A guess at ballparking 'Manual J' heat loss

lsayre · Post by **lsayre** » Tue. Mar. 03, 2020 9:26 pm

The confession of an honest HVAC contractor who applies Manual J:

I thought my scientific approach would impress the prospective client and win me the job, not quite! I presented my proposal to the homeowner along with the heat loss worksheet showing a boiler size of 59,000 BTU/hr. It wasn’t received very well. After he stopped laughing and regained his composure, he showed me the three other quotes he had gotten. The existing boiler was 160K, the quotes were from 160-225K — all from reputable local companies, who at that point in time had been in business a lot longer than me. Long story short, he went with the company proposing the 225K boiler. “That way it doesn’t have to work as hard”

lsayre · Post by **lsayre** » Tue. Mar. 03, 2020 9:50 pm

Another quick and super easy method to do an Ersatz-'Manual J' heat loss computation:

Step 1: Determine your average annual past historical fuel need in BTUH
9,750 lbs. coal x 12,300 BTU's/Lb. x 0.75 efficiency = 89,943,750 total BTU's

Step 2: Divide by the areas average HDD's to compute average daily BTU's per HDD
89,943,750/6692 = 13,440.5

.PNG | 37.3KB | Chippewa Lake HDD's.png

Step 3: Divide by 24 to compute average hourly BTU's per HDD
13,440/24 = 560

Step 4: Determine the desired Delta-T by using the 'Manual J' "design temperature" for your area:
A) Desired inside house temp = 70 degrees
B) Regional 'Manual J' "Design Temp" for the location of my house = 5 degrees
C) Delta-T = 70 - 5 = 65 degrees

Step 5: Multiply the Delta-T times the hourly BTU's per HDD
65 x 560 = 36,400 BTUH

Answer: Ersatz-'Manual J' = 36,400 BTUH

I really like this method.

lsayre · Post by **lsayre** » Tue. Mar. 03, 2020 10:52 pm

Here's how it works out for the 10 years we were all-electric:

Lowest annual KWH = ~24,000
Highest annual KWH = ~33,000
Average annual KWH = ~27,000

Annual KWH demand 1 year after installing the coal boiler = ~6,000 (before getting more efficient appliances and lighting)

Average estimate of KWH for exclusively home heat and DHW = 27,000 - 6,000 = 21,000

Average daily home temp maintained back then was 65 degrees during the heating season (due to the cost of electricity).

Computed extra energy required to raise 65 degrees on a T-Stat to 70 degrees = 14%

21,000 KWH x 1.14 = 23,940 KWH adjusted for 70 degrees

Step 1: Determine your average annual past historical fuel need in BTUH
23,940 KWH x 3,412 BTU/KW = 81,683,280 total BTU's

Step 2: Divide by the areas average HDD's to compute average daily BTU's per HDD
81,683,280/6692 = 12,206

Step 3: Divide by 24 to compute average hourly BTU's per HDD
12,206/24 = 509

Step 4: Determine the desired Delta-T by using the 'Manual J' "design temperature" for your area:
A) Desired inside house temp = 70 degrees
B) Regional 'Manual J' "Design Temp" for the location of my house = 5 degrees
C) Delta-T = 70 - 5 = 65 degrees

Step 5: Multiply the Delta-T times the hourly BTU's per HDD
65 x 509 = 33,085 BTUH

Answer: Ersatz-'Manual J' when all-electric = 33,085 BTUH

Conclusion: My "real world" S-130 Coal Gun efficiency for an entire average heating season must be closer to ~68% as opposed to the 75% efficiency that I was guessing. And if you look back in this forum to the first year or two in which I ran the S-130 I was reporting (crying about) computing ~68% annual average overall efficiency back then for it.

33,085/36,400 x 75% efficiency guess for coal boiler = 68.2% actual efficiency for coal boiler

And interestingly enough: 33,085 x 1.4 = 46,319 BTUH, which is right close to the 46,062 BTUH that our 13.5 KW resistance boiler input/outputs.

lsayre · Post by **lsayre** » Wed. Mar. 04, 2020 5:55 am

At this point you may be asking: How do I find my local 'Manual J' Design Temperature, and what does it represent?

1) Lets answer the last part first: The DB 99% (or "Heating 99% Dry Bulb") value is the average daily outside temperature in your area that is not dropped below 99% of the time. Or in other words, 99% of the time your area is historically as warm or warmer than DB 99%. (There is also a 1% "Cooling DB", which is used to design your A/C needs)

2) To find your local regions design temperature, or DB 99% (Heating 99% Dry Bulb), go here and scroll down to find your state, and then your closest regionally listed DB 99%: https://farm-energy.extension.org/wp-content/uplo ... ns_508.pdf

The closest listed location to me is Akron, OH and its averaged historical DB 99% is shown via this data to be 5 degrees F.

nepacoal · Post by **nepacoal** » Wed. Mar. 04, 2020 6:07 am

The two locations closest to us are 1,000 and 1,500 feet lower in elevation than we are. I'd have to subtract 5 or 6 degrees to be closer to my BB... I wonder if the professionals take elevation data into account when doing the manual J. Otherwise that chart is not very accurate in mountainous regions...

lsayre · Post by **lsayre** » Wed. Mar. 04, 2020 6:15 am

nepacoal wrote: ↑
Wed. Mar. 04, 2020 6:07 am
The two locations closest to us are 1,000 and 1,500 feet lower in elevation than we are. I'd have to subtract 5 or 6 degrees to be closer to my BB... I wonder if the professionals take elevation data into account when doing the manual J. Otherwise that chart is not very accurate in mountainous regions...

It seems prudent that they would take this into consideration.

Post by **Rob R.** » Wed. Mar. 04, 2020 6:57 am

lsayre wrote: ↑
Tue. Mar. 03, 2020 9:26 pm
The confession of an honest HVAC contractor who applies Manual J:

I remember reading that. Here is the rest of the article:

https://hydronicshub.com/heat-loss-calculation-on ... eplacement

lsayre · Post by **lsayre** » Wed. Mar. 04, 2020 7:02 am

Rob R. wrote: ↑
Wed. Mar. 04, 2020 6:57 am
I remember reading that. Here is the rest of the article:

https://hydronicshub.com/heat-loss-calculation-on ... eplacement

Bingo! That is precisely where I learned (late yesterday) how to apply the actual 'Manual J' method which uses past historical fuel data and HDD's, as opposed to my "guess" method as seen in the first post to this thread. As it turns out though, (he said while patting himself on the back) I seem to have made a rather good guess after all.

Now I know precisely why I got that idiotic high-ball 212,000 BTU boiler quote. It's sad that HVAC professionals will more often than not stoop so low and obviously kowtow to an at large ignorant home owner heating needs mentality just to assure making the sale. After getting two similar, but not quite as idiotic, local HVAC contractor quotes, I found a certified Mennonite HVAC contractor in our "Amish" area that is 25-30 miles south of me, and he far more sensibly and honestly assessed the homes heating need, in addition to coming in with a quote well below the other three quotes. The Mennonite guy's quote came in at just under 70% of the average of the three local contractor quotes.

lsayre · Post by **lsayre** » Wed. Mar. 04, 2020 8:15 am

nepacoal wrote: ↑
Wed. Mar. 04, 2020 6:07 am
The two locations closest to us are 1,000 and 1,500 feet lower in elevation than we are. I'd have to subtract 5 or 6 degrees to be closer to my BB... I wonder if the professionals take elevation data into account when doing the manual J. Otherwise that chart is not very accurate in mountainous regions...

The difference in HDD's would perhaps account for most (if not all) of the elevation related difference.

For example, my closest (long duration, plus official as opposed to "back yard") weather stations 1,177 ft. elevation HDD's are 6,692. As the crow flies, Chippewa Lake is about 4 miles from us, and we are within ~+/- 10 feet of its elevation. The 5 degree "Design Temperature" closest to us is sourced from Akron. But downtown Akron city proper (about 20 miles from us) officially has only 5,752 HDD's (being a concrete jungle), and for a better reference the Akron/Canton Airport has 6,154 HDD's.

Post by **Lightning** » Wed. Mar. 04, 2020 9:07 am

lsayre wrote: ↑
Tue. Mar. 03, 2020 9:26 pm
The confession of an honest HVAC contractor who applies Manual J:

Wow.....

I think there is a sentimental value in things that are big and powerful..

lsayre · Post by **lsayre** » Wed. Mar. 04, 2020 9:18 am

Lightning wrote: ↑
Wed. Mar. 04, 2020 9:07 am
Wow.....

I think there is a sentimental value in things that are big and powerful..

It's the American way!

lsayre · Post by **lsayre** » Fri. Mar. 06, 2020 10:32 am

Warminmin, I now believe I have an answer for the difference between my 2.5x rule of thumb, and the Manual J 2.8X rule of thumb.

My 2.5X factor is nominally supposed to ballpark the anticipation of the coal needed to be burned on the coldest single day in any given year, whereas the 2.8X factor is said to be sufficient to cover the coldest single day one may encounter in a decade.

warminmn · Post by **warminmn** » Fri. Mar. 06, 2020 11:19 am

Sounds good to me! It doesnt have to be too close when Im weighing by the pail full instead of the pound

Im narrowing in on the end of my anthracite for the year unless I buy more. I will have to figure it out by dollars as I used some lignite too but I know this was the lowest usage Ive had. Its been a warm winter and I closed in my very large screen porch this year so its hardly fair to compare to other years, but a 24 pound daily average for the 150 days(give or take a day) on anthracite. My previous low was 32 or 33 with my Efel stove. About half the time with my Chubby and the other with the Riteway. Closing in my porch was well worth the 2 days work and it will just take a few hours from now on as the panels are cut to fit the windows. Live and learn... note: not much weighing, just a starting weight.

Hoytman · Post by **Hoytman** » Thu. Apr. 30, 2020 9:25 pm

I really appreciate this thread, and you for posting it, Larry. This fascinates me. Never was much good at stuff like this but I’ve gotten better and become more interested as I’ve gotten older. I followed along fairly well on the first page, got side tracked with wanting to reply on the 2nd page, so I’ll have to read it again at another time.

Since we’ve moved into this house I’ve been keeping track of fuel oil usage at fill up and I have a few records while I was working on the place. I can likely get records from the fuel oil company going back at least 10 -15 years when my grandpa lived here. Would it be worth asking the fuel oil company for those records, Larry? I’d like to run the numbers on this furnace as well as this stove, and for a smaller coal stove.

lsayre · Post by **lsayre** » Fri. May. 01, 2020 5:35 am

Hoytman wrote: ↑
Thu. Apr. 30, 2020 9:25 pm
I really appreciate this thread, and you for posting it, Larry. This fascinates me. Never was much good at stuff like this but I’ve gotten better and become more interested as I’ve gotten older. I followed along fairly well on the first page, got side tracked with wanting to reply on the 2nd page, so I’ll have to read it again at another time.

Since we’ve moved into this house I’ve been keeping track of fuel oil usage at fill up and I have a few records while I was working on the place. I can likely get records from the fuel oil company going back at least 10 -15 years when my grandpa lived here. Would it be worth asking the fuel oil company for those records, Larry? I’d like to run the numbers on this furnace as well as this stove, and for a smaller coal stove.

Yes, given sufficient heating fuel consumption history you can compute the homes heat loss from the data. But I should also remind you that for about as little as perhaps $75 a professional assessment can be made. Having both gives you double assurance that you are on the right track.