A guess at ballparking 'Manual J' heat loss

As a first guess, 'Manual J' heat loss should be about twice your homes average daily BTUH/Hr. heating needs, with proof offered as follows.

For example, lets say that I burn on average during the heating season ~9,750 Lbs. of coal (which is right close to our heating season average for 7 full heating seasons), and the heating season where I live lasts generally ~210 days (such that it does), and my boiler is ballpark 75% efficient at exclusively home heating (my guess), and "as delivered" anthracite typically has 12,300 BTU's per pound.

Thus:
(9,750 x 12,300 x 0.75)/210 = 428,300 BTU's required per "average" heating season day

428,300/24 = 17,845 daily average BTUH output required (which means BTU's per Hour)

17,845 BTUH x 2 = 35,690 BTUH as a presumed Ersatz-'Manual J' heat loss calculation (certainly not official).

Then by applying the ballpark 1.4X factor to cover the BTUH need for the coldest nominal day in 10 years scenario we get:

35,690 x 1.4 = 49,966 BTUH required as output on the coldest single day that is "nominally" expected to happen in any given 10 year period.

We factually heated our home for 10 winters using only a 13.5 KW resistance boiler, and 13.5 KW = 46,062 BTUH (as both input and output, such as resistance electricity is 100% efficient). Only twice in those 10 years did it struggle and barely keep the house warm for a single cold day. So our homes actual 'Manual J' times 1.4 must not be very far off from 46,062 BTUH.

49,966 BTUH is just a tad greater than 46,062. And it likely would have been close to what we actually needed on the 2 days when the resistance boiler struggled to keep up.

So in conclusion my initial ballpark presumption is that 'Manual J' turns out to be close to twice the daily average BTUH needed to heat a home. YMMV

That's one hell of a presumption Larry!!!!!!!!!!!!!!!!!!!!!

freetown fred wrote: ↑

Tue. Mar. 03, 2020 4:56 pm

That's one hell of a presumption Larry!!!!!!!!!!!!!!!!!!!!!

Seems to work for me. But as stated, YMMV.

OK---OK what the hell is a YMMV??????????????????????????????????

freetown fred wrote: ↑

Tue. Mar. 03, 2020 5:01 pm

OK---OK what the hell is a YMMV??????????????????????????????????

Your mileage may vary.

What scares me is I followed along slow and understood it!

Well mostly. Where did the term "manual J" come from? I have never heard that term but am not a boiler person. Or is it just a couple words made up?

And the 1.4X, if Im thinking right, isnt that supposed to be 2.5X? There must be a reason. I am thinking of the 2.5 times normal usage thingy. Now it is 1.4. Perhaps I dont understand it as i thought.

I will never use these as I dont weigh accurately enough but thats ok.

warminmn wrote: ↑

Tue. Mar. 03, 2020 5:17 pm

Where did the term "manual J" come from?

It is a heat load method first developed by the ACCA (Air Conditioning Contractors of America), and later adapted to general HVAC.

AFAICT, it is considered to be the best heat load calculating method currently available. It requires training and certification. It is a code requirement in many places now.

2.5 x average is something I made up.

Manual J is ballpark 2 x average need (or so I'm presuming)

And worst case heat need is "ballpark" 1.4 x Manual J

So therefore:
2 x 1.4 = 2.8 (vs. my presumption of 2.5)

The HVAC ballpark for worst case need is thus more liberal or forgiving than my "Rule of 2.5".

OK, I actually understand what you mean. So instead of taking 2.0 x 1.4 you call it 2.5 sometimes and i think Ive seen you use that figure before. And the 2.8 is usually for gas or oil boiler use in the industry. I can see that being a decent way to figure out furnace size needed. I see it less for solid fuel as they likely stretch it more, the BTU numbers. I wont name companies but I remember a recent one posted that was so far off it was laughable.

You just made that up didn't ya Larry!!!!!!!!!!!!!!!!!!!!! LOL

lsayre wrote: ↑

Tue. Mar. 03, 2020 5:06 pm

Your mileage may vary.

freetown fred wrote: ↑

Tue. Mar. 03, 2020 6:52 pm

You just made that up didn't ya Larry!!!!!!!!!!!!!!!!!!!!! LOL

No, it's quite common.

warminmn wrote: ↑

Tue. Mar. 03, 2020 5:17 pm

Well mostly. Where did the term "manual J" come from?

Manual J is supposed to meet your heating needs on 99% of the days of an "average" year.

365.25 x 0.99 = 361.6

That means it may fall somewhat short of need on an average of about 3-4 days per 'average' year. And that is when "up to" as much as 1.4 X 'Manual J' is "potentially" called for. Or just fire up a supplemental heat source for a couple days per year.

Manual J proves that most homes need far less heat than is typically imagined. But that also presumes that distribution is not an issue.

Rob R. wrote: ↑

Tue. Mar. 03, 2020 7:57 pm

Larry, why would you want to ballpark it when it is free and relatively simple to do a proper analysis?

Because I have so many years of fuel data on heating my home and I like to play with formulas and numbers. Most people don't have the homes fuel demand data and/or they don't know how to mathematically assimilate or formulate it so they don't have a good grasp of how to carry out a "do it yourself" heat loss study. If I was moving into a home that was new to me (no mater how old) then I would assuredly need to get a certified 'Manual J' before installing a heating appliance as I wouldn't be able to properly compute my own.