# Some help dynamically adjusting heater settings

I've set up a high efficiency HRV setup, pretty much automated in Hubitat:

My question is about the most efficient way to handle the wattage settings of the inline electric heaters in the system. I've done some Webcore programming to set variables and use them in Hubitat (pool solar system), however I'd like to control this system all via Rule Machine.

Right now I just have a set of rules that set the heater wattage (dimmer settings) based on the output air temperature so it's bit crude. I compare temps before and after the heater, and set the Zooz dimmers (connected to the heaters) to four different set points. The set points were determined by trial and error, observing the system. The current set of rules cannot take into account the different CFM settings...although ideally I would set the heaters based on both CFM and temperature delta.

I have four scenes:

HRV to 55 CFM
HRV to 60 CFM
HRV to 75 CFM
HRV to 90 CFM

Each scene controls the speed settings (via 0-10 Volt dimmers) for the two fans (intake/exhaust) in the HRV ventilation system.

I have several temperature sensors in the system, but the one that makes the most sense to calculate from is the Fresh Air temp entering the house. I also have a temperature sensor after the inline heaters to measure the heated air temp entering the room. I'd like the target temp (after the inline electric heaters) to be 65 F. So if air temperature leaving the HRV is 60F, I'd like to add 5 F to the temperature. The calculation for the watts needed to do this is ( CFM x 1.08 x Temp delta)/3.41

To raise the air temp by 5 F at 90 CFM would take (90 x 1.08 x 5) / 3.41 or 142 watts.

Air temp leaving the system changes with both outdoor temperatures and CFM settings. I'd like to dynamically calculate and then set the wattage needed based on which CFM "scene" is in use, and the temperature of the incoming fresh air. Then I need to set the Zooz dimmers to the dimmer value (0-100) to match that wattage output. I have measured the Zooz dimmers at various dimmer values and measured actual wattage used by the heaters. For example, a dimmer setting of 24 will see 130 watts used by the heaters.

Any suggestions on the best way to do this? Similar examples in code would be great I got as far as setting up a hub variable, and stopped at the rule doing variable math as I figured some initial guidance would save me a lot of time. In essence I'm building a modulating electric heater based on both input temperature and air flow.

I'm using pretty simple code to set the HRV scenes based on Co2 levels coming from an Ecowitt WH45 indoor air quality sensor;

I'd appreciate any advise on tackling this challenge! I'm a SmartThings refugee so pretty new to HE. I do have Webcore installed and running to manage the pool solar heating system.

Ok, making progress.

I'm stuck trying to define this expression in Rule Machine:

Dimmer_variable = .122 * (((CFM_Variable x 1.08 x (68 - Fresh Air temp))/3.41)/2) + 16

How do you create parenthetical expressions with variable math in Rule Machine?

Disregard...I figured it out.

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