Chestnut
Chestnut public docs
operation
how is it different than a fossil fuel furnace or boiler
Fossil Fuel furnaces operate at high temperatures and fans speed. You can feel the hot air blowing out of the vents. Cold climate heat pumps, like most highly efficient systems, don't heat the air (or water) to such high temperatures. Old style boilers and furnaces have 2 states, off or on full blast. Cold Climate Heat Pumps have Variable Refrigerant Flow (VFR), the compressor and refrigerant run at 10 or more discrete levels, delivering the heat as efficiently as possible. The air won't feel super warm, coming out of the vents, it will be just a few degrees warmer than the space it is heating. Cold Climate Heat Pumps run slow and steady.
on adjusting register vents
Please be very cautious about closing off register vents. If the register vents are too constricted, the compressor can ice up and be damaged. Proper operation depends on the flow of air through the registers.
That being said, there is one register that can be dampened. The register in the front room. Since it is so close to the air handler and right off the supply plenum it may create too much heat in the front room. Let me know if that is the case. There is an internal damper before the register that can be adjusted. For other registers, please let me know if you feel they need adjustment.
on the need for return air
Proper operation of the HVAC system depends on the flow of air through the supply registers, into the rooms and back to the return air grill in the living room. This works best when the doors are open. It should work reasonably well through the space under the doors. So it is best not to have rugs blocking the airflow under the doors. If you find that the comfort level decreases when the doors are closed I can remedy that by undercutting the doors to leave more space under the doors or by putting vents above the doors. Please let me know.
fan speed(inside)
You have a choice on the inside fan speed at the thermostat. The ductwork was designed to provide 68 degrees on the coldest day of the year using the MEDIUM fan speed. I am not sure if that is the best speed under all conditions. You might want to turn down the speed when it is not really cold out. Let me know what works best and is most comfortable. The following refers to the speed at the compressor outside.
setback
https://www.energy.gov/energysaver/programmable-thermostats says Programmable thermostats are generally not recommended for heat pumps.
https://betterbuiltnw.com/resources/avoid-deep-thermostat-setbacks-with-variable-capacity-heat-pumps
...variable capacity heat pumps boast greater efficiency because they are far more efficient when running at low speed than at full power. Also, unlike earlier generations of heat pumps that only operate at one speed, variable capacity heat pumps have a wide range of operating speeds.
Variable capacity heat pump testing has shown that setting temperatures back five to ten degrees initially saves energy by reducing the immediate heating need. However, recovery from a such a deep setback causes the heat pump’s efficiency to plummet. The system will run in the most inefficient full power mode to quickly recover a room or house back to the original setpoint... Instead, allowing a variable capacity heat pump system to maintain a set temperature is the most energy and cost-efficient strategy. This allows the equipment to operate for long periods of time in its most efficient stage. Maintaining a set temperature can minimize, if not eliminate, the need for the equipment to run in its full power mode.
What, if any, setback is ideal? The ideal setback is hard to define, but it is possible to provide homeowners with guidance that will maximize their comfort without increasing their energy bills. For short periods (less than 6 hours), a ductless heat pump is most efficient without a setback. For medium-duration periods (6-12 hours), the best setback is two to three degrees. This prevents the system from operating at full power for an extended recovery period. If the homeowner is away for longer than 12 hours, then a standard heat pump setback of five to eight degrees is the best choice for maximum energy efficiency.
< 6 hours: No setback 6-12 hours: 2-3 degree setback 12+ hours: 5-8 degree setback, depending on desired level of savings
For the old systems we would always provide a setback thermostat. As the tech matures I am sure they will figure out how to do energy efficient set back thermostats for VFR systems that don't trigger the compressor to go into high energy use mode to recover. I will let you know when that is feasible/affordable.
