I am going to start building my shop very soon. I am going to do hydronic heat in the slab. I want to do a heat load calc. How do I go about it? I have looked at some of the online programs, but I keep getting stuck. They ask questions that I dont know the answer to. They ask for the values of the windows that I have not chosen yet. I could not figure out how to answer the slab insulation question? I know I am going to use 2" ridged under the slab and on the perimiter, but the program asked me to add up different dimesions that dont make sense to me. I am confused.
heat load calculation
- Thread starter stealman
- Start date
lzenglish
Well-known member
I swear by the "Carrier System Design Manuals", which were part of my required College text books, when I attended HVAC School in 1973. This one I have pictured is part 1, of 12 books in the set, and is the one I have refered to the most, over the years. Maybe your Library will have a copy?
Wayne
Wayne
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Full Throttle
Well-known member
- Joined
- Nov 8, 2011
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good site here, I have sent them a few buildings that I have done, payed them for the load and design work. I could do your load calc on a scrap 2x4, but does no good if the system is not properly designed and balanced.
http://www.hydronicpros.com/
http://www.hydronicpros.com/
lzenglish
Well-known member
sneezer41
Well-known member
square feet times delta T divided by r value equals Btu.
Ad up all the various surfaces[doors windows walls ] and you will get a total heat loss. Have to factor in air leakage,but if your building is pretty tight it is not much. If you insulate the slab perimeter well you can ignore the floor for most practical reasons.
Ad up all the various surfaces[doors windows walls ] and you will get a total heat loss. Have to factor in air leakage,but if your building is pretty tight it is not much. If you insulate the slab perimeter well you can ignore the floor for most practical reasons.
rburke65
Well-known member
What does that mean....add up all the various surfaces? So....I got 4 walls and a ceiling so that's 5, and I have 6 windows and 2 doors, so my heat loss is 13! What kind of formula is that? Not too helpful. You can't possibly be explaining this correctly! IMO.
Full Throttle
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- Joined
- Nov 8, 2011
- Messages
- 285
you are missing about 99% of the calculation
Stealman - Did you ever get your question answered? I have been attempting to arrive at heat load values for my residential build and have found the topic fascinating, if not a bit intimidating. Here is a good primer on the basic ideas:
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-1
Next comes the actual calculations for your specific project. I have discovered several tools online that I have used with varying degrees of confidence. Most are based on traditional Manual J heat load analysis. However, data out is highly dependent upon proper data in, and here's where experienced HVAC people clearly have an advantage.
For instance, I have been analyzing my garage design for in-floor radiant heating system, and have arrived at load calculations ranging from 49,000 BTU to 105,000 BTU's. One variable I have discovered that is VERY influential in final output is infiltration/exfiltration. Garages, by definition are very lossy, given big doors that open to the outside, and that are not very easy to seal and insulate well. So, does one use a value for air changes per hour of 1.5? (...typical of an "average" home 50 years ago) Or should my value be higher as it is a garage? Or maybe lower, since I will be implementing "best building" practices. I think I remember reading that a home built today to code in Seattle would have and A/C per hour of 0.3 to 0.5.
Plugging these ranges into the various software tools is where I derive most of my variability in my calculations.
Here's a sampler of some online tools I've found:
http://www.shophmac.com/info-center/hvac-calculators/heat-load-calculator.php
--- very simple. Results fall in line with other tools.
http://www.mrhvac.com/free-hvac-stuff/free-heat-gain-and-loss-calculator/
--- lacks proper instructions. Seems to generate highest load calc answers that are rich with "fudge factors"
http://www.loadcalc.net
--- appears maybe a bit more robust? Answers in range of other calc tools.
http://hvaccomputer.com/
--- downloadable software that cost $50. Very robust. Best answers?
http://www.taco-hvac.com/en/products/products.html?current_category=370
--- another downloaded program, though free. Also very robust, but geared towards commercial buildings. Don't know if tool is reliable for residential applications as it gave some of the highest load calc answers.
Hope you get the help you need!
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-perform-heat-loss-calculation-part-1
Next comes the actual calculations for your specific project. I have discovered several tools online that I have used with varying degrees of confidence. Most are based on traditional Manual J heat load analysis. However, data out is highly dependent upon proper data in, and here's where experienced HVAC people clearly have an advantage.
For instance, I have been analyzing my garage design for in-floor radiant heating system, and have arrived at load calculations ranging from 49,000 BTU to 105,000 BTU's. One variable I have discovered that is VERY influential in final output is infiltration/exfiltration. Garages, by definition are very lossy, given big doors that open to the outside, and that are not very easy to seal and insulate well. So, does one use a value for air changes per hour of 1.5? (...typical of an "average" home 50 years ago) Or should my value be higher as it is a garage? Or maybe lower, since I will be implementing "best building" practices. I think I remember reading that a home built today to code in Seattle would have and A/C per hour of 0.3 to 0.5.
Plugging these ranges into the various software tools is where I derive most of my variability in my calculations.
Here's a sampler of some online tools I've found:
http://www.shophmac.com/info-center/hvac-calculators/heat-load-calculator.php
--- very simple. Results fall in line with other tools.
http://www.mrhvac.com/free-hvac-stuff/free-heat-gain-and-loss-calculator/
--- lacks proper instructions. Seems to generate highest load calc answers that are rich with "fudge factors"
http://www.loadcalc.net
--- appears maybe a bit more robust? Answers in range of other calc tools.
http://hvaccomputer.com/
--- downloadable software that cost $50. Very robust. Best answers?
http://www.taco-hvac.com/en/products/products.html?current_category=370
--- another downloaded program, though free. Also very robust, but geared towards commercial buildings. Don't know if tool is reliable for residential applications as it gave some of the highest load calc answers.
Hope you get the help you need!
rburke65
Well-known member
No rebuttal from Sneezer41 yet?
redsky49
Well-known member
Some time back I posted a detailed description of how to perform a load calculation:
http://www.garagejournal.com/forum/showthread.php?t=47200
Seems though that most board members will not take the small amount of time to perform this critical step. Go figure.
As always, offered only as opinion
http://www.garagejournal.com/forum/showthread.php?t=47200
Seems though that most board members will not take the small amount of time to perform this critical step. Go figure.
As always, offered only as opinion
Stealman - If you don't mind my asking, what did your "hired gun" arrive at for an answer? What size garage? How many overhead doors?
Redsky -yes, very nice overview of back-of-the-envelope heat loss calculations. Absolutely agree that this work is something that should be taken more seriously in new construction..
I find arriving at thermal transmission losses through walls, windows and ceilings to be fairly simple. Losses through floors and basements much more complex, however. And exfiltration/infiltration losses are damn near impossible to accurately calculate. Seems the only accurate way I have read about to get exfiltration/infiltration losses is via a blower door test -- not easily done on a house yet to be built!!
Redsky, you mentioned 100cfm for ventilation loss through an overhead door. Just curious, but what is the source of this value? As an example, my garage volume is 24,300 ft^3, and I have 3 OH doors. 100CFM * 3 doors* 60min/hr equals 18,000 ft^3 per hour. Likewise, calculating for 7 windows and 2 service doors I wind up with 31,800 ft^/hr, which yields as ACH of 1.3 through the fenestrations. Seems pretty darn tight for a garage! However, this does not include leakage through wall/floor intersections, ventilation fans, etc.
Do you have a good feel for ACH typical in new garage construction using "best practices" available these days?
Thanks! As always, insights of those more experienced than I are welcomed.
My rudimentary investigation into this issue has also shown me that slab on grade loss is really pretty substantial, exceeding even my ceiling loss by a factor of 2 (heated slab with 2" XPS versus R30/drywall/2x5 24" oc).
And when it comes to calculating loss through a basement here in MN!? Ugghhh!
Anyhow, just some random musings on heat load calculations. Best of luck to anyone venturing into this territory for their build...
And when it comes to calculating loss through a basement here in MN!? Ugghhh!
Anyhow, just some random musings on heat load calculations. Best of luck to anyone venturing into this territory for their build...
JakeKohl
Well-known member
I found this software worth the $50 limited time license. Pretty easy to use and easy to cycle through different build options (even building orientation to the sun).
redsky49
Well-known member
[
Redsky -yes, very nice overview of back-of-the-envelope heat loss calculations. Absolutely agree that this work is something that should be taken more seriously in new construction..
Redsky, you mentioned 100cfm for ventilation loss through an overhead door. Just curious, but what is the source of this value? As an example, my garage volume is 24,300 ft^3, and I have 3 OH doors. 100CFM * 3 doors* 60min/hr equals 18,000 ft^3 per hour. Likewise, calculating for 7 windows and 2 service doors I wind up with 31,800 ft^/hr, which yields as ACH of 1.3 through the fenestrations. Seems pretty darn tight for a garage! However, this does not include leakage through wall/floor intersections, ventilation fans, etc.
Do you have a good feel for ACH typical in new garage construction using "best practices" available these days?
Thanks! As always, insights of those more experienced than I are welcomed.[/QUOTE]
You raise several good points, that will likely take this thread off topic.
Give me a couple days to free up some time and I think I will post a new entry about infiltration - for those so interested
As always, offered only as opinion
Redsky -yes, very nice overview of back-of-the-envelope heat loss calculations. Absolutely agree that this work is something that should be taken more seriously in new construction..
Redsky, you mentioned 100cfm for ventilation loss through an overhead door. Just curious, but what is the source of this value? As an example, my garage volume is 24,300 ft^3, and I have 3 OH doors. 100CFM * 3 doors* 60min/hr equals 18,000 ft^3 per hour. Likewise, calculating for 7 windows and 2 service doors I wind up with 31,800 ft^/hr, which yields as ACH of 1.3 through the fenestrations. Seems pretty darn tight for a garage! However, this does not include leakage through wall/floor intersections, ventilation fans, etc.
Do you have a good feel for ACH typical in new garage construction using "best practices" available these days?
Thanks! As always, insights of those more experienced than I are welcomed.[/QUOTE]
You raise several good points, that will likely take this thread off topic.
Give me a couple days to free up some time and I think I will post a new entry about infiltration - for those so interested
As always, offered only as opinion
BadgerBoilerMN
Well-known member
We use Wrightsoft with the dedicated Uponor module for our radiant floor heating design work. As you can see, all questions answered including specification for PEX tube diameter, length, flow, and water temperature.