6 ms·

The fridge isn't the biggest energy hog in the house, but I'm very sympathetic to the absurdity of heat levels in a house. I often see my HVAC cooling when the

by AlanSE 4y ago

The fridge isn't the biggest energy hog in the house, but I'm very sympathetic to the absurdity of heat levels in a house. I often see my HVAC cooling when the set-point temperature is actually _higher_ than the outside temperature. Logically, the house is a heat generator, it makes sense physically. The roof is black, etc. It would offer a good number of benefits if the system could outright open a duct to the outdoor air, and suck it in whenever the local outside temperature is within the range requested by the user. People who are into optimizing energy use (they exist) can go even further and pre-cool their house during the night in summer. For this to work, all you need a pusher fan, no refrigeration at all. There might be some pressurization problems, like, you may need a duct both for the intake and outlet. Also might require another filter... but air quality would improve significantly. This is a really "dumb" idea, but it's perfectly in-line with all the new ideas being thrown out there. The new ideas just tend to throw in an additional heat storage mechanism, like a water tank (in the article). You can get a lot more efficiency gains by saving the night's cold in a tank and using it through the day. But on a more basic level, you can pump straight into the house when the conditions are right.

ballenf 4y ago
I've often wondered if opening windows while AC is running and outside temp is below inside temp is less efficient than keeping them closed. My thought is I should keep them closed due to extra load on the AC to dehumidify the outside air. Or open windows and turn AC to fan-only mode to prevent stagnant air in rooms without windows.
- soco 4y ago
  Is the AC dehumidifier an extra feature/task, so actively followed, or is it doing dehumidification just as a byproduct happening while cooling?
  - rstupek 4y ago
    It's a byproduct of the process of cooling the air
  - yetihehe 4y ago
    Some AC units have two modes - cooling and dehumidifying. When in cooling mode it will blow a lot of air to keep evaporator as hot as possible, so less humidity is condensing on it (but there will still be some). In dehumidifying mode it will blow less air but keep evaporator cool, so more condensation for the same amount of air cooling.
    - teo_zero 4y ago
      Do you have any source for this? It's a topic I've always wanted to understand better. Especially, it's not clear which mode consumes less.
      - yetihehe 4y ago
        Source - I have Kaisai mini-split at work and I've observed how each mode works (with my hand). There's probably more energy used per cooling amount during dehumidifying, because it's bigger temperature differnce (which means lower COP) PLUS condensing water (water phase change energy is big).
- jdmichal 4y ago
  Unless you have a very strange setup, centralized A/C does not have humidity sensors nor run just to dehumidify. Nor does removing humidity increase the load on the A/C system. Removing humidity is just a happy by-product of how centralized A/C works. So no, you would not be increasing load for that reason.
  - TeMPOraL 4y ago
    Happy in some times, unhappy in others. There are days I'd love if the A/C would have a humidifier built-in, for those winter months when you might want to use A/C to warm the interior.
    - ComputerGuru 4y ago
      A good chunk of the United States (and a number of other countries) uses forced air natural gas heating. If you fall into that group, you can install a "whole-house humidifier" that injects steam into the hot air plenum, distributing it around the house. This makes the air less dry (obviously) but also has the side-effect of actually reducing the need for heating because air holding moisture feels warmer than air without (hence the oppressive heat in the humidity of summer), meaning you can lower the thermostat. Unfortunately I have only seen commercial humidification units that boil water into steam with the use of natural gas. Without exception, everything for sale for home use uses electricity (you'll need 230V minimum, single phase will do just fine) to boil the water, which is costly (though this year natural gas prices have risen or even doubled, but even in states with cheap electricity it's probably still cheaper to use natural gas).
      - mmaurizi 4y ago
        We run our steam humidifier just fine on 120v, but we have a relatively small townhome at 1400sqft
        ComputerGuru 4y ago
        Be careful you’re not running it independently of whether or not the furnace is actually on. 120V generally can’t evaporate enough water in the short window while the furnace is running and many techs will set it up to run at all times (with the blower but not the furnace) to compensate. This lets mildew or mold grow in your ducts.
ComputerGuru 4y ago
I've built/retrofitted and a prototype of mechanically operated louvres with push/pull fans for air exchange in an old school building, tied to the thermostat, ac, and a CO2 detector. The idea of doing the same in my home has been taunting me for years now. Ideally you'd have two such louvres, one with a push fan in the upper floor and the other with a pull fan in the lower floor to simultaneously eject unwanted heat, bring in fresh air, and boost whole-house circulation. They'd be set up to interface with the thermostat/hvac and would operate when the outdoor temperature at intake is lower than the temperature at exhaust and both are above the set point on the ac. The biggest problem is really one of convenience. You'd need a filter on the intake and a rather large and powerful fan to overcome that static pressure - ergo, a noisy one. And you'd probably have to fully dismantle the system in the winter to prevent the cold from getting in (the Midwest is cursed with both hot and humid summers and cold and dry winters). It just end up being the kind of thing where the devil really is in the details and you either do it right and it's a huge undertaking or you do it fast and sloppy and its drawbacks won't be worth it. But I agree, nothing is more infuriating than seeing the AC on and the outdoor air temperature being lower than that of the home. And opening windows just doesn't make a difference since in most 20th century homes there's just poor airflow and no circulation.
- Tijdreiziger 4y ago
  What would the upside be over a standard heat-exchanging mechanical ventilation system?
- landemva 4y ago
  Whole house fans are often used in summer at dusk to rapidly cool house. Air is sucked through house into attic, which also cools the attic. https://www.thespruce.com/whole-house-fan-vs-attic-fan-differences-5114299 https://www.thespruce.com/whole-house-fan-vs-attic-fan-diffe... Crack open a window downstairs before switching it on.
  - sidewndr46 4y ago
    Those usually function as a giant hole in the house insulation when not in use.
elil17 4y ago
That exists, it's called an air-side economizer. It's used a lot on big buildings.
cypherpunks01 4y ago
You are exactly describing a HVAC "Economizer" as they are called. I believe they are almost solely used in commercial HVAC installs, probably due to price/additional install complexity? They are very neat though, and do save significant energy in shoulder seasons, or at night in the summer as you mention.
wffurr 4y ago
You are looking for a ventilation system like a HRV.
stephen_g 4y ago
The function to just bring outside air when it makes sense is an extremely common feature in commercial HVAC systems - it’s called ‘night purge’. Most of these systems have a damper that the system can use to choose how much fresh air is used or how much return air is recirculated so it’s not difficult to use that to just put fresh air in directly with no cooling or heating. Even systems with heat or energy recovery will often have a bypass damper. You need the humidity to be in the right range as well as the temperature, but it can save a lot of energy!

by AlanSE 4y ago

ballenf 4y ago

I've often wondered if opening windows while AC is running and outside temp is below inside temp is less efficient than keeping them closed. My thought is I should keep them closed due to extra load on the AC to dehumidify the outside air. Or open windows and turn AC to fan-only mode to prevent stagnant air in rooms without windows.

soco 4y ago
Is the AC dehumidifier an extra feature/task, so actively followed, or is it doing dehumidification just as a byproduct happening while cooling?
- rstupek 4y ago
  It's a byproduct of the process of cooling the air
- yetihehe 4y ago
  Some AC units have two modes - cooling and dehumidifying. When in cooling mode it will blow a lot of air to keep evaporator as hot as possible, so less humidity is condensing on it (but there will still be some). In dehumidifying mode it will blow less air but keep evaporator cool, so more condensation for the same amount of air cooling.
  - teo_zero 4y ago
    Do you have any source for this? It's a topic I've always wanted to understand better. Especially, it's not clear which mode consumes less.
    - yetihehe 4y ago
      Source - I have Kaisai mini-split at work and I've observed how each mode works (with my hand). There's probably more energy used per cooling amount during dehumidifying, because it's bigger temperature differnce (which means lower COP) PLUS condensing water (water phase change energy is big).
jdmichal 4y ago
Unless you have a very strange setup, centralized A/C does not have humidity sensors nor run just to dehumidify. Nor does removing humidity increase the load on the A/C system. Removing humidity is just a happy by-product of how centralized A/C works. So no, you would not be increasing load for that reason.
- TeMPOraL 4y ago
  Happy in some times, unhappy in others. There are days I'd love if the A/C would have a humidifier built-in, for those winter months when you might want to use A/C to warm the interior.
  - ComputerGuru 4y ago
    A good chunk of the United States (and a number of other countries) uses forced air natural gas heating. If you fall into that group, you can install a "whole-house humidifier" that injects steam into the hot air plenum, distributing it around the house. This makes the air less dry (obviously) but also has the side-effect of actually reducing the need for heating because air holding moisture feels warmer than air without (hence the oppressive heat in the humidity of summer), meaning you can lower the thermostat. Unfortunately I have only seen commercial humidification units that boil water into steam with the use of natural gas. Without exception, everything for sale for home use uses electricity (you'll need 230V minimum, single phase will do just fine) to boil the water, which is costly (though this year natural gas prices have risen or even doubled, but even in states with cheap electricity it's probably still cheaper to use natural gas).
    - mmaurizi 4y ago
      We run our steam humidifier just fine on 120v, but we have a relatively small townhome at 1400sqft
      - ComputerGuru 4y ago
        Be careful you’re not running it independently of whether or not the furnace is actually on. 120V generally can’t evaporate enough water in the short window while the furnace is running and many techs will set it up to run at all times (with the blower but not the furnace) to compensate. This lets mildew or mold grow in your ducts.

ComputerGuru 4y ago

I've built/retrofitted and a prototype of mechanically operated louvres with push/pull fans for air exchange in an old school building, tied to the thermostat, ac, and a CO2 detector. The idea of doing the same in my home has been taunting me for years now. Ideally you'd have two such louvres, one with a push fan in the upper floor and the other with a pull fan in the lower floor to simultaneously eject unwanted heat, bring in fresh air, and boost whole-house circulation. They'd be set up to interface with the thermostat/hvac and would operate when the outdoor temperature at intake is lower than the temperature at exhaust and both are above the set point on the ac. The biggest problem is really one of convenience. You'd need a filter on the intake and a rather large and powerful fan to overcome that static pressure - ergo, a noisy one. And you'd probably have to fully dismantle the system in the winter to prevent the cold from getting in (the Midwest is cursed with both hot and humid summers and cold and dry winters). It just end up being the kind of thing where the devil really is in the details and you either do it right and it's a huge undertaking or you do it fast and sloppy and its drawbacks won't be worth it. But I agree, nothing is more infuriating than seeing the AC on and the outdoor air temperature being lower than that of the home. And opening windows just doesn't make a difference since in most 20th century homes there's just poor airflow and no circulation.

Tijdreiziger 4y ago
What would the upside be over a standard heat-exchanging mechanical ventilation system?
landemva 4y ago
Whole house fans are often used in summer at dusk to rapidly cool house. Air is sucked through house into attic, which also cools the attic. https://www.thespruce.com/whole-house-fan-vs-attic-fan-differences-5114299 https://www.thespruce.com/whole-house-fan-vs-attic-fan-diffe... Crack open a window downstairs before switching it on.
- sidewndr46 4y ago
  Those usually function as a giant hole in the house insulation when not in use.

elil17 4y ago

That exists, it's called an air-side economizer. It's used a lot on big buildings.

cypherpunks01 4y ago

You are exactly describing a HVAC "Economizer" as they are called. I believe they are almost solely used in commercial HVAC installs, probably due to price/additional install complexity? They are very neat though, and do save significant energy in shoulder seasons, or at night in the summer as you mention.

wffurr 4y ago

You are looking for a ventilation system like a HRV.

stephen_g 4y ago

The function to just bring outside air when it makes sense is an extremely common feature in commercial HVAC systems - it’s called ‘night purge’. Most of these systems have a damper that the system can use to choose how much fresh air is used or how much return air is recirculated so it’s not difficult to use that to just put fresh air in directly with no cooling or heating. Even systems with heat or energy recovery will often have a bypass damper. You need the humidity to be in the right range as well as the temperature, but it can save a lot of energy!