Facts about condensing boilers.

1. Is a condensing boiler always running at 95% or rated efficiency when running?
Boiler efficiency during operation can vary dramatically. There are many things that can affect the true efficiency of the boiler. Here is a short list.
  • Oversized boiler
  • Water temperature
  • Near boiler pipe sizing
  • Total number of heating zones.
  • Circulator sizing
  • No Outdoor Reset
2. Sizing of a mod/con boiler is not as critical as it was for a cast iron boiler.
Over sizing is a very common problem with high efficiency boilers. This causes the boiler to short cycle due to much less water volume and the loss of thermal mass such as cast iron. The boiler heats up to fast and can't turn down fast enough and just keeps turning on and off frequently.
3. Frequent starts and stops are normal for high efficiency boilers
Although it is normal in many installations it is not a normal operation. A properly sized boiler should run as long as a thermostat is demanding heat. We realize occasionally a boiler will cycle off during a demand on a properly sized boiler but not often.
4. I hear many boilers are oversized. Is this true?
Most older boilers were oversized when they were originally installed. Go forward from that time twenty to thirty years and they may be even oversized. In today's world a high percentage of home have been made more efficient by upgrading windows, doors and insulation.
5. Most boilers are replaced with the same size boiler as the existing boiler.
This is true, but it is believed that most existing boilers and many newly installed boilers are 50% to 100% oversized. 6. It is best to measure the radiation in the home to properly size the new boiler.
Measuring the radiation is a good practice when replacing the boiler but is no help in sizing the new boiler. Most homes have excess radiation due to the way it was done decades ago or tightening up of the home.
7. All hot water boilers should be sized by a Manual J heat loss calculation.
Absolutely, emphatically yes. This is the only way any heating appliance, not just boilers, or air conditioning system should be sized.
8. Radiation should be measured when sizing a new boiler.
Yes, it should. Once the heat loss for the building is known, the amount of radiation allows us to calculate the proper water temperature to send to the heating system at any given outdoor air temperature.
9. Condensing boilers need to be running below dew point to be efficient.

While this is true if we are talking about maximizing efficiency. But let put efficiency aside a minute and discuss a more important 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 use an algorithm that will negate the use of Outdoor reset and look at run times and water temperature on previous runs. Some high efficiency boilers are installed in domestic hot water production only systems which would 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 the modulation capability of the boilers. The older cast iron boilers fire at 100% any time they are operating. The high efficiency boilers will modulate back reducing the fuel input . The amount of turndown mostly is 5% to 10%. And modulation can be upto 90%. You may see higher turndowns in the near future. 12. Boilers are easy to zone with separate thermostats.
While this is correct, we would want to keep the radiation output above the minimum input of the boiler to avoid short cycling. If this is not possible other measures should be taken to help avoid short cycling of the boiler.

Disclaimer: The information found on this web site is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this web site.