My old Carrier gas furnace heats just fine, but does something I don't
recall it doing in the past. Whenever the thermostat has told the
system to turn on and provide heat, I hear the burner re-igniting
several times while the fan is still running. I assume that since I
hear it igniting that it must have previously shut off each time, but
have not verified this. It used to be that the burner would just
light, fan would then start when heat exchanger reached a certain
temperature, and burner would stay on continuously until thermostat
was satisfied after which it would turn off and fan would continue for
a period, then shut off. What would cause the system to now have the
burner re-igniting several times during the run cycle, and is this
something to really be concerned about? Thanks for any input.
There is a high temperature limit thermostat. If for some reason the heat
exchanger is reaching the high limit, it will shut down the burner until it
drops to a lower temperature. One possible cause for that is reduced air
flow over the heat exchanger. Have you checked the filters?
There are often a number of temperature switches in series on older furnaces,
so if any one closes it kills the burner. High temperature plenum, high
temperature in the flue, flame rollout switch, etc.
Sure enough, problem was reduced air flow. The service guys removed
the squirrel cage blower and cleaned what must have been about 30
years of dust and dirt from the blades. After reinstalling it, plenty
of air flow and no more cycling of burner on and off.
On Sun, 25 Nov 2007 11:54:38 -0800, johnmboy65 wrote:
High limit switch. Does the air feel really warm and is it blowing with
as much force as it usually does? If you don't know what to look for call
someone who does because this could be a portend of something failing.
Check your air filters, check all supply grilles [be sure they're open.]
Check the furnace blower for proper operation. If all of this seems ok,
call a licensed HVAC contractor. If the high limit needs replacement, let
him do it. Most contractors carry general product liability insurance. If
you do it, you won't be insured when something happens.
You didn't say anything about the time interval between
relights. That information would be useful to rule out or in
various poster's suggestions.
You also didn't say anything about the age or type of furnace.
Does it have a pilot or other ignition source?
Does it have a draft inducer (and pressure switches)?
Is it a high efficiency, condensing
furnace? Again this information could confirm or deny
other poster's suggestions.
The time interval between relights is something I've not specifically
noticed...seems like it's around 3 or 4 minutes, but I haven't timed
it. The system is made by Amana ( not Carrier as I originally said),
uses natural gas and has a pilot. It's at least 25 or 30 years old,
so I'm guessing it's not high efficiency. Don't know whether it has a
draft inducer or pressure switches. Service guy is due out to the
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.