Facts about condensing boilers.
Boiler efficiency during operation can vary dramatically. Several factors can impact the true efficiency of the boiler. Here is a brief list.
- Oversized boiler
- Water temperature
- Near boiler pipe sizing
- Total number of heating zones.
- Circulator sizing
- No Outdoor Reset
Over-sizing is a widespread problem with high-efficiency boilers. This causes the boiler to short-cycle due to a significantly reduced water volume and the loss of the cast-iron. 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
Although it is a common occurrence in many installations, it is not a typical operational practice. A properly sized boiler should operate if a thermostat is calling for heat. We realize that occasionally a boiler will cycle off during a demand on a properly sized boiler, but this is not often the case.
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 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 residential existing boilers are 70% to 100% oversized.
6. It is best to measure the radiation in the home to size the new boiler properly.
Measuring the radiation is a good practice when replacing the boiler, but it is no help in sizing the new boiler. Most homes have excess radiation due to the way it was installed decades ago or 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, 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 the dew point to be efficient.
While this is true, it is only relevant when we are talking about maximizing 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 130°F water temperature, there is plenty of fuel savings in non-condensing mode at water temperatures above 130°F.
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 negates the need for outdoor reset, instead analyzing run times and water temperature from 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 their modulation capability. The older cast iron boilers fire at 100% efficiency at all times they are running. The high-efficiency boilers will modulate back, reducing the fuel input. The amount of turndown is mostly 5% to 10%. Modulation can be as high as 90%. You may see higher turndowns shortly.
12. Boilers are easily zoned 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 website is for informational purposes only. All preventive maintenance, service, and installations should be reviewed on a per-job basis. 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 website.