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Question DetailsAsked on 11/23/2016

Furnace blowing only semi warm air. The pilot is on, and the burners are lit, the fan goes on as well.

Pilot is lit and lights the burners fine. The fan comes on as well, however the blowing air is only mildly warm, not almost hot like it use to be. What can be worng?

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4 Answers


Lots of possibilities, which would have to gone through by process of elimination. These are listed as stream-of-thought items in no particular order (actually simpler ones are mostly towards the end of thelist), so you might read through first for the easier ones to eliminate and start working on those first. Some can be eliminated by inspection, some infer a low airflow so can be eliminated if airflow is normal, etc.

1) fan motor is running, but the fan itself has lost its retaining setscrew or bolt or is jammed against the housing but loose on the shaft so it is spinning on the motor shaft and not moving much air if any

2) fan itself is plugged up with lint or piece of plastic oir paper or something like that which is preventing it from moving air as it should

3) burners are operating only on low setting, if multi-stage burner controlled - check also for trouble lights or codes on furnace if yours has them

4) fan is operating only on low speed, if variable or multi-speed fan - this could be because of a controller problem, thermostat mis-set, or a partial blockage to airflow in the ducting, depending on conditions and the types of safety sensors your unit has - check also for trouble lights or codes on furnace

5) if high-efficiency unit, perhaps burners are igniting OK, but the exhaust fan and/or damper are not operating correctly or the exhaust pipe is frost blocked (if having very cold weather) so the airflow or backpressure sensor determines the exhaust is not being vented correctly and shuts the burner off after a very short run time - usually not more than 10-20 seconds.

6) A/C evaporator coil is plugged - with lint, mold, or even ice if A/C is still on - so it is blocking the airflow. If this were happening the amount of air being moved would be down also, not just the temperature.

7) a damper or control vane in the ducting has malfunctioned or been moved, so either the airflow is being blocked, or most of it is going into another part of the ducting (maybe to another floor or wing)

8) if a multi-zone duct system, with individual thermostats controlling each area or floor's airflow, if the battery in the thermostat has died it can fail to operate the control damper for correct airflow and may have basically shut off airflow to one or more areas.

9) some opening in the ducting - access panel to evaporator or air handler, a blanked-off duct end, etc has fallen off, or a duct has come apart at a seam, leaving a wide open exit from the ducting that is stealing much of the heating airflow, causing low airflow at the registers

10) a duct (flexible) has fallen or settled so it developed a kink, blocking much of the airflow at that point

11) if you have a separate A/C air handler that feeds into the ducts, maybe it has not been winterized so a good portion of your furnace air is blowing out that air handler to the outdoors or to wherever its air intake is

12) rare, but happens sometimes with multi-thermostat systems - either air conditioner is still on and fighting with the furnace because one thermostat is still set to A/C instead of Heat or has dead batteries, or a thermostat is still set to A/C even though the A/C is powered off, so the zone or area that thermostat controls is not getting the signal to the furnace or control damper to let hot air through, so all you are getting is a bit of air leakage past the control damper

13) It is possible for burner jets to get plugged or covered in rust. Much more common in water heaters than furnaces but does happen there too in older units or ones with condensing exhaust conditions in the flue piping above the unit resulting in rust cascading down onto the burner

14) Rare but can happen for the gas control valve to malfunction and not be letting the proper amount of gas through, so the main burner might "light" and be burning around the perimeter, but with only a small flame - not full gas flow as it should. Normal reason for this is the thermocouple /thermopile is generating too weak a signal to open the gas cotnrol valve solenoid fully, or the solenoid coil on the gas control valve is going bad so it only opens the gas valve partially. Takes a millivolt volt-ohm-meter to measure the output of the thermocouple/thermopile to see if it is proper amount of voltage.

15) much rarer but I have seen it (nothing like a cranky raccoon being waken up from its nice warm nap/warm air blowdry) isan insect nest (like hornets/yellow jackets most commonly) or rodent/varmit in the ducting. Most commonly occurs on units with outside air intake direct to the furnace and a sealed furnace, so the blockage is restricting air INFLOW to the furnace.

If these possibilities do not solve it for you, then you need to find a well-rated and reviewed Heating and A/C contractor (your Search the List category).

Answered 1 year ago by LCD



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Answered 1 year ago by Member Services


First of all, tahnk you very much for a lengthy reply and your attempt to help me.

Here are my findings/answers:

I've checked all of your suggestions and the only thing that applies to my situation might be #3) There is a good possibiluty that the burners are only running on teh low setting. I dont recall how the flame looked when the furnece was running fine but flames do look low to me. What controlls this second stage and how can I toruble shoot it?

Answered 1 year ago by driftkr6l


Ok - you might or might not get some help from the owner's manual (almost always avaialbe online at manufacturer website if you can't find yours), but perhaps contacting the technical support people at the manufacturer would be a good next step - assuming it is not failing to keep your house suitably warm in current ambient conditions. Obviously, if you are freezing your tails off, then calling a Heating and A/C technician who is familiar with that brand would be your only recourse for a fast fix.

Could be one of several (OK, actually a lot by the time I went through everything I would look for in debugging this issue) things going on:

1) one thing I might not have mentioned - if only barely warm, could just be heat from the unit from being heated by a standing pilot - is it possible the thermostat got set to FAN instead of HEAT (or equivalent terminology) ?

2) the heating demand is low, so your variable output or staged burner system (if you have one - usually on high efficiency units only) is operating at only low heat - figuring it just needs to take the chill off but is not needed to generate a LOT of heat. A variable output system uses a metering control on the burner to regulate the amount of heat generated to correspond to the heating demand - based on the temperature difference between the outdoor and indoor air. Variable is essentially continuously variable output over a specific design range from low output to max rated capacity. A staged system works the same way, just has only 2 or 3 different specific output levels.

3) Then, there are also variable speed fans or blowers - found on both fixed-output burner and variable burner units - commonly operating at full speed (and airflow) when the burner is operating, then dropping to low afterwards to remove the last bit of heat from the heat exchanger. With variable output burners, the fan speed may be regulated to produce a certain output air temp - so lower speed fan operation when burner is on low output, higher with higher output. In some few units the burner is not variable but actually burns for a bit, then turnsoff while the fan evacuates the heat, then turns back on for a short time unless the heating demand is high - generally only on very high-efficiency or pulse / "closed combustion" units.

4) You can see, with possible variable burner output and variable fan speeds, there can be a fairly complex control system with multiple temperature sensors to determine which it is to use at a particular time. If your furnace is standard efficiency (around 80%) it likely is only single-output, single-speed. High efficiency units (90% or more) almost certainly one or both - intermediate 80-90% range may have one or both (variable output / variable speed fan).

5) So - assuming your unit is seeing high demand (like the house is not staying as warm as you have set) could be a control unit issue, which would take some debugging. If a staged fan/blower, could be the motor or regulator for it has partly failed so it is now running only on low speed.

6) However - since you say warm, not hot air - more likely a burner issue than fan, though low fan speed with full burner output could result in only warm air coming out because the air is cooling as it passes through the ducts - especially if the ducts are "outside" the conditioned air space - like in unheated crawlspace or basement.

7) One other more esoteric thing that could be happening - if the fan is not coming up to full speed (due to failure of high-speed part of control, friction from lint buildup making it rub against the housing, worn bearings, etc) then a variable output burner might be firing full power initially, but because the fan is not removing the heat fast enough, is kicking back down to lower output as the heat exchanger and outgoing air exceeds the safety sensor - ditto to a firebox overheating sensor kicking in.

8) Also, many times these safety sensors are bimetallic switches - get too hot and they cut the power to the burners, but when they cooll down they automatically reset and allow it to fire again - so you can get intermittent full burner operation, then nothing for awhile, then as it cools off back to firing again. Takes physical inspection of the furnace from startup onward for sometimes 5 minutes or to track down this sort of failure because it does not result in a detectable permanently "off" circuit like a blown fuse or tripped breaker. This sort of head scratcher is (unfortunately for homeowners, but fairly) a significant money maker for HVAC techs - and many times the control unit does not have sophisticated enough control unit to generate a code for the problem that can be read on the unit (numerically as an error coce or by steady or blinking LED light pattern).

Answered 1 year ago by LCD

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