load calculation

Tim_24

calculations, if done thouroughly, include ALL conditioned spaces in the house. If a designer leaves out portions of the house, they have done an incomplete calculation. Every space need not be treated individualy, but must be covered.

Every method requires the user to be experienced and knowledgeable enough to know how to treat the area(s) in question. Else, the computer programs would do it all. I can't agree with your statement that any type of construction is excluded by any specific method. Never actually seen a completely non-heated basement myself. When calculating the perimeter losses for a slab on grade (not including RFH), the losses figured for the first 3 feet of foundation wall would not typically result in greater losses than if the entire floor is figured over a cold, but windless space. How the heat transfer coefficient for that foundation wall is determined is what will determine if you get accurate results or garbage. GIGO (garbage in, garbage out) very specifically applies to heat loss calcs.

PJC

load calculation

In determining "how much" boiler furnace to buy for a brick, 1857 house, how does one calculate "load" ? What is the exact mathematical formula for determining a load calculation? How much weight is given to each of the variables that go into the calculation?

Constantin

You want to calculate the heat loss?

I presume that you mean to calculate the heat loss of your home, which would represent the load that your boiler/furnace has to carry. Calculating the heat loss by hand would be a project, though some Wallies still do it. Tim, for example, states it takes him a few hours to do a whole house, so it can be done, and he claims better results over common computerized approaches.

I used HVAC-Calc to calculate the heat loss of my home since I do not have that kind of patience and also loved the ability to do scenario analysis. The friendly folk at Slant-Fin also offer a free heat loss calculator on their web-site, just click on the "Free Heat Loss Calcs" button in the menu at left. Any reputable HVAC installer will also perform a heat loss calculation as a matter of course, so it's a good way to help weed out rogue contractors. Don't be afraid to pay for it either as even a computer-assisted heatloss can take a while to perform.

For the exact formulas that go into heat loss calculations, I suggest a class in heat transfer that covers radiation, convection, and conduction, as all three factors will help determine the losses (to a greater and lesser extent, depending on what is losing the heat). The tables that are derived from those formulas (and some empirical testing) simplify the heat loss calculations a great deal.

GMcD

Information needed

You need to first total up and itemize the following basic things about the house:

Window area per room, with window thermal perforamnce (R-value)

Opaque wall area per room, with the overall thermal resistance (R-value)

Ceiling/roof area per room with the overall thermal resistance of the roof/ceilingto the exposed roof.

A check of the infiltration rate of the house either by a blower door test, or a rough assumption (assumptions=larger errors).

The basic heat loss calculation is HL= Area x R value x (inside room temp - outside winter design temp).

Infiltration heat loss= cfm airflow x 1.08 x (Inside room temp - outside winter design temp)

That's the real basics, and most computerized programs will use factors tor thermal storage for the "heaviness" of your construction, thermal bridging assumptions, safety factors and other issues. One thing that all the basic heat loss programs fail to include is the radiant cooling heat loss from the cold inside surface temperature of a window. It's usually fudged in as another safety factor. The heat loss should be done on a room by room basis so you can estimate how much heat you need for each room, and by adding them all up, you get the total heat loss and from that you can choose a heating plant size.

Tim_24

My preference is

to calculate loads manually. I don't know if my method produces "better" results than computerized calculation methods, or not. I know exactly how I determine or derive each variable in the equation and therefore I have no uncertainty as to how "skinny or fat" my numbers may be. For homeowners and contractors who are attempting to bring potentially more accuracy to their work, by all means use the technology available to help you out. Understand that the results may or may not be accurate, depending on the input and assumptions used.

Which variables dominate? Or are the most important? For a simple, steady state heat transmission calculation, the variables in the equation are U, A and dT. A is the physical area of the enevelope elements and dT is established by the design conditions, neither of which require much in the way of judgement or experience. Determination of the heat transfer coefficient, U, and also the infiltration/exfiltraion rates will be where the mistakes, if any, are made. This is also where, the program or the user of the program may not have the ability to determine the proper values to use as input. Examining a wall from the exterior may not give the "designer" enough info. Is there insulation in there, 2 or 3 wythes, concrete block with filled cores, etc, ? A similar judgement must be applied to determine air changes due to infiltration. An entire house, taken as a whole may have 1 air change per hour (ach) or it may have 0.1 ACH. For house with a volume of 20,000 cu. ft, in say Madison, WI, the heat loss due to 1 ach would be 28,000 btuh (at 70 inside, -15 outside).

Bob W._3

Maybe a silly question, but this for is a hot water system, not steam?

Tim_24

Equipment selection

would take that into account (hopefully) but heat loss, is heat loss. Whether you going to heat with electric resistence, forced air, hydronics or steam. If you intend to maintain comfort at design conditions, this bounds the requirements of capacity.

Bob W._3

I couldn't tell from the original post whether it was an existing system or not. The house is old enough it could have installed radiation.

D107

variables

I'm also in the middle of calculating home heat loss and intend to do it manually. I introduced myself to the process via the Hydronic Institutes' i=b=r guide's whole house heating calc sheets, but I have ordered Guide H-22 to get it done right room by room.

But it seems no method can cover the vagaries of every house. e.g. I believe closets, small hallways and stairway areas are not included in the room by room method. Also a big question seems to be the difference between a 1) cold floor over a cold basement or vented crawl space in which case the calc is based on square footage; OR 2) the floor on concrete slab ('cold edge only' they call it) which requires linear feet of the entire perimeter. #2 results in a MUCH higher heat loss. In my case the porch floor is raised 6" off the slab--thereby making a crawl space I believe--and insulated, with heating pipes to two radiators underneath.

I will be working with a heating pro on this so I should be able to combine the practical with the theoretical. Another thing to calculate is potential future home expansion and/or insulation and maybe perhaps directions faced by the cold walls?

David

Unknown

Uh, figuring loads for every linen closet and pantry is really not necessary. Incomplete or not, a load calc must also be comprehensible, and a very small fudge factor can easily cover the non-critical spaces of the home. Unless you're planning on adding zones to keep that linen closet at 70 instead of 75, why bother doing the calc for the room? No one cares.

D107

room by room

thanks Tim and Bob. I guess the pro I hope to work with--who has visited the house--asked me for basic room by room volume, cold wall and window/door measurements and probably would factor other considerations into his computer program. I think I misspoke when I said stairway areas etc wouldn't be factored in.

I just got the H-22 guide and forms in the mail and I can see how detailed the process is. I would obtain the raw data and let the pro make the final calculation.

It is said that an old house can reduce heating costs by 40% with proper sealing and insulation. Add to that changing from a 78% efficient 1981 gas boiler to a 98% efficient condensing boiler with cast iron radiators and a good control system, well there's 60% potential savings right there. If this was done by even half the American homeowners the aggregate energy savings would appear to be astounding.