Air to Water or Air to Air Heat Pump?

by ROGER ABDODec 22, 2022

Air to Water or Air to Air Heat Pump

Which Air Source Heat Pump gets you more benefits? Pros and Cons of Conventional Air to Air Heat Pumps and new Air to Water Heat Pumps

Small Buildings HVAC Systems Market Outlook

North American (US and Canada) Buildings Codes are slowly moving toward NetZero Energy for new constrcutions. The 2025 version of Canadian Building Code will require that every new construction to be NetZero ready (Extremely Energy Efficient Envelope and HVAC system without renewable enery production). In 2030, Canadian Buildings Code will impose the NetZero Energy on every new construction. The delay between issuing Buidling code and implementing it is around five (5) years. So Starting 2030 every new construction will have to have a highly energy efficient enveloppe and HVAC system.

Assuming that the US will follow the same trend, builders and designers will have to look for non conventional means to comply with near future energy requirements of building codes (At Federal, Sates or provincial levels). Air to Air Heat Pumps with DC inverter technology have been very popular since the 90s. The combination of DC inverter and EVI (enhanced vapor injection) technolgies for producing hot water, at temperatures as low as -25°C (-13°F), for both space heating and Domestic Hot Water (DHW) heating using Air to Water Heat Pumps, will gradually take the place of Air to Air heat pumps in new constructions.

Air to Air Heat Pump comparison with Air to Water one

Below are two represenstations of Air to Water Heat Pump for a house (left picture) and a conventional forced air system (air to air heat Pump - right pitcure).

Air to Water Heat pump (HP) System is comprised of a HP, Thermal Storage Tanks (ideally one for DHW and another one for space heating and cooling). Heating and cooling energy is distributed either through infloor heating and cooling pipes, Hydronic Fan Coil Units or a combination of both. Backup heat in this case can be one Boiler using either Gas, Electricity or propane as a heat source.

Conventional Air to Air heat Pump system includes an DX Dcuted central unit with an outdoor air cooled condesner. Depending on Building Size, we can have only one or multiple heat pumps serving the same house. Domestic Hot Water is producted by either an electric, propane or natural gas boiler

To compare Air to Air (System 1) with Air to Water Heat Pump (System 2), we will be analysing Space Heating, Space Cooling and DHW heating for a typical 2000 sqaure foot home in the GTA (great Toronto area) in the province of Ontario. A heating and cooling load calculation has been carried out for this house and the result is 18 Btu/hr/ft² (Using CHVAC Software).

Equipement used for energy simulation are DC inverter heat pumps for both scenarios and EVI technology for air to water heat pump scenario. Simulation results are shown below

System 1: Air to air heat pump scenario

System 2: Air to water heat pump scenario

Annual Power Consumption for the air to air heat pump scenario is 24 548 KWh of electricity for space heating, space cooling and DHW heating. Air to water heat pump scenario has an annual power consumption of 13 327 KWh. The difference is primarly driven by the fact that Air to Water Heat Pump can provide 80-85% of DHW heating demand (which the air to air heat pump can not do), EVI technology allows heat pump to supply more hot water at lower temperatures (between -25°C and -15°C) and below) and that Water is an incompressible fluid and has less friction loss than air (which brings down the annual power required for transporting heat transfer fluid around the house).

The graph on the left compares pumping power required for various types of heat transfer fluids: Water or Water/Glycol mix (called Hydronic in blue), Air (light and dark green) and refrigerant (in red).

For a 350 Linear Feet of ducting, a forced air system will consume more power for transporting energy produced by the heat pump to every room in the house. The exess energy required is equivalent to 15% of annual energy consumed by heat pump's compressor.

Graph Source is Hydronics Industry Alliance.

Simulated Air to Water heat pumps is equiped with outdoor temperature reset. This not only optimises heating power consumption, but cooling power consumption as well. System 2 has a seasonal energy Efficency ratio 70% higher than system1 for cooling.
By assuming an avereage price of 0.15 CAD$ for every KWh of electricty, System 2 represents an annual saving of 1 683 CAD$. That represents 45.7% reduction from System 1.

System 1 & 2: Life Cycle Analysis

Even Though the air to air HP scenario has a lower First Cost (16 497 CAD$ cheaper than air to water heat pump system), it has a higher annual energy and maintenace total cost. Assuming that both scenarios have lifespan of 20 years, The cumulative cost (first cost + recurring annual and replacement cost) of System 1 is higher than System 2 by 32 690 CAD$.

Pay Back for the air to water heat pump scenario is around 6 years when electricty rates are in the range of 15 cents for every KWh. Pay back period will increase in provinces where electricty rates are lower (such as Quebec - for a 10 cents/KWh, pay back period becomes 9 years).

Conclusion

Payback period, Utlity rates, availability of energy sources, cost of capital and other factors can influence the feasibility of possible design scenarios. Retrofit Projects with either old gas furnaces and forced air system or older hydronic heating systems using cast iron radiators are harder to retrofit with Air to Water heat pumps.

The above analysis is more applicable for newly built houses meeting the minimum energy requirements of building code. We recommend getting in touch with our experts, to see which solutions can work better for your project.

Air to Water or Air to Air Heat Pump?