Welcome to Preston's Guide

If the Preston’s Energy Costs Comparator doesn’t show any energy savings over the existing unit…you’ve probably replaced it with the same size furnace!

And, why would you want to do that?

Here’s how you need to size up the situation.

Furnaces are rated by a.f.u.e. or “annual fuel utilization efficiency” the result of the amount of heat energy benefit coming out of the heating device (keeping the homeowner warm) divided by the amount of heat energy that went into the heating device (the amount of dollars per “therm*” the homeowner pays the utility). The difference of this number to 100 is the heat loss going up the chimney, i.e.: you paid for it but didn’t get it.

So, how do we save the customer energy costs when replacing their old furnace.

By installing a smaller, higher efficiency furnace you will no doubt save the homeowner installation costs and certainly save the homeowner energy cost savings for years and years to come.

A smaller furnace?

When you size a furnace your goal is for it to provide sufficient heat output to keep the residents comfortable and since the heat output (benefit) is what matters you’ll work backwards to size the replacement. For example:

Existing furnace A has an a.f.u.e. of 71% and is rated at 64,000 btu/h output. That means that furnace A has an input capacity of 90,000 btu/h or 9/10ths of a therm per hour [64,000 / .71 = 90,000]. Furnace A is replaced with xyz manufacturer’s furnace B that will be selected from their furnace line that have a.f.u.e. ratings of around 90%. NO. WE DON”T REPLACE A WITH ANOTHER 90,000 BTU/H INPUT FURNACE!

We want to size furnace B to perform the same heating task as furnace A did but we want to do it as economically as we can. So, we can size the replacement to a lower input capacity size furnace and it will do the job because its greater efficiency makes it capable of performing the same heating task [64,000 / .91 = 70,330 btu/h]. Therefore, replacement furnace B is approximately 22% smaller (and therefore less costly to purchase) and its doing the same job but lowering the homeowners energy costs by about that same 22%.

• Therm is a unit of fuel (can be natural gas, fuel oil, electricity, propane, etc.) with a heat content of 100,000 Btus. Your local natural gas utility will typically list somewhere on your bill a factor of a few points over or under a cubic foot of gas as having a therm of energy content. You actually pay for the 100,000 Btu increments of heat content and a cubic foot is generally a close match for expressing it in a volume consumers can understand.

Of course we all know that old furnaces just don’t keep on giving the same output/efficiency that they may have done when relatively new. Although there’s no way to set a figure on such a variable we know this deterioration in performance is fact. It’s been said that this figure could be anywhere from a half percent per year to as low as a tenth of a percent per year. That means a twenty year old 71% a.f.u.e. furnace could really be a 69% to 61% a.f.u.e. performer. We’ll leave it up to the individual contractor as to what he or she would consider to be a reasonable appraisal.

A NOTE ABOUT DOE HEATING ZONE MAPS

These are maps published by the US Government’s Department of Energy divided into temperature zones with numbers in each zone representing the average number of hours a heating unit will need to operate during the cold weather season in that zone.

We have yet to see one of these averages that is anywhere near what folks actually experience living in any particular zone. Therefore, it’s a good idea for contractors to make their own heating hour calculation “where you live and sell” to make the estimated energy costs calculations. Do keep that same number as a constant! If you change it you will not be comparing “apples to apples!”



		CONSIDERATIONS WHEN SIZING A REPLACEMENT FURNACE or "Why don't I get any energy savings when I put in a more efficienet furnace?"
If the Preston’s Energy Costs Comparator doesn’t show any energy savings over the existing unit…you’ve probably replaced it with the same size furnace! And, why would you want to do that? Here’s how you need to size up the situation. Furnaces are rated by a.f.u.e. or “annual fuel utilization efficiency” the result of the amount of heat energy benefit coming out of the heating device (keeping the homeowner warm) divided by the amount of heat energy that went into the heating device (the amount of dollars per “therm” the homeowner pays the utility). The difference of this number to 100 is the heat loss going up the chimney, i.e.: you paid for it but didn’t get it. So, how do we save the customer energy costs when replacing their old furnace. By installing a smaller, higher efficiency furnace you will no doubt save the homeowner installation costs and certainly save the homeowner energy cost savings for years and years to come. A smaller furnace? When you size a furnace your goal is for it to provide sufficient heat output to keep the residents comfortable and since the heat output (benefit) is what matters you’ll work backwards to size the replacement. For example: Existing furnace A has an a.f.u.e. of 71% and is rated at 64,000 btu/h output. That means that furnace A has an input capacity of 90,000 btu/h or 9/10ths of a therm per hour [64,000 / .71 = 90,000]. Furnace A is replaced with xyz manufacturer’s furnace B that will be selected from their furnace line that have a.f.u.e. ratings of around 90%. NO. WE DON”T REPLACE A WITH ANOTHER 90,000 BTU/H INPUT FURNACE! We want to size furnace B to perform the same heating task as furnace A did but we want to do it as economically as we can. So, we can size the replacement to a lower input capacity size furnace and it will do the job because its greater efficiency makes it capable of performing the same heating task [64,000 / .91 = 70,330 btu/h]. Therefore, replacement furnace B is approximately 22% smaller (and therefore less costly to purchase) and its doing the same job but lowering the homeowners energy costs by about that same 22%. • Therm is a unit of fuel (can be natural gas, fuel oil, electricity, propane, etc.) with a heat content of 100,000 Btus. Your local natural gas utility will typically list somewhere on your bill a factor of a few points over or under a cubic foot of gas as having a therm of energy content. You actually pay for the 100,000 Btu increments of heat content and a cubic foot is generally a close match for expressing it in a volume consumers can understand. Of course we all know that old furnaces just don’t keep on giving the same output/efficiency that they may have done when relatively new. Although there’s no way to set a figure on such a variable we know this deterioration in performance is fact. It’s been said that this figure could be anywhere from a half percent per year to as low as a tenth of a percent per year. That means a twenty year old 71% a.f.u.e. furnace could really be a 69% to 61% a.f.u.e. performer. We’ll leave it up to the individual contractor as to what he or she would consider to be a reasonable appraisal. A NOTE ABOUT DOE HEATING ZONE MAPS* These are maps published by the US Government’s Department of Energy divided into temperature zones with numbers in each zone representing the average number of hours a heating unit will need to operate during the cold weather season in that zone. We have yet to see one of these averages that is anywhere near what folks actually experience living in any particular zone. Therefore, it’s a good idea for contractors to make their own heating hour calculation “where you live and sell” to make the estimated energy costs calculations. Do keep that same number as a constant! If you change it you will not be comparing “apples to apples!”
	< RETURN TO TOP>°°°°°°°°°°°°°< RETURN TO HOME PAGE >

CONSIDERATIONS WHEN SIZING A REPLACEMENT FURNACE or "Why don't I get any energy savings when I put in a more efficienet furnace?"

CONSIDERATIONS WHEN
SIZING A REPLACEMENT FURNACE
or
"Why don't I get any energy savings
when I put in
a more efficienet furnace?"