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# heat recovery wheel bypass logic

One of my commissioning projects uses an enthalpy wheel on several AHUs. These AHUs have a bypass for the wheel or just shut the wheel off when conditions are more favorable to use straight outdoor air (or a mix of OA and return air.)

The sequence of operation mentions the bypass, but doesn't describe how it becomes active. I have not previously thought about the best control scheme for de-energizing the wheel or bypassing, so I'm reaching out to folk who may have thought it over... (or tested it in a model)

I have some manufacturer literature which suggests a dry bulb economizer-like changeover. I've done a mockup in Excel using TMY weather data and a DB-based changeover looks sloppy compared to a logic-based approach - but I may be missing something.

Can you share any insights?

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There are many control logics one could implement for this specific purpose, the "best" theoretical one being a combination of differential enthalpy and differential dry bulb: you put an "enthalpy" sensor (really it's a temperature sensor and a humidity sensor, with a logic board to calculate enthalpy based off temp and humidity) in the return air inlet to ERV and one on the outdoor air inlet, and decide whether it's more appropriate to use the wheel or not.

I'm specifically mentioning "theoretical" here because the problem with humidity sensors (therefore with enthalpy) is that they can quickly fall off calibration and make the computation of enthalpy completely inaccurate. As I think I recall reading, you should likely replace them every year or so...

This ASHRAE article Economizer High Limit Controls and Why Enthalpy Economizers Don’t Work should prove a most interesting read, even though it doesn't address ERVs specifically but rather economizers. Couple of quotes:

Humidity sensors, on the other hand, are notorious for being difficult to maintain in calibration

[...]

The best option, assuming no sensor error, is the combination of differential enthalpy and fixed dry bulb

[...]

Including sensor error, the worst (or very close to the worst) option in all climates is the differential enthalpy control. This control logic is considered the “best” anecdotally among many design engineers and is required for some climate zones by Standard 189.1. Yet, in practice, with realistic (even optimistic) sensor error, it performs among the worst of all options

In EnergyPlus, relevant fields are:

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Also relevant is the Controller:OutdoorAir input field Heat Recovery Bypass Control Type.

( 2017-06-06 09:19:20 -0600 )edit