Warning to all who choose to DIY their HVAC: 

I definitely recommend hiring a reputable and certified HVAC company to install your HVAC system. Not only is there dangerous voltages in the HVAC equipment and very high pressures in the refrigerant lines, there are risks of severe environmental consequences if you don’t connect and seal the refrigerant lines properly.

I didn’t know it at the time but hiring a certified HVAC technician to install a 3^rd party HVAC unit is next to impossible. They like to include a service warranty for the equipment they sell, install and service. Having a service warranty on equipment they didn’t procure, would be a huge risk to them. Also buying HVAC tools is very difficult if you do not have a certification to handle refrigerants. The local HVAC store wouldn’t sell me tools like a vacuum pump or solder supplies. They even wouldn’t let me buy a 5/16” Allen wrench (none of my sets had one that large). I’d rather support the local economy but when they say no, Amazon said yes.

I bought a vacuum pump, refrigerant gauge set and a hand held ratcheting pipe bender from Amazon.

My son sensing my frustration, 3D printed me the 5/16 Allen wrench bit. What a fine young man.

Unfortunately the break-free torque on the vacuum refrigerant line valve was too high for ABS plastic.

3D Printed 5/16" Allen wrench bit with a little too much torque placed on it

Defeated, the next day I went to Home Depot and bought an Alan wrench set that had this size.

How Does an Air Source Heat Pump perform in Northern Utah?

While not as energy efficient as a ground loop, I am amazed how an air source heat pump is still able to pull energy from the frigid air and use it to keep our house toasty and warm, even when the weather is horribly frigid outside.  



Heat Pump Performance Specs (From the Manual vs Measured)



Referring to the chart above (Expanded Performance Data HIGH STAGE), When it’s a mild 50°F outside, the heat pump can deliver up to 36,000 BTU/hour (10.55kW) while only requiring 2.63kW in electricity to do so. In other words if you put 1 unit of energy in, you get 4.01 units out. Magic?? No, it’s just thermodynamics. However, as the outdoor temperature drops and gets further away from the temperature inside, the less efficient the heat pump gets. 

When its 10°F outside the heat pump can only deliver 18,100 BTU/hour (5.3kW) with 2.16kW in power to run it. Or 1 unit of energy in yields 2.45 units out. Still pretty cool though.

However, by the time it’s only -10°F outside the heat pump is only able to deliver 9,800 BTU/hour (2.87kW) with 1.94kW in power to do so. One unit of energy in to get 1.48 units of heat out is just slightly better than the energy efficiency of just running a space heater (which is 1 for 1).

Sometimes the outside unit has to defrost itself. This is accomplished by switching the direction of the refrigeration cycle around so it runs in air conditioning mode. This causes the coil outside to heat up, melting any ice that has built up. To counteract the cold air that would otherwise be dumped into the house, the air handler (inside the house) has a back-up heating element that kicks on for a few of minutes whenever the outside unit needs to defrost. The heating element runs every 120 minutes of (below-freezing) runtime. This brings down the overall efficiency of the system by a few percent. 



Heat Pump Power Measurements and Calculations



It rarely gets down into the single digits at night in the winter time at our house. Even when it does, the air source heat pump can handle all our heating needs each winter.

The only exception to this is if the temperature set-point is not met within 25 minutes, the auxiliary heating element is programmed to kick on simultaneously with the heat pump so the house can get back to its temperature setpoint faster. These sometimes happens after the following periods when the heat pump is programmed not to run for several hours:

• After the 2-hour winter on-peak period (8-10 AM)
• After the 5-hour peak period (3PM-8PM)

Because of the exceptional insulation and thermal mass qualities of our home, even after not running any heat for 5 hours. The ambient temperature inside the home does not drop more than 2-4 degrees F, Wow!!

However getting all that thermal mass back up even a couple of degrees takes a lot of energy.

I originally installed an auxiliary 10kW resistive heating element into the air handler. That's a good match to the 36,000 BTU capacity of the heat pump. It is comprised of 4 resistive elements, each draws 2500 watts of power. After several months, I decided to disable half of the heating elements so now only 5kW of resistive heating capacity (2500 watts per 120 volt leg) is available. This also helps keep the peak power draw of my house is lower.

After the peak period is over, the heat-pump is automatically activated by the thermostat. On days where it is not super cloudy, the (~5kW of) passive solar heating from the windows (link to that article here) keeps the house at room temperature and the heat pump may not need to be called upon to run at all.

Most days there is some amount of passive solar heating and the heat pump alone is able to satisfy the set point within 10-25 minutes.

On a really cold and cloudy day where there has been little to no passive solar heating, after the on-peak period has passed, the house temperature will have dropped 3-4°F. More energy is required to bring all that thermal mass back up to the set-point temperature. If the heat pump is unable to reach the set-point after a determined amount of time (I set it for 25 minutes), the air handler summons the auxiliary heating element to kick on to help the heat-pump reach the temperature set-point more quickly.

There were a couple of frigid days last winter where outdoor temperatures were in the teens so the heat pump had reduced capacity. After not running for 5-hours, the house setpoint had dropped 4°F. With only the 5kW auxiliary resistive element to help out, it took an hour to reach the set-point.

Total Energy available when it's 15 °F outside.

Considering how little overall energy is being used to bring the house back up in temperature while it's in the low teens outside, that's not bad at all.

How do you size an air source heat pump for an existing home?

Air source heat pump capacity is dependent on the outside air temperature. The enormous efficiencies and performance gains of an air source heat pump quickly diminish when the outside temperatures get well below freezing. Their advertised rating is based on an outdoor temperature of 50°F. In places like Northern Utah where it gets much colder than that, the heat pump needs to be upsized accordingly.

If your home is drafty and poorly insulated, it’s going to take a lot more energy to keep it warm in the winter time and cool in the summer time.

If an existing gas furnace delivers 80,000 BTU and runs 50% of the time when it is 15°F outside, to get the same performance and duty cycle out of an air source heat pump it will also need to be capable of providing 80,000 BTU when the outdoor temperature is 15°F.

In this example, an air source heat pump rated for 140,000 BTU may need to be chosen so it can still deliver the required 80,000 BTU when it is only 15°F outside.

Ground source heat pumps don’t suffer from outside air temperature dependency and can be sized strait across. 

How about for an older, inefficient home?

Houses built in the US today still suffer from severe design negligence when it comes to energy efficiency and passive heating and cooling. Older homes are even worse.

My house does not. The heat pump that keeps my 3400 sq-ft home cozy all year only requires 2546 kWh of electricity energy to heat and 1325 kWh to cool it during the summer months.

To put this another way, the pilot light used in a standard home's natural gas water heater consumes more energy all year than the electrical energy used by the heat pump in my home to keep the house warm all winter.

 Energy in the pilot light flame: 340 watts.

  0.340 kWh x 24 hours x 365 days = 2978 kWh

 Energy used by heat-pump to heat home = 2546 kWh

Now let’s compare the heating energy needs of my home to a similarly sized home that is old, drafty and poorly insulated. Let’s assume this house consumes 150 dekatherms of natural gas to heat it throughout the winter (1 dekatherm = 293 kWh in energy). So 150 dTH x 293 kWh/dTH = 43,950 kWh in natural gas energy.

This house will use 43,950/2546 = 17.3 times more energy (than my home).

Even though natural gas is typically cheaper than electricity, this older home will pay almost 9x more to heat their home. Adding in considerations for electricity time of use rates, the older home might pay 25x more. All while also being less comfortable, more drafty and having unhealthy indoor air quality. 

Insulate and Air Seal prior to upgrading to an air source heat pump:

While the air source heat pump is way more energy efficient than a natural gas furnace, it will only get you part of the way there to achieving a comfortable, luxurious, fuel efficient home.

• COP of an old gas furnace 0.80 (80% fuel efficient).
• COP of an air source heat pump 3.00 (>3.0 when above 25°F outside, <3.0 when below 25°F outside).
• It takes 3 / 0.8 = 3.75x more energy using a natural gas furnace than an electric air source heat pump.
• It takes 5 / 0.8 = 6.25 x more energy using natural gas furnace than an electric ground-loop heat pump.

Based on this poorly insulated, drafty, example home, 17.3 / 3.75 = 4.6 times more energy can also be saved by increasing insulation and sealing up the envelope of the home.

Since replacing the furnace and AC of a home is expensive, I highly recommend tackling the adding of more insulation and air sealing the home envelope first. Then, when you assess what the new energy needs of the home are, a much smaller HVAC system will cover the heating and cooling needs of the entire home.