Design question

Mike Kraft

Chris and Boilerpro,

Chris your explaination is very clear.I understand that.So it would appear to me that when you are doing a heat loss infiltration is not a number that changes room X room.Keeping in mind I am talking about new construction with house wrap, sheathing,siding and insulation.Not a home that is turn of the century and needs a blower door test to be specific.You treat the exposures seperately,panel by panel.The infiltration is a default that remains constant regardless of room X room exposures.

And boilerpro,you seem to be of the school that considers location of rooms (geographicly) and increase the infiltration rate based on the rooms exposures.You duely note the Dead Men and how they planted the larger iron on the first floor (probably didnt want to drag those big monkeys upstairs :) ).Insulation as we know it today was non existent.

I have done renovation jobs in 19th century farm homes and the exterior walls were filled with field stones.The lath was nailed up as they filled it!I dont know how much "new construction" you do boilerpro,but given a new home that has R19 exterior walls,R30 cielings and double pane w/ 1/4" air space would you still use your method of heat loss?

I want to thank you both for your responses.Very sharp and very differed in school of thought!

cheese

Mike Kraft

ACH

When doing a heat loss and you are plugging in the infiltration rates........for convective design example:HWBB are you doing a room X room infiltration rate based on exposures and glass......or are you basing this factor as a default based on general construction of the dwelling?

Then if you are designing for radiant do you lower this rate?and if so how much?Is infiltration considering currents caused by convection?Or is the infiltration rate related strictly to construction?

I havent gotten caught with my pants down and dont want to!But looking to sharpen the lead:)

Thanks Wall........cheese

Chris Maderia

Infiltration

Structural Heat Loss resulta from heat energy that is conducted through the various walls, ceilings, floors, door and windows that exist between the indoor heated space and the outdoors, or another unheated space. The amount of heat that is lost depends upon the indoor and outdoor differential temperature (DT), the amount of surface area exposed, and the specific resistence to heat transfer (R-Value) od the structural materials. To claculate heat loss through a panel, divide the difference in temperature (DT) between the outside and the inside temperatures by the R_Value of the panel. The rsult will be the amount of heat loss (BTUH) transmitted through a single sqft of panel surface per hour. Multiply this number by the total area of the panel to determine the heat loss for that panel. Heat loss for panels with different R_Values such as door, walls, and windows are calculated seperatley.
(Temp Indside - Temp Outside)/R-Value = BTUH per sqft.

In addition to the structural heat losses, a portion of the heat engergy in th eroom will be lost to infiltration and ventilation. The air that exchanges between outdoors and indoors carries with it a certain amount of mass that must be heated to maaintain room confort. Air changes come about by opening and closing doors, leaks around windows and doors, natural and forced ventilation and drawing combustion air for fuel burning heat appliances. In some cases the amount of energy lost to air exchanges is critical. For new construction in resiential bldgs, air changes can be estimated at around 0.5 air changes per hour. In older bldgs with poor fitting windows and poor seals around doors will have a much higher infiltration rates.

Depending on whose heat loss program yo are using your infiltration factor will be different. I use Wirsbo Radiant Express 4.8 for both conventional baseboard and radiant. When using radiant I use a .35 Air Exchange and when doing conventional baseboard I use a .75.

Chris Maderia

Infiltration

Boilerpro

My way of dealing with the infiltration factor

When designing a complete system, I look at room by room. I take into account the quality of construction, but also the exposure of the space. I figuure an average infiltration heat loss rate based on quality of construction, but modify it according to exposssure. Rooms with potentially heavy wind exposure I add about 30 % to the average infiltration rate, and those with more moderate exposures, 15%. On the leeward side of the structure, I either use the average infiltration rate (conservative) or reduce the average rate. I also increase the infiltration rate for 1st floor rooms, since this is where stack effect air leakage will bring in the most amount of cold air, and decrease the leakage for the second floor, since most of the air warmed air from the first floor is leaking out. From looking at some old gravity systems, this appears to be how some of the old timeres did it. Ever notice that in identicaly sized rooms on the first and second floors in old homes built with no insulation, that the rads are bigger by about 10% on the first floor, despite the fact that there is a huge heat loss through the ceiling upstairs that doesn't occur downstairs. Also rads in northwest rooms are protorionally larger. When figuring air leakage volume in large open spaces with air tight ceilings, I only use the outer 15 feet of the space as the area where air exchange occurs.

Boilerpro

Mike Kraft

Point of my post

I'm searching for some input here!Infiltration is a BIG number in the design.This can put equipment sizing up there.I did a loss on a home the other day and I was surprised at the loss.Seemed a bit high.It is a fair bit larger than most homes I size but if I default the home at a fixed infiltration rate it will pull the appliance size down considerably.I am going room by room and defaulting with IBR standard rates.HWBB will be the emitters.

cheese