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Red Squirrel 01-30-2010 03:26 AM

Furnace can't keep up!
I have a bryant furnace model 340mav024060, (installed around 2002 I think) and one thing I've noticed is when it gets very cold out, like -30ish, it seems to have a hard time keeping up. It's been running non stop for about 3 hours and I was only able to bring the house from 12C to 17C and it actually went back down to 16C while it was running, but eventually back to 17 then it stayed long enough for it to stop (I have it set there).

My house is split level, so the higher level is maybe about 21C - I'm not cold or anything, the lower level is cooler, and that's where the thermostat is.

I do feel warm air coming out of the ducts, but the lower level of the house does seem to have less pressure. Like, it will make a piece of paper or cardboard move, but when I put my hand it just does not seem like there's enough pressure and not as much heat.

The two main register vents do seem a bit undersized so I'm wondering if that's the problem. In fact the main duct that goes to the lower level is only warm while the one going up is hot enough to almost burn my hand if I keep it on long. They're both the same size.

The vent dimensions are as follows in inches:

Main return : 8x18
Two returns branch off:
Return going up : 8x13 (sorta, the main return kinda just goes in the ceiling then uses the joists for most of the actual return and there's a 23x6 vent)
Return going down : 8x13

Duct going up: 8x12 (4 registers - 2 basement ones - standard round duct type)

Duct going down: 8x12 (5 registers - 2 crawlspace ones)

Both of those are the only ducts that come off the main plenum. Also I have a few dampers closed, and some registers closed (mostly upstairs as I want to try to push more heat down)

As I was taking measures I noticed it finally stopped and it did hit 17 again (thermostat is set there) so it does eventually heat, but it just seems to take an awfully long time. It actually just started again now.

I also need to check if changing the filter will help, I did vacuum it a while back but maybe it's just reached it's last days and needs to be replaced. I will try that tomorrow and see.

The furnace is dirty but I vacuumed the worse out. The blower does seem to have a thick dust on it, think that would make a big difference to clean that?

Just wondering if there could be anything else I'm not thinking of.

beenthere 01-30-2010 05:00 AM

A 9F temp rise in 3 hours isn't bad at -22F.

Apparently. Its easier for air to travel to the upper level then the lower level.
A clean new air filter will help.

More return from the lower level will help also.

yuri 01-30-2010 06:13 AM

Is it a high efficiency furnace with plastic venting? If so, remove the blower and clean it and check the secondary coil for dirt buildup. Sounds like it really needs it, check the underside of the A/C coil for dirt if you have one.

Scuba_Dave 01-30-2010 08:37 AM

Wow, I don't turn my temp below 55f, usually not below 62
Last nite with the temp going down to 4f last nite I left the temp at 65f
During the day we keep it at 68f = 20c

I'd be doing some insulating if I were you
Can't remember if you have insulated your basement yet
That made a big difference here (rim joist), raising basement temp 10-15 degrees

Red Squirrel 01-30-2010 12:47 PM

Yeah the basement is not insulated so I may be loosing lot of heat that way. I want to do the rim joists at least, and soon I want to do the whole basement, but I want to do it well. Plenty of power/data cable runs etc... As for the upstairs walls not sure how well insulated they are, this is an old house so maybe they did not use much insulation. I'm pretty sure it's fiberglass though, and not stuff like newspaper or something. The house was built in 1965. I will probably add more insulation to the attic as well, I'm at R40-60 range so I want to bring it up to 60+.

I'll start with a new filter and see how that goes, and I'll clean the squirrel cage fan, that looks quite tedious to clean but may be worth it in the end. There's this yellow/brownish scum on the blades. I think it's a combination of very old cig smoke, and spider webs. It's probably never been cleaned. No AC, so no coils to worry about. I had cleaned the inside of the furnace real quick with the vacuum but did not get into the hard to reach areas, I was mostly in there just to check out if there would be any surprises.

beenthere 01-30-2010 01:13 PM

You'd surprised how much of a difference it can make by just caulking around your windows.

Scuba_Dave 01-30-2010 01:54 PM

I went thru a bunch of tubes of caulking here the 1st winter we moved in
My main problem turned out to be a lack of insulation on either side of every window where the old sash weights were from the old windows

Red Squirrel 02-02-2010 06:23 PM

I think I may have found part of my issue with the lower floor heating. I bet all my dampers look like this:

This vent is a straight line so it was easy for me to stick a flash light and my camera down there. The others not so much as they curve. I will need to get them fished out and cleaned I think.

in fact while dampers might be useful as they can do a total block of a vent, they can capture dust like seen here, and get loose. The handle is actually straight, yet the damper is not completly open, so think over time they get loose.

AndrewF 02-03-2010 09:42 AM

Usually the damper bolt that the handle fastens to, has a notch in it that machines the direction of the damper.

Align that notch parallel with the duct and you'll have it open correct. Relying on the handled to be parallel is not always accurate.

If you remove your filter, do you get a noticeable change in airflow?

Handyman4u2fix 02-04-2010 12:35 AM

1- is the thermostat near the return vent?
2- do you have enough air flow by the thermostat? this will help to equelize the temperature, and the thermostat will reach to deciderate point.

Red Squirrel 02-04-2010 12:14 PM


Originally Posted by Handyman4u2fix (Post 394419)
1- is the thermostat near the return vent?
2- do you have enough air flow by the thermostat? this will help to equelize the temperature, and the thermostat will reach to deciderate point.

1: It actually is, there is a return on the same wall as the thermostat, in fact think it was moved at some point since the wire is fished through part of the return, or maybe they were just lazy when they first installed it. :laughing:

2: There is not much air flow due to the issue that the lower level seems to have less air pressure, so air flow is probably not what it should be at. So yeah the temp on the thermostat is probably a few degrees colder then the upstairs area (split level, so not a full level up)

When I removed the filter I did not notice much difference, if any, but there was a funny burning smell so I put it back within about 30 secs, basically the time it took me to go check a vent and come back. I think the blower was running faster then it should, or something.

I just called a HVAC place to see how much it would cost to get my ducts cleaned and it's about $275 if I heard correctly. I think considering how bad the ducts are it might be worthwhile. Hopefully that includes cleaning the furnace itself too. I could do that part myself though but if a pro does it with all the right equipment then even better. They can only come when it's not too cold out though as the cold is hard on their equipment, they normally do this in the summer.

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