Need help with Heat Load analysis

[sands35]

I'm starting preliminary planning work for a garage. About 950-1000 sq feet. Still negotiating with my wife on how much patio area to take. Need to get a survey done as it will be close to the sidewalk easements on the side yard (house is on a corner lot).

Anyway, doing the heat load analysis to make some basic decisions on winter heating. The biggest swing seams to be the "U Value" of the floor. I spent a few hours digging around the internet and didn't find too much.

I'd really like to have radiant floor heat, but it all comes down to investment costs vs. operating costs.

The garage isn't going to be that far from my house, so a gas line would be in the 30 ft. range run. That said, I can always do electric water heat for the floor.

Depending on assumptions, the heat load can swing from ~25kBtuH to ~45BtuH.

Wondering if somebody has a BTU H / Ft^2 F value for different concrete slab designs?

 

[Highbeam]

A common problem with heat load analyis is that no two approaches will yield the same answer. You'll end up guessing in the end. Many folks on this site have come to this conclusion and so you see rules of thumb like 10-30 btu per SF. You want to know your heat load so that you can size your equipment but does anybody have a tried and true program that is available to us for use?

 

[wedge40]

I've been doing this excact same thing for a few weeks now. Nothing generic out there, but hey if you want if right down to a gnat's a** and know Manual J requirements, you're good to go.

I used a free download of looCAD to layout my pex, and it gave a number for the heat load. Seems pretty low though, also used the 30BTU/sqft and it seems better.

These are two things that seem be to magic. Pex layout and heat load.

Wedge

 

[Dick in Wisconsin]

I used a free download of looCAD to layout my pex, and it gave a number for the heat load. Seems pretty low though, also used the 30BTU/sqft and it seems better.

What temp (1) are you trying to maintain in the garage and (2) do you presume outside?

Maintaining 55* in the garage while its 30* outside is a light challenge.

Maintaining 55* in the garage while its -10*F outside is a greater challenge.

Maintaining 72*F in the garage while its -15*F outside is a huge challenge.

At least that is what I'd think. I had done heat loss calcs on houses back in the late 70's and early 80's and while I don't think much has changed ... I never did them with the heat source in a concrete slab on the ground.

While rules of thumb/SF are fine ... you can have 1000sf that is a square (less amount of perimeter) or long and skinny (most amount of perimeter) and I would expect using the same building techniques and insulation, thermal breaks that the heat loss would be greater with the greater perimeter ... even if the SF is identical.

But maybe 30 Btu/sf is in the middle enough for our purposes.

So at 30 Btu/sf, if my garage is 1,000sf ... my "design load" is 30,000 Btu.

Looking at a 50 gal Rheem gas water heater on Menards web site, it is 40,000 Btu and it says it has a "0.67 Energy Factor". Does that mean I put 40,000 Btu of natural gas in the water heater and get 67% out of it in hot water? Or 26,800 Btu.

If that is the case, then this water heater will be "OK" unless the outside temp gets really down where the water heater will run continuously and not be able to maintain my desired temperature; or I want it really hot in the garage and its cold outside.

I am operating on the premise of Btu's in equal Btu's out.

I stand ready to be criticized, corrected, educated, and possibly vilified.

 

[sands35]

Yeah - I've looked at ~3-4 programs and they range from 20k BTU to 45k BTU total heat load.

Located in SW Michigan. Not *that* cold, but we do get lake effect snow. Typical winter day is ~20*F. Occasionally down into the teens. Rarely into single digits.

Working in ~55-60*F is just what I want for the winter.

Garage will be about 24' (16' door) x ~40' deep. *Might* be a ~12x12 cut out at the back left corner to clear a patio and where i think the house electrical is running (need to find out exactly).

Foundation will likely be a frost protected shallow foundation. Haven't gotten close to the step of deciding stuff like insulation packages, etc. Need to do a site plan 1st, then start calling contractors.

The OTHER way is to simply copy somebody else's ~900-1,000 sq ft radiant heat system. More or less just need to know the size of the heat source and the PEX layout. (Thanks for the tip on LoopCAD - 30 day trial for free)

 

[Highbeam]

Ah, you now tell us this. You do not need a heat load for laying out the pex in the slab, there are standard methods used for most buildings. The heat load helps you size the boiler.

In the slab, use 1/2" pex, 12" spacing, loops no longer than 300 feet and all loops within 10% of each other in length. In a garage, in the US, this is standard and sufficient for any normal scenario. The slab part is easy. You will note that at 12" spacing that a 1000 SF building needs 1000 LF of PEX. You'll also note that they sell convenient 300 LF rolls of pex.

 

[Charles (in GA)]

Knowing where you live would help greatly with ideas from GJ members. Slab heating doesn't make sense in Florida, no matter how much you really think you need it, while in Alberta, its a great idea.

Charles

 

[jvitez]

Regarding energy factor ratings of water heaters: it's not combustion efficiency! Steady state combustion efficiency of a regular old gas water heater is about 75%. It reaches steady state after all the cold metal in the burner and the flue get heated up. The time for this varies depending on surrounding air temp.

The 0.67 means only 67% of the NG you burn heats the water over time. It includes combustion efficiency plus standby heat loss. EF's are lower for regular draft gas water heaters because the flue creates a nice pipe for cool air to whistle through cooling the water. So not only do you have standby losses from the exterior of the water heater but also through the flue. Electric water heaters don't have a flue and therefore have higher EF ratings.

 

[sands35]

I live in SW Michigan (post #5 )

I understand that the design of the slab is basically standardized. What I would like to know is the size of the water boiler.

Too big and I spend too much money and it never gets up to full efficiency. Too small and it runs too long and its life is shortened.

I also need to make a basic decision on NG or electric. Run a gas line and provision the roof for a flue or run a slightly bigger electrical panel and the costs that go with more amps into the garage. I'm leaning to NG for a variety of reasons.

 

[wedge40]

You will also need a load calc to help size the pump. At least that's my understanding.

Wedge

 

[Charles (in GA)]

I live in SW Michigan (post #5 )

Sorry, I missed that earlier. You really should update your profile so it will appear on the left below your name and avatar, and with your post count. Makes it easy to find.

Charles

 

[RCStocker]

Don't go electric. It will break the bank. You have gas use it!

 

[sands35]

Sorry, I missed that earlier. You really should update your profile so it will appear on the left below your name and avatar, and with your post count. Makes it easy to find.

Charles

No problem, Updated it.

Still need to know a typical U value for heat loss through the floor and/or what BTU size boiler people use for a ~950-1,000 sq ft garage with radiant heat.

 

[stingry]

Still need to know a typical U value for heat loss through the floor and/or what BTU size boiler people use for a ~950-1,000 sq ft garage with radiant heat.

Never ran across the term "U" value, so I googled it. It is defined as a measure of thermal conductivity and is the inverse of the "R" value, or U=1/R. That being said, the U value for concrete is going to be high because concrete has a very low R value, in other words it conducts heat very well and has little resistance to heat transfer. This is why it is necessary to use insulation under the concrete to prevent heat transfer from the heated space to the ground. The greater the R value of the insulation, the smaller the U value is.
If you will post the following, I will run your building thru a radiant calculator I have and see what falls out!

zip code
building dimensions
Ceiling and wall R values
window and door information, including R values
concrete thickness
Insulation thickness below slab
R value/in of underslab insulation
desired interior temp you wish to maintain
do you plan to use glycol and what ratio

Cheers
Steve

 

[wedge40]

Well, I'll jump on that

zip code: 47468
building dimensions: 40'x56'x12'
Ceiling and wall R values: Ceiling-R40+ Wall-R13+
window and door information, including R values
7 windows - 3'x'3 double pane, one 36" steel entrance door, two 12'wx11' garage doors.

concrete thickness: 5"
Insulation thickness below slab" 2"
R value/in of underslab insulation: R10
desired interior temp you wish to maintain: 60
do you plan to use glycol and what ratio: plain water.

I've had heat loads from 19,987BTU/H to 89,600BTU/H
Thanks.

Wedge

 

[stingry]

Well, I'll jump on that

zip code: 47468
building dimensions: 40'x56'x12'
Ceiling and wall R values: Ceiling-R40+ Wall-R13+
window and door information, including R values
7 windows - 3'x'3 double pane, one 36" steel entrance door, two 12'wx11' garage doors.

concrete thickness: 5"
Insulation thickness below slab" 2"
R value/in of underslab insulation: R10
desired interior temp you wish to maintain: 60
do you plan to use glycol and what ratio: plain water.

I've had heat loads from 19,987BTU/H to 89,600BTU/H
Thanks.

Wedge

I'll try to get to it tonight if possible. Do you know the R value of the garage doors?

Cheers
Steve

 

[wedge40]

Sorry Steve I do not know the r-value the garage doors.
I believe the paper work is there, so I can look and see if it's was included with the specs.

Wedge

 

[sands35]

U = 1/R is what I came up with too. So R10 for 2" = 0.10

The analysis I have suggests a 25k BTU load. But I do see a pretty big swing in the different methods and assumptions.

zip code - 49085
building dimensions - 26x40
Ceiling and wall R values - R40, R19
window and door information, including R values - 1 16' garage door, 1 man door, 3 windows.
concrete thickness - 5"
Insulation thickness below slab - (2" eps)
R value/in of underslab insulation - R10(?)
desired interior temp you wish to maintain - 55-60
do you plan to use glycol and what ratio - probably, 20/80

 

[wedge40]

Steve, took some searching on the net.
Looks like the R-Value of the doors is 8. Not great, but I can either add or in a pinch put panels over the doors, that can be moved out of the way when I need to use them.


Wedge

 

[Highbeam]

Steve, took some searching on the net.
Looks like the R-Value of the doors is 8. Not great, but I can either add or in a pinch put panels over the doors, that can be moved out of the way when I need to use them.


Wedge

That's really not bad for a door since even 2" of straight foam only gets you R-10.

 

[wedge40]

I've seen people add insulation to the door, but what about the sides and bottom? How well are they sealed?

Wedge

 

[brewchief]

Well, I'll jump on that

zip code: 47468
building dimensions: 40'x56'x12'
Ceiling and wall R values: Ceiling-R40+ Wall-R13+
window and door information, including R values
7 windows - 3'x'3 double pane, one 36" steel entrance door, two 12'wx11' garage doors.

concrete thickness: 5"
Insulation thickness below slab" 2"
R value/in of underslab insulation: R10
desired interior temp you wish to maintain: 60
do you plan to use glycol and what ratio: plain water.

I've had heat loads from 19,987BTU/H to 89,600BTU/H
Thanks.

Wedge

I did a quick check with my program and came up with 49k

U = 1/R is what I came up with too. So R10 for 2" = 0.10

The analysis I have suggests a 25k BTU load. But I do see a pretty big swing in the different methods and assumptions.

zip code - 49085
building dimensions - 26x40
Ceiling and wall R values - R40, R19
window and door information, including R values - 1 16' garage door, 1 man door, 3 windows.
concrete thickness - 5"
Insulation thickness below slab - (2" eps)
R value/in of underslab insulation - R10(?)
desired interior temp you wish to maintain - 55-60
do you plan to use glycol and what ratio - probably, 20/80

A quick check on this one showed 28k

Both are ballpark figures, without a ton more details I can't get much closer and honestly I could spend hours on it and not have it change enough to matter much and my program is not designed around doing radiant, it is a pretty decent program however(it is not a freebie off the net, IIRC it was a 1000$-1500$ program).

 

[wedge40]

Thanks Brewchief. I was going to use 50KBTU/H .

Brewchief = You a micro brewer?

Wedge


PS Second try this morning. Looks like GJ is having some issues.

 

[brewchief]

Thanks Brewchief. I was going to use 50KBTU/H .

Brewchief = You a micro brewer?

Wedge


PS Second try this morning. Looks like GJ is having some issues.

No not a micro brewer, name is a holdover from days of snowmobile racing, I made sure we always had plenty of beer for after the races.

 

[stingry]

I did a quick check with my program and came up with 49k


Both are ballpark figures, without a ton more details I can't get much closer and honestly I could spend hours on it and not have it change enough to matter much and my program is not designed around doing radiant

I believe this is one reason people get such a range in the heat loads. You must use a program especially designed for radiant heat since it is fundamentally different from warmed air heat. That being said:

Wedge40, I ran your figures thru the design program I used to design the floor heat for my 3600 sq ft shop. I used a program called Radiantworks Pro that is provided by Watts Radiant, a well know name in the plumbing and heating fields. You can download it yourself from here: http://www.wattsradiant.com/support/radiantworks/
Am I radiant heating expert? NO! I do have a degree in Mechanical Engineering so I do have a fairly good understanding of Thermo and fluid dymanics and I did quite a bit of research when designing my floor heat setup. The program seems to act correctly when design parameters are changed, so I am fairly confident that it is a fairly good program.

Now for your building. I used Bloomington as a location and the program gave a design temp of 0 deg with a 9mph wind. Using your numbers, it spit out a 25,000 BTU/hr heat loss at the design parameters. It also called for 8 300' loops of 1/2" PEX. You would need 2.7GPM of fluid with a head loss of 1.1 Ft. 80 degree inlet water temp would be required to provide a delta T of 20 degrees. I interpret this to mean that if it is 0 degrees, you would need to pump 2.7 gpm of 80 degree water full time to maintain a 60 degree inside temp.

This was interesting, your heat loss would breakdown as follows:
overhead doors 26%
ceiling 18%
walls 43%
windows 9%
door 3%

Sands35, I'll do yours later but now is time to watch Husker football



Cheers
Steve

 

[wedge40]

Thanks Steve.
Now I have two different Loads. One at 50K and another at 25K.
You are right I'll have 8 loops, one zone with 6 loops with the longest being 282 feet and another zone with 2 loops with the longest being 232 feet. I came up with about 9 feet of head though. Guess I'll have to redue using 25KBU/H for the heat load.

Wedge

 

[stingry]

U = 1/R is what I came up with too. So R10 for 2" = 0.10

The analysis I have suggests a 25k BTU load. But I do see a pretty big swing in the different methods and assumptions.

zip code - 49085
building dimensions - 26x40
Ceiling and wall R values - R40, R19
window and door information, including R values - 1 16' garage door, 1 man door, 3 windows.
concrete thickness - 5"
Insulation thickness below slab - (2" eps)
R value/in of underslab insulation - R10(?)
desired interior temp you wish to maintain - 55-60
do you plan to use glycol and what ratio - probably, 20/80

ran your numbers, assumed a couple of things like 10' sidewalls and 3'x3' double pane windows, etc. The program I used calculated a heat loss of approx. 15,000 BTU/hr. Again, if you want to play around with this program, it is available here:

http://www.wattsradiant.com/support/radiantworks/

Cheers
Steve

 

[tdkkart]

With "heat load" right in the title, I would have figured Mr "ya gotta do a heatload" BadgerBoiler would be all over this thread??

 

[wedge40]

With "heat load" right in the title, I would have figured Mr "ya gotta do a heatload" BadgerBoiler would be all over this thread??

Not for Free....

Wedge

 

[tdkkart]

Not for Free....Wedge

Exactly........just the kind of "professional" I want to deal with.

 

[Dick in Wisconsin]

24BTU/SF.

Mr. Sands. I live in 53024. My garage will be very similar to Mr. Wedge's. Could you run the calcs for using his stats and my zip code?

Thanks!

 

[brewchief]

Ok the large differences were bothering me a bit so I spent a little more time on both and found that while I had accounted for the under slab insulation I missed the edge insulation, my program also only accounts for 4' of insulation around the edge of the slab but asks for soil type, I added the edge insulation and changed the soil type to dry and sandy which is as close to full under slab insulation as I could get. I had also run the prior numbers at a 65 degree indoor temp, I dropped that to 60.

Sands35 came in at just under 16k
Wedge40 came in just over 30k

Changing the direction the building faces as well as minor changes in building materials can make a noticeable change, it would be hard to get it perfect without a blueprint and exact bill of materials. Most programs will also have some amount of slop built in.

 

[brewchief]

24BTU/SF.

Mr. Sands. I live in 53024. My garage will be very similar to Mr. Wedge's. Could you run the calcs for using his stats and my zip code?

Thanks!

I reran my numbers using Sheboygan and came in just under 38k

 

[Dick in Wisconsin]

I reran my numbers using Sheboygan and came in just under 38k

Thanks! 1,000sf? 25x40

 

[wedge40]

Ok the large differences were bothering me a bit so I spent a little more time on both and found that while I had accounted for the under slab insulation I missed the edge insulation, my program also only accounts for 4' of insulation around the edge of the slab but asks for soil type, I added the edge insulation and changed the soil type to dry and sandy which is as close to full under slab insulation as I could get. I had also run the prior numbers at a 65 degree indoor temp, I dropped that to 60.

Sands35 came in at just under 16k
Wedge40 came in just over 30k

Changing the direction the building faces as well as minor changes in building materials can make a noticeable change, it would be hard to get it perfect without a blueprint and exact bill of materials. Most programs will also have some amount of slop built in.

Thanks brewchief, but I'll probably still use about 40K for the baseline. If the pump has to run faster or I have lower the water temp then so be it.


Steve I have question? You mention 80 degree inlet water. Is that what they are expecting out of the boiler/heater? Or is that the water temp returning to the heater. Seems pretty low, but I guess it would give me some leeway during the final phases of the barn.

Again I want to thank everyone for taking the time to do this.

Wedge

 

[stingry]

Thanks brewchief, but I'll probably still use about 40K for the baseline. If the pump has to run faster or I have lower the water temp then so be it.


Steve I have question? You mention 80 degree inlet water. Is that what they are expecting out of the boiler/heater? Or is that the water temp returning to the heater. Seems pretty low, but I guess it would give me some leeway during the final phases of the barn.
Again I want to thank everyone for taking the time to do this.

Wedge

I don't really have a good answer for this. It seemed low to me and in reality I think a higher inlet temp is more practical. The only thing I can come up with is that this would be the required inlet water temp to heat your space at 0 degrees outside temp with the system running constantly. Remember, the heat load estimate is for the "worse case" scenario and is used so you have a system design that will allow you enough heat input in extreme conditions. At warmer temps, you will only be running part time.

Cheers
Steve

 

[sands35]

ran your numbers, assumed a couple of things like 10' sidewalls and 3'x3' double pane windows, etc. The program I used calculated a heat loss of approx. 15,000 BTU/hr. Again, if you want to play around with this program, it is available here:

http://www.wattsradiant.com/support/radiantworks/

Cheers
Steve

Thanks! More or less what the different analysis packages have said. The long walls face north and south. I've been debating putting some more windows on the southern wall and getting the eave height right for winter solar gain and summer shade. The bummer is that wall also faces the street. It's a quiet neighborhood, but you never know. 20 years of collecting tools for woodworking and cars adds up to quite a bit of money. Also need wall space for storage and whatnot....

 

[Highbeam]

Interesting that wall accounted for nearly half of the losses. In the radiant world I imagine that walls and ceiling both are equally negative per SF since only hot air rises and not radiation.

Oh and Wedge, I love the "not for free" comment. Not all folks know that vendors participate and might, just maybe, be trying to drum up business along with a healthy bias against DIY. Sometimes though, those vendors have very good knowledge from their experiences if they are willing to let you in on their secrets.

 

[stingry]

Interesting that wall accounted for nearly half of the losses. In the radiant world I imagine that walls and ceiling both are equally negative per SF since only hot air rises and not radiation.Oh and Wedge, I love the "not for free" comment. Not all folks know that vendors participate and might, just maybe, be trying to drum up business along with a healthy bias against DIY. Sometimes though, those vendors have very good knowledge from their experiences if they are willing to let you in on their secrets.

It would seem that the big difference between the ceiling loss and the wall loss was in the R value difference, R19 vs R40. I went back into the program and used R40 for the walls and the heat loss was about equal.

Cheers
Steve

 

[wedge40]

Oh and Wedge, I love the "not for free" comment. Not all folks know that vendors participate and might, just maybe, be trying to drum up business along with a healthy bias against DIY. Sometimes though, those vendors have very good knowledge from their experiences if they are willing to let you in on their secrets.

I read every one of boilerbadger's replies, and given that he does this for a living, I take heed in his advice. But Im a little harsh with anyone who either belittles someones elses choice or wose yet out right tells them it wont work. A prime example is using a hot water heater as opposed to using a boiler. I've read numorous post on here and the net where people are succefully using hot water heaters. I'm glad he's here lending his knowledge. I guess it's his wriging style that bother me. LOL

Wedge