Breaking Down the New Phius Minotair Calculator

One of my favorite tasks in my work here at Phius is the development of useful tools that people in the passive building community can use to improve their designs. I also find it a good practice to try and simplify down some complicated building science into an easy-to-understand and useful tool – it forces me to better understand the concepts behind the math and fundamental concepts that the tool is using. 

I find the same thing applies to teaching a concept. Explaining it to someone else and rephrasing it a few different ways makes me better understand the comment, and nothing is more gratifying than seeing the look in someone else’s eyes once he or she has grasped what I am trying to convey.

For this week, I am pleased to announce the release of our new Minotair calculator. Due to the unique combined nature of the Minotair, we need an external calculator to be able to calculate the performance of the unit and put it into the WUFI model. We have improved and updated our existing version of the calculator, which utilized an hourly simulation method in Excel, to reflect the changes in the Minotair’s performance and capacities.

For those not familiar with the Minotair, it is a Canadian-designed and built compact air treatment unit. It provides space conditioning, dehumidification, and ventilation in one fairly compact box. There is a damper that swings back and forth in the unit to switch between the recirculation mode and ventilation mode, and the unit has some fairly complicated internal controls that make these real time decisions as to the mode and air flow of the unit. 

Recently, the capacities and the efficiency of the unit changed, so we updated our calculator accordingly. The unit is capable of providing 8.7 kBtu/hr of heating and cooling and an average ventilation flow rate of 100 cfm. A 5 kW electric post heater can be added inline to increase the heating capacity of the unit if needed.

Since the performance of the Minotair changes by the hour – depending on the temperature conditions and load in the space – Phius has decided that modeling the performance hourly is critical. 

I took the capacity and efficiencies at the different temperatures and curve fit them to use in the model. For each hour of the year, the calculator estimates a heating load based on the results of the WUFI model, and with the calculated capacity of the unit at that outdoor temperature, we can determine the coverage required by a supplemental heat source, such as the electric post heater or some baseboard radiators, for example. 

The calculator also accounts for the switching between ventilation and recirculation mode. When in ventilation mode, the capacity required to heat the incoming outdoor air is discounted from the calculation, as this is covered in the WUFI model by the ventilation component with a 100% effective sensible recovery. In the end, the calculator determines the coverage by the heat pump versus electric resistance, and the COPs for heating and cooling performance. Other values are fixed by the rating of the unit. Because this calculator requires custom hourly climate data, we do a little bit of gatekeeping, and you need to contact us to request the climate file (no charge) for the calculator. 

I will now share a few screenshots to demonstrate the use of the calculator and provide some notes on the inputs:

1. Fill out the Airflow Calculator tab. This will help in determining the design ventilation flow rate of the unit that needs to be accounted for in the efficiency tab. Once the unit is installed and tested, the rater can fill out this tab to verify the ventilation delivered by the unit is commissioned correctly.
   
2. On the Efficiency Inputs for WUFI tab, input the climate data if not already pasted in for you. This calculator uses the WUFI/WAC hourly weather data format, which we can easily generate for any location in our normal database. If your project requires custom climate data, please let us know and we can provide you with this data and calculator at that time.
   
3. Now fill out the information at the top of the tab. The ventilation flow rate and ICFA are per unit. If you had a 4-unit 3,600-sf multifamily building where each unit has a kitchen and two bathrooms (65 cfm code mandated exhaust), the inputs in the calculator should be 65 cfm, 900 sf – not 380 cfm and 3,600 sf.
   
   The next section requires changing a temperature for the design heating load in WUFI. This helps the calculator estimate the hourly heat load of the building.
   
4. Now the calculation is complete, and you can input the results into your WUFI model. Create a Heat Pump component and an Electric Resistance component and assign the coverage according to the results below. Unlike the previous version of the calculator, an Aux Device is not required.
   
5. Under the Cooling Distribution inputs, fill out the tab in WUFI to look like the section below. The highlighted values are the calculated COPs for ventilation and recirculation mode, while the other values do not change.
   
6. Create a ventilation device for the unit in ventilation mode. It functions like a passive HRV, but with a recovery efficiency of 100%. Input as seen below; these do not change with the calculation.
   
7. In your Heat Pump component, assign it to Heat Pump, not Heat Pump Rated Monthly COP, as this is already the average performance of the unit throughout the year. Input the calculated values as shown below.
   

That’s all folks! Easy as, well, any other Phius Excel calculator. Happy modeling! And as always, Phius is here to back you up. Please reach out with any questions you may have.