Transmission Infilitration Ventilation

So, when we're trying to size our systems, when we're trying to understand the scale of the mechanical system that we're really talking about, here, we're gonna have the transmission issues, the infiltration issues, and the ventilation issues. And when we're trying to put all that together, we're gonna start to see that our transmission is gonna be a series of different wall assemblies, floor assemblies, roof assemblies, foundation wall assemblies. It's gonna be a whole bunch of these as different numbers that we're gonna add together, but each one of them is gonna start with the U-value times the area of that particular wall assembly or roof assembly, whatever it is, times the delta-t, the difference in temperature from the target temperature we're shooting for and the design temperature in that kind of worst case scenario outside.

So, in the Minnesota example, where it might be minus 20 and our target temperature is, say, 68, that means we have a delta-t of approximately 88 degrees. So, that tells us that we need to be putting a lot of heat energy into that heating system in order for it to balance out the loss of heat through those walls, through the transmission through the roof assembly, through the wall assembly, et cetera.

So, U, remember, is the same as one over the R, it's the inverse of the R-value. So, the R is representing the resistance of the heat flow. The U is representing the transmittance, how the coefficient of transmittance, how it's gonna let the heat flow go through.

So, how it's gonna let the heat flow go through times the area of that particular wall system, or whatever it is, times the difference in temperature, and that's gonna give us a total heat loss for that particular wall assembly, and then we would do it for all the wall assemblies, all the floor assemblies, all the foundation assemblies. All those different things, we'd get them all. We'd add them all up, and it would give us an overall transmission number.

Now, remember, that mostly this is done by area, by these different wall assemblies and floor assemblies and roof assemblies, et cetera. But there's a few things, slab edge, for example, where it's done by unit-length. The way that heat is lost from a slab on grade is through the edge of the slab and so they just put a unit-length number on it, and so you just multiply that number times the actual distance of the perimeter, around that slab.

So, there's a couple of other examples like that. Not everything falls exactly into the wall assembly area per square foot type situation, but most of it does. And so, you'd add up all those numbers and that would give us our total heat loss for the transmission in Btuh. And then, same thing with the infiltration. So, this is where we have the cracks and all of that and air is coming in, and when air pushes its way in it means some of the conditioned air is finding its way out.

There's a couple different ways to do this. One is called the crack estimation method. The other is called the air change method. The crack estimation method is a little more old-school and kind of fits a little better with kind of older buildings or situations where you're kind of mixing new construction with old construction, something like that. Air change method is more just kind of a general understanding in certain kinds of buildings there's gonna be an approximate amount of an air change in a certain type of room that we just know is gonna happen by the sheer fact that air will infiltrate through.

And so, it's a way of thinking of it as an air change per volume of room versus the crack method, where the crack I'm actually adding up might be around the windows and then around the doors and then at special seams that we know that are likely to be places where the air is gonna be able to infiltrate. So, I literally add up the length of cracks that I imagine, we don't necessarily know they're there, but we imagine they're there, and add those up and then I give a multiplier number per linear foot of crack, or I think about it as a sort of volume of air moving in and a equal volume of air, of conditioned air, moving out.

And so, therefore, how much of an impact would that make? Like I said, newer buildings tend to be a little tighter. Some buildings are better built than others.

It takes a little bit of kind of an art of the process, here. You're sort of adjusting to what you think is likely to be the case knowing the construction, knowing the age of the building, knowing the quality of the contractors, knowing the level of detailing that the drawings got to. There's a whole series of different ways that might impact the idea of infiltration. So, this is one of those things that takes a little bit of adjusting, but you're gonna adjust it to a point where you, then, feel comfortable and then you're gonna do a calculation and that calculation's gonna lead you to some Btuh.

So, we've now got a couple of those numbers. We don't always have the ventilation in the mix, depends on the situation, but the more that we know that we're going to need fresh air and the more that we know that fresh air is not likely to be pre-conditioned in any way. So, if we know that's gonna be the case, then we would have to make up that heat loss and so that's gonna also give us some Btuh.

So, we're gonna get each of these individual numbers, and then we're gonna add them together in order to understand what our total heat loss is per hour. So, that's gonna give us how much heat that system needs to produce. Now, remember, these are all based on a specific delta-t. So, that's gonna be how much that heating system needs to produce at that worst case scenario in order to maintain the building at 68 degrees, or whatever our design temp was.

We could choose a higher design temp. It just means the system's gonna have to work harder on those cold days. So, if it got colder outside, than what we were expecting, for whatever reason. There's a climate change issue, or it was a particularly cold year, and it was, say, 10 degrees colder, well then, the expectation is, well, your system wasn't really designed to be able to go that much colder and, so, it would probably dip below our design temp, but the expectation is that it wouldn't happen very often.

You're using one of the really cold temperatures, in average, over the years. So, you're probably gonna be pretty close to the coldest temperature of the year, maybe even the coldest temperature of the year. So, it probably won't happen too much, but occasionally it might not be quite powerful enough to keep the system at the target temperature always.

But, the idea is that you're really aiming towards that kind of worst case scenario, generalized worst case scenario, so that it can handle that, but that means its oversized for really all the other days. So, you don't want to just start throwing factors of safety on because if you're throwing factors of safety on and saying, "Well, let's make it 50% more stronger "than the system that, you know, it came out to "when we did the calculation, just to be careful.

"We want to make sure our clients get, you know, stay nice and toasty, warm even on those coldest of days." But, remember, that means the system is already acting inefficiently most of the year because it doesn't need to be as powerful a system in the spring and in the fall as it did in the winter. So, it's already acting inefficiently during those seasons that if I make it act inefficiently in the winter, that means it never is acting efficiently and we're actually reducing our ability to get this system to work efficiently and at a low cost.

It's also gonna make the systems turn on and off, regularly, and that's gonna make them last less long. So, we're gonna have issues. We want to keep it as close to our design temp as we can. We don't want to oversize these systems. We want to keep it fairly reasonable, but just hitting that worst case scenario, it's already unreasonable for most of the year.

So, that means we really don't want to make it even more inefficient at the winter at the sort of worst scenario spot. So, that's why we're aiming towards a cold day, one of the really coldest days, but we're not trying to go way beyond that. It's okay if it drop a few degrees on the absolutely worst day. It's okay if you can kind of get through that system that cold front or whatever it happens to be, but, eventually, it'll be back into the normal range and then it'll be acting more efficiently.

So, we have these three different ways. We're adding them all together. We're getting the Btu's out of all of those. We're understanding what the total Btu loss is and, therefore, that is the total Btu per hour amount that our heating system needs to supply in order to withstand that design temperature target on that design temperature coldest day.

Obviously, the same thing is true when we're talking about cooling in the summer. It just reverses all those numbers. This is the way that we start to size those systems.

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