what is the proper cycle time

[bplinder]

I always here about how important it is to not oversize a furnace and how it might lead to short cycling.

Here is the cycle time for my furnace on a recent cold night, 10 degrees. Coldest day of the year is usually 0 degress (NYC area)

0:00 Flame turns on
0:54 blower turns on
3:25 blower and flame turn off
5:25 Flame turns on again and cycle starts

is this a short cycle?

For a 30 degree night temperature, the cycle lengthened slighly

0:00 Flame turns on
0:54 blower turns on
2:40 blower and flame turn off
6:08 Flame turns on again and cycle starts

I have a 1982 amana 100,000 BTU furnace.

What do you guys think?

[kpsheets0580]

Hard to say for sure without more info. Have you checked your filter recently. May just be a dirty filter.

[beenthere]

If it was just a dirty air filter the blower wouldn't shut off.

It may be oversized, or it could be a stat issue.

You gave no info about the house itself.

[bplinder]

THe house is about 1800 sq ft 1962 house. Insulation is reasonable (R30+ in attic, R11 in walls, windows are standard double pane). The furnace says 100000 input, 78000 output.

The filter is clean. I buy those 90 day filters and change them about every 60 days.


What is a normal cycle time for a cold day? Is my cycle normal, or is this the definition of short-cycling.

[big johnson]

3 minutes or less, that's a short cycle all right. When the burner goes off, are you at setpoint? Just from the little info I have, it sounds more like the burner is cycling because of a control issue, not over-sizing.

[weber]

Your tstat could be set with a 1 deg interval, if so change so the run and off cycles are longer.

[rohalon]

Are the flames and blower really turning off at the same time? If so that sounds like there is some malfunction in the system.

[GasMan II]

if there is an a coil in the plenum, the coil might need to be cleaned, it is possible that the coil is restricting the air flow and setting off the high limit, when the high limit cools down it fires for round 2

[Texas Arrow]

What kind of thermostat do you have?

[gasguy]

Try turning your stat way up (10+ degrees above room temp) and see what it does. It should run continuously until setpoint is reached, or you're cooked.:D
If the unit cycles off and on, then it is either oversized or you have a problem. Either way, you need to get a tech to come out to check your system out as it could be something serious like a cracked heat exchanger. Better safe than sorry!

[wingerman]

Sounds like restrictions in the flow of air may be preventing the set point to be reached. Don't overlooked blocked or shut off supply/return ducts. Some times these get closed by accident, or some move objects or furniture over them. Causes a real decline in air circulation, acts like a very stopped up filter, and causes the furnance to eventually overheat and go into roll-out. Any time air is restricted for extended periods of time or the furny cycles too much (esp on older units), you run the risk of a cracked heat exchanger. Better be safe and call a tech.

[gpjazz1023]

Sounds like a supply vent is too near of the thermostat causing it to reach setpoint too fast then shut off. Then cool air infiltrates the area and t'stat turns system on again.

[dawgtchrr]

What kind of thermostat do you have?

Very Good question, if it has an adjustable heat anticipator it needs to be adjusted for a longer cycle. Very easy call and easy to check.

[4l530]

IF the blower is actually turning off between burner cycles then it's not slamming the high limit. This can also be verified by watching the little silver dial spin if this furnace has a Honeywell fan / limit switch.



Originally posted by gpjazz1023
Sounds like a supply vent is too near of the thermostat causing it to reach setpoint too fast then shut off. Then cool air infiltrates the area and t'stat turns system on again.

I think this sounds plausible, or maybe the heat anticipator is incorrectly set. The cycle rates mentioned in the original post are awful short. As far as sizing, I haven't seen a load calc but based on the info provided the furnace doesn't sound unreasonably large to me. But it still could be.
bplinder, if you do not have your furnace maintanenced regularly, I would also recommend to have a service tech inspect it's operation and safety. 1982's kinda old for a furnace.

[bplinder]

Ok, the thermostat I have is an old analog dial. I plan on upgrading when I add AC to the system (hopefully before this summer).

I'll up the temp 10 degress and see if it runs continuously.

I do have 1 supply vent 70% closed because the flow is way to heavy to one room. That room would be 10 degrees to warm if fully open. I do have 6 other supplies on that end of the trunk, though.

Actually what happened is I divided 1 large room into 2 rooms and added a new supply to the new room. The old room (which is now 1/2 the size) is just way over supplied, so I used a magnetic vent cover to cover about 70% of the vent. By doing that, the temp in that room is equal to the rest of the house. There is no damper in the duct work for that particular vent, and the duct work for that vent is completely enclosed in internal walls.


I'll update after I do the 10 degree test.

Thanks.

[bplinder]

Actually, no supply vent is near the tstat. The tstat is in the main hallway on the 1st floor, and there are no supplies near it. The closest supply is about 10 feet away in a bathroom, and the other supplies are much further away in other rooms.

The return vent, though is very close to the tstat. It is about 5 feet away on a wall that is 90 degrees to the tstat.

[4l530]

Originally posted by bplinder
The return vent, though is very close to the tstat.

That's good, that's how it ought to be. :)

[bplinder]

I turned the furnace down to 60 while I was out. When I came home I turned it back to 68 and it stayed on the entire time without turning off.


and yes the blower and flame turn off at the same time (although the pilot does stay on).

[hetrola]

THe house is about 1800 sq ft 1962 house. Insulation is reasonable (R30+ in attic, R11 in walls, windows are standard double pane). The furnace says 100000 input, 78000 output.

The filter is clean. I buy those 90 day filters and change them about every 60 days.


What is a normal cycle time for a cold day? Is my cycle normal, or is this the definition of short-cycling.

Thermostat heat anticipator setting needs to be calibrated or Thermostat upgrade is needed.

Your furnace is 78% efficient out of the box. So you might be in the 60% range depending on standing pilot, belt driven.....

When you get A/C do yourself a favor and upgrade the system.

[deejoe]

Bplinder; one item to check is...you say; " both burner and fan go off"..

If, as you claim, they BOTH go off at the same time should not be happening at all (a waste of heat). Once the burner cuts off, the fan should remain on for at least a couple of minutes or so in order to deliver the heat that has been built up within the furnace.
Imo, 2 or 3 burner cycles are TOO short. I like a burner to run for at least 6 minutes or so a couple of times an hour when the temperature gets down close to the outdoor design temp.
if your unit has adjustable fan control settings,adjust to; fan on at 120-125, and fan off at 90-95
High limit setting is normally individually set at the 175- 185 range.

[Kevin O'Neill]

I had an Amana furnace when I lived in Pennsylvania. It used a time delay relay for a blower control, much like a heat sequencer. It had a delay to come on, but shut off almost instantly after the burner shut off.

Those 90 day 1" thick pleated filters are very restrictive to air flow. Having done extensive testing with a flow hood, I have found they usually drop 15% to 20% off air flow compared to a fiberglass filter. Unless you have alergies, they are usually a waste of money.

It sounds like your furnace is way too big. What is the coldest outdoor temperature you are likely to see in your area?

Kevin

[sskzekeman]

You are probably somewhat oversized, since the duty cycle at 10 degrees is about .53. It should be closer to .75. But the cycling problem in this case has more to do with the thermostat and the tightness of your house and less to do with the oversize.
To summarize, short cycling is due to any or all of the following:
oversize
tightness
thermostat anticipator setting or low on/off differential

[bplinder]

I had an Amana furnace when I lived in Pennsylvania. It used a time delay relay for a blower control, much like a heat sequencer. It had a delay to come on, but shut off almost instantly after the burner shut off.

Those 90 day 1" thick pleated filters are very restrictive to air flow. Having done extensive testing with a flow hood, I have found they usually drop 15% to 20% off air flow compared to a fiberglass filter. Unless you have alergies, they are usually a waste of money.

It sounds like your furnace is way too big. What is the coldest outdoor temperature you are likely to see in your area?

Kevin



Ok, the coldest temp is typically 0 degrees (or 10 degrees colder than what i quoted). Do others agree with this idea that the 1" pleated filters restrict air flow? What are the filter options in a new furnace.

I'm definitely going to get a new furnace when I buy the AC. I'd like to improve my efficiency. My current furnace has a combustion efficiency of 78% (100000 in, 78000 out). Like i said it is from 1982. What do you think the AFUE is? I want to get a feel for what kind of savings I'm looking at with a new furnace.

Thanks.

[silkysean]

Sounds to me like your furnace is now oversized it's from 1982 and i bet u made home improvements on the windows,doors and insulation in 1982 gas was cheap and no one thought about energyeff. and ever imagined gas would be this high have a energyaudit and manual j calculation done for proper size until that time u can calibrate t'stat to run a little longer.