how quickly can a furnace raise house temp 20F?

Heat a pan on the stove. Quickly smack it with your hand. Now hold you hand for 2 seconds. Now hold your hand on it for 10 seconds. Did the fastest speed give you the most heat?

Same with the air blowing across the heat exchanger. There is only a given amount of heat available and the longer the residence time, the hotter it will get and it can them move the heat to another location. Cool air then replaces the heated air. Check this out with your car heater as it warms up on a cold day. Run the blower speed up and down and see how the temperature changes.

You may have other issues with your furnace too. You mentioned a sagging PVC pipe, kinks in the condensate drain, other parts were replaced. First, it sounds like a hack did the original install. Some of these little errors may be causing the heat exchanger to cycle off and on too frequently or it is not reaching temperature.

While it is nice that the tech can resurrect motors from the dead, I think perhaps, you need a new service company.

So it is not the "blower motor" that was replaced but the "eductor fan" or "combustion purge fan"

Only problem, bubba, is he didn't evenask the right question - it wasn't the blower fan motoer that was replaced. It's the cumbustion purge blower, or eductor fan.

Something called delta T

Delta T is highest when the air flowing over the hot surface is coldest. Max delta T = max heat transferred. This happens when the airflow is fastest because if the air flow is slower, it gets hotter because it is in contact with the heated surface longer. Reduced delta T means less heat transferred from hot surface to air and then to house. Which make me think that you want higher air flow, not lower.

Well, the furnace was installed in April 2003, and that run of PVC pipe is 20 feet long, so five years of gravity apparently became persuasive.

All three terms apply, plus a fourth that I find on my receipts: inducer blower. The tech called it a blower motor, fyi.

Hey Zoot. Think of it this way: Lets take a 100,000 btu furnace and pull the blower and motor out of it. Now install a bathroom fart fan in its place. Turn it all on. What happens? You get an extremely high temperature with almost no air movement. Now, lets take that same furnace and install a 4 foot wide 4 blade fan with a 50 hp motor turning at 30,000 rpm. Now tell me how much heat you feel on the outlet side of that furnace. Answer: None It all needs to be done within a range. Thats why motors have 3 and 4 blower speeds. Its so you can set the heating blower speed and cooling blower speed to fall within a temperature rise or drop across the heat exchanger or cooling coil. Clear as mud now or are you one of those guys with an EE degree? Bubba

Exactly correct. That info applies in automobiles also . You'll always get colder air from your air conditioner if you drop the blower a notch or two. We (as mechanics) would always measure the temp of an air conditioner with the blower on medium, and the mode on recirc. (max)

steve

So, if you have a 50 hp motor turning at 30,000, how hot is the heated surface inside the heater? Real hot? Not very hot because of the blast of air going across it?

If you answered real hot, then how can that be with a blast of air going across it?

If you answerd not very hot, then were did the heat go?

Are you going to tell me next that if you take your thermostat out of your car's cooling system that the engine will overheat?

No it will run cold and under perform due to not maintaining proper operating temperature at least in the winter. You can overheat one by upping the flow of water to the point that it passes through too quickly to absorb the heat.

Hmm, Oversized furnace won't be very efficient.

I guess i just assumed that anyone with any mechanical knowledge knew that. The water actually runs cooler, but the metal parts run hotter. Especially in the back of the heads on a v-8. The thermostat actually provides a necessary restriction even when wide open that is required to make the proper flow to the back end of the block. If you insist on running no 'stat, the it's best to just take the innards from one and install the outer ring.

steve

On SOME cars, they actually WILL.

No- but if it gets to 30 below then the furnace is designed for a 100 degree temp difference. By design, when it is 20 out the furnace is double the size it needs to be. But most of us have just one furnace. [and those of us with 30 yr old furnaces probably only have one fan speed]

Jim [and to save anyone the trouble of posting 'you should replace that old beast' - Maybe I will someday, but it tests at 85%, I know all of its *very simple* parts intimately & I only burn 400 gallons of oil a yr so I'm not in a rush]

a better question is why would one want to raise temperature 20 degrees?

if your leaving the home cool to save energy thats good.

the OP could add some extra heating like a couple gas wall heaters on a master timer switch to help reheat when you arrive home fast.turn knob trips on extra heat for 30 minutes to help with re heat

new furnaces are sized for efficency and lack the BTUs to raise the home fast.

adding a wall heater or other gas heater solves that and is a nice back up if your main heating plant fails

Of course it doesnt get to zero where you are at, or does it.

Whats an EE degree, is that something you just avoided. Here everyone goes for Bachelors, Masters, and Doctorates.