Anyone ever experienced this? I only just recently noticed it but maybe it has always done it. When my oil fired forced hot air furnace first turns on, the temperature at the thermostat will drop 2-4 degrees before it starts to rise again. I figured maybe it's due to cold air circulating around at first but it just seems odd.

If your ductwork is in the attic, the air inside all of the ducts and the ductwork are cold (colder than inside the house). So the furnace then has to move the cold air out of the ducts and the warm air is getting used up by first warming the duct work up for those several minutes before finally bringing warm air into the house. If this is the case, you'd benefit from having them insulated better.

Probably just the fan kicking on early and blowing cold air from the ducting into the house before the heat fires up.

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Originally posted by Gustofer:
Probably just the fan kicking on early and blowing cold air from the ducting into the house before the heat fires up.

This. The same thing happens to my building during start up during the spring and fall.

Maybe set the fan to on instead of auto during really cold weather?

Experts here can probably explain it better but it could be as mentioned just the cold air in the duct work... but then that shows a problem in that it is either not insulated properly or the activator for the fan is set wrong. The air handler (fan) on the furnace should not turn on until the Plenum (area over the firebox/heat exchanger) is hot... opposite end is the fan runs for a short time after the burner turns off to get that last little heat circulated.

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Originally posted by Blume9mm:
Experts here can probably explain it better but it could be as mentioned just the cold air in the duct work... but then that shows a problem in that it is either not insulated properly or the activator for the fan is set wrong. The air handler (fan) on the furnace should not turn on until the Plenum (area over the firebox/heat exchanger) is hot... opposite end is the fan runs for a short time after the burner turns off to get that last little heat circulated.

I dunno, I could see some safety circuitry waiting until it sees air flow before firing off the firebox. Kinda like the thermocouple that has to detect heat from the pilot before it will allow the gas to turn on.

2-4% is a pretty big drop.

Do you have a single unit heating the whole home? Since it's an oil-fired furnace I'm assuming it's in your basement and your basement is heated or at least it's warmer than the outside. So your furnace's 'fresh air' intake is "warm" air, not cold air.

How much square footage does your furnace heat?

Does the furnace have ducting that goes to a second floor?

Trying to get sense for how much cold air gets pushed into the home prior to the warm air getting in.

Is all of your ducting accessible in the basement? You might consider inspecting your duct-work for leaks.

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Originally posted by slosig:

quote:

Originally posted by Blume9mm:
Experts here can probably explain it better but it could be as mentioned just the cold air in the duct work... but then that shows a problem in that it is either not insulated properly or the activator for the fan is set wrong. The air handler (fan) on the furnace should not turn on until the Plenum (area over the firebox/heat exchanger) is hot... opposite end is the fan runs for a short time after the burner turns off to get that last little heat circulated.

I dunno, I could see some safety circuitry waiting until it sees air flow before firing off the firebox. Kinda like the thermocouple that has to detect heat from the pilot before it will allow the gas to turn on.

That'd be a negative ghost rider.

Sounds like a ducting problem or wrong fan temp settings.

I'm really leaning towards the duct work needing better insulation. 2-4F degree drop in the house at the thermostat is a huge drop in air temperature, before the furnace starts heating at the vents and heating up the air temperature inside the house.

How long is it usually from when the unit shuts off and then the thermostat kicks it back on? If it's only cycling off for 30 minutes or less, you definitely need better insulation on the duct work.

quote:

Originally posted by Shaql:
2-4% is a pretty big drop.

Do you have a single unit heating the whole home? Since it's an oil-fired furnace I'm assuming it's in your basement and your basement is heated or at least it's warmer than the outside. So your furnace's 'fresh air' intake is "warm" air, not cold air.

How much square footage does your furnace heat?

Does the furnace have ducting that goes to a second floor?

Trying to get sense for how much cold air gets pushed into the home prior to the warm air getting in.

Is all of your ducting accessible in the basement? You might consider inspecting your duct-work for leaks.

Furnace heats the entirety of a 1300sq' house on 1 zone. The basement is 75% finished and is served by 1 piece of ductwork. The two main ducts and a couple branch ducts are exposed but the rest are within the walls/finished ceiling. There are no cold air returns on the 2nd floor but there are supply registers.

quote:

Originally posted by jimmy123x:
I'm really leaning towards the duct work needing better insulation. 2-4F degree drop in the house at the thermostat is a huge drop in air temperature, before the furnace starts heating at the vents and heating up the air temperature inside the house.

How long is it usually from when the unit shuts off and then the thermostat kicks it back on? If it's only cycling off for 30 minutes or less, you definitely need better insulation on the duct work.

Once the house gets to temperature it usually holds it for at least an hour.

Mine doesn't necessarily drop, but it can take a couple minutes before it actually starts heating the house. I really depends on how cold it is...

Just so I'm clear, you have a 2nd floor, main floor, and basement all being heated, right?

Assuming all your duct work is in interior walls, that's a lot of cold air residing in your home to bring it down that much. My guess is that you have an opening between floors for a vent (say bathroom vent) that is allowing outside air to come in and cool down your duct work. Also guessing you have the old tin duct work, not modern insulated duct work.

What you can do is get one of those laser touchless temp gauges and record the temps coming out of each register when the furnace kicks on. Remember that you will have to do this for 1 register per furnace start-up.

See if you can figure out if one or two of the ducts are much colder than the rest. This will give you a place to start.

But outside of un-insulated or leaking ducts the only other thing I'd look into is the air volume coming out of each vent. Not that this contributes to your cold air problem but you should make sure the majority of your air is making it upstairs. If your basement supply is taping right off the main line, and doesn't contain a dampener, you might be seeing some heat going to waste.

I'm not an HVAC guy, I'm just trying to think through it. Sorry I can't help more.

Is it possible that your thermostat is mounted on a cold air return wall cavity? If so, when the furnace starts up and starts to pull cold air, it could possibly be drawing some of the air through the hole behind the thermostat where the wire comes through. Remove the thermostat from the wall, start the furnace, then do a smoke test to check for draw.

Is a vent blowing directly on the stat? If so that's why it's dropping. But it's probably causing the furnace to shut off before that actual ambient temperature has risen. So the house isn't as warm as the stat says. Monitor that with other thermometers. Maybe you can jury rig a shroud around the stat to test.