I don’t think I can follow your calculation. My version would be:
You are intaking hot wet outside air (wet from both high RH and high temp). You need to cool it and condense a bunch of water out of it. There’s some ratio that’s fixed by the humidity and temperature of the outside vs inside air. I think that’s what you are saying is around 40%? I think actually the number you are giving isn’t what quite this calculation needs, but I’ll run with it anyway.
If all the heat was coming in from outside air (either before turning on AC or from infiltration), then you’d have a fixed ratio of latent to sensible heat removed, so the ratio wouldn’t depend on how much additional infiltration you caused, and we could just ignore humidity when thinking about the efficiency loss.
But in fact some of the heat is coming in from other channels. I guess the other big one is sunlight through windows. That heat doesn’t come with any more humidity. Extra infiltration from 2-hose AC increases how much latent heat you need to remove per unit of sensible heat, by increasing the relative importance of infiltrated air vs sunlight and other sources of heat. So if we just calculate how much extra sensible heat you have to remove, we’ll underestimate the efficiency loss.
The total extra infiltrated heat is about 25% of what the AC removes. At equilibrium, that’s 25% of all the heat gain in the house. If 13% of heat gain is normally from infiltration, then replacing that with 75% normal heat and 25% new infiltration would increase the fraction of heat from infiltration all the way to 35%. (I was super wrong about the 13% going in, I was expecting 25-50%!)
So per unit of heating, you are also increasing the fraction of heat coming from infiltrated air by 22%.
For the heat coming from infiltration, the extra cost of dehumidifying is about 2⁄3 of the sensible heat removed. So per unit of sensible heat removed, you need to remove an additional 15% of a unit of latent heat.
If the AC exhaust was more humid than the inside, then this would be lower, but my sense is that AC exhaust is basically as dry as indoor air?
So the net effect would be to take you from 25% efficiency loss (ignoring humidity) up to roughly 40% efficiency loss, which is pretty huge.
That was a super confusing calculation, definitely beyond my pay grade. I assume I got a ton of numbers/calculations and wrong, that there were much simpler ways to do it, and that this overall computation is likely to be conceptually confused in one or more ways. So I’d be pretty curious for your bottom line estimate or intuition about where it should have ended up.
(But I also understand if you want to stop talking about AC and put this thread to rest...)
I would say that is basically right. AC exhaust is about as humid as indoor air. The fraction of the heating load in the summer due to infiltration really does depend on how tight your building construction is. With the numbers Jeff was assuming for a very old house, infiltration would be a much larger percentage. There are some other sources of heat in a house that come with humidity, such as people and showers, but overall it is much less humidity than bringing in outdoor air (there is heat conduction through the walls, electricity use of lighting and appliances, etc.). So that might mean that it would take you from a 25% efficiency loss (ignoring humidity) up to a 35% efficiency loss, which is still a big deal. But I’m not sure if 85°F in California typically corresponds to 50% relative humidity.
I don’t think I can follow your calculation. My version would be:
You are intaking hot wet outside air (wet from both high RH and high temp). You need to cool it and condense a bunch of water out of it. There’s some ratio that’s fixed by the humidity and temperature of the outside vs inside air. I think that’s what you are saying is around 40%? I think actually the number you are giving isn’t what quite this calculation needs, but I’ll run with it anyway.
If all the heat was coming in from outside air (either before turning on AC or from infiltration), then you’d have a fixed ratio of latent to sensible heat removed, so the ratio wouldn’t depend on how much additional infiltration you caused, and we could just ignore humidity when thinking about the efficiency loss.
But in fact some of the heat is coming in from other channels. I guess the other big one is sunlight through windows. That heat doesn’t come with any more humidity. Extra infiltration from 2-hose AC increases how much latent heat you need to remove per unit of sensible heat, by increasing the relative importance of infiltrated air vs sunlight and other sources of heat. So if we just calculate how much extra sensible heat you have to remove, we’ll underestimate the efficiency loss.
The total extra infiltrated heat is about 25% of what the AC removes. At equilibrium, that’s 25% of all the heat gain in the house. If 13% of heat gain is normally from infiltration, then replacing that with 75% normal heat and 25% new infiltration would increase the fraction of heat from infiltration all the way to 35%. (I was super wrong about the 13% going in, I was expecting 25-50%!)
So per unit of heating, you are also increasing the fraction of heat coming from infiltrated air by 22%.
For the heat coming from infiltration, the extra cost of dehumidifying is about 2⁄3 of the sensible heat removed. So per unit of sensible heat removed, you need to remove an additional 15% of a unit of latent heat.
If the AC exhaust was more humid than the inside, then this would be lower, but my sense is that AC exhaust is basically as dry as indoor air?
So the net effect would be to take you from 25% efficiency loss (ignoring humidity) up to roughly 40% efficiency loss, which is pretty huge.
That was a super confusing calculation, definitely beyond my pay grade. I assume I got a ton of numbers/calculations and wrong, that there were much simpler ways to do it, and that this overall computation is likely to be conceptually confused in one or more ways. So I’d be pretty curious for your bottom line estimate or intuition about where it should have ended up.
(But I also understand if you want to stop talking about AC and put this thread to rest...)
I would say that is basically right. AC exhaust is about as humid as indoor air. The fraction of the heating load in the summer due to infiltration really does depend on how tight your building construction is. With the numbers Jeff was assuming for a very old house, infiltration would be a much larger percentage. There are some other sources of heat in a house that come with humidity, such as people and showers, but overall it is much less humidity than bringing in outdoor air (there is heat conduction through the walls, electricity use of lighting and appliances, etc.). So that might mean that it would take you from a 25% efficiency loss (ignoring humidity) up to a 35% efficiency loss, which is still a big deal. But I’m not sure if 85°F in California typically corresponds to 50% relative humidity.