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What’s the advantage of an air-to-water heat pump as opposed to an air-to-air system? Why are they used for new buildings, instead of a standard A/C indoor unit

What’s the advantage of an air-to-water heat pump as opposed to an air-to-air system? Why are they used for new buildings, instead of a standard A/C indoor unit which can also offer cooling?

sokoloff 4y ago
Water can carry heat around a building more efficiently (in power and space terms) than air. In a heating dominated climate, this can make sense in a new building. (It’s also a good match temperature-wise to in-floor radiant heat.) In a retrofit situation (such as my house), it also makes sense and is cheaper than running supply and return ducts everywhere. It’s not as good a fit in mixed or cooling-dominated climates (though they can cool as well, but then you have to insulate the pipes much better to prevent condensation).
- Nextgrid 4y ago
  > Water can carry heat around a building more efficiently (in power and space terms) than air. Heat is carried by the refrigerant though? It only gets "converted" to air in the room itself by the indoor unit. I am talking about European-style systems where you have A/C units in every room, not the American-style with air ducts distributing air from a central unit in the utility room.
  - WastingMyTime89 4y ago
    > European-style systems where you have A/C units in every room Which European countries are you talking about? There is no European standard regarding A/C or heating.
    - Nextgrid 4y ago
      I didn't mean a formal standard - I checked the definitions and what I'm referring as is that US generally prefers central or "packaged" systems (single big unit in the utility room, air is piped from there into the living space through various ducts) where as Europe prefers "split" systems where you have units in each room with refrigerant pipes going back to the outdoor unit.
      - sokoloff 4y ago
        Most of the US uses split units for central air (split meaning the condenser and evaporator are in separate units). What you’re calling a “split” is more precisely called a “ductless (mini-)split” as even an evaporator in the mechanical room and linesets carrying refrigerant to an outdoor condenser is still a (conventional) split setup.
smileysteve 4y ago
Water is much more efficient at thermal transfer than air. All of those little brittle fins on the outside, less compressor run time. It's also better in cold weather for heating as a water mass rarely gets below freezing. The issues are a water source and corrosion.
- deleted 4y ago
  [deleted]
knolan 4y ago
In our climate there is simply no need for AC as it never gets uncomfortably warm for more than a few days a year. Air to water is used to heat radiators and supply hot water in lieu of gas or oil fired heating. In other climates air to air is more popular and it can be less complex as you don’t need all that extra plumbing. With new builds air to water makes sense as it’s no different to installing any type of central heating system. As a retrofit option an air to air system would seem attractive as the installation is much simpler. Our homes tend to be smaller and there wouldn’t be any HVAC ducting so you would probably only have a since source of warm air with an air to air system.
- crote 4y ago
  I feel like installing a heat pump which cannot cool is a mistake at this point. Heat waves are already getting more intense and more frequent. While AC was a luxury a decade or two ago, it is rapidly becoming a necessity. It is absolutely bizarre seeing houses designed like -20C winters are still a common thing, while completely ignoring summer heat.
- Nextgrid 4y ago
  > With new builds air to water makes sense as it’s no different to installing any type of central heating system. But it's the same level of effort than installing indoor A/C units, so why not install those instead? Plus refrigerant pipes can potentially be thinner depending on the max "load" of that room.
  - sokoloff 4y ago
    In a heating-dominated climate, there may well be no air conditioning installed. In a hydronic-only heating building (underfloor and/or baseboard/radiators), there can be less work to run heating pipes only (much faster, cheaper, and more forgiving than running refrigerant lineset and managing the condensation drain line). For heat-only, I find hydronic to be nearly ideal: low noise, no dust, and high comfort (no air movement/drafts).
  - falcolas 4y ago
    Because it can also heat a house at the same energy efficiency, that is, better than electrical resistance or gas heating.
hanche 4y ago
Not an expert, but the much higher heat capacity (and heat conductivity) of water compared to air is likely one reason. But also, when it’s really cold outside, an air to air heat pump is much less efficient. If you have access to liquid water, that is of course much warmer than outside air.
legulere 4y ago
Heating with air is less efficient in transporting heat with water. So you either need to move more air or heat it up to a higher temperature. The first one is obvious why it’s a bad idea, the second is a bad idea as at a certain point the dust in the air is burning up leaving a bad smell. So practically you can heat a house with air heating alone only when they are extremely well insulated or in a mild climate. The passivhaus-Standard in fact is defined by heating through air being enough.
quintushoratius 4y ago
Other posters are correct that water can transfer heat more efficiently than air, but they're missing or burying the lede: The temperature is very stable once you dig down a couple of meters. Winter or summer, it's probably an even 10-20 C (depending on your specific location). The ground has tremendous thermal mass and can act as an enormous heat sink. Your little house-sized heat pump isn't going to vary the temperature very much. You can optimize your heat pump to the specific temp range and dump/draw as much heat as you like year round.
Ekaros 4y ago
There is also question what is the water in picture. Heat pump efficiency decreases when the temperature delta increases. As such it really does depend is the heating done via radiators or underfloor. As later can be used with significantly lower temperature. Where as specially during cold times radiators need rather hot water. In such cases air-to-air might actually be more efficient.

What’s the advantage of an air-to-water heat pump as opposed to an air-to-air system? Why are they used for new buildings, instead of a standard A/C indoor unit which can also offer cooling?

sokoloff 4y ago

Water can carry heat around a building more efficiently (in power and space terms) than air. In a heating dominated climate, this can make sense in a new building. (It’s also a good match temperature-wise to in-floor radiant heat.) In a retrofit situation (such as my house), it also makes sense and is cheaper than running supply and return ducts everywhere. It’s not as good a fit in mixed or cooling-dominated climates (though they can cool as well, but then you have to insulate the pipes much better to prevent condensation).

Nextgrid 4y ago
> Water can carry heat around a building more efficiently (in power and space terms) than air. Heat is carried by the refrigerant though? It only gets "converted" to air in the room itself by the indoor unit. I am talking about European-style systems where you have A/C units in every room, not the American-style with air ducts distributing air from a central unit in the utility room.
- WastingMyTime89 4y ago
  > European-style systems where you have A/C units in every room Which European countries are you talking about? There is no European standard regarding A/C or heating.
  - Nextgrid 4y ago
    I didn't mean a formal standard - I checked the definitions and what I'm referring as is that US generally prefers central or "packaged" systems (single big unit in the utility room, air is piped from there into the living space through various ducts) where as Europe prefers "split" systems where you have units in each room with refrigerant pipes going back to the outdoor unit.
    - sokoloff 4y ago
      Most of the US uses split units for central air (split meaning the condenser and evaporator are in separate units). What you’re calling a “split” is more precisely called a “ductless (mini-)split” as even an evaporator in the mechanical room and linesets carrying refrigerant to an outdoor condenser is still a (conventional) split setup.

smileysteve 4y ago

Water is much more efficient at thermal transfer than air. All of those little brittle fins on the outside, less compressor run time. It's also better in cold weather for heating as a water mass rarely gets below freezing. The issues are a water source and corrosion.

deleted 4y ago
[deleted]

knolan 4y ago

In our climate there is simply no need for AC as it never gets uncomfortably warm for more than a few days a year. Air to water is used to heat radiators and supply hot water in lieu of gas or oil fired heating. In other climates air to air is more popular and it can be less complex as you don’t need all that extra plumbing. With new builds air to water makes sense as it’s no different to installing any type of central heating system. As a retrofit option an air to air system would seem attractive as the installation is much simpler. Our homes tend to be smaller and there wouldn’t be any HVAC ducting so you would probably only have a since source of warm air with an air to air system.

crote 4y ago
I feel like installing a heat pump which cannot cool is a mistake at this point. Heat waves are already getting more intense and more frequent. While AC was a luxury a decade or two ago, it is rapidly becoming a necessity. It is absolutely bizarre seeing houses designed like -20C winters are still a common thing, while completely ignoring summer heat.
Nextgrid 4y ago
> With new builds air to water makes sense as it’s no different to installing any type of central heating system. But it's the same level of effort than installing indoor A/C units, so why not install those instead? Plus refrigerant pipes can potentially be thinner depending on the max "load" of that room.
- sokoloff 4y ago
  In a heating-dominated climate, there may well be no air conditioning installed. In a hydronic-only heating building (underfloor and/or baseboard/radiators), there can be less work to run heating pipes only (much faster, cheaper, and more forgiving than running refrigerant lineset and managing the condensation drain line). For heat-only, I find hydronic to be nearly ideal: low noise, no dust, and high comfort (no air movement/drafts).
- falcolas 4y ago
  Because it can also heat a house at the same energy efficiency, that is, better than electrical resistance or gas heating.

hanche 4y ago

Not an expert, but the much higher heat capacity (and heat conductivity) of water compared to air is likely one reason. But also, when it’s really cold outside, an air to air heat pump is much less efficient. If you have access to liquid water, that is of course much warmer than outside air.

legulere 4y ago

Heating with air is less efficient in transporting heat with water. So you either need to move more air or heat it up to a higher temperature. The first one is obvious why it’s a bad idea, the second is a bad idea as at a certain point the dust in the air is burning up leaving a bad smell. So practically you can heat a house with air heating alone only when they are extremely well insulated or in a mild climate. The passivhaus-Standard in fact is defined by heating through air being enough.

quintushoratius 4y ago

Other posters are correct that water can transfer heat more efficiently than air, but they're missing or burying the lede: The temperature is very stable once you dig down a couple of meters. Winter or summer, it's probably an even 10-20 C (depending on your specific location). The ground has tremendous thermal mass and can act as an enormous heat sink. Your little house-sized heat pump isn't going to vary the temperature very much. You can optimize your heat pump to the specific temp range and dump/draw as much heat as you like year round.

Ekaros 4y ago

There is also question what is the water in picture. Heat pump efficiency decreases when the temperature delta increases. As such it really does depend is the heating done via radiators or underfloor. As later can be used with significantly lower temperature. Where as specially during cold times radiators need rather hot water. In such cases air-to-air might actually be more efficient.