A guess at ballparking 'Manual J' heat loss

 
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Post by Rob R. » Fri. May. 01, 2020 8:27 am

Historical fuel records are not much help if you don't know what temperature the inside of the house was during that time.


 
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Post by lsayre » Fri. May. 01, 2020 9:10 am

Rob R. wrote:
Fri. May. 01, 2020 8:27 am
Historical fuel records are not much help if you don't know what temperature the inside of the house was during that time.
And also if you don't have decent handle on the efficiencies of the past heating appliances. I was fortunate (albeit that my wallet did not agree with this conception) to have started with electricity, as the presumption of 100% efficiency applies to resistance heat.

 
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Post by Hoytman » Fri. May. 01, 2020 9:29 am

Same furnace for last 21 years is all I can tell you. Didn't live there a big part of that time, obviously.

 
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Post by lsayre » Fri. May. 01, 2020 10:31 am

Hoytman wrote:
Fri. May. 01, 2020 9:29 am
Same furnace for last 21 years is all I can tell you. Didn't live there a big part of that time, obviously.
You can make assumptions regarding the efficiency and the T-Stat settings during those 21 years. Don't over assume the efficiency. Strong bias generally drives us to presume better efficiencies than are actually likely.

As a first guess, to be taken as such, older oil furnaces are likely to be on the order of 50% to 60% efficient. For oil conversions of solid fuel furnaces/boilers the efficiency may be as low as 45%.

 
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Post by Sunny Boy » Sat. May. 02, 2020 9:25 pm

The oil burner and furnace efficiencies were higher than that, at least in the late 70's and later.

One of my customers on LI owned a large fuel oil company. He had his service techs show me how to do a pre-season tune up and how they measure oil burner and furnace efficiency by testing exhaust co2 after doing a cleaning and tune up.

My oil burner back then was a 1920's American coal boiler, converted with a Wayne burner installed in the ash door. After every tune up it registered 80% using the exhaust co2 test.

My Father and a friend both bought the Blue Flame oil burners back in the 80's when they first came out. They are so efficient that they burn #2 home heating oil with a blue flame, Those were rated at around 90 %.

When I moved here in 1992, I had to replace the old furnace because the heat exchanger rusted through. I bought a tune up kit to do the servicing of my new Oneida oil furnace. In the kit it has that same co2 testing equipment. And my furnace gets 80% after a complete cleaning and tune up each season.

And the same for the wet base boiler I installed two years later when I built the radiant heat floor system of my shop.

Both the furnace and the boiler use a Becket retention head burner that is easy to get at least 80% efficiency from.

Paul

 
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Post by lsayre » Sun. May. 03, 2020 6:24 am

My emphasis is upon "real world" (or overall) efficiency, which is emphasizing to not to restrict or delude ones self into the biased viewpoint that whereby under ideal conditions and circumstances one will undoubtedly measure much better efficiency, one should rather attempt be honest with ones self in such an assessment and not let such a bias (often in the specific form of "confirmation bias") get in the way of longer term reality.

Test after test of actual heating appliances placed into real home settings and monitored by experts for periods of generally one or two years does not upon actual measurement and evaluation prove out the initial expectation (I.E., the validity) of such measures as 'AFUE' efficiency.

 
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Post by lsayre » Sat. May. 16, 2020 5:43 am

I'm revisiting the coal based 'Manual J' heat loss computation for my house based upon better information spanning specifically the past 3 years, which have not been very cold here overall vs. longer term historical averages.

New data:
1) Average annual HDD's over the past 3 years here = 6,172 (well below the longer term historical average)
2) Subtracting June through September HDD's, when no home heating is done, leaves an average of 5,921 annual HDD's
3) Average coal burned from mid Oct. through mid May (the heating season here) for the past 3 years here = 8,732 Lbs.
(Note: since I stopped burning coal at the end of March this year, I've added calculated amounts [based upon actual daily HDD's] through mid May for this year to compensate)

8,732 Lbs. x 12,300 BTU's/Lb. x 0.68 overall_efficiency_factor = 73,034,448 Annual BTU's expended as output
73,034,448 BTU's / 5,921 HDD's = 12,335 BTU's per HDD
12,335/24 = 514 BTU's per hour per HDD
70 degree maintained house temp minus +5 degree "design temp" for my region = 65 HDD's
514 x 65 = 33,410 BTUH 'Manual J' (the heat required to keep our house at 70 degrees for 99% of the days in an average year)

33,410 x 1.4 = 46,774 BTUH required as furnace or boiler output by which to annually heat our home on the coldest day expected within 10 years.

This comes very close to matching my earlier calculation made when using our "all electric" era heating data.


 
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Post by Lightning » Sat. May. 16, 2020 8:14 am

lsayre wrote:
Sat. May. 16, 2020 5:43 am
This comes very close to matching my earlier calculation made when using our "all electric" era heating data.
Does it also confirm that your AHS was getting 68% overall efficiency?

 
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Post by lsayre » Sat. May. 16, 2020 8:30 am

Lightning wrote:
Sat. May. 16, 2020 8:14 am
Does it also confirm that your AHS was getting 68% overall efficiency?
It may, but the 68% figure is required in order to bring the electric boiler data and the AHS data in line with each other, and for the electric era data I'm forced to make a bunch of averaging presumptions that in and of themselves may not be fully accurate, and likely are not.

I do believe it is closer to 68% than it is to 80% though. 80% seems to me to be something achieved under idealized conditions. The truth may be somewhere between 68% and 80%. Who knows?

Here is how bad the electric boiler era data is:

I know the least electricity we used in 10 years was ~24,000 KWH
I know the most electricity we used in 10 years was ~33,000 KWH, and that no other single year came close to this outlier.
I therefore merely presumed our average to be ~27,000 KWH, by double weighting 24,000 and single weighting 33,000, as in:
(24,000 + 24,000 + 33,000)/3 = 27,000
I also presumed that everything sans for 6,000 KWH per year pointed to the electric resistance boiler, or 21,000 KWH.
And then I presumed that because we kept the T-Stats way lower back then it would take ~14% more electricity to keep them at 70 degrees.
I also have no clue as to the average of the actual HDD's during the electric boiler era.

Presumption after presumption after presumption....

I had the actual annualized electricity demand data in hand at one time, but at this juncture I would need to call my electric company and request it to get it in hand again. But even if I had the hard data I would never really be able to determine what points to the boiler, and what doesn't. And I will never know the added requirement for T=Stats at 70 degrees verses set back to 62 degrees and set forward to 68 degrees for when we woke up, and/or for when we were at home instead of working, as for the electric boiler era.
Last edited by lsayre on Sat. May. 16, 2020 11:14 am, edited 1 time in total.

 
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Post by lsayre » Sat. May. 16, 2020 8:41 am

Let me play games with the electric numbers again:

I'll presume 21,000 KWH pointing back to the boiler demand for an average year
I'll presume 10% more KWH required for a 70 degree house vs. 62-68 degrees, or 21,000 x 1.1 = 23,100 KWH
I'll presume HDD's of 6,000

23,100 x 3,412 = 78,817,200 BTU's
78,817,200 / 6,000 = 13,136
13,136/24 = 547.3
547.3 x 65 = 35,575 BTUH
35,575 BTUH x 1.4 = 49,804 BTUH

49,804/46,774 x 68% efficiency = 72.4% efficiency for the Coal Gun

I could make the Coal Gun's efficiency come out however I want it to by endlessly playing this game. But it would really be a stretch to have it hit 80%.
Last edited by lsayre on Sat. May. 16, 2020 8:47 am, edited 1 time in total.

 
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Post by Lightning » Sat. May. 16, 2020 8:45 am

Oh okay.. thanks for your effort with the explanation :)

 
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Post by lsayre » Sat. May. 16, 2020 8:49 am

Calling the AHS Coal Gun "around 70% overall efficient" would seem to be a safe bet.

If it was better sized to our house it may have hit 75%. And if perfectly sized perhaps 80%. Who knows? It was grossly oversized for our needs, and idling time efficiency totally stinks. If I burned 13.5 Lbs. per day in the summer, and presumed 3 of those Lbs. for DHW, then ~10.5 Lbs. are needed daily just to keep the fire going when the boiler is not firing, but rather idling.

Lets say I burned 42 Lbs. per day on average during a heating season, and of that, due to idling in the winter, it burned 8 Lbs. per day while simply idling. That leaves 34 Lbs. per day for heating. Lets assume 90% efficient when firing.

34/42 x 90% = 72.9% overall efficiency. But all is merely guessing.
Last edited by lsayre on Sat. May. 16, 2020 9:02 am, edited 2 times in total.

 
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Post by Lightning » Sat. May. 16, 2020 8:58 am

I want to ask a question, that I think I may have the answer to but I'm not sure... We assume that HDDs are linear, in other words as is gets colder you can just add the same number of BTUs per degree difference to predict the amount of energy needed for a forecasted temperature, correct?

So how is it that a hotter object will loose temperature faster than a cooler object inside of the same ambient space. Is it because of radiant heat loss? I assume that that the only radiant heat loss of a house would be out thru the windows since the outsides of walls would acquire the ambient temperature outside.

 
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Post by lsayre » Sat. May. 16, 2020 9:05 am

Lightning wrote:
Sat. May. 16, 2020 8:58 am
I want to ask a question, that I think I may have the answer to but I'm not sure... We assume that HDDs are linear, in other words as is gets colder you can just add the same number of BTUs per degree difference to predict the amount of energy needed for a forecasted temperature, correct?

So how is it that a hotter object will loose temperature faster than a cooler object inside of the same ambient space. Is it because of radiant heat loss? I assume that that the only radiant heat loss of a house would be out thru the windows since the outsides of walls would acquire the ambient temperature outside.

HDD's are linear because BTU's themselves are linear. One BTU is the amount of heat required to raise 1 Lb. of water 1 degree F.

Radiation requires one to know the media into which the heat is radiating, and its nominal temperature. Such as for the HWB presumption that air is 65 degrees F.

 
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Post by Lightning » Sat. May. 16, 2020 9:12 am

Lightning wrote:
Sat. May. 16, 2020 8:58 am
So how is it that a hotter object will loose temperature faster than a cooler object inside of the same ambient space. Is it because of radiant heat loss?
Imagine two 10 pound chunks of steel. One at 600 degrees, the other at 200 degrees. The hotter chunk will lose 100 degrees faster than the cooler one, yet BTU loss of a house is linear. It's as if different physics apply to each until you consider the radiant heat loss..


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