2. Sizing of a mod/con boiler maybe More critical then it was for a cast-iron boiler.
Over-sizing is a widespread problem with high-efficiency boilers. This causes the boiler to short-cycle due to a significantly reduced water volume. The boiler heats up too quickly and can't reduce the fan speed (gas input) fast enough, resulting in frequent on-off cycles.

3. Frequent starts and stops are normal for high efficiency boilers
A propely size modulating boiler shoul opeate the entire time the thermostat is demanding heat. if it sies not either the boiler is oversizd or thee are microzones dmnding heat.
Here are some options for a short cycling boiler.

• Have a Manual-J heat loss performed to detemine the correct boiler size
• If a manual J heat loss was performed calculate the maximum water temperature needed
• Could add a buffer tank for more water volume
• Reduce the maximum fan RPM's to match the Manual-J heat loss.

4. I hear many boilers are oversized. Is this true?
Most older boilers were oversized when installed. Go forward from that time twenty to thirty years, and they may be even more oversized. In today's world, a high percentage of homes have been made more efficient by upgrading windows, doors, and insulation.

5. Most boilers are replaced with a boiler of the same size as the existing one.
It is proven that most existing residential boilers are 70% to 100% oversized.

6. It is best to measure the radiation in the home to properly size the new boiler?
Measuring radiation is a good practice when replacing the boiler, but it is not helpful in sizing the new boiler. Most homes have excess radiation due to how it was installed decades ago or to the tightening of the home.

7. Should a Manual J heat-loss calculation be used to size all replacement hot-water boilers?
Emphatically yes. This is the only way any heating appliance, not just boilers but also furnaces, heat-pumps, mini-splits, or air conditioning systems should be sized.

8.Radiation should be measured when sizing a new boiler.
Yes, it should. Once the building's heat loss is known, the amount of radiation allows us to calculate the proper water temperature to send to the heating system for any given outdoor air temperature.

9. Condensing boilers must operate below the dew point to be efficient.
While this is true, it is only relevant when discussing the maximization of efficiency. But let's set efficiency aside for a moment and discuss a more crucial point: fuel savings. While in condensing mode in a mod/con boiler, which is less than 130F water temperature, there is plenty of fuel savings in non-condensing mode at water temperatures above 130F.

10. All high efficiency boilers should use ODR
Yes, on most installations. Lowering the water temperature, as allowed, reduces fuel consumption and increases efficiency. Some newer-style boilers utilize an algorithm that eliminates the need for outdoor reset, instead analyzing run times and water temperatures from previous runs. Some high-efficiency boilers are installed in domestic hot water production-only systems that do not use outdoor reset.

11. High-efficiency boilers are only good for radiant floor heating systems.
While many contractors believe this, it is inaccurate. High-efficiency boilers are good in any application due to their modulation capability. The older cast iron boilers always fire at 100% input anytime they are running. The high-efficiency boilers will modulate back, reducing the fuel input. The amount of turndown is mostly 5% to 15% of full input.

12. Boilers are easily zoned with separate thermostats.
While this is correct, we would want to keep the radiation output above the boiler's minimum input to avoid short cycling. If this is not possible, other measures should be taken to help avoid short cycling of the boiler.