Hello everyone.  Where I live in the mid Atlantic area of the US, air to air heat pumps are very common.  Air conditioners are a must because of our hot and humid summers, and many people opt for using the same equipment for heating and cooling.  But my house is weird.  It was built in 1978 (old for my area).  There are no ducts.  And the house is Y-shaped (essentially 3 wings 120 degrees from each other), so it's not so easy to install ducts that encounter that geometry.  The previous owners (<2017) installed 4 mini split AC units (sharing 2 outdoor units) and 2 heat pump air to air mini splits sharing one outdoor unit for cooling as well as their old oil boiler for heating with hydronic baseboards and a pellet stove and a cord wood burning stove.  We have opted for green-sourced electricity and are trying to de-carbonize as much as possible.  So in 2019, I had an air source heat pump installed in parallel with my oil boiler.  This is practically unheard of in my area and I had a lot of trouble finding a technician who would even consider working on it.  I had to import my unit from Canada.  After quite a lot of back and forth, I finally got it installed and running in August 2019.  Unfortunately, though, I've been getting low pressure alarms on defrost.  I had a technician come last winter, and they replaced the fill valves and mixed in some UV dye into the refrigerant.  This late fall, on start up, I noticed the heat output wasn't what it was supposed to be and detected a lot of UV glow in the same area.  So they replaced the valves again and put some goop on them.  I didn't get a confident answer when I asked if the guy last year had wiped off the fill valves or whether the florescence there could have just been from when he disconnected the fill tank.  I'm watching closely this winter, both with my UV flashlight and by measuring the time it takes to warm my buffer tank when my house isn't calling for heat.  Knowing the volume of the water, specific heat, etc, I can measure the heat output of my unit and compare it against the manufacturer's table vs. outdoor temperature.  This is how I knew to call the technicians back early this year instead of waiting for a fault during a defrost when the weather really gets cold.

I'm very aware that my heat pump uses R410a with a GWP of just over 2000.  I calculated my loss of refrigerant as about the global warming potential of a month or two of oil heat!  Besides what I'm doing, has anyone else had trouble with leakage of R410a?  Anyone tried the leak detectors or other methods?

If anyone wandered over here from the US and wants to talk to me about my setup, I'd be glad to give a little more information.  

My house is two levels with about 167 m^2 total.  I am working on my insulation and air sealing currently and I don't have good numbers for insulation for you.  My heat pump is a Nordic ATW (Air to Water) 55.  Link below:

https://www.nordicghp.com/product/nordic-products/air-source-heat-pump/air-to-water/

The Nordic ATW has a mode where it will switch to aux heat (my boiler) at a programmed outdoor temperature.  My first full winter, I had it set at its lowest value of I think like -20 deg C (basically not happening).  The heat pump could keep my house at about 20-21 C, but starts to run continuously when the temperatures outside, especially at night, drop below about -3 C or so.  I run an Excel visual basic macro that gathers information from Wireless Tag and IFTTT, processes it, and spits it back to IFTTT to link together all of my smart house features.  When I enable "boost mode" on my Excel macro, it will detect that the house isn't getting up to the setpoint and turn on the air to air heat pump upstairs and pellet stove downstairs to help a little bit, but it looks at the slope of the line and turns them off again before the temperature goes above my house setpoint.  The goal is to get it close but not to overshoot and turn off my house heat.  It's complicated, but it works pretty well!  This winter, I've suddenly realized that I have about 350 gallons of 4 year old heating oil in my underground tank, and I better use it before it goes bad and/or leaks.  So, I've set the crossover at about 2 deg C and the oil boiler takes over before the heat pump runs out of capacity for the house.  

Thanks,

Brian