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View Full Version : Logged winter data, what's the next step?

RandomNick
02-11-2011, 06:35 PM
Hi folks. It's been awhile... hope you all enjoyed the holidays and are doing well.

Some of you may recall I was trying to zero in on my condo's heat loss and furnace needs by monitoring the usage of my existing furnace. I continued to log operation during the coldest periods this winter, and was able to capture peak usage one 5F ish early morning:

BurnerOn BlowerOn BurnerOff BlowerOff
----------- ----------- ------------ ------------
05:43:25 AM 05:46:06 AM 06:00:22 AM 06:03:00 AM
06:13:00 AM 06:15:07 AM 06:29:22 AM 06:32:00 AM
06:41:23 AM 06:44:06 AM 07:00:24 AM 07:03:05 AM
07:13:25 AM 07:16:12 AM 07:30:29 AM 07:33:10 AM
07:43:25 AM

I need to go back over my spreadsheet formulas, but by my current calculations my furnace usage topped out just shy of 60% in order to maintain 70F. What I still don't know is what my furnace actually puts out, so I'm making arrangements to have that measured/calculated/estimated. I could use a sanity check. I've seen a number of you throw out the formula:

BTU/Hr output = CFM x 1.08 x Temp Rise

Where Temp Rise is in Fahrenheit. Is that the correct and best formula to use to estimate output? I think I read somewhere that 1.08 is for Sea Level. FWIW, my elevation is approx 466 ft.

Is measuring temp rise and CFM the best route to a figure for what my furnace puts out? I get the impression that perhaps another route is to measure gas flow and do a combustion analysis. I'm really not familiar with the different approaches and pros/cons. Ultimately I'll be relying on a reputable outfit to do this, but I would like to know what your thoughts are.

garya505
02-11-2011, 07:56 PM
Your burner is on for 2 to 2 1/2 minutes before the blower starts?

euroboy
02-11-2011, 08:51 PM
Yes that does seem like the delay is on the long side - for my furnace it is about 60 sec.

Here is another formula used during the install process to verify if the orifice was selected properly for the rated input:

BTU heating input = Gas rate (in cu ft/hr) * Heating value of gas

You get the gas rate by clocking your meter ie. while the furnace is on and no other device uses gas, measure usage for a period of time, and convert to cu ft/hr - Heating value I believe is different for different areas - in the example I have it is 975 Btu/cu ft

So, your formula give you the output value, and the above is the input value:

BTU out = eff * BTU in

by measuring both BTU in and out you would be able to confirm if you are getting the rated eff. of your furnace - please post back if you do make those measurements.

euroboy
02-11-2011, 09:04 PM
I have a related question for the pros - in the example I have the calculated input rate is shown to be +/- 2% of the furnace rated input - is 2% in fact the value installers try to obtain, and if not what is the realistic difference between rated and actual input rate.

From the info I have, the rate is regulated by the orifice size and manifold pressure - are there other possible adjustments to match the input rate to the furnace rated value ?

RandomNick
02-11-2011, 11:25 PM
Your burner is on for 2 to 2 1/2 minutes before the blower starts?

Thanks Gary, I'm glad you drew my attention to that... I wanted to break something out. This is more like it:

BeginCall BurnerOn BlowerOn EndCall/BurnerOff BlowerOff
----------- ---------- ---------- ----------------- ---------
05:43:25 AM 05:44:10 AM 05:46:06 AM 06:00:22 AM 06:03:00 AM
06:13:00 AM 06:13:45 AM 06:15:37 AM 06:29:22 AM 06:32:00 AM
06:41:23 AM 06:42:08 AM 06:44:06 AM 07:00:24 AM 07:03:05 AM
07:13:25 AM 07:14:10 AM 07:16:12 AM 07:30:29 AM 07:33:10 AM
07:43:25 AM 07:44:10 AM

In response to the call for heat, the combustion fan runs for a bit then the igniter glows for a bit and then the gas valve opens and the burner lights. When I timed it, it took say 45 seconds from call for heat until the burner was lit, then the burner remained lit for 2 minutes, then the blower turned on. Allowing for a 45 second begin call to burner on delay drops usage down a bit (closer to 55% I think) but I was previously just leaving that in there as some buffer. I logged by hand so this isn't precise to the second.

garya505
02-12-2011, 12:52 AM
Here is another formula used during the install process to verify if the orifice was selected properly for the rated input:

BTU heating input = Gas rate (in cu ft/hr) * Heating value of gas

You get the gas rate by clocking your meter ie. while the furnace is on and no other device uses gas, measure usage for a period of time, and convert to cu ft/hr - Heating value I believe is different for different areas - in the example I have it is 975 Btu/cu ft

So, your formula give you the output value, and the above is the input value:

BTU out = eff * BTU in

by measuring both BTU in and out you would be able to confirm if you are getting the rated eff. of your furnace - please post back if you do make those measurements.

I did it like that, by clocking the meter, and the result seemed pretty close to the rating of the furnace.

The other formula
BTU/Hr output = CFM x 1.08 x Temp Rise
ddn't work for me when I tried it.

RandomNick
02-12-2011, 09:55 AM
I did it like that, by clocking the meter, and the result seemed pretty close to the rating of the furnace.

The other formula BTU/Hr output = CFM x 1.08 x Temp Rise ddn't work for me when I tried it.

How did you come up with the CFM and Temp Rise values? Did you use figures published in the manual or attempt to measure them?

The approach you guys discuss would seem viable if you have a new furnace which operates close to its published efficiency. I've heard guesses that a 20+ year old 80% gas furnace could be operating at only 50% to 70% now (wide range). I suppose I could still give what you are talking about a try and see where there results fall.