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Old 07-11-2019, 06:44 PM   #1
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AC blower speed


I have a 2.5 ton AC and I noticed that the furnace blower speed for cooling mode is in its default 4 ton setting (high). The issue I have with this is it's really loud when it kicks on (much louder than the speed setting for heat mode which is medium-low).

I'm tempted to lower the fan speed to the 3-ton setting (which is medium I think). I'm wondering if it's likely to freeze the coil in this situation. There is a TXV installed (unsure if that matters). I'm wondering since the coolant charge was done based on that high fan speed setting if the fan speed can't be changed without adjusting the coolant? (which obviously I cannot do)

Right now it cools pretty much perfectly which is the other reason to leave it, but that thing will wake the dead if it kicks on in the middle of the night!
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Old 07-11-2019, 07:37 PM   #2
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Re: AC blower speed


I'm betting in alberta you have a large furnace relative to a/c due to it getting so cold there. Normally a house that needs a 4 ton drive furnace would have a 3 to 4 ton a/c.

What's the full model number of the furnace?

The ones with the fully featured variable speed blower automatically compensate for ductwork to get proper airflow and you won't freeze the coil of a 2.5 ton slowing it down to 1200 cfm unless your ducts are severely undersized.

Still a good idea to measure the temperature difference between supply and return after adjustments, making sure it's not much more than 20F. Also need to see if frost starts forming on the large insulated pipe.

For the other motor types you have to be more careful.

Fan speed can affect the charge and it's wise to get the it checked* after making adjustments.

*Your tech needs to measure refrigerant line temps, dial it in in accordance with manufacturer's charging chart, not just go by pressures.

A little refrigerant may need to be removed after cutting the fan speed. Too much refrigerant can cause liquid to get back to the compressor which causes damage.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 07-11-2019 at 07:39 PM.
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Old 07-11-2019, 07:39 PM   #3
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Re: AC blower speed


It should be set for 3 tons but you ideally don't want to change it w/o getting the freon pressures checked.

I would change it and get a tech to do a annual maintenance check the next day and talk to him about it. Get the installer company and not some el cheapo company with a "special".
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Old 07-11-2019, 08:05 PM   #4
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Re: AC blower speed


The Furnace is a Lennox 80UHG4-75 from 1999. The AC is a Lennox XC14-030 from ~2012. I've not measured delta T but I probably can if I find a thermometer with the right scale on it.

I've been (perhaps foolishly) avoiding having anyone touch it since I have this (irrational) fear that touching an old furnace could somehow lead to its death (either unintentionally or intentionally by someone trying to sell me a new one).
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Old 07-11-2019, 08:09 PM   #5
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Re: AC blower speed


I just thought of this: If the 75,000 BTU doesn't make sense that's because it's only the upstairs furnace. There is a second 100,000 BTU 80UHG for the main floor and basement (no AC on that one). It seems to work great with only cooling upstairs in the summer.

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Old 07-11-2019, 08:18 PM   #6
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Re: AC blower speed


That furnace has a regular blower motor with no compensation for ductwork and it shouldn't be adjusted without measurements.

75k btu at 80% isn't large, most of this capacity have a 3 ton rated blower, yours has 4. so ignore my comment about that.

There's a static pressure test that can be done to determine airflow using the manufacturer's chart. Many residential people may not know how to do the test.

See attached chart.

There's a way to estimate airflow based on temp rise in heating mode but not overly accurate. You can do this test yourself. I can post details if desired.

You need around 1000 cfm, in your dry climate you can probably go to 1100-1200 cfm and still be okay.

With properly sized ducts it should be on medium-low, but you may need medium or medium-high to get proper airflow.
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AC blower speed-airflow.jpg  
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Last edited by user_12345a; 07-11-2019 at 08:20 PM.
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Old 07-11-2019, 08:24 PM   #7
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Re: AC blower speed


Quote:
I've been (perhaps foolishly) avoiding having anyone touch it since I have this (irrational) fear that touching an old furnace could somehow lead to its death (either unintentionally or intentionally by someone trying to sell me a new one).
You won't wreck it unless you do something really dumb. Changing the fan speed is pretty simple.

At 20 years old just keep in mind it should have annual inspection anyway for cracks.
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Old 07-11-2019, 08:34 PM   #8
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Re: AC blower speed


Medium-low is the setting the heat is on, and it's so much quieter when it runs the heat for sure. It doesn't look like there's a way to use the same speed for heat and cool on this model though because there's the connection posts for heat and cool on the board and they each get a different wire to the fan for its speed (unless I missed something there). Even the 3T/medium setting ought to be quieter though I'd think.

If I had the guy out to adjust the coolant for the new speed, is there any chance it won't work as well as is it does now for cooling?
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Old 07-11-2019, 08:37 PM   #9
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Re: AC blower speed


Quote:
Originally Posted by user_12345a View Post
At 20 years old just keep in mind it should have annual inspection anyway for cracks.
I suppose that's very true. If there's no change in flame color when the blower first comes on is that a good sign? or is it utterly meaningless?

I have quotes for replacement of the two furnaces but I am conflicted over replacing something that's still working and could go a while yet, versus one of them dying on say December 22nd and really being up a creek. I have no idea how to time this one at all.

For what it's worth I only see one house on the street with a new furnace based on the combination of new pipes on the side of the house and the smaller raincap on the roof from the reduction from 6" to 4" flue for just the remaining water tank(s). I guess they're lasting so far?
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Old 07-11-2019, 08:45 PM   #10
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Re: AC blower speed


It's possible distant rooms won't get enough air with the lower speed - but given it's serving one floor, it's unlikely. It's more of a problem when you have one system doing 2+ floors.

You can actually use one speed for both heating and cooling but requires a splitter on one of the terminals and extra wire.



I wouldn't bother with that -> it should be okay on medium and may actually need medium high as I said before.

Without measurements it's a guess.

At a minimum, temperature drop should be checked. There are charts indicating what the temperature drop should be based on indoor humidity level -> too high and you need to speed the fan up.

Ideally, get airflow verified* and charge checked.

*You need a tech that can measure static pressure or has an alternative way to measure flow -> ie anemometer that can be inserted into duct.



Quote:
If I had the guy out to adjust the coolant for the new speed, is there any chance it won't work as well as is it does now for cooling?
You'll get best performance when it's properly charged and airflow is within manufacturer's normal limits. (350 to 450 cfm per ton)

Having airflow too low hurts capacity, having it too high aside from being noisy reduces dehumidification. In the dry climate you can go to 450 cfm per ton.

----------
Quote:
I suppose that's very true. If there's no change in flame color when the blower first comes on is that a good sign? or is it utterly meaningless?
It's a good sign but no substitute for a visual inspection.

With a coil right on top of the furnace the blower needs to be pulled to check it properly. With the coil well above, you can make an inspection hole and cover it with sheet metal fastened by screws.

Inspect every spring after heating season so you have the whole summer to get quotes and make a decision.
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I am not in the business of any trade I give advice on. I have non-professional hvac experience + good knowledge of theory. Attempt repairs at your own risk. Never jump out safeties - especially pressure switches - on a furnace for testing with fuel supply on; use a meter. Do not troubleshoot with live line voltage present unless there's no alternative.

Last edited by user_12345a; 07-11-2019 at 08:52 PM.
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Old 07-11-2019, 08:52 PM   #11
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Re: AC blower speed


thanks so much for your help and time explaining all this
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