# Heating fuel usage diagnostics

Posted Feb 9, 2018 2:21 AM ET
Edited Feb 9, 2018 10:34 AM ET

1.

Is this a boiler or a furnace ? Ducted or baseboard / other hydronic system ?

Posted Feb 9, 2018 8:42 AM ET

2.

Its a Rheem Prestige series forced air furnace, when I referenced "econet" that is Rheems fancy touchscreen t-stat, has runtime hours available to view but only total and last 2 weeks.

We've been in the house a year and a month, I never really tracked propane useage but it seemed like it didnt use much, then all of a sudden we went through 40% in a month, Im interested in a useful metric that can be applied to a house like cfm50 or ACH50 can be.

Posted Feb 9, 2018 9:26 AM ET

3.

T. Carlson,
To double-check your math, I'll do my own analysis.

Your furnace is probably delivering heat at a lower efficiency than its nameplate rating. Let's call it 92% efficient.

Since your propane kitchen stove contributes heat to the house, we'll consider all propane use as part of your space heating fuel use.

200 gallons of propane burned at an efficiency of 92% provides 91,600 * 200 * 0.92 = 16,854,400 BTU

Btus per square foot per heating-degree-day:
16,854,400 / (4500*1794) = 2.09

The book Residential Energy notes: "Btus per square foot per heating-degree-day ... This factor -- often called the Home Heating Index (HHI) -- gives a means of comparing buildings with different sizes, climate, and energy prices. The home heating index varies from HHI-2 in very efficient homes to HHI-20 or more in the most inefficient existing homes. Three other energy indexes are used for homes in warm climates, electrically heated homes, and larger residential buildings. These indexes are expressed in annual Btus per square foot, dollars per square foot, or kilowatt-hours per square foot. These indexes are more useful for buildings whose energy costs are not dominated by heating and not so directly related to heating degree-days. Extremely efficient residential buildings may use 5,000 Btus/square foot or 2 kWh per square foot annually for all uses ..."

Here is a table -- I forget the source:

Table: Distribution of Single Family Homes in the Northeast, Midwest, and West Census Regions using Propane or Natural Gas as the Primary Space Heating Fuel in 1997

Range of Energy Use............................. Relative Frequency
Under 5.0 BTUs/HDD/Sq.Ft............................ 12.2 percent
Between 5.0 and 10.0 BTUs/HDD/Sq.Ft......... 39.4 percent
Between 10.0 and 15.0 BTUs/HDD/Sq.Ft......... 23.9 percent
Between 15.0 and 25.0 BTUs/HDD/Sq.Ft........ 14.7 percent
Over 25.0 BTUs/HDD/Sq.Ft. ............................ 9.8 percent

Posted Feb 9, 2018 9:51 AM ET
Edited Feb 9, 2018 10:21 AM ET.

4.

Another way to look at it is calculating the 99% heat load using these methods, and coming up with the heat load per square foot:

Comparing a the fuel use load calculation against an I=B=R or Manual-J can also be useful. If they're more than 15-20% apart it could be telling you something, especially if the fuel-use calc comes in significantly higher than the IBR/Manual-J. A higher number from the fuel use calc could be attributable to leaky ducts, leaky propane fuel likes, etc.

If your math works out (and I haven't checked it), the ~2 BTU/HDD-ft^2 is about 1/3 what my 1920s 2x4 bungalow is using per square foot if I don't count the insulated but unfinished basement and ~40% of my use if I count the basement (which stays at or above 65F all winter on the standby and distribution losses of my mechanical systems.)

Posted Feb 9, 2018 3:07 PM ET

5.

T. Carlson,

A few things to consider

1 The gages on most propane tanks are far from precision instruments. I do not think you can say you used 200 gallons of propane based solely on tank gage readings.

2 When I watch my Econet heat pump thermostat I often find the thermostat calling for “low heat” but the heat pump is running at full speed and supplying high heat. I do not think you can assume that a hour of low heat logged by the Econet thermostat equals 1 hour low output of the furnace.

If you want to find the leaks in your thermal envelope I think the best way is the have a blower door test on a very cold day and have the inspected with an inferred camera while the blower is running. This will make your leaks show up very brightly. This test is part of a quality energy audit.

Walta

Posted Feb 9, 2018 10:59 PM ET

6.

Thank you Martin, Dana and Walter for your responses. HHI sounds interesting, but wouldnt it be more accurate/useful tied to conditioned volume?

Anybody know if a combustion analyzers efficiency ratings are accurate to determine true furnace efficiency?

@Dana, great article, I'll give it a go.

@Walter. I hear you on the propane gauge, hard to tell with a 1 1/2" dial marked at 5%, it would be nice to have an inline flow gauge or something, I dont trust propane companies.

My econet seems to be somewhat accurate, I can hear it ramp down and rpm and cfm react in real time, I havent measured the static pressure to check against the display but I have manipulated it and watched the tstat react to it. Only thing its a modulating furnace, there is supposed to be many stages of output, not just high and low so Im not sure whats up with that.

I guessed 200 on my poor memory, I just looked at the ticket and they only filled me to 40% because of the "cold snap" (or to sell me higher priced winter propane, I mentioned I dont trust propane companies right?) So I used approx.150 gallons total but will adjust for accuracy as soon as they fill me up in the next day or two.

Posted Feb 10, 2018 10:03 AM ET

7.

Heat load isn't a function of volume any more than it is of square footage. It's about exterior surface area. A sprawling single story had more exterior surface area than a 2 story of equal volume. A house with a lot of ells & bump-outs has a lot more surface area per unit volume or square feet of floor han a simple rectangle.

Posted Feb 10, 2018 4:10 PM ET

8.

Hows that, more volume is more exterior surface area, no?

Posted Feb 10, 2018 6:47 PM ET

9.

If you lived in a sphere you would have about 20% less surface area that a cube, a rectangle could be closer to 40%, add some crazy bump outs you could double the surface area.

https://math.stackexchange.com/questions/43573/surface-area-of-a-sphere-...

Walta

Posted Feb 10, 2018 10:43 PM ET

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