I’m having some trouble with my forced air gas furnace . It’s a Rheem, horizontal in the crawlspace about 15 years old. The fan was not coming on after the burners lit.
I replaced the fan control unit (Honeywell), but it still take 5 minutes after ignition for the fan to kick on. The little tabs on the dial are set right. The dials (on both the old unit and the new one) appear to slowly move along as the heat builds up, but can’t seem to make the ‘jump’ to actually cause the fan to switch on. When they finally do get over this hurdle, the fan kicks on and the dial then rapidly advances about 40 degrees or so.
Any clues or tips? Hopefully this makes sense to a real HVAC person, I’m just a dumb cabinet maker. Thanks
Jim
Replies
Sounds like the wrong unit for a horizontal furnace. Should have a timer.
(Problem is that there's no air moving and so the control never gets warm. In an updraft furnace this is handled by convection, but in a horizontal I think you need a timer -- the fan control is just used for overtemp.
Dan H:
Thanks for your idea. You may be right, it certainly makes sense to me, though I have the original paperwork/instructions and they do indeed specify this exact type of control for the unit. Who knows? I may just bite the bullet and get a new high efficiency unit to save on heating costs and do my bit for global warming.
Is this a unit that can be installed either horizontally or vertically? If so then I'll bet that if you read the instructions carefully you're supposed to add a timer in the horizontal position. Or there may be alternate mounting spots for the fan stat, based on whether this unit is laying on one side or the other. The stat location you use should be as high as possible in the unit.In any event, the thing to do in the interim is to turn the temp range way down on the fan stat. Set the low lever (fan off) at about 100F and the middle lever (fan on) tight against the lower lever. Move the top (overtemp) lever down below 140 -- as low as it can be without tripping in normal operation. (Note: ALWAYS hold the dial while moving the levers. If you don't you can trash the unit.)
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Dan:
Again thanks for you attention. This baby is made to be horizontal & the unit is mounted high and to the rear. The main thing is that since 1983 it has been working ok. I'll give your suggestion a shot tomorrow after I digest all of this turkey I ate today. Thanks for all your help.
PS I'll probably buy a new high effeciency unit, think this baby has sen better days, the booming from the firebox delayed ignition (flame frequently won't cross the spreader bar to the other burners for a few seconds, then BOOM it does) combined with this screw up is getting to be too much. I removed the burner plate, blew out the rust n dust, but have to admit the little holes in the heater tubes appear to be of variable size which is not how it was manufactured.
I'll betcha that the booming blew out a baffle that helped direct the heat to the fan control. If so then yeah, the unit should go to the junk heap.Would this perhaps be a unit originally for propane & converted to natural gas? If so then they probably failed to change over the pressure regulator gizmo on the gas valve to be a step-opening variety. That'll cause the booming.
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A fan control usually does two separate functions. When the burner kicks on it takes awhile for the heat to build up enough to activate the control which makes the fan go on. You don't want cold air coming up from your ductwork. So in effect the control has two settings on (which I have never altered). If the temp inside the furnace is too cool then the fan won't come on. If the temp gets too hot the fan keeps running but the burners shut off. In an overheating situation you want to disapate the heat fat but you don't want the burners to keep adding to the problem.
At a quick glance to your problem the furnace is cooling down quick and shutting off the fan ( I'm assuming the burner keeps going). This can be normal (to a point) because the house is cold and there is too much cold air but as the temp of the house increases the furnace with shut off, by the fan control, less and less.
Some things to look for : are all the burners coming on? maybe you are not getting enough heat.
: maybe the fan is going too fast and cooling down the control but as the house warms it would equal out.
: maybe your crawlspace is so damn cold it's lowerin the return air so much it's affecting the control.
As I said on a cold house the control will shut off the fan but as the house warms it will shut off less and eventually work by the wall thermostat.
Off the top of my head you might have a faulty anticipator in your wall stat which makes for eratic furnace operation. It doesn't hurt the furnace but makes for some uncomfortable swings in room temp.
Good luck.
roger
I think you misread his post. It takes 5 minutes after burner startup for the fan to come on, then the dial "jumps" upward (since hot air is now reaching it thanks to the fan). Clearly the heat simply isn't reaching the fan control by convection.
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