Problem with oil burner central heating system

[NH_Wood]

Hi all,

I'm essentially 100% wood heat. Haven't burned a drop of oil since 2009-2010 for heat, and even then, I was supplementing as much as possible with pellets, so rarely used my oil heat. Anywho - had to use the oil today because the entire family was going away for about 12 hours and it was supposed to be quite cold - figured I'd set the t-stat to 70*, build a fire, and let the oil forced hot air kick in for heat if the temps dropped in the house. Here is the problem. We have a digital programmable thermostat - I set it at 70* (house was at about 68*, so it kicked on), set the switch from 'off' to 'heat', and set the fan on auto. Furnace kicked on, but the fan never blew any hot air through the ducts. Only when I turn the switch for the fan from'auto' to 'on' would the fan bring hot air through the ducts. Problem is that once the room hits 70*, the fan remains on all the time, and ends up blowing cold basement air into the house. So.......does this sound like a thermostat problem? Relay problem? What do you all think? I'm going to get working on it tomorrow and would appreciate any advice. Thanks and Cheers!

 

[heat seeker]

There is a switch on the furnace that measures the temperature of the plenum, so the system doesn't blow cold air when the furnace is off (auto mode). It's probably stuck after a period of non-use. It should be mounted on the furnace itself. A few judicious taps on it may free it up. The switch itself could be bad, too.

In short - when the plenum heats up, fan on. Plenum cools down, fan off.

 

[NH_Wood]

There is a switch on the furnace that measures the temperature of the plenum, so the system doesn't blow cold air when the furnace is off (auto mode). It's probably stuck after a period of non-use. It should be mounted on the furnace itself. A few judicious taps on it may free it up. The switch itself could be bad, too.

In short - when the plenum heats up, fan on. Plenum cools down, fan off.

Excellent - thanks! Cheers!

 

[heat seeker]

If a couple of taps doesn't free it up, turn the power off, and take the cover off the housing for the switch. It's fairly obvious how they work. Most have a "finger" that sticks into the plenum with a bimetal strip that twists to activate the switch. The pivots may be junked up or even rusted.
Keep us posted?!

 

[NH_Wood]

If a couple of taps doesn't free it up, turn the power off, and take the cover off the housing for the switch. It's fairly obvious how they work. Most have a "finger" that sticks into the plenum with a bimetal strip that twists to activate the switch. The pivots may be junked up or even rusted.
Keep us posted?!

Heat seeker - will do! Today ended up not a good one for getting to it, so I'll have to get at it this week. I did look on the blower unit and didn't see an obvious fan control switch on the outside, so I'll remove the cover and look. Do you know if there is a special switch if the same thermostat control heat and a/c? Thanks for your help! Cheers!

 

[mrfjsf]

For a/c the indoor fan is controlled by the stat. As soon as the T-stat calls for cooling, it will bring the indoor fan on. As soon as the call for cooling ends, it shuts the fan off.

 

[heat seeker]

An added thought - my hot air system was a pretty old one, and I expect things have changed over the years. They may well use a solid-state sensor and switch for the fan, but the principle is the same. There needs to be a sensor on the plenum, and a way for it to turn the fan on.

The sensor won't be on the blower unit, it will be where it can measure the temperature of the plenum itself. The blower unit is basically just a fan.

On my central AC, the fan runs for a bit to extract as much "cold" from the evaporator as possible, so it runs for a couple of minutes after the 'stat is satisfied.

 

[NH_Wood]

An added thought - my hot air system was a pretty old one, and I expect things have changed over the years. They may well use a solid-state sensor and switch for the fan, but the principle is the same. There needs to be a sensor on the plenum, and a way for it to turn the fan on.

The sensor won't be on the blower unit, it will be where it can measure the temperature of the plenum itself. The blower unit is basically just a fan.

On my central AC, the fan runs for a bit to extract as much "cold" from the evaporator as possible, so it runs for a couple of minutes after the 'stat is satisfied.

Okay - I'll check tomorrow and see if I can find the sensor - would it normally be hidden within a panel, etc., or very accessible? Thanks again! Cheers!

 

[heat seeker]

I'm guessing that you might have to remove a panel on a newer unit. My old hot air system had the mechanical switch located a couple of feet above the burner itself. YMMV. It was a metal box about 1½" X 3". A solid state unit would probably look different and be smaller. A Google search for one might show you some modern units.

Pics:

plenum switch

plenum switch setup

 

[NH_Wood]

I'm guessing that you might have to remove a panel on a newer unit. My old hot air system had the mechanical switch located a couple of feet above the burner itself. YMMV. It was a metal box about 1½" X 3". A solid state unit would probably look different and be smaller. A Google search for one might show you some modern units.

Pics:

plenum switch

plenum switch setup

Thanks for the pics - more similar to my setup - turning out to be a busy week and since I almost never run the furnace, haven't had a chance to get to it - hope to in the next few days. Again, you've been really helpful and I appreciate it! Cheers!