Furnace Output - DIY Chatroom

MinervaSue · 11-03-2009, 10:33 PM

I am wondering about the temperature of the heat coming from my floor registers. It feels lukewarm. Outdoor temperature was about 56 but I guess that shouldn't matter.

Putting a thermometer to the air coming out of register gives a reading of 107 degrees (after about 30 minutes of running) and that is as high as it will go. Does anyone know if this is normal? My furnace is proper size for sq footage of house, if that matters.

I had thought the heat exchanger heated the air and that once it starts blowing, the temperature doesnt slowly work its way up, over a period of 30 minutes. Is that incorrect? As I say it takes mine 30 minutes to get to 107 degrees. My previous furnace seemed to put out hot air within a few minutes.

Replies would be appreciated.

Houston204 · 11-03-2009, 11:17 PM

Is this a gas furnace?
What is the air temperature entering the furnace?

beenthere · 11-04-2009, 07:17 AM

No such thing as sized right for the square footage.

As above. is it a heat pump or gas furnace. What is the air temp entering the return. Do you have a basement that your ducts are in, or is your floor a slab.

MinervaSue · 11-05-2009, 08:16 PM

I have Downflow multiposition gas furnace in utility room. House has insulated crawl space. The morning air going into furnace was 59 degrees . Air coming out of closest vent (15-20 feet from furnace) was lukewarm for about 15 minutes then got to 100 degrees and then to 105 after 5 more minutes , then slowly to 107 and that seems to be the limit. Mayabe 107 is all I can get from this furnace or maybe the heat exchanger (? ) releases the air to the blower while it is still lukewarm. (Blower cames on 1 minute after themostat kicks on. ) Outdoor temps have been 40 at night and 60-70 daytime. I have never had a furnace that didn't have nice warm heat at the vent within just a few minutes. This one gradually works its way up and one day it took 19 minutes for it to go from 104 degrees up to its limit of 107. Also the transformer hums. The technician said transformers are supposed to hum, but he replaced it and the new one hums too.

beenthere · 11-05-2009, 08:34 PM

Does the utility room have its access door on the outside of the building.
Or does it have combustion air intakes that run outside. If yes for either. If the air filter access is in the same room. Can it be pulling in cold outdoor air in the filter access.

Houston204 · 11-05-2009, 08:35 PM

Your furnace should display the rated temperature rise in the burner section near the model and serial number. A 35 to 60 degree rise in temperature across the furnace is a common rating.

MinervaSue · 11-05-2009, 09:09 PM

Access door from kitchen, no combustion air intakes from outside. Brochure indicates heat rise 35-65 for this furnace.

drewhart · 11-05-2009, 09:45 PM

i was just about to ask this same question. my furnace feels the same way. i just installed it, and i hired a guy to build the plenum and the main trunk. when i test ran it the air felt luke warm. it could be because my house was only 45-50 deg. when i ran it. but i am thinking the trunk this guy installed is only 8X10", maybe this has something to do with it?

MinervaSue · 11-05-2009, 10:01 PM

My previous furnace worked fine in this house. It was an oversized (so I was told by a friend)---120,000 btu for a ranch under 1200 sq ft. This one is 75000 and seems fine for this house except for the humming transformer and the slowness of heat to the vents.

Yoyizit · 11-05-2009, 10:33 PM

Does the flame look the same while the air gets slowly warm?

Does your lukewarm feeling correspond to your thermometer readings? Thermometers have a thermal time constant, which may run to a few minutes depending on their thermal mass.
When the thermometer gets to 107 put it in a room at 72. The time it takes to go from 107 down to 85 is the time constant.

Houston204 · 11-05-2009, 10:34 PM

Sounds like you have at least a 41 to 46 degree rise. This is within range.
You can't expect it to heat as much as your oversized furnace.

Is your ducting well insulated?

Yoyizit · 11-05-2009, 10:47 PM

If your furnace is right-sized it could take a long time to reach the t'stat setpoint. And your furnace would run almost continuously.

What kind of cycle times did your larger furnace have? I think it reached 107 in 6 minutes, which I think is the time constant for your house and the air that's in it.

MinervaSue · 11-05-2009, 11:30 PM

I cant see the flames in this furnace. They are behind a closed panel. The ducts are not insulated. The house has a masonry covering.

As to the 107 degrees, when it finally gets to that point at the vent, it feels warm. But it doesn't seem like the exchanger wants to send 107 degree air to the vent. It is more like the exchanger wants to "think about it" for 15-20 minutes while it sends cooler air .

I don't really expect this furnace to heat the house as quickly as the bigger one did, but I assumed the properly heated air would come out of the exchanger to the vents within 2 to 3 mintues as in every house I have lived in, seemingly regardless of size of home or size of the furnaces.

My bigger concern now is what happens when really cold temperatures hit my locality if the warm air is this slow getting into vents now. The velocity of the air seems okay, but not the temperature, at least not until it gets to 107.

Yoyizit · 11-05-2009, 11:44 PM

Quote:

Originally Posted by MinervaSue

I cant see the flames in this furnace. They are behind a closed panel. The ducts are not insulated. The house has a masonry covering.
1

I don't really expect this furnace to heat the house as quickly as the bigger one did, but I assumed the properly heated air would come out of the exchanger to the vents within 2 to 3 mintues as in every house I have lived in, seemingly regardless of size of home or size of the furnaces.
2
My bigger concern now is what happens when really cold temperatures hit my locality if the warm air is this slow getting into vents now. The velocity of the air seems okay, but not the temperature, at least not until it gets to 107.
3

1 You could check that your gas meter runs as fast at turn-on as it does minutes later.
2 My houses too, but furnaces may routinely be oversized.
3 I guess your furnace should be sized for the 97.5 percentile temperature.

**Scuba_Dave** · 11-05-2009, 11:45 PM

60-70 in the day & you have the heat on ?

We've had frost here, only up to 50 during the day & haven't turned the heat on yet
Supposed to be close to 60 thru Tues - so no heat until after then at least

The new furnace will lower your heating bills
On colder days it will run longer & more often
In a "perfect" system the air would be coming out constantly at about the Temp you want the house to be
No wasted fuel "overheating" the house