HVAC - impact of closing off unused rooms

In an effort to be a little greener and to keep my heating cost down, I have closed the registers in several rooms that are infrequently used. I am noticing that my gas furnace is now frequently "short cycling" i.e. it runs 2 back to back short cycles instead of one normal cycle. I presume the cycle is being shortened because the furnace is heating up more quickly because less air is moving through it (because I closed the registers).

Does anyone know if this will potentially damage the furnace? Is there some way to adjust the furnace to prevent this?

Steve Q

Not an expert by any means. But yes, short cycling is not good for it. In addition, by closing the registers, you increase the static pressure in the ducts, causing more stress on your fan/blower. If you have variable speed blower, then it doesn't matter as much as it controls the motor drive automatically. Otherwise the life is shortened some.

Given how expensive these units are to repair I don't think I would continue running it that way. Set the temp 1 degree cooler and wear warmer clothes instead .

I wish there was an easy way of figuring out if there are pressure issues, without hiring a pro. My entire basement is not in use (not finished yet), and my livingroom, kitchen, dining room are all 1 space, so I decided to shut most basement ducts down, so the huge open space upstairs would warm up a little better. I don't think it is short cycling, and I have been doing this for a few years now, but I am concerned that my system isn't configured properly.

do you know if your system has a bypass installed?
i have a system (for a/c since i'm in florida) instead, and that's what solved my problem, that way it doesn't strain the system due to the closed vents.. i ended up going full blown with a rcs zoned system. which has saved me a ton on cooling costs, since all the dampers for the living room, kitchen, dining room etc, stay closed at night, and the cooling is directed at only the rooms people are sleeping in, resulting in less run time for the unit.. the rcs unit also does heat as well and can be controlled by the elk, and if memory serves also has a feature to prevent the units from cycling too often and wearing themselves out..

while i'm no expert either though, i know (at least in a/c systems) for zoned units the bypass valve is mandatory because if all the zones are closed (same effect as registers being closed) and the unit is run, pressure builds up in the system (just like blowing up a balloon).. maybe instead of closing off the vents just open them halfway to semi restrict some of them.. but FWIW, i wish i'd have gone zoned sooner.

How large is your bypass tube compared to the rest of your system?

I've been trying to figure out a few things...but haven't gotten any information back from the company about my system yet.

As in, how many CFM does it produce, how much backpressure it can take, etc.

--Dan

forgets said:

while i'm no expert either though, i know (at least in a/c systems) for zoned units the bypass valve is mandatory because if all the zones are closed (same effect as registers being closed) and the unit is run, pressure builds up in the system (just like blowing up a balloon)..

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I'm no expert either, but my house has a 5 zone Trane system that doesn't use a bypass. It uses a "Relief" algorithm that sends excess air to other rooms.

Brian

Dan and Steve Q -

I spent a number of years in the heating/AC business which leads me to not want to try and answer this by answering a forum post, but maybe some background will help. That being said, and with all appropriate caveats of not knowing the make of unit, the model, etc, etc. there are usually at least a couple of things you can do but please know you are working with a piece of equipment that can cause extreme fire dangers and health hazards.

Dan, if you have a variable speed fan and a couple of stages of heat you are probably be good to go. In that type of a system the fan speed will be controlled by a static pressure sensor in the main duct and will vary the speed of the fan based on the build up of pressure in the main duct. If you close the registers, or better yet close off the balancing dampers in the run you want to shut down, the pressure will build up and the fan will slow down. These systems typically also have a couple of stages of heat because the by-product of slowing the fan down too much without also cutting back on the gas flow is that the heat exchanger doesn't transfer as much heat and therefore heats up too fast and will likely short-cycle or shutdown. That type of shutdown or short cycling could be caused by the high-limit temperature switch tripping and then reseting when things cool down (doesn't sound like Steve Q's situation).

For Steve Q's situation it would be helpful to know if the thermostat is still calling for heat when the furnace is short-cycling (I assume it is) rather than the thermostat turning the system on and off.

With a single stage heat system there are still some things you can do. Most fans are multi-speed, just not automatically controlled. In other words there are various points on the control panel that you move the wiring to in order to set the fan permanently on high, med, low, etc. If there are significant areas of the house that will be unused for a long period of time you can probably close off the areas and then rebalance the airflow of the system. Hopefully there are balancing dampers installed in the branch runs which is how you should close off areas rather than using the registers. If you close off the balancing damper(s) to those areas you either need to be able to open up another area(s) dampers to allow the static pressure to stay the same in the main duct, or you need to change the fan speed, or both. Problem with closing one balancing damper and opening another one even wider is that the amount of air through the open duct is now way more than originally intended and you'll probably get really tired of the amount of noise coming through the register. That is when you slow the fan down but understanding that slowing the fan down can cause it's own problems with too much heat at the heat exchanger.

If you don't feel comfortable messing around with all of the above please hire a professional. As you can imagine, an improperly working furnace is a fire hazard and a health hazard. Forced air furnaces are a delicate balance between balancing the air flow throughout the whole system (static pressure), temperature control in the appropriate spaces, noise, and efficiency and making sure it all is done in a without blowing up your furnace and house or creating other health hazards. If all the built-in safeties are working that shouldn't be an issue but you get my point.

Thanks for the detailed response beerguy! I gave up on zoning as it is something a pro would have to help me with (I understand enough to realize I can't and shouldn't do this myself). Right now, I am just worried about too many dampers being closed. I only have a basic single stage non-variable fan unit.

Dan (electron) said:

Thanks for the detailed response beerguy! I gave up on zoning as it is something a pro would have to help me with (I understand enough to realize I can't and shouldn't do this myself). Right now, I am just worried about too many dampers being closed. I only have a basic single stage non-variable fan unit.

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Dan, it's been awhile since I did this for a living (like 20 years) but if I remember correctly you can use an ammeter to measure the current the fan is drawing to get an indication if it is overworking due to registers/dampers being closed (too much static pressure in the system). If you have an ammeter open all of your registers and measure the current the fan motor is drawing and then as you start closing registers/dampers you will likely see the current draw go up as the fan motor runs up its curve. Once you've closed everything you want to close compare the current draw with the listed current range for the motor. Your furnace owners manual should have this info. If you are within the range, the system isn't too noisey, and you aren't short-cycling anything you should be just fine. Just remember to change your filters often and the system should last a good long time.

Pat

Dan (electron) said:

I gave up on zoning as it is something a pro would have to help me with (I understand enough to realize I can't and shouldn't do this myself).

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Then I recommend you discuss your situation with a pro and get an estimate for whatever it would take to modify your system for top performance. Try to get an answer that includes payback period. Zoning is a good thing; I wish we had more. We built our new house with an unfinished walkout basement on its own zone. We do spend some time down there, so that zone is set to a really low temp unless we're down there. We also spec'd a top of line (two years ago) Carrier Infinity system. Our gas company has an on-line analysis feature which consistently indicates we use far less gas than similar homes in the area. Over the summer they replaced our gas meter for "routine" reasons. Today a gas company tech came to the house and asked us to turn up the thermostat so he could observe the meter when the furnace fired. He said it appeared the meter really does reflect our gas use as he was leaving. The house has other features for energy conservation such as 6-inch exterior walls and minimim windows on the north side.

Heating fuel isn't likely to get cheaper, and I'm pleased we upgraded our energy systems when we built the new house. You could do worse than to upgrade your system, but at least you should know what the costs would really be. Go ahead and splash more advertising on this board if you need to, but get an estimate.

Regards. . . . John

beerguy, thanks for the (very good) tip! Will check it out.

Photon, I'll probably get a pro involved once I have the budget for this, as I don't expect this to be cheap.

Thanks for all the great information and suggestions. After a few more days of ON/OFF cycling I have observed that the short cycling occurs when the furnace comes ON after the setback turns off; i.e. it is colder in the house. I have also noticed there is a blinking red LED on the furnace circuit board after the first short cycle. The code that it blinks corresponds to a "high temperature limit switch". So, I guess the furnace is running longer when it first turns ON in the morning and because I have closed the registers it is over heating. I don't really have many options for dumping the heat somewhere else. But I might be able to add another take-off to the main plenum and add a motor controlled damper that would allow me to dump heat into the utility room upon first startup on the furnace in the morning.

I think I will also measure the load on the fan motor as suggested by beerguy.
But I think the simplest thing would be to turn down the flames at the burner. Is this possible? Is there an adjustment screw somewhere?

Steve Q

SteveQ said:

Thanks for all the great information and suggestions. After a few more days of ON/OFF cycling I have observed that the short cycling occurs when the furnace comes ON after the setback turns off; i.e. it is colder in the house. I have also noticed there is a blinking red LED on the furnace circuit board after the first short cycle. The code that it blinks corresponds to a "high temperature limit switch". So, I guess the furnace is running longer when it first turns ON in the morning and because I have closed the registers it is over heating. I don't really have many options for dumping the heat somewhere else. But I might be able to add another take-off to the main plenum and add a motor controlled damper that would allow me to dump heat into the utility room upon first startup on the furnace in the morning.

I think I will also measure the load on the fan motor as suggested by beerguy.
But I think the simplest thing would be to turn down the flames at the burner. Is this possible? Is there an adjustment screw somewhere?

Steve Q

Click to expand...

Steve, unless you have a multi-stage gas valve, which would be doing it automatically, I personally wouldn't mess with the gas valve. The blinking red light is the clue to the problem. You need to get more air flow across your heat exchanger. It is surprising that closing off registers would cause that. Did you close a huge percentage of the system's registers (which I'm still not sure would do it)? Generally the fan is going to push the air right through the closed register and you are going to get whistling. Registers aren't meant to be used for balancing the system (regulating total airflow) but rather directing where the airflow goes. They generally don't close off that tight that a fan motor wouldn't still push the air through. That what has me wondering if that is really the cause.

Check your fan speed setting on the control board. There are usually multiple lugs that the motor can plug in to to control low-medium-high speed for both heat and cool. Some newer models also have some in-between speeds. If yours is on the lowest speed you might try speeding it up one setting.

Just thought of another duh question that I should have asked before. Are you certain that your filters are clean and that there are no obstructions to the airflow on the return air side of the system?

Pat

Steve,

Open those zones back up and see if it stops. If so maybe just dont close as many. If not you may need to call in a pro. (problems just in time for the winter! always....)

Pat, The filter is part of the issue because if I remove the filter the system will not short cycle. It is a 16x25x4" filter and it is hard to find a replacement. Most of the replacement filters are now the "allergen free" type which cut the air flow even more. I'm tempted to just remove the filter or replace it with a flimsey 1" fiberglass one. I will also check on the fan speed. I think there is a DIP switch which I can select a different speed.

Todd, before I open up the registers and start heating unused space, I think I will try adjusting the set back to a higher temperature. That way the furnace won't have to work so hard when it starts up in the morning.

Thanks for the suggestions.

Steve Q