Forced Hot Air Furnace Gurus Technical Question

[Bob_VT]

Okay this has been bugging a couple of us and since the heating season is around the corner.........I have a real technical question.

I can set the proper burn on the nozzle and measure the exhaust gases too to insure the burning is correct HOWEVER I can do that with a .80 nozzle or a 1.0 or a 1.10 to get the burn correct now how long does the fan run run in conjunction with the burner?

The fan blows the heat into the house while the jacket is hot but for how long?

Should the nozzle burn time match the fan time? Should the fan run longer?

Here is the big one - should a smaller nozzle be used? to match the fan time and create a more efficient furnace?

I have looked for web sights to explain this and have come up empty?

And help? Recommend a larger or smaller nozzle? Too much cycling will cause ash and soot which will create chemicals that are bad for the metal?

Thanks!

 

[Limited-Time]

It's my understanding the blower/fan time is controlled by plenum temp. Kicks on at one temp off at a lower temp. So it's kind of independent of the burner. It starts after the burner starts and the plenum heats up. It shuts off after the burner turns off and the plenum cools down.

 

[GA_Boater]

Fan cycle time is independent of burn time. It's controlled by the plenum temp and starts blowing after the plenum warms so less cold air is blown out of the vents. When the burner shuts off, the fan still runs until the temp drops to use the "free" heat of the still warm plenum.

The fan delay helps maintain an even Admiral comfort level and slightly longer time between burns and slightly shorter times when it is burning.

Can't help with changing the nozzle size.

 

[dwco5051]

The less a furnace cycles the more efficient it will be. When the flame first starts it is firing into a cold combustion chamber and takes a short amount of time to burn efficiently. That being said there are a lot of variables of which the biggest is outside air temp. The ideal size nozzle would be the one that would keep your home at the desired temperature when the outdoor temp is the coldest that is expected in your area by running constantly.

Energy codes specify design temperatures. You have to know what the calculated heat loss for the structure is. The new energy codes will usually specify much smaller heating units than what were installed forty or fifty years ago.

Example;

Design temps 68 F inside 0 F outside Building heat loss 90,000 BTU?s

.8 GPH nozzle X 85% efficiency X 137,000 BTU?s/gallon = 93,160 BTU?s

All that being said do not install a nozzle larger or smaller than specified for the furnace as burns rates are matched to the combustion chamber size and shape.

 

[MTboatguy]

GA is a great one to get information about "Hot Air"

LOL!

 

[Bob_VT]

Thanks for the input. I have some figuring out to do.

I was thinking that the "burn time" and the fan running should be almost the same.

 

[Tnstratofam]

The furnace fires and then after the heat exchanger reaches a warm enough temp ( usually around 120?) the fan starts. After the desired temp is reached on the control or thermostat the burner shuts down and the fan continues to run until it cools the heat exchanger below a certain temp ( usually 100?).

If the fan stops as soon as the burner shuts off the heat exchanger will actually continue to heat up from residual heat. This will shorten the life of the heat exchanger considerably.

 

[GA_Boater]

GA is a great one to get information about "Hot Air"

LOL!

Thanks for the kind words, MT. :brick: LOL

 

[dwco5051]

The furnace fires and then after the heat exchanger reaches a warm enough temp ( usually around 120?) the fan starts. After the desired temp is reached on the control or thermostat the burner shuts down and the fan continues to run until it cools the heat exchanger below a certain temp ( usually 100?).

If the fan stops as soon as the burner shuts off the heat exchanger will actually continue to heat up from residual heat. This will shorten the life of the heat exchanger considerably.

Not to complicate things but many thermostats have built into them a "heat anticipator" that shuts the burner off a little early so the residual heat doesn't cause the controlled space temperature to go over the thermostat setting along with a differential setting so eliminate short cycling of the burner.

 

[Tim Frank]

Best "household hint" I've ever received is to switch your fan to manual in the winter so that it runs constantly.
Gives a more uniform temp throughout the house and helps noticeably if you have any moisture/condensation on windows.

The fan motors are designed to run "lots" and this will not shorten their service life.