Fan Run Time After Burner Goes Off FAQs

Can we adjust the furnace fan run time ?

Blower Runs On After Furnace Burner Shuts Off FAQs

Tip: Also see the diagnosis and repair procedures at A/C or FURNACE FAN WONT STOP

How can I shut the extended furnace blower fan on-time down to turn off sooner?

Can I shut the extended blower down sooner, by adjusting these switches, to leave some heat in the aprtment?

I dont know if you need more info about model and such. Its a small efficiency Trane 80%.

The schemetic says the blower is already set med low, but havent confirmed this directly on control board.

Just looking not to do any damage. Have research knowledge, but no practical knowledge of HVAC.

Thank you kindly

I have an apt furnace 40000/32000 that is a bit oversized for the sq ftg and cubic footage of air.
It also has short return duct runs of 3-4 ft that make the return suck very powerful.

After the burners shut off, the furnace blower runs for about 90-100 secs Sucking every last bit of warm air out of this smaller apt every darn cycle keeping it constantly chilled.

But the blower fan has coll off and heat off adjustments of 60, 100 ( factory setting) etc, etc and I wouod like to shut down

That blower soon since this is a smaller space so it keaves some warmth in apt.

The furnace is in an outside closet, so dont think it will hurt the exchanger with overheating since its exposed to constant outside air, but am not certain this will help, or how to do this.

Can you lend a hand please? Its only three switches as I have looked at the electrical schematic, but just dont know if im looking at the correct thing, how to set it, or the consequences.

Thank you - On 2018-12-09 by Pete

by (mod) - why the furnace blower fan runs after call for heat has stopped - the heat purge cycle

it's normal and proper for the blower fan to continue to run for a time at the end of a heat on cycle. That's necessary in order to extract the remaining heat from the heat exchanger

Watch out: If you don't allow the blower fan to run on after the burner shuts off - I call that the "cool down" period - then the heat exchanger may be overheated and might crack which would make it dangerous risking the release of flue gases into the building air - a potentially fatal carbon monoxide hazard.

But I want to Shorten the Blower Cool-Down Cycle Time on my Trane Gas Furnace

Our Trane gas furnace is version TUD1A040A9241A

But can I not SHORTEN this from the standard set 100 secs to the lower 60 secs?

I can send the schematic if you want to see it? Three switches one for cool and two for heat.

Shortening the fan cycle time will leave more heat in the apt, yes.. Because it will be sucking hot air oit for a shorter

Length of time,plus the firnace is an an outside closet exposed to outside air, so woukd help cool fhe exchanger quicker?

Any thougnts on doing this? Do you need anymore dpecific i fo like model number and the schematic to help see if
We can safely shorten the blower time after burners go out?

Thank you for responding. Pete

Controlling the furnace fan purge cycle duration: wires & color codes

Is there a way i can confirm what temp its registering at to shut off?

Would it be listed in either the owners manual or Installers manual I have access to, to help Make a safer better informed decision of possibly turning it back to 60 sec From 100 secs?

Reminder: furnace is in an open to cold air closet, whichShould help with cooling quickly.

Built on outside wall of building.

2 ton, updraft furnace set to low? Does this help with how much Air is moving through, to decide if we can turn back after burner blower time?

I looked at this today:

CONFIRMED BY SIGHT:

1) furnace has RED on heat, which by schematic Corresponds to

LOW for fan, which is not standard setup for this furnace.

Yellow on cool.

Black in park 1

Blue in park 2

2) blower fan is on factory setting Of 100 secs for fan to stop after burners go out.

All switches pushed to ON.

Let me know your thoughts,

Appreciative of your help and opinion. On 2018-12-12 by Pete

by (mod) - Wiring diagram for the Trane XR80 - its temperature, not time, that usually controls the blower fan temperature purge cycle duration

Pete

The furnace blower fan heat purge cycle is not run by time but by temperature.

That is I expect the fan to run until the limit switch senses that the temperature has dropped to a safe level.

You wouldn't want to shorten that interval - as you'd risk an unsafe heater.

The schematic showing the fan switches is on page 15 of the Trane XR80 manual (and is illustrated above).

Wiring diagram above, cited by Anon, for the Trane XR80 - found in

TRANE XR80 GAS FURNACE MANUAL [PDF](2015) Upflow/Horizontal Right or Upflow/Horizontal Left Induced Draft Gas Furnace, Models: TUD1A040A9241A, TUD1A040A9301A, TUD1A060A9241A, TUD1A060A9361A, TUD1B060A9361A, TUD1B080A9241A, TUD1B080A9361A, TUD1B080A9481A, TUD1C080A9601A, TUD1B100A9361A, TUD1B100A9451A, TUD1C100A9481A, TUD1C100A9601A, TUD1D100A9721A, TUD1C120A9541A, TUD1D120A9601A, TUD1D140A9601A Single-Stage Fan Assisted Combustion System, Trane 6200 Troup Highway Tyler, TX 75707 www.trane.com

by (mod) - Honeywell ST9120U Universal Electronic Fan Timer Manual

What's the brand and model of the limit control here? This doesn't sound familiar to me.

I have seen this separate control from Honeywell: HONEYWELL 69-0644-1 ST9120U UNIVERSAL FAN CONTROLLER TIMER UNIT [PDF]

Excerpts:

The ST9120U Universal Electronic Fan Timers integrate control of all combustion blower and circulating fan operations in a gas warm air appliance. This control is the central wiring point for most of the electrical components in the furnace.

The basic purposes of the ST9120U are to monitor the thermostat for heat, cool and fan demands, run the induced draft blower motor and run a circulating fan (up to two speeds) as required.

The electronic fan timers also monitor limit switch strings and energize separate ignition control systems through spst pressure switches.

The ST9120U can replace any ST9101 or ST9120 listed in Table 2 below.

The electronic fan timers feature a field-adjustable heat fan-on delay, a field-adjustable heat fan-off delay, a field-adjustable cool fan-on delay and a fixed cool fan-off delay.

by Pete

Any clarification on what you need, and what you dont recognize?
The wiring setup seem screwy to you? Is that that your saying?

Pete

The brand of furnace? TRANE XR 80
Or
do you want me to look inside the furnace for a brand name on
The wiring control panel?

by (mod) - identify the fan limit controller in order to understand its wiring and find its manual

Again: What's the brand and model of the limit control here? This doesn't sound familiar to me.

It's the limit controller wiring that we need to examine - that's not the furnace brand, though perhaps your furnace manual does include instructions for wiring the limit control. Take a look and let me know.