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Question DetailsAsked on 10/27/2015

What to do if new furnace installed is too large for my house?

Three months ago I had my 25-year old furnace and AC replaced during the warm summer months. Initially the AC seemed real cold in pockets of my home. Now that we have hit the colder months, it has become obvious that this new furnace is too large for my home and ductwork as it always short cycles. The installing contractor has removed the filter and cover to allow for more return air and stop the short cycling to heat the house and even with this there are some rooms that are hot and others that are freezing. They have suggested either cutting a perminent return duct in the furnace opposite the filter/bypassing the filter or changing the heat limit switch to a higher temp. Both suggestions seem to be unacceptable in my opinion. I think this furnace needs to be changed and sized appropriately. Any advice?

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3 Answers


I am sorry to hear you are having this problem. It seems you have a contractor that has missed the mark a bit. There are several factors to consider when sizing air conditioning and heating equipment and your contractor may not be proficient in this area. They may have the responsibility to replace your system if the guidelines set by your state and the federal energy commission were violated. I would seek the assistance of a reputable HVAC contractor with some engineering background to guide you in this matter.

The next approach would be to downsize the furnace. To accomplish this would entail replacing the burner orifices with smaller size orifices and ensuring the fan speed and CFM output is coordinated. The air conditioning fan speed would remain on medium high or high speed. To do this will again require the services of a reputable firm with some engineerig background or consultation with the manufacturer of the equipment.This would not solve any problems you may have with the air conditioning. That is another matter requiring the guidance of a reputable contractor.

I would not trust this work to any contractor alone, unless he gets the right information from the manufacturer or the manufacturer's representative. To do otherwise may endanger the welfare of the residents and cause a dangerous situation.

DO NOT let anyone guess the right procedure; make sure they have the manufacturer or manufacturer's representative's blessing. Your problem can be resolved with the right approach. The best to you.

Source: Poppy Ross

Answered 4 years ago by PoppyRoss


If the furnace is actually oversized by more then 15%, and is not a two stage furnace then it should be replaced. Old size versus new size furnace?

However the symptoms described could be due to undersized ducts alone. Todays efficient furnaces move more air then te older ones,of the same size.

There are likely better alterations that can be done to "fix" the duct system.

The need to know the ESP(external static pressure) of the supply and the return ducts ,for starters. Ask about the use of turning vanes in both supply and return,to reduce the ESP.

You may need to get an expert on duct systems , to solve the issues.


Answered 4 years ago by BayAreaAC


Good responses by PoppyRoss and BayAreaAC as usual - though I admit I am stumped on BayArea's thinking thst undersized ducts could be the problem - perhaps he was keying in on the some cold and some hot rooms part, rather than the short cycling (which implies the heat is certainly getting to at least the thermostat in a hurry, at a minimum). Several possible causes here, as they said, plus a few I am adding to the discussion mix -

You did not say if this unit is still under installer warranty - if so, you might be able to force him to correct the problem, which might include replacement or changing its output capacity if that is the only solution he can make work.

As PoppyRoss says, you need a Heating and A/C/ contractor with experience in system configuration and modifications, not just an installation tech - and getting the manufacturer involved is VERY important if looking at resizing the furnace. Many use the same basic body and guts for various heating capacity units, just changing the burners, blower, and perhaps a few control settings for the different capacities. So, it may be quite easy and not too expensive to downsize the capacity of the unit (called "derating") - especially if this is a high-efficiency furnace which is the condensing type (plastic exhaust pipe rather than metal ducting). If metal exhaust flue ducted it might cause an issue if the derating reduces the airflow and flue heat to the point it becomes a condensing unit, which might then require downsizing the exhaust flue pipe or adding an exhaust blower. Not a big thing if it is a straight shot to the roof to downsize the pipe, but is an added cost to do that. Nominally derating a condensing unit should not affect exhaust size, except might need a different rated exhaust blower and/or sensor.

Some modern units with variable burn rate as well as variable speed fans may be able to be reprogrammed on-site to just bypass the maximum burn rate and run at a lower output, effectively derating your unit with only a settings change on the controller.

The some cold - some hot (or some too cold in the summer) room issue sounds like improper airflow to or from the rooms, or too much air escaping from some or not enough from the too cold/hot ones (which I would guess were the same rooms in each season) - would be an airflow adjustment/splitting issue or possibly an issue with where the thermostat is located rather than a furnace sizing issue. I think it is significant that this happened with the A/C too - to me that says duct issues, unless the A/C was replaced with a higher output unit and is short cycling also. As one comment said, improving airflow by reducing resistance, adding or modifying return ducts, changing thermostat location or going to two thermostats with a controllable splitting damper, etc might solve that if airflow distribution is the problem.

On what has been done already - removing the filter and cover is NOT a solution and is an illegal modification of the manufacturered unit that could void your warranty. In fact, if this is a relatively new unit with cover interlock switch (which shuts off the unit if the cover is removed, to prevent backdrafting) defeating that switch would be grossly improper and should be reported to the state contractor's board as a blatantly unsafe modification. That is a hick solution that is bad for your system and house, because taking the filtration out of the system means you will be crudding up your heat exchanger and ducts and air conditioner evaporator (if you have yours in the ducts - "central air" system - which it sounds like you have). In my book that should get that company fired right off the bat. Also - doing what he did makes no sense - providing more airflow (unless the exit air temp was excessive due to inadequate airflow to the heat exchanger) would transfer MORE heat (if there was any measureable change at all) to the house in a given time, worsening the short cycling issue rather than helping the situation.

Permanently bypassing the filter by short-circuiting the return duct is the same bad solution - you need the filter in the system to trap the airborne contaminants, otherwise your furnace passages, heat exchanger surface, ducts and house will just get layered with the household dust and fibers - and the dust burning onto the heat exhanger will reduce the unit efficiency and can cause premature exchanger failure as well, as well as eventually reducing airflow - exactly the opposite of what he was trying to accomplish. I guess his thought was to remove airflow resistance and increase the airflow so the air would be cooler on exiting the heat exchanger to make the cycle longer - that is the sort of "fix" you commonly see from a handyman or homeowner, but that is not a professional way to approach a problem.

Changing the heat limit switch to a higher temp (I presume he means the outgoing temperature limit switch) - especially if not in the recommended manufacturer instructions or if it would be beyond rated level - is also a bad idea, and rather than a solution sounds like perhaps an indication that maybe your short cycling is due to an overtemp sensor killing the system rather than the house getting up to temp. You should be able to check that at the thermostat if digital, to see if it is kicking off before the thermostat room temp reading gets up to the programmed temp. If that is the case, then the furnace or blower is malfunctioning, or you might have a blockage in the ducts that is limiting airflow. An airflow pressure test would indicate if that is the problem.

Sounds like you have been dealing with a maintenance tech, not a fully qualified HVAC man. You definitely need a system assessment - both furnace operating temps and pressures as BayAreaAC said to see if unit is operating within design limits, and also have ACCA Manual J, S and D calculations done to see if your system is actually oversized, and the calculastion then an inspection to see if your ducts (or the air splitting between different duct branches, either fixed (based on duct sizing or variable with a dampear) are part of the issue or not.

Any dampers or airflow controllers in the ducting should also be checked - your problem could be due to a secondary damper-controlling thermostat not working right, or a damper not functioning or set incorrectly (if manual). It might even be a simple as a wire to a damper not having been hooked back up with the new furnace install, so one branch of your ducting is not getting enough (or any) warm air.

One other thought in rereading your question - you did mention hot pockets (not the munching type), so if you have both hot and cold pockets it is possible that during each heat cycle the unit is not actually short cycling because it is putting out too much heat overall, but rather because is not heating all the areas it is supposed to so is short cycling because the airflow is not heating the entire house (or zone if zoned). This could be due to "short circuits" (doors or lack thereof letting airflow from one area to another of the house when it should not), improper duct damper sizing or settings for a branch, adjustable registers set wrong in some rooms (on inlet or return air side), clogged up dusty registers, open/leaking attic hatch or door, etc.

One other thing I have seen a few times after a new forced air furnace install is the tech opened up duct dampers all the way to check and adjust airflow to each room at the registers, but did not check that each branch was coming up to temp (or cooling with A/C) at the same time, so the split between each branch which the damper was installed to accomplish was never adjusted at the end of the test - letting one branch (usually the shortest one) cannibalize most of the air.

If you figure out which rooms ran cold or hot in the summer and now, and trace out the ductwork (if visible, or by sound by having someone tap on the ducting in the affected rooms) you may find that the cold summer and hot winter rooms are the same rooms and all fed off one branch of the duct, and the summer hot / winter cold rooms on another, indicating either that not enough airflow is being sent to that second branch; or that the thermostat is representing only the overfed rooms so the underfed ones are not getting to the desired temp before the thermostat kicks off the unit.

A thermostat at the top or bottom of stairs can also cause this if it is getting exposed to air moving up and down the stairs (generally due to lack of a door to block up/downstairs flow). Results in cold air flowing down the stairs in the summer and shutting off downstairs thermostat prematurely, and heated air rising from downstairs and prematurely shutting off the upstairs thermostat in the winter. In this case, the summer and winter rooms that are getting over cold in summer / overheated in summer will generally be the OPPOSITE in the two seasons, not the same ones like where branch splitting is improper. Ditto to basement stairs, especially in summer if the basement is not part of the air conditioned space with its own ducting.

Do you know if the new unit was sized based ACCA Manual calculations - or did they just replace with same or larger size than the existing unit ? It could be they put in the same gross Btu unit as before without assessing the size actually needed, and the combination of a new unit operating at peak efficiency and its likely higher heat transfer efficiency resulted in more heat output than the old unit, making it oversized. It is also not unheard of for the former unit to have had the gas pressure to its burners turned down or a reducing orifice installed to reduce its output, then the new one was bought at the same rated capacity just based on nameplate rating.

I ran into that a couple of years ago - a friend was replacing their furnace and asked my advice about why the contractor was proposing a vastly larger unit than several of their neighbors with much larger houses had. They had a 140,000 Btu forced air furnace in about a 1500SF house ! Easily about double the needed size - then I saw that half the burner orifices had been replaced with a plug, so instead of 6 it was running on only 3 burner tubes. A new 140,000 Btu unit would have broiled them alive and been short-cycling like crazy.

So - as BayAreaAC and other contributors here frequently say - the prime consideration is the quality and expertise and experience of your contractor, because no matter how good the materials are, without professional quality installation you are likely to be dissatisfied with the result.

Answered 4 years ago by LCD

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