Help with heat load pro

fwmech · September 24

Hello everyone,

I am a new heating contractor in Anchorage Alaska. I spent my career up to the present as a journeyman and foreman plumber/fitter. Since I am on my own now I am trying to learn the design side of the business.

This leads me to my request for the professionals on this site. I recently bought the software from hydronicpros.com and I used it for the first time this week in calculating the heat load of a triplex that my customer needs a new boiler at. The total heat loss number I received is 64,914 BTU/hr which seems suspiciously low considering the old boys I ask who multiply the square footage (3,240sq' for this building) by 60. That old school math gives us 194,400 BTU/hr, or that's how they size boilers. Maybe I'm wrong assuming that the 64,914 number is what I use when I size a boiler??? I would like to give my inputs so someone can double check my result if that's not too much to ask! I used -18F for the outdoor temp and 70F for the room temp.

Upper level: the upper level has exposed ceilings. 1/2" drywall, 10" fiberglass batts, no attic floor, 2x6 wood 16"OC. The wall assembly for the entire building is 1/2" drywall, no interior sheathing, fiberglass batts 5.5", 2x6 wood studs 16" OC, exterior sheathing is 1/2" plywood, hardboard siding .44"

UL kitchen: one exposed wall 8' ceiling 22' long. Two windows 4'x8' each. All windows in the house are of the double pane style, I chose the generic double glass r 1.81, no removable glazing panel. 22'x23' exposed ceiling

UL living room: 30'x22' exposed ceiling. 8'x22' exposed wall, no windows. 8'x30' exposed wall with two windows 5'x7' each and two exposed doors. The doors are each double glass 1.81, 2'x5' glass area, steel door r1.0, steel door panel r 1.67.

UL master bedroom: 30'x24' exposed ceiling. 8'x24' exposed wall no windows. 8'x30' exposed wall with one window 3'x10'

UL bathroom: 8'x12' exposed ceiling. 8'x12' exposed wall with one window 3'x5'

UL bedroom 2: 12'x12' exposed ceiling. Two exposed walls 8'x12' with each having one window 4'x8'

Home foyer: 12x20 exposed ceiling. One exposed wall 18'x18' high with one window 3'x5', one exposed door 2'x5' glass.

For the mid and lower units I was not allowed access so I used the outside dimensions and counted windows. The mid and lower levels have three exposed walls, the fourth is adjoining a garage that has a separate heating system.

Mid wall 1: 8'x27' exposed wall with one 4'x5' window and one 5'x6' window

Mid wall 2: 8'x28' exposed wall with one 2'x3' window

Mid wall 3: 8'x27' exposed wall with one 3'x6' window

For the lower level I used for the exposed floor: carpet 1/4" nylon r 1.44, under slab insulation expanded polystyrene r1.9. The house was built in 1965 so I am actually unsure on the floor construction or its insulation. The Lower level is a daylight basement so I used a masonry wall construction detail even though the upper third of the wall is framed. The wall assembly I configured as: 1/2" drywall, no interior insulation, 8" masonry block, 1/2" polystyrene exterior insulation, aluminum sheet exterior siding.

Low wall 1: exposed floor 27'x28', 8'x27' exposed wall with one 3'x5' window, one 4'x5' window

Low wall 2: 8'x28' exposed wall, two windows 6.5sq' each

Low wall 3: 8'x27' exposed wall with one 3'x5' window

Please help me out, guys. I'm struggling with this one!

Thanks, Mark

EdTheHeaterMan · September 24

I didn't take the time to read all the inputs, but the gist is that you believe that the numbers are low for the size and location of the place being heated. That happens a lot.

I have used different Load Calc software and each one is a little different but the bottom line is that they all use the ACA Manual J numbers for the calculation. You need to know how to work those numbers so you can get comfortable using the software you have. One of the big factors is called "Infiltration". That is actually a number of air changes per hour. That is outside cold air replacing inside heated air. In new construction that number can be as low as .25 air changes per hour. In an old barn that can be as high as 5 times per hour. So you need to decide how tight the building is based on experience and inspection of the structure. Do you feel cold drafts near the closed windows or not? Is there insulation in the walls? I would remove an outlet cover or switch plate and stick a hook made of a wire coat hanger in there to see if I could pull some insulation from behind the wall. It won't tell you the R value but it will tell you that there is something there and based on how old the building is, you can make an educated guess as to what the building code required back then.

Using a fixed BTU per sq ft number lumps tight new construction in the same category as a 150 year old uninsulated building. That can't be right. You will need to find a number that fits the structure and once you have that, you can bet your reputation on it. I also didn't use the recommended 14°F as my southern New Jersey design temperature. That is because I didn’t want to get a call from someone about the heater being too small when it went to -5°F as it does once in a while around here.

So you need to use the software with a little common sense and not follow the letter of the law because that may end up with a heater that is too small. A little bigger is probably OK for now. Eventually you will get some experience and start to get more jobs based on lower price smaller heaters that work just fine.

fwmech · September 24

https://forum.heatinghelp.com/discussion/comment/1865884#Comment_1865884

Hi Ed,

That sure helps a lot and I appreciate your comment.

I bumped up the ACH to 2, a middle ground number considering that there is insulation in the walls and while the windows are crummy, they are next on the customer's list to be replaced. Using the 2 ACH I have a much more realistic number to use.

Thanks again!

Kaos · September 24

A quick check for load on existing structure is last years winter fuel use. Heat load is approximately Therms*42.

If this number is near your man J, the calculations are correct.

60BTU/sqft is the heat loss for a pole barn with no insulation. Houses have come a long way, even 20BTU/sqft is high for newer construction.

EdTheHeaterMan · September 24

Great. 1965 construction might be closer to 1.5 ACH but go with your gut. If you get the job and install the system, You will want to observe it when it gets really cold. if it cycles off when the outside temperature gets close to design temperature that will tell you how much over sized the boiler is.

10 minutes on, then 10 minutes off by the high limit = 100% oversized

15 minutes on and 5 minutes off by the high limit = 50% oversized.

If you get anything longer than a 20 minute run time before it gets to go off by the high limit, I would call that a success. if you get a 30 minute run and a 3 minute off on the coldest day of the year, then You hit the nail on the head. Perfect load calculation and boiler selection.

Of course if you are installing a modulating boiler that adjusts the input based on the load, then you will never gat short cycling because the input adjusts based on the needs of the building.

fwmech · September 24

https://forum.heatinghelp.com/discussion/comment/1865889#Comment_1865889

Ed, that is some really great information!

It's a conventional boiler, a Weil Mclain CGa. I understand that there is an optional outdoor reset that I was planning to install. I'm hoping that the outdoor reset will let it "modulate" but I'll have to get real world data, like you're suggesting!

EdTheHeaterMan · September 24

With a CI Boiler like the WM CGa the burners are off/on. The flame does not modulate or change input. That means the if you use ODR, the burner will operate at full input capacity until the water temperature reaches the desired set point and the burners will shut off. That is not a bad thing… I recommend ODR for fuel savings. It just limits the boiler temperature from getting up to the high limit setting of let's say 190° when all you need is 140° water in the radiators to heatthe home on a mild day.

With a CI Boiler you must create a higher temperature return as quickly as feasible to prevent flue gas condensation form corroding the heat exchanger. This means the the ODR must be set with a minimum return temperature of about 130°. That means when you only need 120° radiators, the boiler must operate at 140° and cycle off of that minimum reset water temperature. Once your water temperature requirements are 140° and above, then the circulator may run more with the burners off when the water temperature gets to the reset temperature of say 145° or 150° or 160° as needed. You may never get the actual boiler temperature to the 190° limit setting with this energy saving ODR control.

Help with heat load pro

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