'Perfect' Heat Recovery on Ideal Air Systems?

Hi everyone,

I am hoping that someone might know the answer to a funny effect I’m seeing in a recent model - I’ve put together a quick sample .gh file that illustrates the effect and attached it here.

I’m modeling a simple ‘Ideal Air Loads’ system and trying to look at the effect of Heat Recovery on the ventilation air. I’ve set up the example here to isolate just the mech vent (I’ve zero’d out all the windows, economizers, infiltration, other loads, etc… ) and I’m using just a simple ‘Office’ program with default schedules.

My question is why it might the case that even with ‘perfect’ (or near perfect) 'sensible and latent heat recovery I still see a fairly substantial amount of ‘Mechanical Ventilation Energy’ in the ‘readEPResult’ component?

I have tested it at several different efficiency % values and compiled just a few in a excel I’m attaching here as well. Just looking at 3 steps (0%, 50%, 100% HR) I am getting results like this:

As you can see in the output, even at ‘perfect’ HR it still registers quite a bit of energy loss over the year. Does anyone know what might be going on here and why I’d still be getting Mech Energy output even with ‘perfect’ HR applied? I double checked the csv output and I do believe I don’t have any other source of air into the zone other than the ‘mechanical’ on this zone.

Does anyone have an idea what might be causing this to occur?

Thanks in advance for any help!
-Ed

Mechanical Hear Recovery Sample File.gh (574.5 KB)
Mechanical Heat Recovery Sample Output.xlsx (55.4 KB)

Hello
Did you get any light about this?

Hi @JuanPabloTrujilloLev,

No I never did figure out what was going on there. If you ‘0’ the Ventilation Loads you can eliminate any ‘Mechanical Ventilation’ losses from the results, but as soon as you include any ventilation airflow, even with a 99.99% effective HRV you still see ventilation losses in the final results. Granted, its a small amount compared to the transmission losses - so maybe its just that last 0.01%?

Energy Balance with 99.99% Efficient HRV

Still seems like its more than that to me though, just looking at the total kWh compared to something like a 50 or 70% effective ERV.

Just wish I knew I was modeling it correctly and understood why I’m seeing the results I’m seeing.
best,
-Ed

@edpmay

If I’m understanding your question correctly, I think the ventilation losses you are getting are correct as from a thermodynamic perspective, you can only recover a fraction of the heat from exhaust air.

For example, if you have a system where your exhaust air is recirculated into a mixing chamber with fresh outdoor air, and assuming the box is perfectly insulated, you will have the following relationship between the input and output air:

  1. MassAir1 + MassAir2 = MassAir3
  2. MassAir1 * Enthalpy1 + MassAir2 * Enthalpy2 = MassAir3*Enthalpy3

Rearranging 2 and substituting 1 for MassAir3 we get:

Enthalpy3 = (MassAir1 * Enthalpy1 + MassAir2 * Enthalpy2)/ (MassAir1 + MassAir2)

So the resulting enthalpy is the weighted average of the exhaust and outdoor air, by their air mass flow rate (same thing also applies to temperature assuming the same specific heat capacity, and humidity ratio). Enthalpy is the total heat content of our air, so this equation proves that you can’t transfer all your heat from one air mass to another. Assuming equal exhaust and outdoor air rates, the best you can recover is half the heat content.

In your bar chart, you can also see that going from 0 to 99.99% heat recovery is reducing the mechanical ventilation more then 50% which is something I can’t explain. I’m not an expert on HVAC systems, so there may be something I’m missing on how mechanical heat recovery systems work, but this is how I would think about it.

S