Heat loss calculation for atypical types of rooms.

ntonkin

I have an 840 sq. ft. attached garage with heated room above. Walls are R-28, ceiling is 9' with R-20 between a heated room above and R-48 above where the heated room is not overhead.
floor has 2" of foam board around the perimeter with an insulated blanket under the concrete two continuous loops of 3/4" pex within the concrete for floor heating. Two 8 x 10 doors insulated to R=20 and two 40" x 80" south facing double glazed windows. I keep this garage at about 50 degrees.

I have done the heat loss calculations for the rooms in the house but can't find anything to approximate this garage situation.

SuperJ

Assuming your not in Alaska, you heat loss will probably be sub 15,000btu/hr. But the seals on your garage door are probably going to be the biggest wildcard.

have you looked at loadcalc.net?

-You will have to model the garage doors as walls
-I would probably play with the construction quality to compensate for the air leakage of the doors.
-Make sure you enter the correct indoor and outdoor temperatures (top right corner).
-The ceiling area won't contribute to the heat loss since it is a heated area.
-The concrete edge insulation is about R7-10 depending on the foam used. The blanket isn't near as good but it provide a moisture barrier which is useful.

Where are you located? What is your 99% design outdoor temperature?

Here is a sample run.

Brewbeer

The heat loss to the ground is going to be challenging. What is the insulation blanket between the concrete slab and the subgrade?

Gordy

Probably "the barrier" under the slab.

http://www.thebarrier.com/under-slab-insulation.php

SuperJ

Looks like a lot of the R value in "the barrier" comes from having 6" of dry gravel.
http://www.thebarrier.com/pdf/brochure.pdf

Gordy

Yes. Define dry gravel. In the winter.....

hot_rod

Dry gravel doesn't conduct heat? Does dry concrete?

How does one know or assure the gravel is always dry, if you wash the truck in the garage the sub-base could get wet. What about base rock that has fines in it, is that considered gravel?

I'd use R-5 per inch for foam products, if the blankets are 1/2" actual thickness it would be a R-2.5

Gordy

Well dry concrete is more conductive than dry gravel. However we are getting into aggregate graduations, and compaction percentages.

Xps is always a sure thing.

ntonkin

Thanks SuperJ,
I'm in the western end of the Upper Peninsula of Michigan - in Watersmeet township. The design outdoor temp for here is -8.4 degrees. No I haven't looked at loadcalc.net yet, but I will, thanks. The blanket is something I bought at the time (almost 14 years ago) that was supposedly designed to insulate under concrete slab floors. The garage sat unheated for almost 10 years before I hooked up the pex loops to make another zone on the system when I installed a Lochinvar Knight KBN106 boiler to replace my old Weil-Mclain HE-4 about 6 years ago. Since I have a high speed boiler now I can't do everything myself anymore as I don't have the software and instruments required so I've had several "professional" techs to do the initial start-up on the boiler and to try to figure out an issue with one of my zones that had me stumped. Anyway, I'm preparing to get a replacement for the Lochinvar when it's time comes. I've not done a heat loss calc on the house as it is now, but I will give that a try - it has no garage feature though. One of the techs I've had in seems to question if the KBN106 has adequate capacity for my house now. So I'm trying to find out if he's right.
Though we've had a period of below zero weather now for a week or so (25 - 30 below) and the house seems to have no trouble staying warm. We've been burning quite a lot of propane the last couple months and I'm starting to wonder if I don't have the Lochinvar's setup tweeked optimally.

Gordy

Watersmeet......I remember going to one hell of a Boy Scout camp there many moons ago. Does it still exist?? A time I’ll always remember.

ntonkin

Hi Gordy,
What was the name of the camp or what lake was it on? I've lived here since 1980 and went to forestry camp near here in 1972. There are several organizational camps around here but I can't remember if one is a "Boy Scout" camp. My wife says there used to be a Boy Scout camp over on Record Lake near the Sylvania Wilderness.

ntonkin

As I remember, the blanket was a padded affair somewhat like a furniture pad. The sub-grade is about 4 feet (in the back at least) of compacted sand or very sandy material that the excavator hauled into the poured concrete foundation that goes up 5 feet above ground level in the back and less than a foot in the front

Gordy

ntonkin said:

Hi Gordy,
What was the name of the camp or what lake was it on? I've lived here since 1980 and went to forestry camp near here in 1972. There are several organizational camps around here but I can't remember if one is a "Boy Scout" camp. My wife says there used to be a Boy Scout camp over on Record Lake near the Sylvania Wilderness.

Clear water was one of the lakes. There was a chain. It was called North Woods.

Gordy

The compacted sand can have very different insulation properties when wet, or dry.

SuperJ

Another thing to consider, is that it may only take 20kbtu to maintain 50f once everything is up to temp, but the slab and the ground under it may absorb 100kbtu/hr for days before it reaches equilibrium at 50deg F.

At least you have 2inches of edge insulation hopefully it goes down below the slab? That's the most important detail (not that under slab isn't).

Gordy

Unless you have a high water table. Even an uninsulated slab will work. It just loses a lot of energy heating everything it doesn’t need to. Up in your neck of the woods that would be more of a fight with well below zero temps.

I’ve seen a slab ongrade with sand fill accommodate -27 below, and maintain setpoint. All the while having a snowmelt feature around the perimeter of the slab

ntonkin

The slab sets inside an 10" poured foundation with an R-10 foam insulation sheet running from above the slab all the way down to the footing of the foundation. Foundation has graded drainage along the base of the footing on the outside. I don't see how the sand fill could be anything but dry. The slab has a center drain that goes through the foundation to a filtered dry well

hot_rod

really any slab will heater long as you have adequate BTUs to dump and overcome downward losses and heat loads. Uninsulated snowmelt have been installed for years. If we can melt snow while it falls, certainly we can heat and insulated building.

Limiting downward loss will reduce energy consumption and eliminate some "fly wheeling" both up and down.

Some areas now require 3" under slab insulation for their codes, maybe more oil some locations?

ntonkin

I used the Weil McLain heat loss calculation guide and the result (without including the garage) was 78.8 MBH. I know this is high as there were several factors which couldn't be addressed which make my house more efficient. My KBL106 Lachinvar Knight boiler is AHRI rated at 82 MBH Net and even with night temps of 20 - 30 degrees below zero for the last couple weeks, it has had no trouble keeping the house warm (including the garage). Right now the boiler sensor says it's -21 outside and I'm sitting here in the dining room in a tee shirt - perfectly comfortable. The more I look at the sizes of available boilers and the heat loss calculation for the house, I'm becoming convinced that I won't be able to get any closer to the actual heat loss requirement for this house & garage than I have with this boiler I currently own.