Welcome! Here are the website rules, as well as some tips for using this forum.

Heat loss calculations

Options
Member Posts: 1,935
why would you try to calculate solar gain?

it would be of no value in a heatloss calculation, reason being, it has no reliabilty in providing heat when needed.

• Member Posts: 33
Options
Heat loss calculations

I have been doing heat loss calcs for my whole career (22yrs) and have never used rules of thumb. Recently I started looking at my calcs vs. actual energy costs and I am finding that the projected energy costs based on my calcs are quite a bit higher than the actual energy costs. I am starting to question my sizing. Anyone else dealing with this? I want to make sure I am putting in the best thing for my customer. Any thoughts would be appreciated. Thanks John
• Member Posts: 53
Options
Reality versus Theory

I am curious what degree of delta you are seeing, John. Let's compare notes.

If you are confident in your ability to calculate a heat loss (you after 22 years and me after 30 years, we should stipulate "yes"), then let's see where the variables lie:

1) Infiltration is a huge wild-card... Crack method gets you one number. ACH gets you another..If you had a blower door test that is one thing. Otherwise, the ACH rate used should be what now? 0.50? 0.35? 1.0? Hey it's Tuesday. What's Paris Hilton's IQ?....

2) Are your homes and buildings of late being built with higher integrity? Low-e glazing but you are using older tried and true u-factors?

Better air barriers? Icynene and other spray foams versus fiberglass batt wall-air filters? Less infiltration these days because of that alone.

3) Plant efficiency; the "e" part of the denominator in the degree-day formula. Sure the boiler says one thing. Could be another. Up or down 20% right there...

4) The Cd factor, that roughly 0.60 multiplier applied to the raw fuel units used, to compensate for internal gains, solar gains, setbacks... maybe that has to be re-thought.

So, what you are finding, John? My experience has been, absent better information, a function of the assumed plant efficiency.
• Member Posts: 1
Options

So what ACH # do you guys use. I use a 4 but I have wondered. I do not know how to come to a better number in new construction because the home isnt built when I do a heat loss.
• Member Posts: 33
Options
heat loss calcs

The jobs I have been questioning this on are older homes that are theoretically "loose" and not very effiecient. I try to be as accurate as I can with all the details. I am starting to think about getting a blower door, just to set me apart from my competitors. Yes, it is difficult to determine the existing plant efficiency. I am especially interested in this when quoting geothermal heat pumps, if I can down size my equipment, it would have a great impact on ROI. Thanks, John
• Options

• Member Posts: 2,440
Options
My interest is purely scientific, Ray.

When dealing with less than whole integers and seeking a numerical standard for finite measurements, I find the use of the pH scale quite useful. Other than that, I never think about her.
• Member Posts: 6,928
Options

I've found a number of sources (generally European) that suggest "heat loss" calculations--good ones--will overstate the actual heating load by between 40% - 55% over a "typical" heating season.

There are many potential causes:

1) The numbers involved in heat loss calculations are based primarily on laboratory measurements--not real-world materials in real-world situations. NOBODY wants to under-estimate so it seems that everything is rounded up at each and every step of the calculation (including ones you don't see even if doing "by hand").

2) Infiltration is a true wildcard. As Brad White and I both like to point out, structures are both infiltrating and exfiltrating in the real world. In a structure made up of rooms, some rooms see primarily infiltration of outside air while some see mainly exfiltration of conditioned air. Infiltrations calculations (even if you've used a blower door test and have a "real" value) assume that EVERYTHING is infiltrating... On extremely windy days, I'd suggest that infiltration may actually be higher than the estimate, but unless extreme wind is the "norm" for your location, infiltration is going to be significantly lower than estimated most of the time.

3) Heat loss calculations have no provision for solar gain.

4) While we include occupancy gains (lights, appliances, people, etc.) in heat GAIN calculations, every source I've seen specifically prohibits reducing your heat loss because of occupancy gains. (Highly engineered commercial structures are likely somewhat of an exception here, but I've never worked with such--I just have good ideas about how I would include and compensate.)

-------------------------------------------------

The moral: Trust yourself, your experience and your calculations. Do not oversize heating equipment above your heat loss calculation--most especially with mod-cons! If the difference between models is say 20% "under-sized" compared to 30% "over-sized", I'd sincerely opt for the "under-sized"!
• Member Posts: 6,688
Options

really puts the oversized-boiler situation in the glaring light it deserves. If a heat-loss calc will probably result in an oversized boiler, the label method and other rules of thumb are much, much worse.

There's really no excuse for not doing heat-loss calcs anymore, not when a simple laptop-based program will do it.

• Member Posts: 299
Options
Maybe you shouldn't calculate...

Maybe you shouldn't try and calculate the heatloss for existing structures and instead just test them?

I totally agree with Mike's numbers. My Manual J proved to be exactly 50% overstated (the true load is 2/3rds of the calcs).

For existing structures, could an instrument-type boiler be developed that could test heatloss? It wouldn't be cheap but neither are blower doors, and even using their results will not make a Manual J accurate.

Perhaps a tiny propane fired boiler on wheels, exhausted out a window insert with a special 6½" wide spacing connection so that it gets bolted in to the flanges where the existing system pump normally is so that it can use the existing piping and emitters.

It could measure the heat gain from multiple internal reference points simultaneously verses exterior temperature and that way it could be used whatever the season. It might be a case where they are rented from wholesalers or manufacturers. Bolt it in, run it for 30 minutes and take it out. It can be scaled so that it could be used at any outside temperature (the deadmen could test output in the summer!) although sunny summer afternoons wouldn't be ideal. ;-) The whole test could take an hour. How long does a Manual J take on a complex building?

Either that or accurately clock and analyze the current boiler's combustion efficiency and measure heatloss up the flue - the rest should be heating the house.
• Member Posts: 1,935
Options
infil= exfil

mike,

for all practical purposes infiltration will equal exfiltration, other wise the house would implode or explode.

can't really effect calculations/measurements, can't have one without the other, thats the blower in "blower door test."

• Member Posts: 6,928
Options

Of course infiltration will nearly equal exfiltration (and vice/versa). Most structures don't exist in a pressurized or depressurized state compared to the outdoors...

Heat loss calculation do however consider wind as arriving from all directions at once with all rooms infiltrating. A blower door does the same thing in reverse.

In reality in a home with rooms, the windward rooms will experience the vast majority of the infiltration while the leeward rooms experience much of the exfiltration.

• Member Posts: 2,440
Options
And (But)....

Mikes point is clear: Every room will at some time have INfiltration. Obviously the others will have EXfiltation at some point as well.

Regardless, each room has radiation sided to each room's potential heat loss. It is when the sum of those rooms radiation-stipulated as sized to a calculated heat loss- is taken, that the accumulation of heat loss overlaps.

This is why I will routinely size radiation room-by-room for transmission and infiltration, then go back to the total of the whole house transmission (back out each room's infiltration component) and add back in the infiltration to the house as a whole unit. This usually cuts the infiltration by about 40 percent. It is this number I use to size the boiler. Sometimes it makes a differnce, sometimes not.

JP knows that only the flimsiest of FEMA Katrina Trailers have that explode/implode problem
• Member Posts: 6,928
Options

A nasty storm front moved through two days ago with a huge cold front behind.

Right now we have glorious weather. Beautiful blue skies with some nice fluffy clouds. 66F but quite windy (about 10 mph costant with gusts twice that). Somewhat variable direction from the E to the NNW. Have had windows open all day.

Just closed them all save the W facing window in my office--left the storm open a crack. Smoke test shows some pretty decent exfiltration with only occasional infiltration. Fully closed the window and storm and still enough exfiltration for the smoke to sometimes be drawn down into the little cracks at each end where the meeting rails of the double-hung sash come together (the one place I haven't weatherized the original 1903 windows). That part of the windows and the south-facing main front door are my intentional "breathing spaces". It's old and has lots of large windows and doors to the outside, but it's not drafty in any weather.
• Member Posts: 1,935
Options
and but, well......

I think what often goes forgotten here is that the heatloss is a worse case.

and often i think people forget about condition for max infiltration which is the 'design condition', thus claiming the heatloss calcs are grossly off.

after all what is your true infiltration rate on a calm day, Zero mph wind speed? compared to the design day 25 mph "constant" wind and constant design temp?

and then is there infiltration with outside/inside temps the same, no wind of course? then there needs to be an adjusted infiltration rate dependant on delta T for more accuracy. but then WHY bother, you can't get a 37,65o btu boiler anyway
• Member Posts: 53
Options
Well, obviously but

I think Mike's point goes to the operating gains over time (actual net energy usage) versus the calculated heat loss as an indicator/predictor of fuel usage.

3) Heat loss calculations have no provision for solar gain. I read into this "nor should it", to complete the thought.

Mike's point #4 expands on that, that no credit can be taken (again, nor should it) when determining the net heat loss.

The difference is probably best expressed with the "Cd" factor and the delta between a shaded house on the north side of a hill and the same house unshaded on the south side of a hill could be huge in exactly the same degree-day climate.
• Member Posts: 6,928
Options

Am not saying that it's practical to calculate solar gain and certainly not saying it's possible to predict it nor that you should generally try to compensate for it when sizing boilers.

BUT, solar gain is there and it happens and over time it certainly makes a contribution to the load.

Just as with occupancy gains, rounding up and assumed constant (and omnidirectional) wind, solar gain is an "uncompensated contributer to the load".

Again, a heat loss calculation--even a very good one--will overstate the seasonal load by a significant degree. Your "fudge factor" for unusual weather and different occupancy habits is already built-in. Resist the urge to add even more.
• Member Posts: 53
Options
To be clear

(and if not obvious, I agree with you, Mike, and support your assertions), the "Cd" factor I cite is a reducer of the gross load. I am suggesting by this that the nominal 0.55 to 0.65 range the factor tends to take in temperate climates could in fact be lower.
• Member Posts: 6,928
Options

John:

I've deeply torn into quite a number of old homes (many quite simple) and have never seen what I would consider "loose" original construction.

That said, the "loosest" old homes in my experience have a highly irregular footprint with lots of inside corners and (often) 45 degree corners (both outside and inside). Such come from the Victorian (romantic) era and were ALWAYS considered "hard to heat".

Additions and enclosures of former porches are another matter--I've seen some truly loose and crappy construction.

I've never completely built a new home from the foundation up, but I love to walk and love to snoop through and critique homes under construction. While I suspect the trend is about to end, many of those new homes are "romantic" with highly irregular footprints. While I believe that all modern construction should be considered at least "good", I'd have a hard time calling MANY "average" even compared to an old home in decent shape with a simple footprint.

Suggestion: Unless the old home was hideously maintained (think holes/large cracks in walls visible from the outside) or a Victorian monstrosity with non-stormed windows in poor shape, I'd sincerely consider it "average" with regards to infiltration. Use your best judgment regarding additions/enclosures. If necessary compute them separately from the main house and with "poor" infiltration.
• Member Posts: 5,884
Options
ph ?

Are we talking a litmus paper test here or more along the lines of How many times does a Brad go into a Hilton ??

I figure .... 45 seconds ?

Scott

• Member Posts: 53
Options

Total.
• Member Posts: 955
Options
Uni R is right

My own home, new unit 2 years ago year, and I did the heat loss to a tee, true test done in operation, 33% to big on the COLDEST actual day last heating season, and that was 20° colder than my design. From design day it was 2x bigger

THe fudge factor in heat loss is astro-friggin-nomical.

Since then I have dropped my final results from Heat loss calcs to 75% of final calc./..no complaints so far.

Mitch
• Member Posts: 4
Options
does Manual J size for heatloss or heatup?

Does Manual J calculations provide sizing requirements for heating a cold structure up or just maintaining an already heated structure? Also is there a time limit for reaching desired temperature and is the desired final temperature part of the calculations also?
• Member Posts: 6,688
Options
Interesting question

not sure what the answer would be.

The design final temp is part of the calc, for example an inside temperature of 70 degrees F when it's 0 degrees outside.