# Can I use a thinner furnace filter?



## ilyaz (Jan 22, 2019)

It's been pretty cold lately but even with that in mind, our house is unusually cold, even with the thermostat set to 73-74 degrees. I am wondering if a big reason for this is the type of filter I've been using. A few years ago I started using MERV 13 filters because I suffer from seasonal allergies. Also, it's 16X25X4 so it's pretty thick. I am thinking or trying a much lower MERV rating. But can I also use a thinner 16x25 filter? If so, do I need to have any sort of adapter for it to fit snugly into the space?

Just in case it matters, our HVAC unit is less than 10 years old. It's a TRANE XV80 system, but I don't how to tell if it's powerful enough for our house: a 2-story split-level built around 1960.


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## bud16415 (Jan 23, 2019)

I don’t think if the filter is clean it is having any impact on how well your furnace heats. I wouldn’t use a filter that doesn’t fit properly.


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## pjones (Jan 23, 2019)

The thicker filter actually helps you with airflow. You just need to look for a lower MERV rating. Try MERV 8 and see if that helps. If you can’t find a lower MERV in a 4” frame then you can get creative to prop the filter into position and hold it in place, in order to test your theory. Just put the prop on the air entering side of the filter so if it comes loose then it won’t get sucked into the fan motor. 

Usually low airflow issues in older homes is caused by improper ductwork. Older homes were designed for low airflow and high temperature rise systems. New homes and systems operate opposite to that. Things to check for if it’s a recent change, make sure all floor vents are open fully and none of the return air vents are blocked. Move the couch that is in front of them or the book case that fits perfect in that spot, it will destroy your airflow and hinder your performance. If the room has a supply air vent then it needs a method of return air getting to the return grill. Either the doors need to be undercut about an inch or a return air grill needs to be in each room. Air won’t go where it can’t escape. 
Those are the simple solutions... if they don’t work then you may need to have someone come in and preform a heat load calculation and determine how many BTUH are required to heat your home and how many CFM are required to heat each room. Following that you can discuss if new ductwork will resolve the issue or if you need to increase the size of the unit. 

Does the unit ever shut off and satisfy the thermostat? If it does then you have hot and cold spots that can be resolved by balancing your air distribution. 

You may also want to check to make sure walls and roof are insulated properly. 

Is this system gas heat only or a heat pump?


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## ilyaz (Jan 23, 2019)

pjones said:


> Does the unit ever shut off and satisfy the thermostat? If it does then you have hot and cold spots that can be resolved by balancing your air distribution.
> 
> You may also want to check to make sure walls and roof are insulated properly.
> 
> Is this system gas heat only or a heat pump?



Yes, it turns off and on. The thermostat is in the middle of the house, and temperature is definitely different in different corners of the house.

Not sure about the 2nd question. The furnace is gas. Is this the answer, or can it be a heat pump in addition to that? How can I tell?

I am fairly sure insulation is original from the 60s -- so pretty bad -- in the walls. We did some work a few years ago insulating the attic and it might have helped a bit, not sure if adding more insulation there would make much sense.

Can the same company that would do a local calculation also perform some sort of detailed analysis -- with infrared cameras or other tools -- to see which surfaces of the house lose the most heat? I am hoping for some "low hanging fruit" that could be mitigated rather than having to buy a bigger HVAC system or having to insulate all walls in the house.

Thanks!


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## pjones (Jan 23, 2019)

ilyaz said:


> Yes, it turns off and on. The thermostat is in the middle of the house, and temperature is definitely different in different corners of the house.
> 
> Not sure about the 2nd question. The furnace is gas. Is this the answer, or can it be a heat pump in addition to that? How can I tell?
> 
> ...



If you have a heat pump with a gas package then you would also have A/C. It sounds like it have a gas furnace. 

It sounds like your first step is to balance the system better. Close off the registers that are close to the thermostat, to about 3/4 open, and open all other supply vents fully. What’s probably happening is that the room that the thermostat is located in is heating up quickest and turning off your heating system. You want that room to heat up slower so that the other rooms can get more hot air into them. Every time you make an adjustment let it run for a day or two so that you can get an accurate feel of how the change effected the temperature. Remember you need to heat up the walls, tables, floors, lamps, chairs, etc... those take time to heat up so running the system and using its results after one or two heat cycles isn’t going to be accurate. Remember to keep those return air registers open and free, doors open if they don’t have undercuts to allow air to escape the rooms, etc...  if the change wasn’t effective then try closing those couple vents closest to the thermostat from 3/4 down to 1/2 and repeat above process. 

You don’t want to block off too much air since that can cause the temperature rise through the furnace to be too great. Too high of a temperature rise can damage your heat exchanger. Closing off a couple registers on a properly designed system shouldn’t hurt any but we don’t know if your system was designed proper or if it was ever set up correctly so proceed with caution. The temperature rise of the air allowed for your unit should be marked on the nameplate. You can check what you have vs the nameplate specs to see if you are still ok after finding your balance. 

If your ductwork is exposed then you can look for balancing damper handles on your supply and return and make sure they are opened up. Those are not as often found on the return duct but sometimes they are there and those are the things that can get bumped closed over years of people working around them. 

Try that first before having heat load calculations done. Balancing the system out is going to be tour low hanging fruit that you are looking for. Most everything else will cost money. An HVAC contractor should be able to preform a heat load calculation and help you determine heat loss if you decide to go that route, just let them know what you are looking to achieve before they come out so you can be sure they send the right person with the right tools. They are going to be there a while as they get the measurements and information that they need so you may as well try and help them get that info on the first trip out instead of having them come back with the tools that they didn’t expect they would need.


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## ilyaz (Jan 23, 2019)

pjones said:


> If you have a heat pump with a gas package then you would also have A/C. It sounds like it have a gas furnace.



Hmm... So I have a fairly standard set up: gas furnace, electric AC. So does it mean I also have a pump?

Thanks for all the suggestions. The first thing I want to try is the same size filter but with a lower MERV. Say 7 or 8. Other than this rating and the size, do you have recommendations on what to pick? Like type or brand or model? I am less concerned about capturing more dust (at least, until Spring) and more with heating the house better.


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## pjones (Jan 23, 2019)

ilyaz said:


> Hmm... So I have a fairly standard set up: gas furnace, electric AC. So does it mean I also have a pump?
> 
> Thanks for all the suggestions. The first thing I want to try is the same size filter but with a lower MERV. Say 7 or 8. Other than this rating and the size, do you have recommendations on what to pick? Like type or brand or model? I am less concerned about capturing more dust (at least, until Spring) and more with heating the house better.



Anything low efficiency in a 4” would be good. If you can’t find it then go with a single 2” filter and make a frame to occupy the remaining 2” of unused space. Do not double up the filters. 

I think that might help but I really think you have a balancing issue rather than a filter issue. 

Stay away from those electrostatic foam washable filters, or anything in the pleated 1” size. There is too little surface area on the filter material to pass enough air. 

What is the make and model number of your AC unit? Not all condensers provide the heat pump option. If you are in heating mode and your gas furnace is firing then you aren’t in the heat pump heating mode even if you do have that option so it wouldn’t have much effect. If you are getting heat but the gas isn’t actually operating then you might be using the Heat Pump to provide heat and depending on your outside temperature right now it may be having problems because of its operating conditions. That would require a different solution, not worth getting into right now until we determine if you have a heat pump. 

When you put it in heating mode does the compressor outside operate? If it does then you can feel the copper pipes that connect the compressor to the indoor units coil and see if they are hot. If they are then you are running in heat pump mode. If you put it in heat mode and the gas heat fires up immediately then you are not using the heat pump if you have it.


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## ilyaz (Jan 23, 2019)

I dug up the receipts from the original install in 2010 and, in case it helps, from the additional job we did in 2012 adding more registers. The extra registers was for the living and dining room areas that are colder than most of the rest of the house. The 3 dampers were installed near the furnace and have two positions (summer/winter) that are all in winter now. But they are quite difficult to flip and so I am not 100% sure if they are not stuck somewhere in between and of course i can't check that since they are inside the ducts.

The outside unit does not run in winter, only in summer when the AC is on.

What additional info would I need to determine if this set up is enough to heat my house?


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## pjones (Feb 21, 2019)

ilyaz said:


> I dug up the receipts from the original install in 2010 and, in case it helps, from the additional job we did in 2012 adding more registers. The extra registers was for the living and dining room areas that are colder than most of the rest of the house. The 3 dampers were installed near the furnace and have two positions (summer/winter) that are all in winter now. But they are quite difficult to flip and so I am not 100% sure if they are not stuck somewhere in between and of course i can't check that since they are inside the ducts.
> 
> The outside unit does not run in winter, only in summer when the AC is on.
> 
> ...



You would need to know heat loss and heat load calculations for each room, duct sized supplied to each room and main duct sizes, and return sizes from all locations, 

Have you had any success with replacing the filter and it’s a lower efficiency type?

Have you had to try balancing the supply air grills around they thermostat yet?


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## sadavis80 (Feb 21, 2019)

Try setting your fan to ON if it's in AUTO now. Constant movement of the air should improve your warm/cold area immensely. 
Steve


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