Long furnace warm-up


  #1  
Old 12-05-10, 02:23 PM
J
Member
Thread Starter
Join Date: May 2010
Location: Canada
Posts: 11
Likes: 0
Received 0 Likes on 0 Posts
Long furnace warm-up

I have an old 3-bedroom house with old forced air furnace. At the current outdoor temp of -19 degC the furnace is cycling 9 minutes fan running, 12 minutes off. The gas fires up for 2min 45 secs before the fan starts running. This seems a very long time. Although some of the heat is going into the ductwork by natural convection, it seems very wasteful.

I have my thermostat set for a long cycle. On the standard forced air setting the cycle is 5 minutes on, 6 minutes off. On that setting, the 2min 45 secs is a huge chunk of the cycle time.

Is this long warm-up time an indication that the temperature control to start the fan is out of whack?
 
  #2  
Old 12-05-10, 02:27 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,499
Likes: 0
Received 1 Like on 1 Post
Is this oil. propane, natural gas?

What is the age of the furnace approximately?

What is the model number of the furnace which is on the rating plate in the burner compartment of the furnace?

Is the fan shutting off, or is it running most or all the time with the burner turning on while the fan is still running?

Have you checked to see if the filters need to be changed?
 
  #3  
Old 12-05-10, 03:10 PM
J
Member
Thread Starter
Join Date: May 2010
Location: Canada
Posts: 11
Likes: 0
Received 0 Likes on 0 Posts
It's at least 30 years old, Furnasman HG 125 36, Natural Gas, 125,000 BTU/hr.

My present long cycle is broken down into consecutive events as follows:
Thermostat gives signal to run
Gas runs for 2min 45 secs
Fan starts up and runs for 9 minutes (including cooldown after thermostat up to set temperature)
Fan Off
System is completely idle for 10mins 15secs

The circuit is quite simple. The solenoid valve is powered by 24VAC and connected to the thermostat. There is a 200 degF switch on the primary side of the transformer to shut off gas on over-temperature.

The fan is controlled by a switch. The dial is 80-120 degF, currently set at 110 degF and the dial is labelled "Fan Off". I do not have any indication at what temperature that switch turns on the fan. I assume that if I reduce the temperature setting, the fan will start up sooner but run longer on cooldown.

The filter is fine.
 

Last edited by JohnDIYS; 12-05-10 at 03:11 PM. Reason: typo
  #4  
Old 12-05-10, 05:38 PM
SeattlePioneer's Avatar
Banned. Rule And/Or Policy Violation
Join Date: Jul 2007
Location: Seattle, Wa
Posts: 5,499
Likes: 0
Received 1 Like on 1 Post
I take it that the fan switch and limit switch are separate parts.



Look carefully at the fan switch. Usually there is an adjustment that allows the temperatuire at which the fan turns on to be set, and a second adjustment that allows the temperature at which the fan shuts off to be selected.

Sometimes that's a dial, other times it's a lever.

Sometimes there is no adjustment and the setting and the differential temperature is set by the design of the switch.

That's probably what you have, since you very likely would have mentioned adjustments had they been there.

If you have a fan switch without that adjustment feature, you're stuck. Replacing the switch with one that does have those adjustments is pretty easy and a new part is $50 or so.

That's probably worth doing if you plan to keep the furnace a few more years.
 
  #5  
Old 12-05-10, 08:15 PM
J
Member
Thread Starter
Join Date: May 2010
Location: Canada
Posts: 11
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by SeattlePioneer View Post
I take it that the fan switch and limit switch are separate parts.
Yes. The fan switch has space for another adjuster but not installed. It must be a fixed deadband. I tried lowering the "fan off" setting. The fan started up 15 secs earlier but that's not worth the bother. I have next Summer earmarked for replacing the furnace so I'll live with what I have for now.

Keeping the thermostat cycle long reduces the percentage of time the gas is on and the fan is not running. 9mins per hour with the long cycle (2 min 45 secs x 3), 15mins per hour with the short cycle (2 min 45 secs x 5). The room temperature will vary more but that's not a problem.

Thanks for the help.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: