how quickly can a furnace raise house temp 20F?

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Usually the heat strips come on anytime you are more than about 3 or 4 deg low. If the heat pump is turned up about 2 deg at a time they will not come on , but if it is moved from 45 to 65 deg then they probably will. Also at 20 deg the heatpump is not too efficiant and it might as well be using the heat strips.
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Funny that you ask this question.....I have to have done the expereiment on three houses
Two in Orange COunty, Ca.
One is a 1 1/2 story 1930 ....no wall insulation but ~R30 in the attics.....on cold (for SoCal) winter day of about 50F outside, about 5 or 6F degs per hour
Second on is a one story ranch iwth blown in cellulose attic insulation......about the same performance
the last home is an eastern sierra two story lots of wall & ceiling insulation, propane heat...takes about 3 or 4 hours to go from 55F to 70F
your temp rise of 2F deg per hours seems a bit on the low end in my experience but not hugely unreasonable...a furnace needs to be able to maintain the house at a reasonable temperature......how quickly it gets there isn't really all that important.
what is the out side temp (I'm guessing in the 20's?)
an important thing to check would be the temp rise across the furnace
But I'm sure Bubba will jumping right in with a helpful addition (along with his typical serving of insults) to this thread being the furnace expert that he is
cheers Bob
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If I turned my furnace off & let the house get to 45- then turned it on [while it was 20 with wind] I imagine it would take 1 -1 1/2 hours of running to bring the house up to 65.
But my furnace was designed to keep the house at 70 when it was 30below outside- and that was with old, drafty windows & no insulation.
If 20 is about as cold as it gets where you live- and if it was 45 for some time so all the mass had to be re-heated- and if you've got room for improvement in your windows and insulation- then don't worry about your furnace. Work on the insulation.
You say you "had the blower motor replaced". That implies that some guy who knows a whole lot more about your setup was in your house. What did he/she say?
Jim
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With the house thoroughly chilled to around 50, with an outside temp in the 30s, I'd say my forced air will do about 4 or 5 deg an hour. So, I'd agree that 2 deg an hour sounds on the low side.
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wrote:

Yes: when I find time, I'm going to be calling for a polyurethane closed cell insulation install quote for the box band of the house, which should be fairly easy/inexpensive, all factors considered.

The HVAC company that installed the furnace has been here four times in just over 2 weeks. The first time was when the inducer blower died on a Friday night; Saturday morning the tech checked a bunch of things, decided the motor wasn't dead, and propped up the long horizontal PVC exhaust pipes to correct a sag that apparently was preventing gases from exhausting properly.
That got the motor going for another two weeks until this past Saturday night when it died again. A second tech came over, checked the system, determined the motor was dead, but was able to get it going again. He told me to call again if it didn't keep working. It died overnight, so I called Sunday morning (yesterday) and told him we really needed to replace the motor, per his diagnosis. He was able to get one and came over to install it, and all was more or less well, except that the house took 7 hours to warm up when it had taken about 3 with the previous motor the previous episode.
This morning a third tech came over and replaced the condensate pump (gratis as he broke off something on it) plus the vinyl condensate discharge tubing (which I asked for, as ours was pretty gunked up after almost 6 years in service). We agreed to do this after he found several kinks that I'd asked the first tech about, which that tech dismissed. Anyway, all seems well now; I will be watching tomorrow morning to see how quickly the furnace brings the house to the called-for temp (it took almost 2 hours to bring the house up 5 degrees F this morning and used to take one hour with the old motor). Tonight will be low teens, tomorrow mid teens, so pray for us all!
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So it is not the "blower motor" that was replaced but the "eductor fan" or "combustion purge fan"
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On Mon, 26 Jan 2009 22:28:40 -0500, snipped-for-privacy@snyder.on.ca wrote:

All three terms apply, plus a fourth that I find on my receipts: inducer blower. The tech called it a blower motor, fyi.
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Depends how many BTUs the furnace output is, as well as how much heat loss the house has. Also depends how much thermal mass. If it needs to heat up a lot of masonry it will take longer than heating up a lot of stud-framed walls with good insulation.
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You do know you have asked a trick question? With the information you have given, it cant be answered. Here let me give you and example or two so you might understand. Where I live, we design furnaces to maintain an indoor temp of 70 at a 0 degree outdoor design temp. What that means is: If my furnace is sized properly and my indoor temperature is 70 and the outdoor temp is 0, then my furnace will run 24/7 until the outdoor temp begins to raise. If the outdoor temp continues to drop to -10 or -20 below 0 then my house will begin to get colder and colder and I will need to add some type of supplemental heat. On the other hand, with that same furnace, if it is 50 degrees outside and 60 degrees in my home and I want to raise it to 70 I would probably take much less than an hour. What you are interested in is if the motor replacement you just got is set properly. It should be set so that you get the proper "temperature rise through your furnace as stated on the furnace equipment label. Usually a temp in the range of 35 - 70 degrees. More blower speed will lower this temp range. Less blower speed will raise this temp range. Bubba
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wrote:

Why would more blower speed lower this temp range? It seems to me that the faster the air, the cooler the air blowing across elements/heat exchanger will be, and the more heat is getting transferred in.
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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.
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wrote:

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.
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On Mon, 26 Jan 2009 17:09:24 -0800, "Zootal"

Something called delta T
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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.
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On Mon, 26 Jan 2009 17:09:24 -0800, "Zootal"

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
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Bubba wrote:

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
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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?
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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.
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x-no-archive:

CORRECT
NOT CORRECT
Mark
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On Thu, 29 Jan 2009 10:06:35 -0800 (PST), snipped-for-privacy@yahoo.com wrote:

What are your qualifications for claiming this is not true?
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