way. For example, 43% means you are getting 430 Btus for every 1,000 Btus you’re paying for.
At 84%, you are getting 840 Btus for every 1,000 Btus you pay for.
Most utilities have records (or you can find the information on the internet) showing the average number of Btu’s used in each section of the country. This is often broken down by loose house, average leakage rate, and/or Energy Star ratings. Next you can look up your local fuel rates. Then you can come up with an energy savings dollar figure. This can be broken down per 1,000 sq. ft. and fuel to fuel. On oil and propane you can convert to gallons.
Again these are approximate calculations and are meant to be conservative.
Another way to calculate percentage of savings is using the combustion analyzer and comparing O2 and flue temperature readings. Because most equipment is underfired, these are the numbers that will be used.
O2 and Flue Temperature Comparison
- Creating a 1% decrease in O2 by increasing fuel = 2% efficiency increase
- Creating a 30° increase in flue temperature by increasing fuel = 2% efficiency increase (natural gas, induced draft, and natural draft equipment)
- Creating a 35° increase in flue temperature by increasing fuel = 2% efficiency increase (propane-fired equipment)
- Creating a 40° increase in flue temperature by increasing fuel = 2% efficiency increase (oil-fired equipment)
- Creating a 5° increase in flue temperature by increasing fuel = 2% efficiency increase (condensing equipment)
Example: Induced Draft Furnace
|Starting:||02 = 12%||Final:||02 = 8%|
|Flue = 240°||Flue = 330°|
O2 = (Starting O2 – Final O2) × 2
O2= (12% – 8%) = 4% × 2 = 8%
So this gives us 6% + 8% = 14% increase in Flue temperature.
To take this a step further we know that the maximum efficiency of this furnace is 72%. Now divide 14% by 72% and you will see an overall 19% increase in efficiency.
Example: Condensing Furnace
|Starting:||02 = 11%||Final:||02 = 6%|
|Flue = 95°||Flue = 115°|
O2 = (Starting O2 – Final O2) × 2
O2= (11% – 6%) = 5% × 2 = 10%
This gives us 10% + 8% = 18% increase in flue temperature.
Maximum efficiency of a condensing furnace is 88%. Therefore 18% divided by 88% gives us a 20% increase in efficiency.
We say increase in efficiency which, in turn, means reduction in usage.
- A 19% increase in efficiency means that to deliver the same amount of heat that used 100 cu.ft. of gas will now be accomplished with 81 cu.ft. of gas.
- A 20% increase in efficiency means that to deliver the same amount of heat that used 100 cu.ft. of gas will now be done using 80 cu.ft. of gas.
The O2 and Flue temperature method is the simplest way to compute savings and it is also the most conservative, so no one will be disappointed.
If a customer knows their annual fuel costs, then an approximate dollar amount savings can be figured. This method has been used for almost 30 years and has never under-delivered.
Both methods are explained in more depth as part of the materials NCI provides attendees of the Combustion Performance & Carbon Monoxide Safety Certification class. If you are interested in learning more about this class, click here.
Jim Davis is the senior trainer for NCI. His HVAC career began in 1971 when he went to work for an HVAC contractor which led him into sales, and he became involved in selling digital combustion analyzers. Today he is recognized as one of the foremost authorities on combustion and one of it’s unfortunate side-effects — carbon monoxide. To learn more about Jim, visit the NCI Website.