Author: Jesse

Definitions

Definitions

Air

  • Combustion air: Air required to complete combustion (primary air and secondary air)
  • Ambient air: This is the air that surrounds us
  • Dilution air: The ambient air admitted to the draft hood, draft diverter, or draft regulator. It’s used to cool the venting system and vent the flue gases
  • Excess air: The portion of the combustion air supplied to ensure complete combustion can happen. This is added to combustion air
  • Primary air: The portion of air added to the air/gas mixture prior to ignition
  • Secondary air: The portion of air supplied prior to ignition around the flame

Aldehydes
Aldehydes are a product of incomplete combustion. They’re irritating to the eyes, throat, and nose and smell terrible. Tastes somewhat metallic and are likely accompanied by carbon monoxide

Ambient
The ambient air temperature is the dry-bulb temperature of the surrounding air

Applied voltage
The voltage supplied or applied to the electrical circuit
(also known as supply voltage) continue reading...

Plenum

Plenum

Supply Air

Sheet metal box connected directly to the supply outlet of the HVAC appliance that directs the heated or cooled air to the ductwork.

Return Air

Most units have multiple, or at least one return duct in the house moving conditioned room air to the HVAC unit. When air is forced through the supply ductwork, conditioned room air gets pulled throughout the structures return ductwork into the central return air plenum to be moved through the appliance to be heated or cooled. continue reading...

TH, TR and TH/TR

TH, TR and TH/TR

You’ll run into these terminals commonly on older standing pilot furnaces. TH/TR is a common connection, but that doesn’t explain much in itself.

We’ll overview which each of these terminals is intended for:

TH – The 24v hot leg from the thermostat on a call for heat (R+W closing on the thermostat) to the gas valve (TH terminal) to open the solenoid to allow gas to flow, assuming the transformer is working and the high limit is closed (allowing for the flow of electricity) continue reading...

Tankless water heater

Tankless water heater

The following sequence is for the Rinnai tankless water heater. If you can learn this sequence, you will have a good understanding across the board for all brands out there as they all follow a similar sequence of operation.

Step 1. Upon a call for hot water, the water flow sensor will sense water flow and send a pulsing signal to the PCB (control board). The gallons per minute minimum to start the unit start trial for ignition will differ across models and brands, but 0.4 gpm is common continue reading...

90%+ Furnace

90%+ Furnace

The following is an example of a 90%+ furnace. Things may differ slightly from manufacturer to manufacturer, but they’ll all be pretty close, and theory will remain the same.

  • Furnaces equipped with a control board are continually checking to confirm rollout switches and the high-limit switch are all closed. It also sends 80V – 110V to the flame sensor to prove there’s no flame present before ignition as well with no call for heat
  • The thermostat has a call for heat and closes R+W switch
  • The control board completes a safety check to ensure rollouts and high-limit are closed (completed circuit). If a high-limit or flame rollout is open, or a pressure switch is closed before running the combustion blower, the control board will display an error and run the blower to get the attention of the homeowner
  • Combustion blower starts up to ensure exhaust venting, primary heat exchanger, secondary heat exchanger, combustion air, and drain lines are clear. The contacts of the pressure switch will close (to complete the safety circuit) if no blockages exist. If the combustion blower cannot close the contacts of the pressure switch, an error will be displayed. The combustion blower will try a couple more times before going into a lockout
  • The igniter will start to warm up for around 10 – 20 seconds
  • The trial for ignition begins. The gas needs to ignite the burners and flame sensor sense the flame. The gas valve will energize for about 7 seconds and ignite the burners
  • The flame will be proven using flame rectification. If the flame is not proven within a few seconds, the gas valve de-energizes and the process starts again. The furnace usually tries three times before going into lockout and displaying an error
  • Assuming flame is proven, the blower will begin after the heat exchanger has had time to warm up. Waiting for the heat exchanger to warm up helps not to blow cold air on the homeowner. There’s usually a delay of 30 seconds or so
  • Once the thermostat is satisfied, the gas valve stops the flow of gas, and the blower will continue to run for about 90 seconds to cool off the heat exchanger and use the rest of the heat from the heat exchanger
  • The combustion blower runs for a short time as well to endure the exhaust is cleared
  • continue reading...

    Thermostat

    Thermostat

    The thermostat is the command center for an HVAC system. The thermostat is what will switch the equipment on and off. Most people are familiar with the thermostat on the wall for their heating and cooling, but they’re in almost everything from water heaters to freezers and kitchen equipment.

    Modern thermostats sense temperature using a thermistor. The thermistors resistance value changes depending on the surrounding temperature. A small microcontroller converts the measured resistance to temperature and decides based on desired settings if heating or cooling should be switched on. continue reading...

    Condensate drains and traps

    Condensate drains and traps

    Improper condensate trapping can cause a lot of property damage. It’s necessary that a drain line is installed correctly to avoid damage to a residence or business.

    Evaporators coils are installed to allow a blower to either push or pull air across the coil. The pressure, either positive or negative applied across the coil is also the same applied pressure to the drain line connected to the drain pan of the evaporator coil. continue reading...

    Thermistor

    Thermistor

    Thermistors are used to sense/measure temperature. You will find them in ductless splits to detect evaporator coil or condenser temperature, temperature measuring devices such as a digital thermostat, ambient air temperature, PTC start relay, and supply or return air temperature. The name thermistor is a combination of “thermal” and “resistor.” continue reading...

    Pressure switch

    Pressure switch

    The plastic orifice shown in the image above is a snubber. Sometimes the appliance will experience sudden pressure changes such as wind against the exhaust or the burners igniting. The snubber is designed to only allow a designed amount of pressure to be pulled against the switch at one time.

    The insert should not be removed to make the appliance function. If you remove the insert and the appliance works, you have another larger problem such as a plugged intake, plugged exhaust or drain. continue reading...