Designing an HVAC monitoring system

Ideas - things to do

The HVAC (Heating, Ventilation and Air Conditioning) system, as many other systems, has the bad habit to broke down when most needed.

The idea is to design a monitoring system for HVAC capable to predict problems, with the less possible impact on the system, meaning that the monitoring system should be as much as possible independent from the HVAC itself.

In a way it would be easy to "hook up" to the HVAC, monitoring directly some parameters in order to determine if is working properly.
However there are some major problems to this approach :

expenses
Hooking up some parameters directly, like the refrigerant pressure, or the temperature/humidity inside the evaporator, imply to use specific sensors, usually expensive and to modify the HVAC, creating places for the sensors and cables.
If errors are made is possible to have the HVAC damaged and also removing the sensors can be more difficult and expesive.
not universal
Monitoring specific HVAC parameters, imply to customize the results for each HVAC.
Different brands/models can have different parameters, ways to hook up sensors, etc.
This imply the need to customize the range of every parameter and design specific interfaces for each HVAC to monitor.
legal
Adding sensor hooked up to the HVAC itself, can have a legal implication.
In some cases, like hooking a sensor to measure the refrigerant pressure, it could be considered an alteration of the plant. If something goes wrong, warranties can be dismissed because "the HVAC was tampered with".

Define the problem

The HVAC system, has the purpose to change the temperature/humidity of the house.
In winter it will generate heat, in summer it will cool down.
The purpose of the monitoring system is to determine the working status of the HVAC, so to be able to forecast possible problems BEFORE they actually happens.

In order to do so, the idea is to monitor different characteristics of the HVAC, and calculate the probability to have a problem and in what area.

Problems to be identified

Here a list of the problems that the system should be able to early detect:

AC - low pressure refrigerant gas

air not cold enough or same room temperature

compressor not starting

airflow limited or missing

presence of ice in the evaporator

V - air filter dirty

limited airflow

HVAC - not working

fuse blown out

Requirements

Here some requirements to meet for the project :

the HVAC and HVAC plant will not be modified/touched.
i.e. the monitoring system must be transparent/independent to the HVAC system
the monitoring system need to be installed permanently but it can be easy removed if necessary
it must be cheap as possible
it must be easy to install
it can be connected to a computer in order to obtain data and graphs
it can generate alarms using different media
it must be able to give reasonable predictions about the possible fault, however since many direct parameters can be only inferred, it will be much lower the 100% accuracy

Detection system

In order to detect problems, the monitoring system has to acquire data.
It will be the air flow/pressure, temperature, humidity, leaks, etc.

There are different problems that can occur in a HVAC system. The blower can broke down (no air movement), ice can be form in the evaporator (blocking the air flow and creating a leaking), the coolant pressure can drop for leaks or other reasons, the burner can stop to work, etc.

The goal is to set up a system acquiring the less possible number/type of parameter in order to assess the status of the HVAC and possibly make predictions based on the parameters history.
Some parameters can be difficult to acquire or they can require a modification to the HVAC.

Possible data to acquire

temperature

ambient
external (outside)
around condensation unit (ice presence)

air flow

pressure
air flow supposed to reach

humidity

internal (home)
external (outside)

water

presence in the discharge pipe
presence outside the condensation unit (leak)

HVAC on or off
Cooling or Heating mode

Measurement units

temperature

Celsius

air flow

kg/m

humidity
water

Presence/absence

Strategy

There are different ways to monitor an HVAC system:

comparing the read data with a set of predefined/theoretical values
comparing the read data with a previous "assumed to be correct" values
a mix between the previous two strategies

The first method is much more precise but has some impractical limits.
It is assumed to know the theoretical data about the HVAC system.
Also, older plants can maybe never reach the theoretical performance, even if they are still considering "working".

The second method is less precise but can be set up easily and adapted to different plants/environments.
So it will be adopted for the project. The monitoring system will be able to "capture" a set of data at a certain moment and use them as "referral" for the monitoring.

A degrading read values from the "saved" ones will be considered a problem.

Possibly an integration between the two previous strategies can improve the monitor capabilities, acquiring or assuming theoretical values only for some parameters.

Monitoring system

The monitoring system will read the defined parameters (temperature/air flow/water levels/etc.) and will use them to decide if the system is working properly or if problems exists.

Different parameters will concur to determine problems, but single parameters alone can be indicative.
For example if sensors are placed to detect leaks, then the presence of water will indicate the existence of a problem, regardless the fact the temperature and air flow are still in the correct range.

What to monitor

It would be nice to monitor every possible aspect of an HVAC but is impractical and expensive.
So lets discuss about what is possible to monitor and what can be exert from that.

Airflow

The main component of an HVAC is the Ventilation.
Without ventilation is not possible to change temperature efficiently.
So monitoring the airflow is mandatory.
The absence of ventilation however is not an indicator of a problem "per se".
It depends about many other factors. On nice warm "not too hot" days is possible the HVAC is not working simply because there are not the conditions to work.
The airflow presence/absence alone is not enough to determine if a problem exists.
But the amount of airflow when the fan is working can indicate two possible problem conditions, if the measured value is lower than previous readings :

the air filters are really clogged and needs to be replaced
ice is present in the evaporator preventing the airflow to go over the evaporator

The second condition can happens if the pressure of the coolant (like freon) is too low (presence of a leaking)
(see how to prepare a cheap and easy airflow sensor)

Temperature

The temperature is another mandatory parameter to monitor.
Actually more than 1 temperature is needed.
Ambient temperature and airflow temperature needs to be acquired and integrated with other measurements, like the airflow and the time, i.e. temperatures need to be monitored considering the variations rather than the instant value.

Water presence

It could be valuable to monitor some "spots" for the presence of water.
This especially for the air conditioning.
A sensor can be easily placed in the discharge pipe to see if water is generated by the HVAC.
Other sensors can be placed where possible leaking occurs.

Humidity

One of the main result of the HVAC is to reduce the humidity in the house.
Measuring the humidity can be beneficial but is not mandatory.

Ice

Normally when an HVAC has refrigerant problems, ice is forming over some pipes.
Be able to detect if ice is present can be a good indicator that exists a refrigerant pressure problem.

Tools

Air flow/temperature

In order to measure the air flow, it is necessary an airflow meter.
The sensor has attached also a temperature sensor so to measure also the air flow temperature.
In order to test the system and have some data to work on, I bought a standard thermo-airflow meter.

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Saturday, July 7, 2018