"call for heat" in my carrier furnace?

[juicymixx]

Hey guys,

We just moved to a new home with a Carrier 58PAV up-flow gas furnace. The furnace checked out fine before we moved in, so I'm not sure if the way that it's currently operating is normal (and it works in a way I don't expect), or if something is wrong:

Say the temperature in my house is 68 degrees, and the thermostat is set to HEAT at 71 degrees. Well, the furnace stats circulating the air (that is, air is moving through the system and into the house so the blower fan turns on), however it's just the ambient temperature (whatever temp it is in my garage and ducts - but not hot/warm). So, effectively I have this cool breeze blowing through my home! If I set my thermostat up to 90 degrees, then the burners click on and start heating the air. I'm not sure why the 'call for heat' isn't heating for low differences in temperature..?

am I totally nuts? Thanks in advance for your help...!

[Art Todesco]

This is the way some/many of the newer units work. I hate it. It's dumb.. My 18 year old 90+% recouperative unit doesn't turn the blower on until the plenum gets hot. AND, even then, for a second or two, you feel the cold draft.

[juicymixx]

So... This is 'normal' operation? I have to set the thermostat up to

90+ degrees to get any warmth? Grrr..! The other night we had the thermostat set to 71 degrees, and the outdoor temp got to the mid to low 60's - I'm fairly certain that the system just circulated the air throughout the night (that is, it didn't warm the air, just that cool breeze all night). However when I woke up (shivering - indoor temp was below 70 degrees) and pushed it up to 90+, the house got warm pretty quick...(then, of course, I turned the system off...)

Is there something I can set/change to change this? As I mentioned before, the furnace is a Carrier 58PAV up-flow. Also, my thermostat is made by Totaline - although I'm not sure of the type, it seems pretty low end...

Thanks again!

[Mark Lloyd]

Looks like its a good thing my furnace DOESN'T need replacing.

[bigjim]

No it's not normal. When a house is inspected usually only operation is checked. Not quality of operation. Get a HVAC tech to check it out. Your t-statmay be bad or another component isnt working

snipped-for-privacy@mail> Hey guys,

[Al Moran]

Is the thermostat digital or analog? If it is analog then the heat anticipator is defective. Open the furnace up and remove the red and white wire from where ever they connect. Take a piece of wire and connect it to the red and white post/screw that you just took the wires off of. Does it fire up? if so then your thermostat is defective.

[jolt]

The furnace has a two stage gas valve, the low stage uses less gas thus produces less temperature rise at the registers. When you set the thermostat to 90 the second stage comes on thus increasing gas usage and resulting is warmer temperature from the registers.

The fan speed may be adjusted too high for the low stage burn or your expectation of the delivery temperature doesn't allow for the low stage burn. If you measure the temperature of the supply air I think you'll find it is warm ay low stage burn but not hot as it will be at high stage burn.

[mm]

Mine works that way. I'm sure you know that it's inevitable that you'll feel cold air at first because that's the air in the ducts.

By coincidence, today I took apart the control unit for my furnace.

23 years ago it seemed pretty complicated somehow, but today I noted it had only 2 resistors, a capacitor, and two simple 3-wire transistors, plus a power relay, and a cutout relay that functions when there is no flame, for example. It's simple. For example, the cutout relay only has 4 wires. I think the power relay also has only 4 wires.

So how *does* it manage to delay the fan from coming on until the plenum is hot, and delay it from turning off until the plenum is cool? Maybe those two things, related, are the only things it does. I guess it has enough parts to do that.

The small number of parts makes it all the more annoying that they wanted 200 dollars for a replacement 23 years ago. Of course that one would have also had the 110 to 24 volt transformer, but when I groaned at the price, they sold me the transformer only for 10 or 20 dollars.

That hasn't needed repair since. (Remember that weekend, Paul?)

Last night around 2:30 I thought my carbon monoxide detector went off. I woke up and was too groggy to remember if the green light was supposed to be on, and my eyes couldn't tell if the red led segments lighting up were a, b, c, a, b, c, which is normal, or a, c, a, c, a, c which isn't. So I turned off the furnace. Gradually I reallized there was no CO, the alarm didn't say there was, and the noise was from the upstairs wireless doorbell which sqawks loudly, twice, after every power failure. (I had noticed a vcr and a digital clocks that were flashing 12.

[Art Todesco]

The typical furnace has a temperature sensor on the plenum. when it gets hot, it starts the fan, when cool, it stops. Some of the sensors are actually dual; one to run the fan and one as an over-temperature safety .... some are separate. Some are adjustable, most are not. Mine originally (non-adjustable) waited too long (too high temp) to turn on and waited too long to shut off (cold air blowing). I call the company, they replaced it; the new one was worse than the original. I finally bought a new, adjustable one; set it where I wanted; works perfectly for the last 18 years.

[Stormin Mormon]

The older furnaces had a temperature sensor that extended into the heat exchanger. Called a fan limit switch. One totally hot summer day I had the furnace blower kick on. Wasn't sure what was going on, until it finally occured to me the furnace was hot enough it thought there was fire in the firebox. Ah, well.

[Stormin Mormon]

Since it does heat if the temp is way up, sounds like a thermostat problem. Please let us know what you find.

[juicymixx]

Stormin Mormon wrote:

okay... sorry it took me so long to get this reply out. Thanks to everyone who contributed to the explanations! I just have to say that I'm really glad that manuals exist and are easily downloadable on the internet! The problem was indeed in the thermostat. It turns out some of the settings in the thermostat (Totaline P474-0100) were not right (and the previous owners of the house must have just suffered with it). The settings in question were the 'electric heater fan' option, which was mis-set to ON. This was causing the blower in the [gas] furnace to blow cool air through the house. The other setting was the 'dT' setting (or the 'wiggle factor' setting) which gives a 'buffer' to the temperature set point. It was set to 5 degrees (wow!), I set it to 1 degree. This may make more sense if I explain how the thermostat works... For this thermostat, the 'call for heat' is set up in two stages: (say my house is at 66 degrees and the temp I've set the thermostat to turn the heat on at is 71 degrees) stage 1) when the indoor temp reaches the setpoint (71 degrees) then 'stage 1' has been reached. A red LED turns on on the thermostat indicating that it's reached the set point, and if the 'electric heater fan' option is set to ON, the blower is turned on. stage 2) if the temperature gets to the setpoint - wiggle factor - 2 more degrees then the furnace is told to turn on.

So, what I was getting was the blower turning on at 71 degrees because the thermostat thought my furnace was electric. This horrible breeze would circulate through the house until the thermostat reached 64 degrees (71 degrees - 5 degrees of wiggle - 2 degrees), but since the house was sitting at 68, the thermostat would never turn on the burners! The result for me was exactly what I was experiencing. It appeared that for small differences in temperature (between the heat setpoint and the house temperature) I was just getting this cool breeze, but if I boosted the heat setpoint up (say to 90) then the stage 2 criteria were satisfied and the thermostat would send an appropriate 'call for heat' to the furnace...

On top of this, the thermostat was measuring the wrong temp to begin with, so I went ahead and recalibrated the temperature sensor... What a difference!

So now my furnace turns on at exactly 3 degrees below the heat set point since I can't set the 'wiggle factor' to 0 and 2 extra degrees are hardcoded into the thermostat, but I no longer get the cool breeze. This is MUCH more acceptable! Tthe previous owners must have always just compensated for this problem! I'm really glad that the manuals were online! Thanks again for everyone's suggestions!