Home furnace timing question
November 11, 2006 5:49 PM   Subscribe

I just moved into a house that I bought, and the heater makes this 700 square-foot house nice and toasty, but the timing seems off. First something comes on, it makes a humming, sounds like a motor, but no air is coming out yet....that goes on for a minute or two, then the igniter clicks on and glows for about a minute, then flames come on for 3-5 minutes, then the fan comes on and blows heat up throughout the house. Inspector said...

The inspector said it has a timing issue....is this the case, what is likely the problem, and is this an expensive fix...?

Also, is this that big of a deal? It would seem, for fuel conservation purposes, not to be very efficient to have the flames on for a few minutes with no fan blowing...because once it blows it does so for only a few minutes before tripping the thermostat and shutting off...

i'm just trying to get a handle on how much it might cost to get this thing timed right.

the heater is a 1994 Fraser-Johnston.
posted by Salvatorparadise to Home & Garden (9 answers total)
If you just moved in, and IF it's not a safety problem, a month or two of operation will tell you if it's inefficient or not.

I wouldn't jump to conclusions on the manner of operation. My forced air oil furnace runs for several minutes before the fan comes on, too. It may be a characteristic of your furnace.

Consult an owner's manual if you have one, and if not, contact the manufacturer and obtain one. While you have them on the phone, ask them if this is normal operation or not. Don't spend money on repair until you are sure you have something to repair.
posted by FauxScot at 6:15 PM on November 11, 2006

This is the correct theory of operation. The exhaust blower goes on first, to clear any potential build-up of leaked gas, then the gas is turned on and ignited. The delay before the house blower then comes on allows the heat exchanger to get up to temp first, which prevents the furnace from blowing cold air into your house for several minutes. It also increases (!) efficiency, because the heat exchanger is most efficient when it's hot.

3-5 minutes for flame without blower does sound a bit on the long side (ours is about 60 seconds, I think), but it may be within normal variation, depending on the size of the heat exchanger, BTU rating of the furnace, and temperature of the actual furnace location. Have you actually timed this? This is the only bit of timing that is controlled by the furnace itself, and if I had to bet on it, I'd say there's nothing wrong with your furnace.

The rest of the timing (length of cycle, when to turn on and off) is controlled by the thermostat, which is an easy DIY to replace with a snazzy programmable one ($50-$60) that will let you do all sorts of things to increase comfort and efficiency.
posted by trevyn at 6:44 PM on November 11, 2006

I'd like to add that our oil furnace behaves in the way you described. Now, I don't know if it's normal, since I don't know anything about oil furnaces - and your post made me question whether it might be broken.

trevyn's explanation of clearing gas fumes makes sense, though.
posted by odinsdream at 7:51 PM on November 11, 2006

the igniter clicks on and glows for about a minute, then flames come on for 3-5 minutes...

I'd guess that the igniter glowing for about a minute before oil starts burning is the timing problem your inspector was referring to. Burning for a while before the blower comes on is normal, mine does that too, but it ignites instantly.
posted by sfenders at 8:09 PM on November 11, 2006

thanks for the comments, those timings were unscientific and very rough, so it may not even be as long as 3-5 minutes, tho it seems like it when i stand there and wait, ie watched pot never boils....but it sounds like the furnace is OK


posted by Salvatorparadise at 8:10 PM on November 11, 2006

trevyn has it. My forced air/gas combo behaves just like trevyn's, and my thermostat controls the fan timing.
posted by Opposite George at 8:22 PM on November 11, 2006

There's a physical interlock that prevents oil/gas from flowing immediately after the pilot is lit. It's sensing the temperature directly using a bulb of something-or-other which has a wire coming off it. After the pilot is burning, that bulb has to heat up so that whatever it contains expands.

I think that such an interlock is required by the fire code. Anyway, it's normal for there to be a delay of up to a minute after the pilot lights before fuel starts to flow.
posted by Steven C. Den Beste at 9:44 PM on November 11, 2006

FYI, the air handler blower (the one that starts up last and finally blows hot air at you) is usually completely independent from the ignition and flames and all that it. It simply runs on an internal thermostat: if it detects a high temperature heater core, it runs the fan, and if the core has cooled down then it turns itself off. It's a separate circuit for safety reasons.

Forced air heat is mostly a southern thing, right?
posted by intermod at 8:06 PM on November 12, 2006

Forced air a southern thing? Only for values of 'southern' which include Michigan and Wisconsin. I think it's a function of building age, and maybe a bit of east/west (saw more radiators on the east coast).
posted by Goofyy at 3:21 AM on November 13, 2006

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