How does the variable speed blower fan on recent furnaces work?
Webpages say one speed for heat and another for AC.
But they just say "the right amount".
Which is lower, for heat or AC?
And how does the lower one compare to the lowest** possible speed on my old furnace? I only use one speed now, so would the lower speed of a new furnace be lower than what I have now, or would the higher speed be higher than that?
**My current furnace has a 3-speed fan, set by moving a wire around, and I have it set on the lowest speed.
I apologize for repeatedly posting about home repair. When I get this furnace stuff done, annd maybe some plumbing, I'll stop.
Even old furnaces had a high blower speed for cooling and a lower one for heating. The variable ones are continuously variable.
Depends on what you have and what you buy. The variable ones start off very slow and ramp up over some number of seconds so it's less noticeable when it starts. They are also typically programmable at the furnace, at least for decent furnaces. If you're going to piggyback a new heat pump onto an old oil furnace, I'd be concerned about whether the low blower speed for say a 100K BTU furnace is still too high for a heat pump in moderate climate and that the air coming out could feel cold.
Also, a variable speed fan can be used with a two stage AC that can do a better job of removing humidity and of getting the whole place to an even temp compared to a fixed speed blower.
I have had 2 variable speed air handlers (Trane and Carrier/Bryant) and when you dig down in the tech literature, that is just 4 speeds in both of them.
I still have a 4 speed motor from my Trane TWE that crapped out but I only used 1 of them. We don't have the heat on enough to rewire the thermostat for two. I did play with a circuit using some CMOS and SSRs to duplicate the variable speed thing but I never bothered to screw with making it.
BTW if you are out of warranty, you will never get the cost back of replacing that motor or the card and they usually recommend doing both because a bad card will take out a motor and a bad motor will take out a card. The pair for my TWE was almost as much as a new air handler ($1600). I threw a regular motor at it and replaced the whole system a year or two later when the compressor crapped out.
That's why I bought a Rheem, parts are apparently less expensive. It is a expensive though if one of those variable speed blowers dies. 9 years on the one here, still fine, probably saved about enough on electric by now to pay for a motor if it craps out. It's not just the variable speed motors that are expensive, the condenser had an ECM fan motor. That dies about 4 years in. It was ~$350 for a new one. I replaced that with a PSC one for $90. That's an application where it makes no sense. It's small, not enough power usage to make any significant difference, it's outside exposed to the elements, etc. But I'm sure it helps meet the energy eff standards.
I really doubt the variable speed fan does that much either. Buried in the cost of a new air handler is wasn't that much more and you might see the difference but I know my bill didn't change in any significant way when I threw the PSC motor in there in place of the variable speed. It certainly wasn't anything like the $1400 difference in price for the replacement.
Generally heat is lower than AC I don't have RPM specs for mine, but it is a 8X10blowerand at .3" WC it varies from 651 to 1289 CFM from lowest to highest speed.
Direct drive blowers generally run 1025RPM,belt drive motors generally run1725 (nominal 1200 and 1800 less slip) On my old furnace with 2 speed motor that was 1/2 and 1/6 HP and 1725 RPM on high. A 2 speed motor is generally for heat only apps with the low speed being for continuous circulation. 3 speen moptors are generall High for cool, Medium for heat, and low for constant circulation.
With "variable speed" the speed is adjusted to give a specific heat rize for heat and a specific temp drop for cooling.
Generally that would result in a low amount of very hot air being circulated instead of ahigher volume of kooler air. Ideally you should not get a "desert breeze" effect when the furnace comes on and the registers should not feel hot after the furnace has run for several minutes.
On mine when I changed furnaces from a 30year old with belt drive AC motor to a new Medium Efficiency with DC blower the gas bill didn't change, but the electricity use dropped. The motor used less electricity (significantly) and the motor ran much cooler - so the furnace had to produce more heat from the gas. Sending less heat up the chimney and burning more efficiently it produced the extra heat without burning more gas.
And the heat from the blower motor is a nit you'd never see in the gas bill compared to a 70 or 120k btu furnace. The electric bill here declined with the new variable speed blower too, but like you say, it was an old psc motor, not a new, more efficient one. It's a crap shoot getting the ECM or variable speed. If it lasts, it's great. If it fails out of warranty in a few years, you're hosed.
Very good to know. The previous owner saved the furnace manual for me, and I found it online too, so it's in my computer. I can't remember but I'll see if it talks about fan speed.
Amazing to me, I can't remember if my blower is belt or direct. It was making noise once and I had no time to fix it myself so I let the oil company do it. They changed the motor and it was still making the noise, which meant it was the squirrel cage. I got the good motor back from them, though I don't have much use for it, and I think they gave me a new squirrel cage**. I figured that coudln't start a fire while I was out of town so I did nothing until I got back. Two weeks after I got back, it stopped making noise, and 12 years later, it still makes no imporoper noise. It was just loose on the hub and it somehow got forced on tightly again.
**Years later I talked to the owner for some reason and he said I should have called him at the time. Instead I had changed oil companies.
That sounds good, but as long as I have the old oil furnace, I won't have that. I gutess that's true whether I got new AC or a heatpump, so why am I thinking about it.
It's what I called "toasty hot". Feels good on my feet when I'm barefoot in the bathroom.
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