I would like to lower the output capacity of my furnace if possible. How can I do this? Is it worth doing, or should I just live with it? Is there an easy way? Some ideas: replace burner nozzles with smaller nozzles, replace burner with smaller burner if this exists, close the left of the four nozzles on the burner and move the flame sensor to the third. Since the burner and heat exchange seem to be designed to work together I assume that adjusting the burner output would screw with the efficiency of the system.


I purchased a house with furnaces that have a higher capacity than necessary for the space they heat. I found this out recently when I closed a small vent to one room that is not used often. Closing this one vent, not fully maybe 75% closed, triggered the high limit switch a few times and eventually the lockout. After opening the vent and resetting the system things have been fine. The cycles are still pretty short, but the high limit has not been triggered again since. The system is a Trane 80k BTU 80% efficient, forced air unit. Blower seems to be set to highest setting for heat rather than a lower setting. The heated space is around 1,100 square feet, upstairs with decent insulation and on the south side of Atlanta GA where it does not get too cold. Square footage is potentially lower due to the system not actually interacting with the hall if doors are closes. Returns and positive vents are all in rooms; hall stays warm from lower system. I did some rough estimation and figure I need somewhere between 30 and 40 thousand BTUs of output and am getting around 64k BTUs.


I figure tinkering with the furnace to lower its capacity is not safe and will keep this in the forefront of my mind. No lectures necessary.

  • Furnaces, as in multiple? – isherwood Jan 15 '16 at 16:42
  • @isherwood, yep there are two furnaces. I only care about adjusting the upper one. Lower one does not seem to be as over-sized and can handle closing a vent or two. – zerpsed Jan 15 '16 at 18:23

Cut the gas back some by slightly closing the gas valve (the one on the pipe, not in the unit), however be prepared to have to reset the furnace occasionally if your local gas pressure drops intermittently. Watch the flames as you do it. Just, "take the edge off".

This will cost you more in electricity, as it'll run longer to come up to temp, but it will bring you a more even comfort level. I'm unsure what effect this will have on the efficiency of the heat exchanger, but at least it will work.

The last place that I lived where I had to do this, I also kept the fan set to on in the winter. My concern was comfort over efficiency. Note, part of my problem was the T-stat becoming satisfied before it should have, but I think the same solution applies - unless you want to start spending money.

A quick hop online* tells me efficiency probably drops like a rock if you do this, but it sure is one way to lower the output of an oversized furnace.

*...adjust your thermostat for the widest differential your comfort will tolerate to obtain the best efficiency and equipment service length. –Define "Short Cycling", hvac-talk.com

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  • Wow, it is such an obvious solution to lowering output. The valve is right there.. right next to the unit. Bright red, can picture it in my head. – zerpsed Jan 27 '16 at 20:17

If partially closing a single supply vent caused your high limit to trip you probably have an restriction problem.

I would recommend investigating a bypass damper with a return and supply temp. sensor. These devices are typically just a part of a whole house zoning system, but can be used to easily resolve supply air restrictions in situations where modifying existing duct runs is not practical. Essentially, the bypass damper dumps excess air back in to the return. The two sensors ensure that you don't exceed safe operating temperatures without reaching the max temp of the high limit in the winter or freezing the system up in the summer. The only brand I have used EWC Controls which work just fine, but there are other brands available.

footnote: reducing the static pressure of your system will reduce energy usage.

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You can't. I have the same problem. My house has a 125,000 BTU furnace for a 25,000 BTU heat load. My solution? I set the thermostat at 69 and turn it on manually when the temperature falls below 65 or so. This ensures that the furnace runs for a nice long time when it's on, and then stays off the rest of the time.

But you don't really even need to do this at all. If you have a 64,000 BTU output furnace but your rough calculated heat load is 30-40,000 BTUs, that's not really so bad as furnace oversizing goes. I wouldn't worry about it. In 20 years when you need to replace the furnace, get a smaller two-stage model.

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  • 2
    Instead of turning the thermostat on & off, there are thermostats where you can adjust how big the swings are. I know my digital thermostat has it; I imagine it's a common feature. – David Yaw Jan 15 '16 at 18:28
  • I have mine set to +/- 3 degrees F to accomplish this. It is a bit uncomfortable sitting through a 6 degree rise in temp but I don't really want to kill my system with 5 minute cycles. – zerpsed Jan 15 '16 at 18:38
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    A programmable thermostat with a large night-morning-away-evening swing will prompt the furnace to be on for long periods and then stay off for long periods, which accomplishes some of the same things as a 3 deg jump – gbronner Jan 15 '16 at 19:42
  • The longer it runs the more likely it is to trip out on over heat. Having a T-stat with a larger differential might work against the OP. – Mazura Jan 27 '16 at 6:52

I know this is an old post, but thought I could help future visits. My house is about 1900sf and my furnace is a 135k BTU (a bit much). In my case I had a cracked heat exchanger and the crack was very close to the high limit switch, which of course kept shutting the unit off. I also found that the inside of the a-coil was very dirty/if not partially clogged. So I would say if you are having high limit issues, have the fan side of the a-coil cleaned and check for a crack in the exchanger by simply removing the high limit switch and inspecting the heat exchanger for cracks, which is pretty common on over-sized furnaces.

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I solve it slightly different way, using Macromatic TR-6512U repeat cycle relay. This relay has independent ON time and OFF time settings from 5Sec. to 100 Hrs. It is very important that relay is ON First. It works on any voltage from 24V-240VAC. My thermostat drives the relay and contacts turns on gas valve. I observed my Hi limit first it usually took about 12 minutes on average for the Hi-limit to trip, so I set my ON time to 8 minutes ON followed by 35 Sec. OFF time. Your setting may have to be different. My problem is the way my daughters arranged furniture in their rooms, every single register is blocked and as you can guess it is not negotiable.
I play with it for about three days. Basically you must turn gas valve OFF before Hi limit dose and turn it back ON when temperature of the heat exchanger drops about 10-15 Deg. This way you don't even notice the fluctuation at the registers. This is working 100% for me. Good luck.

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  • This is interesting. Does it effectively bypass much of the safety logic programmed into the furnaces control board by directly controlling the gas valve or do you have it series to interrupt the control boards gas valve ON signal? My igniter cools after a flame is registered, so I'm guessing that if the gas flow is interrupted, it will not re-ignite when gas flow is restored. Actually, thinking further, the flame sensor would shut my system down when the gas flow is interrupted. To try this, I think I would have to jump the flame sensor and re-energize the igniter somehow. Seems risky. – zerpsed Nov 16 '18 at 14:13

My Trane (5 yr old) has different possible blower settings for the heat and non-heat periods. My house was getting chilly in spots. So, I turned the fan up during the periods when the thermostat was not calling for heat. That circulates the air and has made the house a more even temperature. Meanwhile, the blower setting when the furnace is on has been set to lower than max. There are instructions for this on the panel door, but I had the installer do it.

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Having an oversized furnace compared to the needs of the conditioned space should not cause furnace limit problems unless the duct system is too small. If the furnace was installed by the same people as the duct system they should be allowed to address the problem. There are limits to how many outlets can be closed without causing this problem.

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  • Good chance the duct system is too small. Both parts probably installed by same people. People probably do not exist anymore though. Everything is roughly 24 years old. Ignitor has been replaced, but the rest looks stock. All very clean though. – zerpsed Jan 27 '16 at 20:12

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