Which equipment plan has lower lifecycle emissions?
Alder Climate Team helps a fictional school group compare two ventilation plans.
The question
Both plans provide the same ventilation service. Start with a 15-year study period and constant annual operating emissions. Other effects are equal under the opening assumptions.
| Retain and refurbish | Replace | |
|---|---|---|
| Upfront emissions (tCO₂e) | 100 | 300 |
| Annual operating emissions (tCO₂e) | 30 | 15 |
Count upfront and operating emissions over the same period.
“Compare the totals and explain which assumptions change the ranking.”
Not saved by this site. Your browser may restore text when you navigate back.
A strong answer, step by step
Open each step in turn. This is one strong answer, not the only one.
Step 1: What is the 15-year total?
What would you do here? Open the move you’d make.
Choose refurbishment using upfront emissions alone
Tempting, but it misses something. That leaves out 15 years of operating emissions.
Compare total upfront and operating emissions for both plans
The strong move. Each option delivers the same service and uses the same 15-year boundary.
Treat annual operating emissions as a one-time amount
Tempting, but it misses something. The equipment operates for each year in the study period.
See how a strong candidate says it
“Refurbishment totals 550 tCO₂e. Replacement totals 525 tCO₂e. Replacement is 25 lower over 15 years with the stated inputs.”
Why it works: The lower upfront option has higher operating emissions each year.
Step 2: When does the ranking change?
What would you do here? Open the move you’d make.
Divide the upfront difference by the annual difference
The strong move. This gives the time until lower operating emissions equal the extra upfront emissions.
Divide the new upfront total by the old annual total
Tempting, but it misses something. The decision depends on differences between the two plans.
Use the same ranking for any length of service
Tempting, but it misses something. The amount of annual savings depends on the service period.
See how a strong candidate says it
“Replacement adds 200 tCO₂e upfront and avoids 15 each year. The totals meet after 13⅓ years. Replacement has the lower total after that point under the stated assumptions.”
Why it works: The crossover is the upfront difference divided by the annual operating difference.
Step 3: What if the site needs ten years?
The school has no confirmed lease renewal and asks for a 10-year service comparison. All other inputs stay the same.
What would you do here? Open the move you’d make.
Compare the first ten years for each plan
The strong move. This uses the same service requirement for both plans.
Keep the 15-year comparison because it has more savings
Tempting, but it misses something. The school needs a decision for the confirmed ten-year period.
Compare ten years of refurbishment with 15 years of replacement
Tempting, but it misses something. Those totals describe different amounts of ventilation service.
See how a strong candidate says it
“At ten years refurbishment totals 400 tCO₂e and replacement totals 450. Refurbishment is 50 lower for the revised service period.”
Why it works: The confirmed period ends before replacement makes up its larger upfront amount.
Step 4: How does extra work affect the choice?
A condition survey finds that refurbishment needs more work in year seven. The estimate is 60 tCO₂e, with a range of 40–80. Replacement inputs are unchanged.
What would you do here? Open the move you’d make.
Keep refurbishment at 400 because the work occurs later
Tempting, but it misses something. Year seven is inside the ten-year study period.
Use 460 as proof that replacement is always lower
Tempting, but it misses something. If the extra work is only 40, refurbishment totals 440.
Report the range and test the 50-tonne decision threshold
The strong move. A better estimate of the later work can resolve the choice.
See how a strong candidate says it
“The central refurbishment total becomes 460 tCO₂e. Its range is 440–480. Replacement stays at 450. The plans tie if the extra work is 50, so I would confirm the work before deciding.”
Why it works: The additional work occurs inside the study period. Its uncertainty can reverse the ranking.
The answer, in short
- Decision
- Compare equal service over the same time and include known later work.
- Because
- The difference between upfront and annual emissions can change the ranking.
- What would change it
- A narrower estimate of the extra refurbishment work could settle the comparison.
Common traps
- Use the same period and service level.
- Include later work inside the chosen period.
Check your answer
- I added upfront and operating emissions.
- I calculated the crossover.
- I used the revised service period.
- I stated the threshold for the extra work.
Last reviewed Nov 11, 2026