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#2886399 - 01/27/13 11:52 PM Oil Furnace Blows too strong
expat Online   content


Registered: 05/12/09
Posts: 3852
Loc: Canada
Last week I was here with a problem with an old Gun type oil furnace @ 1961.
As a result of some good advice I received, yesterday I replaced it with a modern, more efficient Olsen furnace of the same BTU output.

I works great, But!
Early this morning, the furnace burner cut in (OK) Then the blower cut in like a hurricane!

The output (and noise) seems far too strong.
Worse, the sequence runs like this:
Burner cuts in.
Soon after the fan cuts in (with gusto).
Later burner cuts out.
Soon after Fan cuts out.
30 seconds later Fan cuts back in again (I assume scavenging some residual heat from the heat exchanger) then cut out again a few seconds later.

All this repeats until the house temp has stabilized.

My feeling is, if the fan speed were slower, noise would be reduced, and the fan 'cycling' could be cut down to a point were, the cooling of the heat exchanger (via the fan) would be more in tune with the Burner output.
Any thoughts on this?

Warranty is not an issue here.

The fan motor appears to be direct drive (sadly) what sort of speed control could be used?

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#2886406 - 01/28/13 12:07 AM Re: Oil Furnace Blows too strong [Re: expat]
Quest Offline


Registered: 12/19/04
Posts: 6427
Loc: beaver land EH?
not sure what model you have but some gas (or oil) heater furnace has board or jumpers where you can set with a lower RPM (thus lower noise).

One of the difference I've noticed recently (witnessed a lot of retro work done in our area to 2-stage condensing natural gas heater furnace, some higher end ones (AFUE >97%), house whose previously run fine with older single-stage heater with belt-driven squirrel cage may suddenly experienced air-rush sound due to high fan speed on newer replacements. Turning down the motor speed definitely helps but ultimately, one may have to consider going through some of the ducting, esp. the inlet side where sometimes expanding the inlet pipe to reduce resistance from the intake grille/baffle helps.

Q.
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#2886516 - 01/28/13 06:40 AM Re: Oil Furnace Blows too strong [Re: Quest]
greenjp Offline


Registered: 05/19/04
Posts: 1008
Loc: Germantown, MD
Originally Posted By: Quest
not sure what model you have but some gas (or oil) heater furnace has board or jumpers where you can set with a lower RPM (thus lower noise).

Have to be very careful with this, too few CFMs across the heat exchanger can cause it to overheat and eventually fail. It may in fact be the solution but not something to be attempted lightly.

jeff

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#2886566 - 01/28/13 08:09 AM Re: Oil Furnace Blows too strong [Re: expat]
rjundi Offline


Registered: 03/16/04
Posts: 6362
Loc: New England
The only problem you potentially have is (same BTU output).

Back in the 60's it was the norm bigger is better irrelevant to energy use and BTU requirements. Currently modern thinking is undersize it for the average colder day not coldest. On the coldest it may not do it. You typically should a boiler or furnace sized instead of throwing a v8 in where a 4 cylinder works perfectly.

How are the returns in your home?
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#2886589 - 01/28/13 08:29 AM Re: Oil Furnace Blows too strong [Re: greenjp]
ARCOgraphite Offline


Registered: 05/17/09
Posts: 8613
Loc: N.H, U.S.A.
Originally Posted By: greenjp
Originally Posted By: Quest
not sure what model you have but some gas (or oil) heater furnace has board or jumpers where you can set with a lower RPM (thus lower noise).

Have to be very careful with this, too few CFMs across the heat exchanger can cause it to overheat and eventually fail. It may in fact be the solution but not something to be attempted lightly.

jeff
I was thinking the same thing. Cant just lower the fan speed/CFM without possibly overheating the exchanger. Proceed with caution and a professional at your side.
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#2886710 - 01/28/13 10:29 AM Re: Oil Furnace Blows too strong [Re: greenjp]
expat Online   content


Registered: 05/12/09
Posts: 3852
Loc: Canada
Originally Posted By: greenjp
Originally Posted By: Quest
not sure what model you have but some gas (or oil) heater furnace has board or jumpers where you can set with a lower RPM (thus lower noise).

Have to be very careful with this, too few CFMs across the heat exchanger can cause it to overheat and eventually fail. It may in fact be the solution but not something to be attempted lightly.

jeff


Yes, this had occurred to me previously.
I also realize that slowing the fan may reduce the efficiency a little (air will spend more time in the ducts)

One indication that (IMO) the fan really is too strong, is, the fan cuts in a second time for a few seconds after it has shut down.
I imagine the heat exchanger being hot, the buner shuts down.
Fan keeps running until the fan control senses cool air. And shutsts down.
BUT the heat exchanger still has enough internal heat to re-trigger the fan control again for a few seconds.
I feel a little more gentle, slower air speed over a little longer period could prevent this.
I'll look into it today.
Unfortunately the local furnace guy is a bit of a Prima Donna.

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#2887098 - 01/28/13 03:55 PM Re: Oil Furnace Blows too strong [Re: expat]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
I have a similar problem.
There often is wiring option [moving a couple of wires] that will make the fan run slower. Pop open some covers and check the wiring diagram. Specific questions on Google will give advice
.
I did this, but is not enough.
A slower fan motor is what I need.

A loud fan is truly annoying, isn't it?

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#2887220 - 01/28/13 05:48 PM Re: Oil Furnace Blows too strong [Re: expat]
expat Online   content


Registered: 05/12/09
Posts: 3852
Loc: Canada
A loud fan is truly annoying, isn't it?

I think I prefer being cold!

I have since discovered that my blower motor has Taps on it's windings for various speeds. Now, the problem is, Medium is still too high (blows too much) and Low is too little (I fear overheating the heat exchanger.
But the Motor is a PSC Permanent Split Capacitor type, so it should be possible to fit a variable speed control :-)

Now I just have to find one for my 7amp motor.

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#2889259 - 01/30/13 11:57 AM Re: Oil Furnace Blows too strong [Re: expat]
Thermo1223 Online   happy


Registered: 04/18/05
Posts: 1767
Loc: Easton, PA
The problem starting with buying the same BTU output. Then not having a HVAC company come out and actually calculate your BTU loss. You need to do this to see whether you duct-work is adequate in flow(it usually is not). Undersized duct-work leads to exactly what you have now, noise.

Get an HVAC company in there.
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#2889918 - 01/31/13 12:06 AM Re: Oil Furnace Blows too strong [Re: Thermo1223]
expat Online   content


Registered: 05/12/09
Posts: 3852
Loc: Canada
Originally Posted By: Thermo1223
The problem starting with buying the same BTU output. Then not having a HVAC company come out and actually calculate your BTU loss. You need to do this to see whether you duct-work is adequate in flow(it usually is not). Undersized duct-work leads to exactly what you have now, noise.

Get an HVAC company in there.


I a perfect wold, sure.
But this house is either in for a total Reno or Teardown in 3-5 years.
Hence my attempt to keep the old furnace going.

The New furnace is the same BTU as the old unit. I'ts just that 70k BTU will normally heat a much larger house these days, so the new furnace's Fan output IS too much for our old duct work.
And I'm not about to re-duct the house!

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#2890481 - 01/31/13 03:36 PM Re: Oil Furnace Blows too strong [Re: expat]
mechtech2 Offline


Registered: 09/05/06
Posts: 19479
Loc: Chicago Area
there should be a high temperature cut off for safety.
If the fan blows too slow, the temps may go up, but this device should prevent problems. if it keeps tripping, then you know you are too slow for your system.
But, many can be adjusted up a bit and still be safe.

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#2918349 - 02/24/13 11:00 AM Re: Oil Furnace Blows too strong [Re: expat]
expat Online   content


Registered: 05/12/09
Posts: 3852
Loc: Canada
I have fitted a speed control to the direct drive fan motor, it's been used for the last few weeks now.
Huge difference ! The speed did not have to be reduced much in order to reduce the noise considerably.
Glad to say I am no longer being woken in the early hours of the morning by a noisy furnace fan.

I anyone wants further info on what I did, PM me. :-)

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#2918360 - 02/24/13 11:13 AM Re: Oil Furnace Blows too strong [Re: expat]
oldhp Offline


Registered: 06/28/12
Posts: 838
Loc: Southern Illinois
The airflow has to balanced between output vents and the cold air returns. When my furnace was installed they calculated the fan output and then measured the air going into the cold air return. It took them 2 trys to get the balance right,but thats how it works. If your system is too loud, you need to insulate the system.
The direct drive blowers are the way to go. Mine has 10yr. warrenty so it has to be good.
Hope this helps.
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