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Posted by jsf on January 07, 2006 at 17:45:18:

In Reply to: Re: BOILER RUNNING posted by Heatpro on December 27, 2005 at 18:32:26:

Competent designers select the boiler capacity so that the boiler will run almost continually at the design load (heat loss in Btu’s per hour).

Design loads are calculate the heat loss of a structure given the following variables- outdoor design temperature (varies by location), indoor design temperature (typically 70F) and the effective R-value of the structure (resistance to heat transfer, varies by construction type, insulation levels, # type and size of windows etc.) of the structure.

The same home in different locales around the US will require different sized heating systems due to varying design temperatures.

Local outdoor design temperatures are based on 30 yr weather data and vary by location.
The design values calculated representing 2.5% of the heating season temps. Design temperatures can be found in the ASHRAE Fundamentals Handbook. Examples of approx. design temperatures:

Denver, CO 1.4 degree Fahrenheit
Washington, DC 18 degree Fahrenheit
Anchorage, AK -35 degree Fahrenheit
Charlotte, NC 22 degree Fahrenheit
Tampa, FA 40 degree Fahrenheit

Design temperatures are not the absolute coldest day of the year, but a “safe” average, which allow designers to size heating systems which can maintain the design indoor temperature (typically 70F or 21C) most of the winter. In extreme cold winters there may be several days of the year where the heating system will run continually and never be able to heat the home to the design temp of 70F, but would still be able to maintain an acceptable 68F.

This system allows for the highest energy efficiency without sacrificing overall seasonal comfort.

What all this means is that anytime the outdoor temperature is less than the design temperature the boiler will run less than 100% of the time. How much less depends on the how much less the outdoor temp is below the design temp.

Given the variables of the outdoor design temp and the fact that many systems are oversized makes any attempt to guess how much your system should be running pointless and even a little irresponsible.

A better question to ask would be how large should my boiler be? Invest about $100 bucks (you may even get this for free) to have a professional heat loss calculation performed on your home and compare that figure with the boiler output. The boiler output should be between 100% -140% of the heatloss of your home.

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