Air Source Heat Pump Coil Frost/Freeze

edit

Hi there,

I'm modeling a system with an air to water heat pump for heating and a boiler as the backup. The heated water always passes through the heat pump first. If the leaving water temperature is lower than the setpoint temp, then the boiler will start to work. I put them in series (HP first, then boiler), and add a SetpointManager: Scheduled on the outlet of the HP and the boiler as shown in the figure below.

I got a lot of warnings of possible coil frost/freeze of the air source heat pump and it led to a huge drop in the energy consumption of space heating. If I disabled the heat pump, the boiler will have an energy consumption of 56 GJ annually. If I enabled the heat pump, the heating loop will cost only 1.1 GJannually which doesn't make sense.I’m wondering if there’s a way to avoid this? I read a similar question (https://unmethours.com/question/8515/...) But still, have no idea how to solve my problem.

"* Warning * CalcDoe2DXCoil: Coil:WaterHeating:AirToWaterHeatPump:Pumped "HPWH DX COIL" - Full load outlet air dry-bulb temperature < 2C. This indicates the possibility of coil frost/freeze. Outlet temperature = -7.01 C. * ~~~ * ...Occurrence info = RUN PERIOD 1, 01/01 01:11 - 01:15 * ~~~ * ... Possible reasons for low outlet air dry-bulb temperatures are: This DX coil * ~~~ * 1) may have a low inlet air dry-bulb temperature. Inlet air temperature = 0.300 C. * ~~~ * 2) may have a low air flow rate per watt of cooling capacity. Check inputs. * ~~~ * 3) is used as part of a HX assisted cooling coil which uses a high sensible effectiveness. Check inputs."

" *** * Warning * CalcDoe2DXCoil: Coil:WaterHeating:AirToWaterHeatPump:Pumped="HPWH DX COIL" - Full load outlet temperature indicates a possibility of frost/freeze error continues. Outlet air temperature statistics follow: *** * ~~~ * This error occurred 19273 total times; *** * ~~~ * during Warmup 0 times; *** * ~~~ * during Sizing 0 times. *** * ~~~ * Max=1.999849 Min=-10.178238"

" *** * Warning * Plant loop falling below lower temperature limit, PlantLoop="AIRTOWATER HW PLANT LOOP" *** * ~~~ * This error occurred 24049 total times; *** * ~~~ * during Warmup 134 times; *** * ~~~ * during Sizing 0 times. *** * ~~~ * Min=16.697391"

When I changed the "minimum inlet air temperature for compressor operation" from -5°C to 13°C, the coil freeze warning is missing, but the "plant loop falling below lower temperature limit" warning increased from 24049 times to 117064 times. The Boiler energy cost is 50GJ but the heat pump energy consumption is still only 0.65 GJ. BTW, the shop drawing specified the minimum entering heat pump air temperature is -6.67°C.

Thank you very much for your reply in advance.