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# Calculating Heat Load - Seems unusually small2

## Calculating Heat Load - Seems unusually small

(OP)
Recently started a new job as an assistant project manager bit of a career change so no real experience in construction. Owner/lead engineer is putting an 8 unit addition onto one of his houses, and typically hires out to get designs stamped by engs who specialize in whatever, but is having me review things just so I can get up to speed and hopefully stamp off when I feel I'm competent enough. The structure is built, and now just being finished. He asked me to calculate the heat load, and don't have anything beyond the web / an HVAC course I did through school 15 years ago.

Dimensions of building are 28 x 74 x 35 high, with a lip that extends 38' x 28 x 9' high over the existing building. (So a forth story on current 38 x 28 building, plus building/extending those 4 stores 74' back)

so 7000 sq ft of wall space (8" ICF) R = 24
only 100 sq ft of windows R = 4
3200 sq ft ceiling, R 30, 5" spray foam
105 sq ft of doors, R 10
2100 sq ft floor space, foam insulated concrete R 30

Assume 70 deg F difference ( 0 to 70) during peak heating load yields:

21k BTU/hr through walls
2k BTU/hr through windows
8k BTU/hr through ceiling
1k BTU/hr through doors
5k BTU/hr through floor

So not even 40k BTU/hr which seems extremely low. Add in another 50k BTU/hr or so to heat incoming air based around .5 changes per hour, so not even 100k. - But still seems low. More so when you consider that a good deal of the heat will be recovered via HRVs. I've read a good deal about how efficient ICF can be, but the owner who's built other places based on rules of thumb figured he would need 2 heating units, one 200k BTU/hr, and a 100k BTU/hr as a compliment/backup. Is there something obviously wrong? It seems like a fairly straight forward calculation and additionally I've been reading about how common it is for heating units to be criminally oversized, since it's a culmination of "better safe than sorry", people who don't know how to do the calcs themselves, and the financial incentives to sell bigger units.

I also filled out the ASHRAE excel work sheet, and that took into account several other factors such as inefficiencies in the heating system, and 0 HRV recovery, but even still it came in around only 120k BTU/HR. Am I missing something obvious here?

Replies continue below

### RE: Calculating Heat Load - Seems unusually small

Mechanical or natural ventilation? You'll want to account for that as well.
Make sure your design temperature is accurate; I use 90-95 delta T for heating in the dead of winter where I am.

### RE: Calculating Heat Load - Seems unusually small

Correct your R values to account for thermal bridges. Add infiltration based on enclosure surface.
Unless this is really great contruction, your windows look very optimistic.

### RE: Calculating Heat Load - Seems unusually small

(OP)
Thanks for the feedback. We are planning on mechanical ventilation, 8 x HRVs at 65 cfm which is that other 50k BTU/hr I accounted for, even though supposedly up to 80% of the heat will be recovered. What sort of thermal bridges are going to be present in ICF construction? the Plastic webs? Re-bar extending into the footings? I'm just going by https://www.nudura.com/media/2529/nudura_whole_wal... where an engineering firm did a physical test of its R-Value.

The windows have a sticker quoting their R value. While it may be optimistic, even taking it at half doesn't really add a material number to the calculation since the area is so small. A couple k BTU/hr missing vs a couple hundred k BTU/hr missing per "rules of thumb".

### RE: Calculating Heat Load - Seems unusually small

It seems as if your calculation is theoretically correct. 300 MBH might be a bit oversized, I would go for 2 x 100 MBH for some back-up and redundancy.

### RE: Calculating Heat Load - Seems unusually small

The A/C salespeople use 500 sq ft/ton of A/C when they calculate capacity requirements. Since your floor space is only 2100 sf,, then 4 tons of A/C, which is 48,000 BTUh. I could see heating being double, since the temperature delta is possibly double.

TTFN (ta ta for now)
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### RE: Calculating Heat Load - Seems unusually small

(OP)
The floor space of 2100 sq ft is the portion directly in contact with the ground for purposes of heat loss through that mechanism. Total living space will be on the order of 4 x 2100 + 1064 on top of the existing or roughly 9500 sq ft of new living space.

Hence 20 tons, which seems overkill. Especially when you take into account that heating and cooling aren't quite the same thing, so a 20 ton cooling capacity heat pump would likely be producing 25-50% more heat - which seems like almost an order of magnitude more than needed based on rough heat loss calcs.

### RE: Calculating Heat Load - Seems unusually small

2
I recommend consulting ASHRAE fundamentals chapter 17/18 Cooling and Heating Calculations. ASHRAE 90.1 appendices have good tables for typical construction U-values taking in account thermal bridges. Your code will tell you more about your local required ambient design temperatures.

You can't estimate load based on area only. Saying 500ftÂ²/ton is not an actual load calculation. If it was that easy, all engineers would be unemployed. I'm not a rocket scientist just because I know to go to the Moon I need a "rocket ten times the size of the capsule" because I looked at some NASA pictures and scaled from a previous space flight. There is more to it.

I don't know your building and climate, and I'm not designing for you. But most numbers mentioned in this thread are wild guesses at best.

I would question the assumed values as they don't account for thermal bridges. I also question you have R4 windows. At least it would be unusual. There are windows that have a U of 0.25, but they are not common in standard construction. "Don't trust, but verify". I also have not seen R10 (U 0.1) doors in real life. the very best doors I've seen have a U-value of 0.29, but standard doors are closer to 1. Are you reporting metric or IP values?
the only value that is somewhat realistic is the ICF, but that also is optimistic. The siding is ventilated, so that doesn't really count as insulation. Your ceiling with sprayfoam is verified to be 6"of high-density foam? and what about the wood or metal joists and trusses and the thermal bridging?

### RE: Calculating Heat Load - Seems unusually small

I'd caution a main/backup setup. This often leaves one unit to rot to death and leaves it non-functional eventually. If backup is desired I'd run two equal units and have the control flip-flop them so they both work thru their lifetimes and are both equally exercised. Often you can also get a quantity price break you won't get with different models.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Calculating Heat Load - Seems unusually small

There is a reason we use specialized programs for calculating the cooling load for a building. There is A LOT that goes into calculating the cooling load, starting with the location on the Earth, general environmental conditions (tropical/arid/etc.) and which way the building is facing, to finer details such as the amount of electronics in the space, to the number of people and what those people are doing.

How much of that 7,000 SF of wall is facing north? south? east? west?
3,500 SF "ceiling" over 2,100 SF floor indicates to me a sloped roof (is there an attic?). What is the slope and how much is facing North/South/East/West?

HVAC-Novice's advise to go to the ASHRAE handbook is a good start, but you really need to hire a HVAC professional.

### RE: Calculating Heat Load - Seems unusually small

As other have suggested, I'd recommend taking a second look at your infiltration values. It's a finicky science/art and I'd say a good deal of calculations takes into account 'good' judgement. How old is your building? Depending on the age, there may be more specific CFM/SF rates for infiltration through your exterior walls and through your roof. Other than that, I'd say your heating load calculation does a decent job of covering the basics. Did you put a safety factor into of your calculations? If not, they may be smaller than expected. Depending on the age of spray-foam, I know the R-values tend to drift off as they age. If the insulation is very old or already halfway through its half-life, you may not be getting R-30 values for thermal purposes. Yes, you want your system to work the day of install, but you also want it to work ten years down the line. Unless you are doing this every day, there are things you will not know about. Safety factors in the calculation help account for the unknowns. But, unfortunately without expertise and experience, it's hard to know what to look out for. Best of luck.

### RE: Calculating Heat Load - Seems unusually small

(OP)
Thanks everyone for the advice. I looked over ASHRAE 17/18 and ASHRAE 90.1, but again, even in the worst case scenarios, this only shows bridging effects to the tune of 10% or so from say R30 insulation to R25 overall. Same thing with whether it 12 vs 16 people, vs 8 TVs or 10. Minor points that will assist in accuracy, but changes that will collectively likely be in the order of +/-25%. I'm looking for explanations as to why I'm calculating 40k BTU/hr, while rules of thumb like 30-50 btu/sq ft (x 9500 sq ft living area) gives you on the order of 8-10x as much. Even the guy who owns the place, another P. Eng was planning on 300k BTU/hr.

While appliances or heat leakage through a slightly colder N facing wall will surely impact things, it's not going to be by an order of magnitude.

I've looked at several bits of software, both for envelope calculators by ASHRAE, as well as efficiency improvement calculators looking at energy use over the year, and they all essentially seem like glorified spread sheets which give me similar values.

As for the building, it's a 3 floor addition behind an existing building, with a 4th floor going on top of both new, and existing, which is why floor and ceiling are different - I'm only calculating for the addition. Ceiling is flat, 5" spray foam isn't in yet. Building is built, just needs inside work done and will hopefully be done by next spring.

It's a new build. Windows are a very small at just over 1% of wall space, and U of .26 was taken from stickers on window. R value for 8" ICF was taken from an independent engineering test on manufacturers site.

I'll concede doors are low. But again, 100 sq ft of R1 doors on 13000 sq ft of R25+ won't jump you from 40k BTU/hr to 400k.

What's typical infiltration on ICF builds like? I would have assumed that the concrete/styrofoam would largely be air tight? The engineer who designed and largely oversaw the build is trying to make it fairly high end, so would assume it's otherwise well sealed beyond the HRVs. But as you say, I suppose it's the sort of thing you appreciate only after you've seen 100 buildings and 100 energy bills. But, if that's the case, that ultimately it's reliant on an experience based "X" factor, what's the use of the software and calcs beyond the most rudimentary idea that you're going to x10 for good measure?

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