Temp at the Split

[Ben H]

1200 SF ranch, Gas furnance. No info what so ever on it. None. The return temp was around 65-68.

I had a VERY large gap in temps around the house. Ranging from 88-131[:-bigeyes. The 88 seems ok, it' the three rooms that have a vent temp of 125-131 that I'm concerned with.

What can cause this high of a output temp?

[Jim Katen]

1200 SF ranch, Gas furnance. No info what so ever on it. None. The return temp was around 65-68.

I had a VERY large gap in temps around the house. Ranging from 88-131[:-bigeyes. The 88 seems ok, it' the three rooms that have a vent temp of 125-131 that I'm concerned with.

What can cause this high of a output temp?

The normal temperature rise across a gas furnace can be anywhere from 35 degrees to 70 degrees. Look at the data plate; it'll tell you what's acceptable for that furnace.

If there's a large variation from duct to duct, I'd be looking at problems with the duct system.

- Jim Katen, Oregon

[Ben H]

That's one of the problems...No data plate. There wasn't anything anywhere that told me anything about the unit. [:-blindfo I'm kinda blind here.

[kurt]

You're not blind. It sounds screwed up simply because it doesn't have a data plate.

The ductwork is surely a mess; check location of supplies and returns, and simply feel if there's satisfactory air flow.

This amuses people, but one of the tech's I've worked with said "take a 30 gallon garbage and hold it over the register; if it can lift and essentially fill the bag, it's good, if it doesn't, it's bad".

As ridiculous as it sounds on the front end, it's a remarkably cheap and effective way to figure it out on our end. Expect folks to laugh @ you.

After a while, you can mostly ditch the bag; you begin to get the idea of the general air flow reasonably well with your hand in most cases.

Returns should be up high, and as many as possible. Nothing in the kitchen.

Does it sound like the system is "wheezing? If it is, you probably have incorrectly sized supplies and returns, or the filter's crap.

Pull the filter out when you're checking air flow.

[Terence McCann]

Older gas furnaces have a bi-metal style of fan/hi-limit switch. The normal settings of this switch are hi-limit=200°-burner off, fan on=150° and fan off=100°

I'd be more concerned with lower duct temps in your setup. With the lower temps in the ducts - look for dampers in the duct work that may be fully or partially shut, not at the register but at the branch leaving the supply plenum of the furnace.

[Marc]

I wonder if some of the discrepancy in temperature readings is a result of taking some readings immediately after the fan switch has turned on, and other readings at a later point in time?

Marc

[Ben H]

I wonder if some of the discrepancy in temperature readings is a result of taking some readings immediately after the fan switch has turned on, and other readings at a later point in time?

Marc

I don't think so, The furnace had been running for about 20-25 minutes when I took the readings. Such a small house took less than 30 seconds to check all the rooms.

[Jim Katen]

I wonder if some of the discrepancy in temperature readings is a result of taking some readings immediately after the fan switch has turned on, and other readings at a later point in time?

Marc

I don't think so, The furnace had been running for about 20-25 minutes when I took the readings. Such a small house took less than 30 seconds to check all the rooms.

What were you using for this check?

- Jim Katen, Oregon

[Ben H]

Fluke 62 infared thermo held up to the vent. There's only 6 vents in the house. I was wrong about he SQ. Edit***Turns out the house was just a tad over 1000.

[Bill Kibbel]

Fluke 62 infared thermo held up to the vent.

Next time, use a thermometer that measures air temperature. Also, get an anemometer to determine if air flow is the problem, or just temp. drop.

[Jim Katen]

Fluke 62 infared thermo held up to the vent. There's only 6 vents in the house. I was wrong about he SQ. Edit***Turns out the house was just a tad over 1000.

Then you were infering air temperature, not measuring it.

In terms of furnace operation, the only meaningful measurements are the ones that you take as close as possible to the furnace, preferably at the supply and return plenums.

The temperatures at the registers tell you about the duct system, not the furnace.

The 131-degree measurement (if it was really the temperature of the air) is probably closest to what was coming out of the furnace. If so, that doesn't seem all that high to me.

The 88-degree measurement (if it was really the temperature of the air) is really low. Were the ducts insulated? Were there gaps in them? Dampers as Terry mentioned?

Other things that can dramatically affect the temperature at the registers include a clogged filter as Kurt mentioned and an undersized or oversized furnace. But if the temperatures are inconsistent, it's the ducts, not the furnace, that's to blame.

- Jim Katen, Oregon

[asihi]

Is this house slab on grade? If so, were the ducts in the slab? I would be looking for ducts that are filled with water if this is the case.

Tony

[Ben H]

Is this house slab on grade? If so, were the ducts in the slab? I would be looking for ducts that are filled with water if this is the case.

Tony

Nope. Crawlspace. Ducts in the attic. The home has an addition with the cheap 6" flex ducts. One would think they would be the rooms with the low temp, but thats not the case. It's the original hard lines that are the low ones. I wrote it up as needing a HVAC guy. The lack of an info plate made it impossible to give the lady any idea of anything... 32 year old short sale foreclosure that hasn't been touched in a long time.

[Jim Katen]

. . . It's the original hard lines that are the low ones. . . .

If they aren't insulated, steel ducts can radiate a whole lot of heat into a cold attic. . .

[kurt]

Yes, multiple studies have shown that uninsulated ducts in unconditioned crawlspaces or attics are extremely inefficient.

Not to mention, the attic ducts I usually see are all non-linear. On average, the attic ducts I see have at least 2 extra 90's by taking the heat to the attic to distribute it; lottsa air flow gets lost in 90 degree bends, and adding a couple extra can really screw stuff up.

Put the two together, and you have really lousy heat performance.