Dear CC. There are 2 ways to look at this issue. If you want to do your part to prevent climate change, you should change the boilers. They will reduce your CO2 output by at least 30%. If economic considerations are foremost, do the following:

• Add up the gas consumption for the boilers. If there are separate boilers for domestic hot water, take the consumption for July, August, and September (the summer months, because the invoices will be for the month before the invoice date). 
• Multiply that figure by 12/3, or 4, to get the amount of gas consumed to heat domestic hot water. Subtract that from the total gas consumption to get the consumption for heating only. If there is makeup air that is heated by separate gas-fired makeup air units, deduct 20% from the gas consumption for heating. 
• The result is a rough measure of how much gas your boilers use for heating.  Assume that higher efficiency boilers will have 15% of the gas consumed for space heating. Take this number and multiply it by your current cost of natural gas. Make sure that all of the charges are included – delivery, storage, etc.  It should be around $0.30 per cubic meter (M3). 
• Multiply the amount of gas that you estimate you will save in M3 by the cost of gas in $ per M3 to get your savings in $. Divide the cost of replacing your boilers by the savings in $ to get the simple payback. If this amount seems financially attractive – it will be worth while having an engineer do a more detailed analysis. 

It use to be that building managers were looking for a simple payback of 5 years or under. However, many will accept a longer payback now because the interest rates are so low. For example, an 8 year simple payback is equivalent to an interest rate of 12.5%. When you can borrow money at less than 6%, and when your money is earning peanuts in the bank, this looks like a good investment.