This post is about how I successfully configured a bufferless HP system using a simple Chinese air to water monoblock hybrid heat pump - Alberta, Canada install.  Note - I am very new to doing these installs .. so any experienced pros please feel free to comment as needed!

Heat pump suppliers recommend buffer tanks as they greatly simplify the installation for installers and they help protect the manufacturer against warranty/ installation issues as a result.  It works, but buffertank systems cost more, they reduce efficiency, and they are often simply not needed.

Design - Ensure system thermal mass and/or heat exchange is sufficient.  This mass / exchange can be from any combination of system water volume, mass volume (concrete in a hydronic slab), or heat exchanger capacity (ability to exchange heat).  As long as the heat pump can exchange enough heat it won’t short cycle .. this is physics 101.

Sizing - Many installers in the HVAC industry are used to oversizing gas furnaces by anywhere from 20-50%.  This is fine for a gas furnace, but heat pumps should be sized for the minimum justifiable load.  Oversized units cost more to run and they will likely short cycle.  In very cold regions like Alberta, back up heat is necessary anyway for when conditions fall below -35C.  For economic efficiency, it is actually better to use -20C as the cutoff as COP drops below 2 typically at this temperature.  In Alberta, 90% of the seasonal BTU heating load is above -20C (this is accurate btw).

Controls - Heat pumps use different strategies and controllers to turn on / off and modulate.  When using a buffer tank, the heat pump heats water to a target tank temperature and then turns off once the tank reaches the target temperature.  In a bufferless system, installers will either place the buffer tank’s sensor onto the return water line, or just ignore it and leave it in the unit.  Either way tank temperature is arbitrarily set high so that it is never reached. Set this way, the HP would never turn off .. instead, a home thermostat is used to control the on- off function and the return water line is used to modulate output.  Ambient temperature modulation curves are in turn linked to return water temperature.

Zones - bufferless systems work best in simple, full flow designs with one circulating pump for the whole system.  No zones, no extra pumps, no /few auto valves, etc.  You try to set / balance your flows mechanically to ensure the system always has full flow available.  This is a “must” as the system needs to be ready to exchange heat at any time .. especially for defrost cycles where the system will run at full capacity.

On/Off hybrid system control - Depending on your HP, it may have sufficient ambient temperature control and features to not even need an on/off thermostat.  If it needs one, here are some solutions.

You can set up separate thermostats to run both the HP and gas furnace or use just one.  How you set this up will vary by your HP controller and thermostat logic.  For the system I just installed in Alberta I used a separate thermostat for the gas furnace and simply set it 1C lower than the HP so it performs as a “peaker”.  This is not ideal as both units could run at the same time and this can cause too much heat / errors depending on your design.  A better solution is to use one thermostat or system to coordinate both units.

You may find you need to use a dual transformer compatible / high DT thermostat in this case (like an Ecobee Premium), and you may find that you need to wire the thermostat W1 and Y to the same on/off switch on your HP unless your HP controller has both terminals.  The cheap Chinese unit I just installed didn’t.  If you do this, the current will probably backfeed and cause damage to your HP and/or thermostat.  A cheat is to install diodes on W1 and Y to prevent backfeeding the circuit.  A better solution is to wire W1 and Y to separate dry contact relays to control the HP.  OB control (reversing valve) may be switched manually / seasonally or it may be automated depending on your HP.  Good luck!