Ok, here's the real story, I fix things for a living but I get stumped once in a while. My own home's forced air gas furnace is giving me a run around. A few years ago, I installed an electronic pilot ignition system in it to replace the original standing pilot. I also raised the size of the blower fan pulley a little to try to boost performance. The ignition has worked flawlessly ever since but I have subsequently lowered back the pulley size..too noisy etc. The issue is, mostly in cold times, when the (digital) thermostat calls for heat, the ignition triggers, flame detection responds and main valve fires...all normal. The main burner continues for awhile heating the box until the temp / fan switch raises and triggers ultimately starting the blower...again all normal. Then, once the blower is running, while watching the temp / fan switch, it slowly begins to lower (presumably because the blast of cooler / cold return air coming into the box). Unfortunately it continues and gets to a point where it then shuts the blower back off to keep the unit from blowing cold air into the house. The burner stays on...never goes out...because the thermostat is still calling for heat. Then, we wait for the box to again raise in temp (remember burner is still on) to a point where the temp / fan switch will once again raise and turn on the blower moving warm air into the house....but also cooler / cold air back to the box via the return duct. The cycle continues, blower on for a little bit, temp sensor gets cooled by incoming air, blower shuts off....repeating on, and on, and on, The temp / fan switch was replaced a couple of years ago. Filter is new...actually no filter right now, but access door for the filter is sealed shut to prevent outside air from entering the return duct. The high limit setting on the temp / fan switch it is about 180 to 190....it never gets to that point, The trigger on (fan on) set point is about 125 to 150, the trigger off (fan off) point is about 65 or 70. The burners look good...but all I can think of is the unit is not putting out the level of heat it should and can't keep up with the temp of cooler air returning back to it. In thinking about this, I think the problem has been occurring probably since the conversion to the electronic ignition (maybe before) which included a new main combination valve. I am thinking of bringing PG%26amp;E in to look at the flame to see if it seems too low...but in my years of working on every kind of equipment around...including many furnaces...the flame looks fine to me...short bright blue flame rising about 3/4" from the top of the burner with a longer tail of blue and yellow raising 4 inches or so above that. Maybe it needs to be higher...I am not familiar with a level adjustment of the flame on the control valve...just the air mixture via the burners adjustable air dampners. OK experts...got any suggestions?|||Ok, here we go. The flames sound fine to me and the adjustment for the gas valve is on top of it, there is a silver screw cap covering the adjustment port. Remove cap and look down in there and you will see it. Turn CW to increase and CCW to decrease, they come factory set so you shouldn't have to mess with it, was just letting you know where it was.
Now for the limit switch. Is it the right one/size? They come in 5,8,10,and 12 inch. Referring to the bimetallic shaft that goes into the heat exchangers. Make sure it is right.
Now the settings on the on. The limit should be set to around 210-215. The set point the fan on should be between 170-190. The set point for fan off should be between 90-110. Make sure you hold onto the dial while adjusting the pointers.
One more thing. Make sure your fan is running on low speed for the winter and on high for the summer. Hot air is lighter and does not take much force to move. Hope this helps|||Your sucking heat off faster than the furnace can put out.
Try to change the lower limit to a lower temp.
The fan speed was designed to work with the heat output.|||It hates you.|||That was a very thorough description. And the problems can be resolved by using just the basic methods of trouble shooting and what we perform as routine furnace maintenance. First, the gas pressure at the gas manifold should be at 3.5 in of water column (checked with a gas pressure gauge), then the fan limit control stem should be long enough to reach the middle of the heat exchangers. The set points on the limit should be enough to maintain the correct temperature rise according to the specs on the units data tag.(30 to 40 degrees). This is tested with a digital thermometer using an air probe. These tests should make it operate properly.|||wow,
I wish i could help you,
I don't get all the technical stuff you mentioned, but let me see if i got something, maybe we have the same problem...
I will be very simple...
when the weather is too my furnace is on the highest which is 90 and is still cold
and when it's not that cold and the inside temp is for example 72, the furnace still runs even more then it should...
no repair man admits that it has a problem, but I have to live with it, the winters are just too cold with my furnace
anyway, can you answer my other question about my furnacehttp://answers.yahoo.com/question/index;鈥?/a>
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