30 Mechanical Minutes: Episode 16 "Air-to-Water Heat Pumps"

HPAC Magazine's editor Doug Picklyk sits with hydronics industry expert John Siegenthaler to talk about Air-to-Water Heat Pumps for the residential market in Canada. The discussion covers exactly what air-to-water heat pumps are, what's available today, how they operate and some system design considerations.

[@tlangdon12]

Brilliant video! So much ground covered in just 40 minutes.

[@hpacmag]

Glad you enjoyed it!

[@tlangdon12]

In Europe, many manufacturer are recommending Anti-freeze Valves instead of anti-freeze fluid. Anti-freeze fluid lowers the performance of the system whereas anti-freeze valves don't. The valves need to be good quality and protected from water & snow. They are fitted on the supply & return lines to the outdoor unit outside of the home. They release some water from the system if the outside temperature drops below 38F. If the outside temperature is below 38F, you can be certain that the heat pump will be running UNLESS it has a fault or there has been a power cut. If the heat pump is running the valves won't open, but in the even of a fault or power cuts they will open just enough to let a little water out to stop the outdoor unit from being damaged. The system needs to be topped up when the fault or power is restored, but you only need to top it up with fresh water, not a solution of water and anti-freeze.

[@tlangdon12]

Panel radiators are very common in Europe and the UK in rooms where the home owner want thick carpets and thick underlayment. Combined with simple Thermostatic Radiator Valves, to avoid overheating due to thermal gain, they can be a very cost-effective solution were cooling isn't required. Radiant ceiling or fan coils are the best emitters for cooling applications. Radiant floors or panel radiators are the best for heating. Radiant walls might be a good in a few limited situations, but my sense is that there is more risk of a nail through a wall than through a floor or ceiling.

[@tlangdon12]

Another solution to the two buffer tank question is two heat pumps: one for space heating/cooling with a single buffer, and a ducted Heat Pump Water Heater for hot water. The ducted Heat Pump can be a CO2 model, so able to heat domestic water to a point where Legionella is not a concern. This avoids the need for the space cooling heat pump to have to switch to heating mode to keep a supply of hot water ready for use. This configuration also allows the heating heat pump to operate at maximum efficiency in heating mode because the flow temperature can be controlled by outdoor reset.

[@tlangdon12]

If the designer follows the manufacturers recommendations for minimum fluid volume in the system, defrosting should not produce a noticable drop in termperature indoors, but North America typically has homes with much lower thermal mass than the UK or Europe, so it could be more of an issue in North America. A system volumiser or buffer tank is a reasonable solution, but in the UK some experts are actively trying to design out buffer tanks (and zone valves) from their systems. Perhaps the ultimate buffer tank configuration is no buffer tank! Great to hear that in John's experience home owners don't experience any cooling when the system is in defrost.

[@tlangdon12]

The Global Warming Potential of R410a is 2088, for R32 it is 675, for R290 (Propane) it is 3, and for R744 (CO2) it is 1. Propane is pretty much the ideal refrigerant for small residential monobloc systems where all the refrigerant can be kept outside of the home; although CO2 is better for high temperature applications. For split systems and larger commercial systems, Propane is not ideal and an alternative refrigerant is preferable.