Question on home heating furnace

[Flyinfool]

My furnace always seems to run fairly short cycles. These short cycles are being commanded by the thermostat.

My question is, How short is short?

As is a typical cycle lasts about a minute from the time the Furnace blower kicks in to the time the thermostat is satisfied and ends the call for heat. It will then be just 1 to 10 minutes depending on outside weather till this repeats. I would think it would be more efficient to have the furnace run fewer longer cycles.
In the setback mode, I have it set back 10°F at night, It only takes about 10 minutes to have the house back up to temp at the end of the setback.
Does mean my thermostat is to sensitive?
Or maybe is my furnace to big for the house?

I have looked inside of the thermostat for any kind of anticipateor type of setting, there are no internal adjustments of any kind I can find as seems to be the case with most electronic units.

 

[mmcmdl]

I had this problem also , oversized furnace .

 

[Flyinfool]

At the time I got this one it was 10 below zero and kind of had to take what they had in stock.

What is the best way to determine what size I should have?

 

[mmcmdl]

I don't have that answer but I know they have calculations .

What happened to me ......................

I bought a house 2 houses up from my original house and shop back in 93 . The house was approx 2300 SF . The furnace was only heating half of this house and short cycling like yours . I asked my co-worker what was going on and he said my furnace was larger than his and he heated a 3500 SF space .

What I ended up doing was ripping all duct work out of the basement to the upstairs , and added ducts to the other side of the house . We generally don't use the electric baseboard heaters any more because the furnace can breathe better and spread the heat thruout most of the house now .

 

[Alexander McGilton]

I think the hysteresis of the thermostat is too low, or it is placed too close to an adjacent vent. Too many on/off cycles can shorten the life of your furnace motor and any moving parts of the gas regulator or electrical contacts.

 

[mickri]

On my thermostat you can set a temperature range to turn the furnace off and on. For example if the thermostat is set for a 1 degree range and is set for 70* The furnace will turn on at 69* and turn off at 71*. This is a pretty typical factory setting. If set for a 2* range it turns on at 68* and off at 72*. Where the thermostat is placed is another factor. My thermostat is place 4' off the floor and close to the return air vent. No heat vents are close. So the thermostat is governed by the temp of the air getting sucked into the furnace.

 

[talvare]

It's possible that it is not the thermostat that is causing the short-cycling. Other safety devices like the thermal cut-out switch or the fan proof switch could be defective. May pay to have an HVAC tech troubleshoot it for you.
Ted

 

[pdentrem]

How modern is the furnace? Could be the thermostat as mentioned.
If a hi efficiency with sealed air system.
Check for a trouble code light that will blink the number. Usually have to open the access panel. The codes are typically printed on the panel or near by.
Check for blockage on the air supply pipe and exit pipe. There are sensors on each. Also check the water trap that is under the condenser that is before it drops to the drain line. They do get plugged with debris and need to be emptied every couple years. Have a bucket ready to catch the water if so.
Pierre

 

[RJSakowski]

Thermostats have different setting for the type of heat that you have. Depending in the thermostat, the adjustment varies. A shorter cycle rate will provide more uniform temperature control at the expense of more wear and tear on the equipment and lower operating efficiency. My Sensi thermostat has three settings; fast, medium, and slow. My old programmable Honeywell had a two position switch for fuel or electric heat. I believe the still older round Honeywell thermostats had a mechanical adjustment.

 

[Flyinfool]

It is defiantly the thermostat. I can hear the relay open and close that commands heat on and off, and the furnace is following that exactly.

I found an online calculator for furnace sizing Using the worst case load factor I should have a 48,000 BTU, I have an 80,000 BTU in a house that is 1,100 square feet.

I wonder if that is part of my problem too.
I have 3 of the 4 bedrooms with the heat shut off cuz I don't use them for anything other than storage. That means I have 80K BTU feeding about 2/3rd of the house.I wonder if it might be more efficient to open those rooms back up to have better use of the furnace.

The cold air return is directly under a picture window, that would be a bad place for the stat.The heat vent in the room is nowhere near the stat nor is it blowing toward the stat. the stat is very close to centered in the house, on an inside wall that does not go to the attic.

The drain was just replaced last year and now has a sensor to warn me if water is backing up.
It is not quite new enough to have trouble codes. Just a bunch of relays.

It is sounding like my furnace is just to big.

This old gas furnace is only 38 years old and was a high tech at 92% efficient, great for the day.

I had been looking at replacing it in the not to distant future, but every time it stops working I have been able to find parts to fix the problem.
Since my house was built in the late 1800s, still has the original single pane windows, and only as much insulation as I could sneek into the walls, I am thinking of a 60K BTU for next time.