Newbie question about cold air from heater

[dynamos2000]

First of all, I'm glad I found this place! I'm a new home owner and a computer geek and not much else...

When my heater kicks on, I get cold air blowing. After about 90 seconds, the air gets moderately warm and doesn't get any warmer. When we first started using the heat about two months ago, the air seemed to warm quickly and was definitely warmer air blowing through the vents.

Any ideas? THANKS to all and happy holidays

[Hube]

Welcome to the forum. I would suspect the air speed is too fast, but without any info such as type of heater, furnace, ductwork ,etc its hard to pinpoint any cure. Also, it was a little warmer 2 months ago, and the air would seem to be warmer because of any return air coming into the system. Post back with some more info and we will try to advise further, thanks, Hube

[a46geo]

The more air you move past the heat exchanger, the more heat you will pick up. The more heat that you pick up, the more efficient the operation. The more efficient the operation, the cooler the operating temp of the heat exchanger. Generally speaking, this should extend the life of the furnace.

As long as the thermostat maintains the set temperature, I wouldn't worry too much about air flow temperature.

[ngarfield@mindspring.com]

I am also having trouble with cold air coming through my vents in very cold weather but what is interesting is that in slightly warmer weather the air is warm. I am wondering what this has to do with return air and what that means. I believe that was part of the response given earlier. Any help would be greatly appreciated. Thanks

quote:Originally posted by dynamos2000

First of all, I'm glad I found this place! I'm a new home owner and a computer geek and not much else...

When my heater kicks on, I get cold air blowing. After about 90 seconds, the air gets moderately warm and doesn't get any warmer. When we first started using the heat about two months ago, the air seemed to warm quickly and was definitely warmer air blowing through the vents.

Any ideas? THANKS to all and happy holidays

[ellerbi]

Hube
I'm glad there is a forum out here for this stuff!!
I have been holding off remodeling a bathroom (Chimney is buried in closet next to it)
Can I vent a Lennox Whisper (New 1994) out the side of my foundation??
Furnace is in basement 20" below where I would vent it out- total lenght to vent out is about 5 feet from furnace to wall

Safe or unsafe Appreciate the input!!
Bill
[?][?]

[LazyPup]

Perhaps a little bit of HVAC theory will help everyone understand the question a bit better.

Many people erroneously feel the discharge air from a duct expecting the air temp to be whatever the thermostat setting is. This simply is not the case.

Both a furnace and an AC have a differential temperature, which is the amount of temperature change that will occur as the air passes through the system.

Let us assume for the moment that the designed differntial temperature for your furnace is 18degF. (A differential of 12 to 14 degF is typical for AC or heat pumps)

Now let us assume your system has been off and the house has cooled to 40degF.

When you turn the furnace on, the return air will go into the heat exchanger where it is heated 18degF, which means the discharge air temp in now 58deg.F which would still feel quite cool if you were to put your hand near a supply duct.

The main fan and air ducting system is designed to effect three or four complete air changes per hour. Thus for the first 15 minutes or so the dischare air would be 58degF.

By then end of the first 15or 20 minutes the air in the room should now be 58degF which means it would then be heated another 18 degF on the second pass, thus the discharge air will now be 76 degF. This cycle continues until the air temp in the room is equal to the temperature set on the thermostat. When the calling set temp is achieved the thermostate then turns the system off until the room air cools and the cycle begins again.

The furnace or AC runs at a constant differential temperature regardless of what temperature is set on the thermostat. The only difference being: if the calling temp were higher the system would have to run longer to meet the demand.

When diagnosing a system an HVAC tech will often measure the temp of the return air, and the discharge side of the plenum to determine the differential temperature. A radical difference in differential is a primary indicator of a blocked air flow. Most common cause, a dirty air filter.